**2. Structure and properties of nanostructured oxifluoride glasseramics**

The story of the oxifluoride glassceramics production technology starts in 1970-es. In [14] was made an attempt to synthesize the oxifluoride glasses, containing Ln2O3 (Y, La, Gd, Lu), Yb2O3, PbF2, MnOm (M = B, P, Te, Si, Ge), activated by Er2O3 or Tm2O3. This attempt has resulted in production of the nontransparent glassceramic materials, containing the microcrystals with diameter of about 10 m. The efficiency of luminescence, revealed by these media, was in several times larger than that of the etalon luminophors LaF3 : Yb : Er. Later on, in 1993, there was published the first paper, devoted to synthesis of the transparent glassceramics, containing the cubic fluoride phase, activated by erbium and ytterbium ions [15]. There were for the first time produced the materials, which combine all the advantages of the glass-like alumosilicate matrix and the optical features of the lowphonon fluoride crystals.

Since recently the transparent fluorine-containing glassceramics matrices, containing the rare-earth ions, included into the fluorite-like nanocrystalline phases, are drawing the

unique properties. For example, nano-size of crystalline phase results in quantum-size, resonance, and other effects. Such materials can possess unique properties, that can't be realized in traditional materials. Following new materials can be synthesized on the base of

Currently, optical transparent glassceramics are of the great interest for the modern element base of photonics, because they stay at intermediate state between crystalline materials and glasses. These glassceramics combine the best properties of crystals (high emission crosssection, quantum yield of luminescence, mechanical and thermal strength etc.) and glasses (possibilities of pressing and molding, spattering, pulling optical fibers, and carrying out ion

In this paper a new type of glassceramics, namely the nanostructured lead oxifluoride glasseramics, which was developed by the authors have been discussed. In addition some examples of using the novel nanoglassceramics for photonics applications have been

a. b. c.

Fig. 1. Transition from millimeter-size crystals (a) to a micrometer-size crystals (b) and to

The story of the oxifluoride glassceramics production technology starts in 1970-es. In [14] was made an attempt to synthesize the oxifluoride glasses, containing Ln2O3 (Y, La, Gd, Lu), Yb2O3, PbF2, MnOm (M = B, P, Te, Si, Ge), activated by Er2O3 or Tm2O3. This attempt has resulted in production of the nontransparent glassceramic materials, containing the microcrystals with diameter of about 10 m. The efficiency of luminescence, revealed by these media, was in several times larger than that of the etalon luminophors LaF3 : Yb : Er. Later on, in 1993, there was published the first paper, devoted to synthesis of the transparent glassceramics, containing the cubic fluoride phase, activated by erbium and ytterbium ions [15]. There were for the first time produced the materials, which combine all the advantages of the glass-like alumosilicate matrix and the optical features of the low-

Since recently the transparent fluorine-containing glassceramics matrices, containing the rare-earth ions, included into the fluorite-like nanocrystalline phases, are drawing the

**2. Structure and properties of nanostructured oxifluoride glasseramics** 

nanoglassceramics: plasmonic materials, photonic crystals, metha-materials.

exchange to fabricate waveguide structures).

nano-size crystals (c) in the glass host.

phonon fluoride crystals.

outlined.

attention do to the series of spectroscopy advantages. It is obvious that from the point of view of laser active media development optimal are the materials, which are characterized by the low-frequency phonon spectrum and by the low content of the OH-groups, because in this case one can reduce the excitation energy losses due to the multi-phonon quenching process. For a long time there was a common opinion that only the fluorine-containing materials (like fluoride glasses and crystals) are optimal for the said problem solution. However, since recently the synthesis of the glassceramics materials like the oxifluoride silicate glasses has become the priority direction of studies [16-24]. Such composite materials combine the optical parameters of the low-phonon fluoride crystals and the good mechanical and chemical features of the silicate glasses. It was also revealed that some of the oxifluoride glass-like materials have a feature of forming the fluoride nanocrystals, doped by the rare-earth ions, during the process of heat treatment of the raw primary glass. Hence such a materials combine all the positive features of the fluoride nanocrystals, which control the optical properties of the rare earth ions, with that of oxide glasses like easy production technology and excellent macroscopy features like chemical and mechanical strength and the high optical quality. It is well known that in the case of production of the optically transparent glassceramics, including those, which are used for optical waveguide fabrication, it is very important to minimize the light losses for absorption and scattering. The Rayleigh scattering by the micro-inhomogeneities with the size about the radiation wavelength is a factor, limiting such materials use. It imposes the strict limitations over the size of the separated crystalline phase. According to the Rayleigh theory for the visible spectral range, the radius of the crystals, dispersed around the glass, has to be not more than 15 nm. The refraction indexes of the crystalline phase and of the amorphous matrix are to differ in not more than 0.1. Later on these limitations were somewhat softened. In [16] on the case of a model it was shown that one can produce the transparent glassceramics with the size of nanocrystals up to 30 nm and with the refraction indexes difference in not more than 0.3.

#### **2.1 The glassy matrix for glassceramics production**

The separation of the crystalline phase in a silicate glass is a traditional way of the opalescent glass production. It is well known that insertion of the fluorides into the glass, whose content is similar to the window glass, leads to forming of a large number of microcrystals in the glass volume. It leads to a drastic increase of a light scattering and to the so called opalescence effect. Hence, the fluorine content in a glass leads to intense phase separation; very often the second phase is represented by the introduced fluorides or by their derivatives. This fact has became the basis for the numerous studies, devoted to fabrication and investigation of the silicate nanoglassceramics, based on the fluorite-like nanocrystals like Ba(Sr,Ca,)F2 or of the hexagonal LaF3, activated by small concentration or rare-earth or of transient elements.

The separate group is represented by the glassceramics, fabricated on the basis of the glasslike systems with a big amount of fluorides. One can fill into this type the alkali-less germanate and silicate systems like GeO2-PbO-PbF2, SiO2-PbO-10PbF2 SiO2 -Al2О3- CdF2 - PbF2- ZnF2:(YF3). In such systems the fluoride concentration can be as high as 60-70 mol.%. The process of crystalline phase formation in this case is not so obvious and needs a more detailed study.

New Nanoglassceramics Doped with Rare Earth Ions and Their Photonic Applications 109

a.)

Fig. 2. Thermal curves of the starting glass (a) and of the glass, which was thermally

b.)

the mixed fluoride crystals with partial replacement of Pb2+ on Cd2+.

Thus the precipitation of the fluorite-like crystalline phase of β-PbF2 doped with ErF3 has been earlier observed in various glass hosts. However the sizes of the elementary cell of the crystalline phase separated in glasses were differ from ones of the solid solution models. In all cases the sizes of elementary cell of the crystalline phase in glass host were much smaller than that observed in the solid solution models or in the crystals of β- PbF2 doped with ErF3. In [20, 21] there was revealed the dependence of the cell constant of the nanocrystals (а =5,72 - 5,81 Ǻ) in the glass of system of 30SiO2 -5ZnF2-(29-х)CdF2-(18+х)PbF2-7,5Al2О3-3 ErF3 from the ratio of lead and cadmium fluorides concentration. There was made an assumption about formation of

In the present work a complex analysis of the precipitated crystalline phase has been carried out. The precipitation of the crystalline phase during heat treatment of glasses with the content of 30SiO2-29CdF2-18PbF2-5ZnF2-7,5Al2О3, containing the fluorides of yttrium and of the lanthanum group elements, including the case of simultaneous addition of yttrium and

processed for 2 hours (b).

## **Production of the starting glass and fabrication of the glassceramics on its basis**

The glasses of the system of 0.3SiO2-0.15AlO3/2-0.29CdF2-0.18PbF2-0.05ZnF2-0.03 (Ln,Y) F3 (the content of the glasses is given in molecular percents before synthesis), where Ln=La, Pr, Dy, Nd, Tb, Eu, Er, Sm, Tm, Но, have been synthesized. The synthesis was carried out in platinum or corundum crucibles in an air or argon atmosphere, making it possible to produce glasses with a high transparency in a visible spectral range. With the purpose to prevent the spontaneous crystallization, the glass was produced in a gap between two cool glass-carbon plates. Hence the thickness of the produced glass was not more than 2 mm.

Heat treatment with the purpose of evaluation of the crystallization temperature and of the glassceramics production was carried out at the temperature of the crystallization start. It was determined from the thermal curve, measured by means of the differential scanning calorimeter (DSC), see Fig.1. This curve has two separate crystallization peaks, which is the obligatory condition for the transparent glassceramic production. The observed exo-peaks are produced by the bulk (522oC) and by the surface (607oC) crystallizations. The choice of the heat treatment temperature at the start of the first peak makes it possible to prevent completely the surface crystallization, which can lead to the uncontrolled growth of the large crystals. Basing on the DSC data (Fig.2), the specimens of glasses with all contents were exposed to temperature of 480-500oC for 0.5-10 hours.

In the Fig.2 are shown the thermal curves for the starting specimen (a) and for the glassceramics (b). The elimination of the first exo-peak, which is observed, is an evidence of the complete separation of the volume crystalline phase after heat treatment of the glass during two hours.

Hence on the base of DSC data we have determined the regimes of heat treatment of the starting glass, necessary for the nanocrystalline phase production: temperature 500oC, duration 2 hours.

#### **2.2 The content of the crystalline phase and its growth kinetics**

One has to note that, disregarding a large number of papers, considering the studies of the lead fluoride glassceramics, the chemical content of the nanocrystalline phases is still discussed. In [17] on the base of spectroscopy and X-ray phase data was concluded that the erbium-containing crystalline phase is a solid solution of erbium fluoride in β-PbF2. In [18] there was investigated the separation of the crystalline phase during heat treatment of the glasses 50GeO2-40PbO-10PbF2. Their heat treatment has resulted in separation of the fluorite-like phase. Change of the processing temperature from 350 to 395oC has resulted in crystals size growth from 11 to 16 nm. The size of the elementary cell was within the range of 5.82-5.83 Å, i.e. it was preserved constant within the experimental error limits of ±0.005 Å. It was supposed that the separated crystalline phase is comprised by β-PbF2, doped by ions of Er3+. In a search for confirmation there were synthesized the monocrystals of β-PbF2 with various content of ErF3. For the admixture of 20% of ErF3 the size of the elementary cell was reduced from 5.94 to 5.816 Å. In the oxyfluoride crystals [19] the heat treatment of the glass with the content 32(SiO2)-9(AlО1.5)-31.5(CdF2)- 18.5(PbF2)-5.5(ZnF2):3.5(ErF3) has resulted in separation of the crystalline phase with the fluorite structure and cell constant а= 5.72 Å.

The glasses of the system of 0.3SiO2-0.15AlO3/2-0.29CdF2-0.18PbF2-0.05ZnF2-0.03 (Ln,Y) F3 (the content of the glasses is given in molecular percents before synthesis), where Ln=La, Pr, Dy, Nd, Tb, Eu, Er, Sm, Tm, Но, have been synthesized. The synthesis was carried out in platinum or corundum crucibles in an air or argon atmosphere, making it possible to produce glasses with a high transparency in a visible spectral range. With the purpose to prevent the spontaneous crystallization, the glass was produced in a gap between two cool glass-carbon plates. Hence the thickness of the produced glass was not

Heat treatment with the purpose of evaluation of the crystallization temperature and of the glassceramics production was carried out at the temperature of the crystallization start. It was determined from the thermal curve, measured by means of the differential scanning calorimeter (DSC), see Fig.1. This curve has two separate crystallization peaks, which is the obligatory condition for the transparent glassceramic production. The observed exo-peaks are produced by the bulk (522oC) and by the surface (607oC) crystallizations. The choice of the heat treatment temperature at the start of the first peak makes it possible to prevent completely the surface crystallization, which can lead to the uncontrolled growth of the large crystals. Basing on the DSC data (Fig.2), the specimens of glasses with all contents

In the Fig.2 are shown the thermal curves for the starting specimen (a) and for the glassceramics (b). The elimination of the first exo-peak, which is observed, is an evidence of the complete separation of the volume crystalline phase after heat treatment of the glass

Hence on the base of DSC data we have determined the regimes of heat treatment of the starting glass, necessary for the nanocrystalline phase production: temperature 500oC,

One has to note that, disregarding a large number of papers, considering the studies of the lead fluoride glassceramics, the chemical content of the nanocrystalline phases is still discussed. In [17] on the base of spectroscopy and X-ray phase data was concluded that the erbium-containing crystalline phase is a solid solution of erbium fluoride in β-PbF2. In [18] there was investigated the separation of the crystalline phase during heat treatment of the glasses 50GeO2-40PbO-10PbF2. Their heat treatment has resulted in separation of the fluorite-like phase. Change of the processing temperature from 350 to 395oC has resulted in crystals size growth from 11 to 16 nm. The size of the elementary cell was within the range of 5.82-5.83 Å, i.e. it was preserved constant within the experimental error limits of ±0.005 Å. It was supposed that the separated crystalline phase is comprised by β-PbF2, doped by ions of Er3+. In a search for confirmation there were synthesized the monocrystals of β-PbF2 with various content of ErF3. For the admixture of 20% of ErF3 the size of the elementary cell was reduced from 5.94 to 5.816 Å. In the oxyfluoride crystals [19] the heat treatment of the glass with the content 32(SiO2)-9(AlО1.5)-31.5(CdF2)- 18.5(PbF2)-5.5(ZnF2):3.5(ErF3) has resulted in separation of the crystalline phase with the

were exposed to temperature of 480-500oC for 0.5-10 hours.

**2.2 The content of the crystalline phase and its growth kinetics** 

fluorite structure and cell constant а= 5.72 Å.

**Production of the starting glass and fabrication of the glassceramics on its basis** 

more than 2 mm.

during two hours.

duration 2 hours.

Fig. 2. Thermal curves of the starting glass (a) and of the glass, which was thermally processed for 2 hours (b).

Thus the precipitation of the fluorite-like crystalline phase of β-PbF2 doped with ErF3 has been earlier observed in various glass hosts. However the sizes of the elementary cell of the crystalline phase separated in glasses were differ from ones of the solid solution models. In all cases the sizes of elementary cell of the crystalline phase in glass host were much smaller than that observed in the solid solution models or in the crystals of β- PbF2 doped with ErF3. In [20, 21] there was revealed the dependence of the cell constant of the nanocrystals (а =5,72 - 5,81 Ǻ) in the glass of system of 30SiO2 -5ZnF2-(29-х)CdF2-(18+х)PbF2-7,5Al2О3-3 ErF3 from the ratio of lead and cadmium fluorides concentration. There was made an assumption about formation of the mixed fluoride crystals with partial replacement of Pb2+ on Cd2+.

In the present work a complex analysis of the precipitated crystalline phase has been carried out. The precipitation of the crystalline phase during heat treatment of glasses with the content of 30SiO2-29CdF2-18PbF2-5ZnF2-7,5Al2О3, containing the fluorides of yttrium and of the lanthanum group elements, including the case of simultaneous addition of yttrium and

New Nanoglassceramics Doped with Rare Earth Ions and Their Photonic Applications 111

took place; it is also indirectly confirmed by the spectroscopy data. In the Fig.5 is shown the

**1 3**

**b.)**

**500 750 1000 1250 1500 1750**

**Wavelength, nm**

**0 2 4 6 8 10**

Fig. 5. Dependence of the crystal size on the heat treatment time at T= 480°C. ErF3

**Secondary heat-treatment time , h**

Fig. 4. Attenuation spectra of the (*1*) initial glass containing 3 mol % PrF3 and (*2*, *3*) nanoglassceramics obtained after heat treatment at *T* = 475°C for 4 (*2*) and (*3*) 32 h and (b) a fragment of the absorption spectra for the *3H4→ 1G4* transition (normalized spectra).

**1300 1400 1500 1600 1700**

**Wavelength, nm**

**2**

kinetics of the crystals growth during heat treatment.

**3**

**a.)**

**0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0**

**1**

**Absorption coefficient, cm-1**

**2**

**0**

**0**

**100**

**200**

**Cristall size, A**

concentration is 1.5 mol %.

**300**

**400**

**20 40**

**60**

**80**

**100 120**

**Absorption coefficient, cm-1**

**140 160 180**

of lanthanum group elements has been studied. The phase content and the cell constants have been determined with the use of XRD analysis. It was found out that thermal procession of the oxyfluoride matrix glass, lacking yttrium or lanthanum group elements, which was carried out for 2-3 hours under the temperature of 480-520oC, did not lead to crystalline phase separation. Only the glasses, containing the yttrium fluoride or lanthanum group fluoride (or their arbitrary concentration) has revealed separation of the crystalline fluorite-like phase (Fig.3).

Fig. 3. X-ray diffraction curves for the angle range 2θ from 20 to 35o, 1 – thermally processed glass without YF3 и lanthanum group fluorides, 2 – thermally processed glass, containing 3% of YF3.

The presence of REI (rare-earth ions) in the crystalline phase is also confirmed by the absorption spectrum data. It is obvious, that redistribution of REI between the glass-like and crystalline phases has to result in variation of REI absorption and luminescence spectra. In the Fig.4 are shown as an example the absorption spectra of the starting glass, activated by erbium, and after its thermal processing.

The size of the crystals was evaluated by means of X-ray scattering to small angles. It was determined from the diffraction reflection width with the accuracy ±5%. After heat treatment of the glass at T= 480oС at 2-3 hours the size of the crystals has reached 250-300 Å. In such a regime the phase volume was close to the equilibrium one, i.e. it was not increasing in the case or increasing the heating duration or temperature. The further heat treatment has resulted in crystal size increase, while the integral intensity of diffraction reflection was practically constant. I.e., in this case mainly the re-crystallization processes

of lanthanum group elements has been studied. The phase content and the cell constants have been determined with the use of XRD analysis. It was found out that thermal procession of the oxyfluoride matrix glass, lacking yttrium or lanthanum group elements, which was carried out for 2-3 hours under the temperature of 480-520oC, did not lead to crystalline phase separation. Only the glasses, containing the yttrium fluoride or lanthanum group fluoride (or their arbitrary concentration) has revealed separation of the crystalline

Fig. 3. X-ray diffraction curves for the angle range 2θ from 20 to 35o, 1 – thermally processed glass without YF3 и lanthanum group fluorides, 2 – thermally processed glass, containing

The presence of REI (rare-earth ions) in the crystalline phase is also confirmed by the absorption spectrum data. It is obvious, that redistribution of REI between the glass-like and crystalline phases has to result in variation of REI absorption and luminescence spectra. In the Fig.4 are shown as an example the absorption spectra of the starting glass, activated by

The size of the crystals was evaluated by means of X-ray scattering to small angles. It was determined from the diffraction reflection width with the accuracy ±5%. After heat treatment of the glass at T= 480oС at 2-3 hours the size of the crystals has reached 250-300 Å. In such a regime the phase volume was close to the equilibrium one, i.e. it was not increasing in the case or increasing the heating duration or temperature. The further heat treatment has resulted in crystal size increase, while the integral intensity of diffraction reflection was practically constant. I.e., in this case mainly the re-crystallization processes

fluorite-like phase (Fig.3).

3% of YF3.

erbium, and after its thermal processing.

took place; it is also indirectly confirmed by the spectroscopy data. In the Fig.5 is shown the kinetics of the crystals growth during heat treatment.

Fig. 4. Attenuation spectra of the (*1*) initial glass containing 3 mol % PrF3 and (*2*, *3*) nanoglassceramics obtained after heat treatment at *T* = 475°C for 4 (*2*) and (*3*) 32 h and (b) a fragment of the absorption spectra for the *3H4→ 1G4* transition (normalized spectra).

Fig. 5. Dependence of the crystal size on the heat treatment time at T= 480°C. ErF3 concentration is 1.5 mol %.

New Nanoglassceramics Doped with Rare Earth Ions and Their Photonic Applications 113



The conclusions of the Section 2.2 are confirmed by the luminescence and spectral studies. If the rare earth ion environment has changed - for instance, when it has moved to the crystalline phase - the energy gaps between its levels and manifolds change due to changing of the electric and magnetic field tension around the ion. These changes, obviously, are to reveal themselves in the shape and position of the absorption and luminescence spectra, and

In the novel lead oxyfluoride nanoglassceramics the rare earth ion shifts from the glass-like phase to a fluoride crystal one, i.e. its environment changes drastically. Let us consider these

In the Fig.6 is shown the modification of the europium ions luminescence after the isothermal heat treatment (480oC) in its dependence upon the time of thermal treatment. The europium ion is traditionally used as the sounding ion for interpretation of its environment modification both in crystals [21] and in glasses [22], because this ion transitions reveal intense and well theoretically explained dependence upon the ligands fields. The analysis was carried out on the basis of modification of three bands in the three-valence europium spectrum, corresponding to the transitions from manifold *5D0* to *7F0* (extremely sensitive),

The starting glass is characterized by the spectrum, where the most intense line corresponds

cm-1. This can be an evidence of improvement of environment symmetry of rare earth ions and of general transfer of europium to a crystalline phase [22]. Treatment for 90 minutes results in yet larger modification of the europium ion luminescence spectrum and reveals the complete transfer of REI to the nanocrystal. The analysis of the "extremely sensitive" transition line confirms the assumptions, which were done earlier. The frequency shift in the

*7F1*. It is a triplet line, making thus the basement for the conclusion that Eu3+ is positioned in the low symmetry environment, usual for the fluoride glass-like media. Heat

*7F2*. The second intensity is revealed by the transition

*7F1* (590 nm). Heat treatment for 60 minutes leads to drastic

*7F1* is replaced by the doublet with the maxima at 16875 and 17150

*7F2* (620 nm)

**2.3 The luminescence and spectral properties of the lead oxyfluoride** 

changes on the example of the europium, erbium and neodymium ions.

treatment for 30 minutes results in some decrease of intensity of transition *5D0*

consequent heat treatment or its temperature increase;

elements of lanthanum group;

are also to modify the radiation probabilities.

*7F1* (magnetic dipole) and *7F2* (electric dipole).

to the degenerated transition *5D0*

in comparison with *5D0*

changes. The triplet *5D0*

*5D0*

**nanoglassceramics** 

fraction of the crystalline phase approaches equilibrium, i.e. it does not change after

There was observed the definite correlation between the concentration of the introduced fluorides of the lanthanum group elements and the integral intensity of the fluorite-like phase diffraction reflection. Hence one can conclude that the presence of fluorides of rareearth elements and of yttrium totally determines the crystalline phase separation, and their concentration in the starting glass completely determines the volume of the extracted phase. One can treat it as an evidence of one and the same stoichiometry of the separated crystalline phases.

All the produced nanocrystalline phases have the fluorite-like structure – cubic, edgecentered, spatial group Fm3m. The lattice constants depend upon the REI radius (Table 1). The size of the elementary cell of the phase, produced by heat treatment of the glasses with YF3, was equal to 5.74 Å. It differs significantly from the size of the elementary cell of the yttrium oxyfluoride of lead PbYOF3, which is equal to 5.792 Å. The reduction of the size of the elementary cell of the phase separated in the studied glasses with YF3 can be explained by presence of Cd. Hence the significant variation of the elementary cell size is connected with the formation of the solid solution


#### Pb1-хCdхYOF3.

Table 1. Size of the elementary cell of the fluorite-like crystalline phases, separated in the investigated glasses, containing the fluorides of various lanthanum group elements and the ion radiuses of such elements and yttrium, R, according to Goldschmidt.

One the base of the investigations, which were carried out, one can make some conclusions and outline some recommendations for the synthesis of the lead oxifluoride glassceramics with the required spectral and luminescence properties:


There was observed the definite correlation between the concentration of the introduced fluorides of the lanthanum group elements and the integral intensity of the fluorite-like phase diffraction reflection. Hence one can conclude that the presence of fluorides of rareearth elements and of yttrium totally determines the crystalline phase separation, and their concentration in the starting glass completely determines the volume of the extracted phase. One can treat it as an evidence of one and the same stoichiometry of the separated

All the produced nanocrystalline phases have the fluorite-like structure – cubic, edgecentered, spatial group Fm3m. The lattice constants depend upon the REI radius (Table 1). The size of the elementary cell of the phase, produced by heat treatment of the glasses with YF3, was equal to 5.74 Å. It differs significantly from the size of the elementary cell of the yttrium oxyfluoride of lead PbYOF3, which is equal to 5.792 Å. The reduction of the size of the elementary cell of the phase separated in the studied glasses with YF3 can be explained by presence of Cd. Hence the significant variation of the elementary cell size is connected with the

Pb1-хCdхYOF3.

group element R, Å а, Å

1 Pr3+ 1 5.83 2 Nd 3+ 0.99 5.82 3 Sm 3+ 0.97 5.81 4 Eu 3+ 0.96 5.8 5 Tb3+ 0.89 5.765 6 Dy 3+ 0.88 5.75 7 Ho 3+ 0.86 5.735 8 Er 3+ 0.85 5.725 9 Tm3+ 0.84 5.715 10 Yb 3+ 0.81 5.7 11 Y3+ 0.97 5.74 Table 1. Size of the elementary cell of the fluorite-like crystalline phases, separated in the investigated glasses, containing the fluorides of various lanthanum group elements and the

One the base of the investigations, which were carried out, one can make some conclusions and outline some recommendations for the synthesis of the lead oxifluoride glassceramics



Lanthanum

ion radiuses of such elements and yttrium, R, according to Goldschmidt.

with the required spectral and luminescence properties:

starting glass;

crystalline phases.

formation of the solid solution

fraction of the crystalline phase approaches equilibrium, i.e. it does not change after consequent heat treatment or its temperature increase;


#### **2.3 The luminescence and spectral properties of the lead oxyfluoride nanoglassceramics**

The conclusions of the Section 2.2 are confirmed by the luminescence and spectral studies. If the rare earth ion environment has changed - for instance, when it has moved to the crystalline phase - the energy gaps between its levels and manifolds change due to changing of the electric and magnetic field tension around the ion. These changes, obviously, are to reveal themselves in the shape and position of the absorption and luminescence spectra, and are also to modify the radiation probabilities.

In the novel lead oxyfluoride nanoglassceramics the rare earth ion shifts from the glass-like phase to a fluoride crystal one, i.e. its environment changes drastically. Let us consider these changes on the example of the europium, erbium and neodymium ions.

In the Fig.6 is shown the modification of the europium ions luminescence after the isothermal heat treatment (480oC) in its dependence upon the time of thermal treatment. The europium ion is traditionally used as the sounding ion for interpretation of its environment modification both in crystals [21] and in glasses [22], because this ion transitions reveal intense and well theoretically explained dependence upon the ligands fields. The analysis was carried out on the basis of modification of three bands in the three-valence europium spectrum, corresponding to the transitions from manifold *5D0* to *7F0* (extremely sensitive), *7F1* (magnetic dipole) and *7F2* (electric dipole).

The starting glass is characterized by the spectrum, where the most intense line corresponds to the degenerated transition *5D07F2*. The second intensity is revealed by the transition *5D07F1*. It is a triplet line, making thus the basement for the conclusion that Eu3+ is positioned in the low symmetry environment, usual for the fluoride glass-like media. Heat treatment for 30 minutes results in some decrease of intensity of transition *5D07F2* (620 nm) in comparison with *5D07F1* (590 nm). Heat treatment for 60 minutes leads to drastic changes. The triplet *5D07F1* is replaced by the doublet with the maxima at 16875 and 17150 cm-1. This can be an evidence of improvement of environment symmetry of rare earth ions and of general transfer of europium to a crystalline phase [22]. Treatment for 90 minutes results in yet larger modification of the europium ion luminescence spectrum and reveals the complete transfer of REI to the nanocrystal. The analysis of the "extremely sensitive" transition line confirms the assumptions, which were done earlier. The frequency shift in the

New Nanoglassceramics Doped with Rare Earth Ions and Their Photonic Applications 115

**2 1**

**1450 1500 1550 1600 1650**

**2**

**Wavelength, nm**

**77К b.)**

**1450 1500 1550 1600 1650**

**Wavelength, nm**

glass, containing 0.4 mol.% of ErF3 (curve 1) and of nanoglassceramics, obtained after heat

One can see that after the heat treatment the relative intensities of luminescence transitions have changed. For instance, the intensities of transitions with the maxima at 478, 530 and 540 nm have reduced in comparison with 660 nm, while a new transitions in an UV band (365 nm) have arose. In addition, the intensity of the transition in a blue range (405 nm) has grown up. The arise of additional bands became possible due to increase of probability of

The heat treatment results in a significant modification of the spectral and luminescence features of the rare earth ions. The spectral and luminescence studies provide the independent confirmation of the X-ray phase analysis conclusion that the rare earth ions

**300К**

*4I13/2* in the

**a.)**

**0**

**0,0**

**0,2**

**0,4**

**0,6**

**Intensity, a.u.**

radiative transitions from these levels.

shift to the crystalline phase.

treatment (curve 2).

**0,8**

**1**

Fig. 7. Absorption (a) and luminescence (b) spectra for the transition *4I15/2*

**1,0**

**2**

**4**

**6**

**absorption coefficient, cm-1**

**8**

**10**

process of the glassceramics formation is practically absent, confirming thus the assumption that the europium ion is in a completely fluoride environment. The half width of the transition *5D07F0* is a measure of the rate of inhomogeneity of environment across the ensemble of the activating ions. Hence its reduce in nearly 2 times in comparison with the starting value after treatment for 90 minutes is an evidence of a nearly complete transfer of europium from glass to a crystalline phase. The modification of the ratio of the characteristic bands is also an evidence of a radical re-building of the europium ion environment, accompanied by its symmetry improvement.

Fig. 6. Luminescence spectra of the glass, activated by 3 mol.% of EuF3 before and after isothermal heat treatment (Т=480 оС)

Similar modifications are observed in the absorption (Fig.7,a) and luminescence (Fig.7,b) of erbium ions. One can see that the heat treatment results in a strong deformation of the erbium ion absorption and luminescence contours. Transition from glass to nanoglassceramics definitely reveals the Stark structure of erbium, which is usual for the crystalline media. Such a behavior of the absorption and luminescence contours is an evidence of modification of the activation ion environment during transfer from glass phase to a crystalline one.

In addition the rare earth ion transfer to a crystalline phase results in modification of the radiation probabilities from different levels. It reveals itself in intensity growth of some bands in the luminescence spectrum and in reduces of some other bands. Let us consider this effect on the example of the glassceramics, activated by erbium (Fig.8).

process of the glassceramics formation is practically absent, confirming thus the assumption that the europium ion is in a completely fluoride environment. The half width of the

ensemble of the activating ions. Hence its reduce in nearly 2 times in comparison with the starting value after treatment for 90 minutes is an evidence of a nearly complete transfer of europium from glass to a crystalline phase. The modification of the ratio of the characteristic bands is also an evidence of a radical re-building of the europium ion environment,

620 600

5 <sup>D</sup> <sup>0</sup> - <sup>7</sup> F2

W avelength, nm

5 D0 - <sup>7</sup> F1

**15800 16000 16200 16400 16600 16800 17000 17200 17400**

cm-1

Fig. 6. Luminescence spectra of the glass, activated by 3 mol.% of EuF3 before and after

Similar modifications are observed in the absorption (Fig.7,a) and luminescence (Fig.7,b) of erbium ions. One can see that the heat treatment results in a strong deformation of the erbium ion absorption and luminescence contours. Transition from glass to nanoglassceramics definitely reveals the Stark structure of erbium, which is usual for the crystalline media. Such a behavior of the absorption and luminescence contours is an evidence of modification of the activation ion environment during transfer from glass phase

In addition the rare earth ion transfer to a crystalline phase results in modification of the radiation probabilities from different levels. It reveals itself in intensity growth of some bands in the luminescence spectrum and in reduces of some other bands. Let us consider

this effect on the example of the glassceramics, activated by erbium (Fig.8).

0min 30min 60min t=90min

*7F0* is a measure of the rate of inhomogeneity of environment across the

transition *5D0*

accompanied by its symmetry improvement.

**0**

isothermal heat treatment (Т=480 оС)

to a crystalline one.

**2**

**4**

**6**

**8**

Intensity, a.u.

**10**

**12**

**14**

**16**

Fig. 7. Absorption (a) and luminescence (b) spectra for the transition *4I15/2 4I13/2* in the glass, containing 0.4 mol.% of ErF3 (curve 1) and of nanoglassceramics, obtained after heat treatment (curve 2).

One can see that after the heat treatment the relative intensities of luminescence transitions have changed. For instance, the intensities of transitions with the maxima at 478, 530 and 540 nm have reduced in comparison with 660 nm, while a new transitions in an UV band (365 nm) have arose. In addition, the intensity of the transition in a blue range (405 nm) has grown up. The arise of additional bands became possible due to increase of probability of radiative transitions from these levels.

The heat treatment results in a significant modification of the spectral and luminescence features of the rare earth ions. The spectral and luminescence studies provide the independent confirmation of the X-ray phase analysis conclusion that the rare earth ions shift to the crystalline phase.

New Nanoglassceramics Doped with Rare Earth Ions and Their Photonic Applications 117

Erbium ion activated glasses are widely used for creation of the mini-microchip and fiber lasers, emitting at 1.5 and 3 m. The lasers and amplifiers at 1.5 m are used for data transfer via fiber-optical communication lines and also in range finders, because their radiation fills into the so-called third window of the fused silica transparency and to the eyesafe spectral range. In the telecommunication tasks a very important feature of the fiberoptical amplifying medium is the width of its amplification spectrum, because the wider amplification range makes it possible to fill into and to provide simultaneous amplification of the big number of spectral channels. Unfortunately, disregarding a very good technology (from the point of view of fiber production), optical and luminescence spectral features of the erbium doped silicate and phosphate glasses, the halfwidth of their amplification spectrum is not large. It is equal to =20-30 nm, thus limiting significantly the number of spectral channels in the amplifier. The fluoride glasses ZBLAN, doped by erbium, have a wide amplification spectrum of =50-80 nm. However, they meet some problems and limitations – first of all from the point of view of the fluoride fiber production and exploitation. One can overcome the said disadvantages and limitations in a new leadfluoride nanoglassceramics, where erbium is in the crystalline fluoride phase and the glass

For instance, Fig.9 illustrates the spectra of erbium ions luminescence in a novel lead-

1450 1500 1550 1600 1650

 **Silicate glass Lead-fliuoride** 

**glassceramics**

**Wavelength, nm**

Fig. 9. Luminescence spectra of erbium ions in silicate glass and lead-oxifluoride

fluoride nanoglassceramics and in the commercially available silicate glass.

matrix contains oxygen.

0,0

0,2

0,4

0,6

**Intensity, a.u.**

nanoglassceramics.

0,8

1,0

Fig. 8. Luminescence spectra of the (*1*) initial glass containing 0,2 mol % ErF3 and (*2*) nanoglassceramics samples obtained over 10 hours at heat treatment. The spectra are normalized at peak of 660 nm

#### **3. Application of new lead-oxifluoride nanoglassceramics for lasers and amplifiers**

Various laser media are used for various types and applications of lasers and optical amplifiers. During the design the luminescence and spectral properties of the medium are chosen so as to provide the best fit to the solved problem. One has also to mention that the use of some rare earth ions imposes its own limitations over the medium use. For instance, the praseodymium ion practically does not luminescence in the oxygen glasses, and that is why the oxide-less fluoride glasses like ZBLAN are used in a praseodymium amplifiers. One has also to mention that the important feature of laser medium is its applicability for design of either super compact mini- or micro-chip lasers or amplifiers, when one has to increase the activator concentration, or of the extended fiber-optical lasers or amplifiers with the length of several meters. Hence the definite requirements are imposed onto the laser material. For instance, the material (glass, glassceramics etc.) has to be possible to accept a high concentration of the activator, while the factors, leading to the luminescence quenching (the presence of OH-groups or the nonlinear up-conversion) are to be minimized. In other case of fiber-optical lasers and amplifiers the material is to be possible to be used as the preform for the activated fiber production. The lead fluoride nanoglassceramics, activated by the rare earth ions, which was developed, can be used as an active medium for the miniature lasers of for the fiber-optical lasers and amplifiers. Let us discuss the laser features of the lead-fluoride nanoglassceramics on an erbium ion example and let us compare it with the other well known glass analogs.

**Glass (1)**

**1** 

 **Glassceramics (2)** <sup>х</sup><sup>5</sup>

**350 400 450 500 550 650 700**

Wavelength, nm

Fig. 8. Luminescence spectra of the (*1*) initial glass containing 0,2 mol % ErF3 and (*2*) nanoglassceramics samples obtained over 10 hours at heat treatment. The spectra are

**3. Application of new lead-oxifluoride nanoglassceramics for lasers and** 

Various laser media are used for various types and applications of lasers and optical amplifiers. During the design the luminescence and spectral properties of the medium are chosen so as to provide the best fit to the solved problem. One has also to mention that the use of some rare earth ions imposes its own limitations over the medium use. For instance, the praseodymium ion practically does not luminescence in the oxygen glasses, and that is why the oxide-less fluoride glasses like ZBLAN are used in a praseodymium amplifiers. One has also to mention that the important feature of laser medium is its applicability for design of either super compact mini- or micro-chip lasers or amplifiers, when one has to increase the activator concentration, or of the extended fiber-optical lasers or amplifiers with the length of several meters. Hence the definite requirements are imposed onto the laser material. For instance, the material (glass, glassceramics etc.) has to be possible to accept a high concentration of the activator, while the factors, leading to the luminescence quenching (the presence of OH-groups or the nonlinear up-conversion) are to be minimized. In other case of fiber-optical lasers and amplifiers the material is to be possible to be used as the preform for the activated fiber production. The lead fluoride nanoglassceramics, activated by the rare earth ions, which was developed, can be used as an active medium for the miniature lasers of for the fiber-optical lasers and amplifiers. Let us discuss the laser features of the lead-fluoride nanoglassceramics on an erbium ion example and let us compare it with

**0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0**

**2** 

normalized at peak of 660 nm

the other well known glass analogs.

**amplifiers** 

**Intensity, a.u.**

Erbium ion activated glasses are widely used for creation of the mini-microchip and fiber lasers, emitting at 1.5 and 3 m. The lasers and amplifiers at 1.5 m are used for data transfer via fiber-optical communication lines and also in range finders, because their radiation fills into the so-called third window of the fused silica transparency and to the eyesafe spectral range. In the telecommunication tasks a very important feature of the fiberoptical amplifying medium is the width of its amplification spectrum, because the wider amplification range makes it possible to fill into and to provide simultaneous amplification of the big number of spectral channels. Unfortunately, disregarding a very good technology (from the point of view of fiber production), optical and luminescence spectral features of the erbium doped silicate and phosphate glasses, the halfwidth of their amplification spectrum is not large. It is equal to =20-30 nm, thus limiting significantly the number of spectral channels in the amplifier. The fluoride glasses ZBLAN, doped by erbium, have a wide amplification spectrum of =50-80 nm. However, they meet some problems and limitations – first of all from the point of view of the fluoride fiber production and exploitation. One can overcome the said disadvantages and limitations in a new leadfluoride nanoglassceramics, where erbium is in the crystalline fluoride phase and the glass matrix contains oxygen.

For instance, Fig.9 illustrates the spectra of erbium ions luminescence in a novel leadfluoride nanoglassceramics and in the commercially available silicate glass.

Fig. 9. Luminescence spectra of erbium ions in silicate glass and lead-oxifluoride nanoglassceramics.

New Nanoglassceramics Doped with Rare Earth Ions and Their Photonic Applications 119

a.)

b.)

**1450 1500 1550 1600 -0,9**

**1450 1500 1550 1600 -1,0**

**Wavelength, nm**

**100%**

**70% 50% 30%**

**0%**

**1450 1500 1550 1600**

**Wavelength, nm**

Fig. 10. Gain/loss spectra of initial glass (a), glassceramics (b) and commercial silicate glass (c), doped with erbium, for various pumping ratio N2/NEr. Pump wavelength 980 nm.

**Wavelength, nm**

**100% 70% 50% 20%**

**0%**

**100%**

**70% 50% 20%**

**0%**

**-0,6**

**-0,5**

**-0,15**

**-0,10**

**-0,05**

**0,00**

**Gain coefficent g, cm-1**

**0,05**

**0,10**

c.)

**0,15**

**0,0**

**gain coefficient g, cm-1**

**0,5**

**1,0**

**-0,3**

**0,0**

**gain coefficient g, cm-1**

**0,3**

**0,6**

**0,9**

One can see that for the nanoglassceramics the spectrum halfwidth is equal to = 66 nm, while that for the silicate glass it is equal to =20 nm. One has also to note that the new nanoglassceramics reveal the high (>80%) quantum yield of luminescence for the transition *4I13/2 4I15/2*.

In the Fig.10 are shown the amplification cross-sections for the new nanoglassceramics in comparison with the starting glass (before treatment) and with the commercially available silicate glass.

One can see that the transfer from the starting glass to the nanoglassceramics by heat treatment results in increase of the amplification range from 48 to 64 nm. It is accompanied by the increase of the maximal amplification gain for the same pumping level. For instance, for the starting glass for the pumping level 70% the gain is equal to g=0,35 cm-1, while for the glassceramics it is equal to g=0,42 cm-1. One can also see that the amplification spectrum for the commercial silicate glass is much worsens than that of both of the untreated fluorinecontaining glass and of the nanoglassceramics on its basis.

Let us briefly consider the medium for lasers, emitting at 3 m, which are used first of all in medicine. Today the oxygen-less crystals like LiYF4 or garnet crystals like YAG are used as the matrices for erbium ions. However, it is impossible to fabricate the optical fiber and fiber-optical amplifier on the base of these crystals. Hence the search of new media and realization of fiber-optical lasers, emitting at 3 m, are today a very important task. The new lead-oxyfluoride nanoglassceramics, activated by erbium, can become an interest object from the point of view of realization of a fiber three-micron lasers.

In the Fig.11 is shown the luminescence spectrum of the erbium activated nanoglassceramics for the transition *4I11/2 4I13/2*.

One can see from the Fig.11 that the heat treatment leads to broadening of the luminescence spectrum. It, for instance, makes it possible to tune the laser wavelength within the range 3- 3.15 m. The life time for transition of the *4I11/2→4I13/2* is a very important parameter for lasers operated at 3 m. The life time for new nanoglassceramics achieves 5 ms. In comparison, for crystal of Er:YAG (that very often used as a laser media at 3 m) the life time decay achieves 1 ms. Thus, the new nanoglassceramics is a very attractive candidate for 3 m-fiber lasers.

Hence a novel lead-fluoride nanoglassceramics, activated by erbium, reveals the luminescence spectral and laser features, which are not worse than that of the well-known commercial glasses, and exceeds them for such parameters, as the amplification spectrum width and the lifetime of the metastable manifold. At the same time the structure and the content of the nanoglassceramics makes it possible to subject it to a traditional technology the optical fiber production. So one can treat the novel lead-fluoride nanoglassceramics, activated by erbium, is the prospective medium for fiber-optical lasers and amplifiers.
