**8. Extended X-ray Absorption Fine Structure (EXAFS) experiments**

Extended X-ray absorption fine structure (EXAFS) experiments were performed at the Gd *L*III edge to obtain further detailed information on the local surroundings of the chemical elements of the Gd-oxide doped (0.5 mol%) nano-glass composite system calcined at different temper‐ atures, namely, 700, 800, 900, and 1200°C (henceforth referred as Gd05-7, Gd05-8, Gd05-9, and Gd05-12, respectively). The broad dielectric anomaly in Gd2O3:SiO2 can be plausibly attributed to oxygen vacancy defects, which are implied by the EXAFS measurements. Of particular interest are the coordination number of oxygen around the Gd atom and type of neighbors, the interatomic distance between the Gd atom and the surrounding oxygen, and the Debye– Waller factors which contain the mean-square relative displacements due to static disorder or thermal vibration. Figure 16 depicts the room temperature experimental EXAFS spectra of samples Gd05-7, Gd05-8 and Gd05-9. The EXAFS signals weighted by *k*<sup>2</sup> [i.e., *k*<sup>2</sup> *χ*(*k*)] are shown in Figure 16(a), and the moduli of their Fourier transforms |*F*(*R*)| are presented in Figure 16(b). The EXAFS spectra in Figure 16(a) look very much the same, on the first glance, and thus one would not expect a major difference in the Fourier transform data. The Fourier transform modulus (Figure 16(b)) exhibits two main coordination peaks: the first one, located at *R* ~ 1*.*8 Å corresponds to the interatomic distances of Gd3+–O (first coordination shell), while the second one (not growing properly) is located at *R* ~ 2.8 Å, indicating mostly the amorphous environment of Gd [21], as expected for a Gd–Gd coordination (second coordination shell) in a crystal. However, the TEM results confirm the presence of NPs Gd2O3 with localized crystallinity in the glass specimen calcined at 700o C. The fitting results of Gd3+–O interatomic distances obtained from Figure 16 are summarized as a function of calcination temperatures in Table III. The values obtained from the EXAFS spectra are not significantly different in all the glass specimens with very low doping concentration. However, on close inspection it is evident that the average Gd3+–O interatomic distance of Gd05-7 is shorter than that of Gd05-8, Gd05-9, and bulk Gd2O3 (unsupported SiO2 glass matrix), suggesting that oxygen ions surrounding the oxygen vacancies around Gd ions should concurrently involve a relaxation toward their adjacent vacancies and, as a result, shorter bond length can be attributed [43]. This argument can, presumably, also be applied to the Er2O3:SiO2 NP-glass system. It has been found that the porous glass are formed at ~ 400°C and annihilation of pores start at ~ 700°C, completed at ~ 800°C [44]. Therefore, silica gel-glass doped with low concentration of rare earth ions are subjected to systematic heating, the collapse of the silica pores is initiated near 700°C (Si–OH groups condense to Si–O–Si bonds). In this calcined glass, agglomeration of individual rare earth oxide ions loosely attached to the pores to be detached to form clusters. The dimensions of the clusters reside in the nanometer range. At 700o C calcined sample, in rare earth oxide NPs, oxygen vacancies could be easily created by loss of oxygen at low oxygen partial pressure, according to *O*0<sup>↔</sup> V0 <sup>+</sup> <sup>1</sup> / 2O2. Based on these considerations, the formation of oxygen vacancies inside the NPs influence the bond-length change, and the number of oxygen atoms surrounding the Gd atom should also decrease. This argument is in fair agreement with the result of a relative decrease in the number of nearest neighbors (Table III) in Gd05-7 in comparison with other high temperature calcined samples. At higher temperature, namely, 800°C, collapse of larger pores also takes place, agglomerate more RE2O3 to form bigger size NPs [18]. This implies that annealing at higher temperatures increase the particle size with lower *V*0 concentration. These results are consistent with dielectric studies, resulting in the decrease in dielectric constant along with DPT behavior by annealing the sample at higher temperatures. Likewise, the Debye-Waller factor increases with decreasing NPs size, indicat‐ ing that the disorder in oxygen environment increases with lower calcination temperatures. In this connection, it is relevant to refer to the important findings of RE3+ doped EXAFS studies by Rocca *et al*. [45] on the densification of silica xerogels as a function of heat treatment up to 900°C. Their measurements have shown that the densification of xerogel without evidence of clustering of RE3+ induces a decreasing of the co-ordination number and a shortening of the main RE–O distance and absence of RE–RE correlation. Nevertheless, our TEM studies in densified glass specimens (700°C and above) have revealed clear detectable NPs with separa‐ tion. So, it is reasonable to assume that the shortening of Gd3+–O and the decreasing of the coordination number with lower calcined nano-glass specimens depends on NPs size (or, more correctly, the oxygen vacancy) for which further experiments of all the rare earth with different doping concentrations are still necessary. Therefore, our EXAFS findings provide additional insight into the origin of the unusually colossal room-temperature dielectric response in rare earth oxide NPs embedded oxide glasses.

the interatomic distance between the Gd atom and the surrounding oxygen, and the Debye– Waller factors which contain the mean-square relative displacements due to static disorder or thermal vibration. Figure 16 depicts the room temperature experimental EXAFS spectra of

in Figure 16(a), and the moduli of their Fourier transforms |*F*(*R*)| are presented in Figure 16(b). The EXAFS spectra in Figure 16(a) look very much the same, on the first glance, and thus one would not expect a major difference in the Fourier transform data. The Fourier transform modulus (Figure 16(b)) exhibits two main coordination peaks: the first one, located at *R* ~ 1*.*8 Å corresponds to the interatomic distances of Gd3+–O (first coordination shell), while the second one (not growing properly) is located at *R* ~ 2.8 Å, indicating mostly the amorphous environment of Gd [21], as expected for a Gd–Gd coordination (second coordination shell) in a crystal. However, the TEM results confirm the presence of NPs Gd2O3 with localized

distances obtained from Figure 16 are summarized as a function of calcination temperatures in Table III. The values obtained from the EXAFS spectra are not significantly different in all the glass specimens with very low doping concentration. However, on close inspection it is evident that the average Gd3+–O interatomic distance of Gd05-7 is shorter than that of Gd05-8, Gd05-9, and bulk Gd2O3 (unsupported SiO2 glass matrix), suggesting that oxygen ions surrounding the oxygen vacancies around Gd ions should concurrently involve a relaxation toward their adjacent vacancies and, as a result, shorter bond length can be attributed [43]. This argument can, presumably, also be applied to the Er2O3:SiO2 NP-glass system. It has been found that the porous glass are formed at ~ 400°C and annihilation of pores start at ~ 700°C, completed at ~ 800°C [44]. Therefore, silica gel-glass doped with low concentration of rare earth ions are subjected to systematic heating, the collapse of the silica pores is initiated near 700°C (Si–OH groups condense to Si–O–Si bonds). In this calcined glass, agglomeration of individual rare earth oxide ions loosely attached to the pores to be detached to form clusters. The

earth oxide NPs, oxygen vacancies could be easily created by loss of oxygen at low oxygen partial pressure, according to *O*0<sup>↔</sup> V0 <sup>+</sup> <sup>1</sup> / 2O2. Based on these considerations, the formation of oxygen vacancies inside the NPs influence the bond-length change, and the number of oxygen atoms surrounding the Gd atom should also decrease. This argument is in fair agreement with the result of a relative decrease in the number of nearest neighbors (Table III) in Gd05-7 in comparison with other high temperature calcined samples. At higher temperature, namely, 800°C, collapse of larger pores also takes place, agglomerate more RE2O3 to form bigger size NPs [18]. This implies that annealing at higher temperatures increase the particle size with lower *V*0 concentration. These results are consistent with dielectric studies, resulting in the decrease in dielectric constant along with DPT behavior by annealing the sample at higher temperatures. Likewise, the Debye-Waller factor increases with decreasing NPs size, indicat‐ ing that the disorder in oxygen environment increases with lower calcination temperatures. In this connection, it is relevant to refer to the important findings of RE3+ doped EXAFS studies by Rocca *et al*. [45] on the densification of silica xerogels as a function of heat treatment up to 900°C. Their measurements have shown that the densification of xerogel without evidence of clustering of RE3+ induces a decreasing of the co-ordination number and a shortening of the

[i.e., *k*<sup>2</sup>

C. The fitting results of Gd3+–O interatomic

C calcined sample, in rare

26 27

42 43 44

*χ*(*k*)] are shown

samples Gd05-7, Gd05-8 and Gd05-9. The EXAFS signals weighted by *k*<sup>2</sup>

dimensions of the clusters reside in the nanometer range. At 700o

crystallinity in the glass specimen calcined at 700o

194 Ferroelectric Materials – Synthesis and Characterization


**Table 3.** Results of the quantitative analysis of the first coordination shell derived from EXAFS filtered data of Gd2O3:SiO2 nano-glass composite systems at different calcinations temperatures. *N*, *R* and *σ*<sup>2</sup> are the average coordination number, interatomic distance, and relative mean square displacement (Debye-Waller factor), respectively. Colossal dielectric and MD response of RE2O3 nanoparticles in SiO2 glass matrix 19

Figure 16. (Color online) Gd *L*III-edge EXAFS spectra of Gd3+-doped SiO2 21 glass samples calcined at different temperatures. Spectra are vertically shifted for clarity. (a) *k*<sup>2</sup> 22 -weighted EXAFS signals. (b) 23 Fourier transforms moduli radial distribution functions. Both experimental data (symbols) and the best-fit theoretical curves (dashed) are also reported. The transformation range is *k* = 2.5–8 Å-1 24 for all **Figure 16.** (Color online) Gd *L*III-edge EXAFS spectra of Gd3+-doped SiO2 glass samples calcined at different tempera‐ tures. Spectra are vertically shifted for clarity. (a) *k*<sup>2</sup> -weighted EXAFS signals. (b) Fourier transforms moduli radial dis‐ tribution functions. Both experimental data (symbols) and the best-fit theoretical curves (dashed) are also reported. The transformation range is *k* = 2.5–8 Å-1 for all the spectra and the range for the first coordination shell fit is *R* = 1–3 Å.

25 the spectra and the range for the first coordination shell fit is *R* = 1–3 Å. Figure 17 depicts the room temperature experimental EXAFS spectra of LGS systems ((La, Gd)2O3:SiO2 NP-glass composite systems) with different doping concentrations (Table II) and

 Figure 17 depicts the room temperature experimental EXAFS spectra of LGS systems ((La, Gd)2O3:SiO2 29 NP-glass composite systems) with different doping concentrations (Table II) and particles size (different calcined temperatures). The first coordination peak located at ~1.8 Å (Figures 17(a), (b)) with the interatomic distance of La3+-O/Gd3+ 31 -O looks very much the similar without any perceptible shift at different doping concentrations. However, the interatomic distances of the first coordination peak (~1.8 Å) of LGS4 with different calcination temperatures (Figures 17(c), (d)) are shifted significantly even with very low doping concentration of La2O3/Gd2O3 34 . It reveals significantly that the average La3+–O/Gd3+ 35 –O interatomic distances of LGS4 samples at lower calcination temperature is shorter, suggesting higher oxygen vacancies around La/Gd ions, supported with our previously reported article [16]. Therefore, the dielectric value decreases by annealing the sample at higher temperatures (or, more correctly, with higher NPs size) with identical molar concentration of dopant element. However, identical particle size (magnetic and/or non-magnetic NPs) with concentration dependence does not affect the oxygen vacancies. In other words, oxygen vacancies

41 depend only on the particle size but not its magnetic phase.

11

particles size (different calcined temperatures). The first coordination peak located at ~1.8 Å (Figures 17(a), (b)) with the interatomic distance of La3+-O/Gd3+-O looks very much the similar without any perceptible shift at different doping concentrations. However, the interatomic distances of the first coordination peak (~1.8 Å) of LGS4 with different calcination temperatures (Figures 17(c), (d)) are shifted significantly even with very low doping concentration of La2O3/ Gd2O3. It reveals significantly that the average La3+–O/Gd3+–O interatomic distances of LGS4 samples at lower calcination temperature is shorter, suggesting higher oxygen vacancies around La/Gd ions, supported with our previously reported article [16]. Therefore, the dielectric value decreases by annealing the sample at higher temperatures (or, more correctly, with higher NPs size) with identical molar concentration of dopant element. However, identical particle size (magnetic and/or non-magnetic NPs) with concentration dependence does not affect the oxygen vacancies. In other words, oxygen vacancies depend only on the particle size but not its magnetic phase. Gd)2O3:SiO2 13 NP-glass composite systems) with different doping concentrations (Table II) and 14 particles size (different calcined temperatures). The first coordination peak located at ~1.8 Å (Figures 17(a), (b)) with the interatomic distance of La3+-O/Gd3+ 15 -O looks very much the similar without any 16 perceptible shift at different doping concentrations. However, the interatomic distances of the first 17 coordination peak (~1.8 Å) of LGS4 with different calcination temperatures (Figures 17(c), (d)) are shifted significantly even with very low doping concentration of La2O3/Gd2O3 18 . It reveals significantly that the average La3+–O/Gd3+ 19 –O interatomic distances of LGS4 samples at lower calcination 20 temperature is shorter, suggesting higher oxygen vacancies around La/Gd ions, supported with our 21 previously reported article [16]. Therefore, the dielectric value decreases by annealing the sample at 22 higher temperatures (or, more correctly, with higher NPs size) with identical molar concentration of 23 dopant element. However, identical particle size (magnetic and/or non-magnetic NPs) with 24 concentration dependence does not affect the oxygen vacancies. In other words, oxygen vacancies 25 depend only on the particle size but not its magnetic phase. 26 27 28

Colossal dielectric and MD response of RE2O3 nanoparticles in SiO2 glass matrix

Figure 16. (Color online) Gd *L*III-edge EXAFS spectra of Gd3+-doped SiO2 5 glass samples calcined at different temperatures. Spectra are vertically shifted for clarity. (a) *k*<sup>2</sup> 6 -weighted EXAFS signals. (b) 7 Fourier transforms moduli radial distribution functions. Both experimental data (symbols) and the best-fit theoretical curves (dashed) are also reported. The transformation range is *k* = 2.5–8 Å-1 8 for all

12 Figure 17 depicts the room temperature experimental EXAFS spectra of LGS systems ((La,

9 the spectra and the range for the first coordination shell fit is *R* = 1–3 Å.

21

**Figure 17.** (Color online) Room temperature Fourier transforms moduli radial distribution functions EXAFS spectra of LGS systems at (a) La *LIII*-edge with different La2O3 concentrations, (b) Gd *LIII*-edge with different Gd2O3 concentrations calcined at 700°C, (c) La *LIII*-edge of LGS4 sample at different calcined temperatures, and (d) Gd *LIII*-edge of LGS4 sam‐ ple at different calcined temperatures. Spectra are vertically shifted for clarity.

### **9. Magnetic measurements**

The *dc* magnetization (after diamagnetic correction) of LGS5 sample under zero-field-cooled (ZFC) and field-cooled (FC) condition as a function of temperature (2–350 K temperature 22 **Ferroelectrics**

range) in the presence of an applied magnetic field of 200 Oe are shown in Figure 18. It is noteworthy that the observed temperature dependent magnetic feature of LGS5 sample is attributed from magnetic transition of Gd2O3 NPs only. On close inspection, the divergent behavior between ZFC and FC data occurs at low temperatures (irreversibility temperature, *Tirr*) with a rounded maximum for ZFC curve is obtained at 58 K (identified as the blocking temperature, *TB*). A spread in the blocking temperature may be rightly assumed with NPs size distribution. Such behavior is akin to superparamagnetic phase of Gd2O3 NPs in the nano-glass composite system similarly observed in oxide glasses containing Gd2O3 [46]. This typical characteristic temperature would be unlikely arising from a magnetic transition in oxygen contaminant [47]. The FC magnetization increases continuously with the lowering of temper‐ ature below the irreversibility temperature, consistent with ferromagnetic-like ordering of Gd2O3 NPs. The shape of the inverse susceptibility data (right axis of Figure 18) reveal a quite different behavior in comparison with usually found Curie-type behavior in bulk Gd2O3 [48]. 14 **Magnetic Measurements** 15 The *dc* magnetization (after diamagnetic correction) of LGS5 sample under zero-field-16 cooled (ZFC) and field-cooled (FC) condition as a function of temperature (2–350 K temperature 17 range) in the presence of an applied magnetic field of 200 Oe are shown in Figure 18. It is noteworthy 18 that the observed temperature dependent magnetic feature of LGS5 sample is attributed from magnetic transition of Gd2O3 19 NPs only. On close inspection, the divergent behavior between ZFC and FC data occurs at low temperatures (irreversibility temperature, *Tirr* 20 ) with a rounded maximum for 21 ZFC curve is obtained at 58 K (identified as the blocking temperature, *TB*). A spread in the blocking 22 temperature may be rightly assumed with NPs size distribution. Such behavior is akin to superparamagnetic phase of Gd2O3 23 NPs in the nano-glass composite system similarly observed in oxide glasses containing Gd2O3 24 [46]. This typical characteristic temperature would be unlikely arising 25 from a magnetic transition in oxygen contaminant [47]. The FC magnetization increases continuously 26 with the lowering of temperature below the irreversibility temperature, consistent with ferromagneticlike ordering of Gd2O3 27 NPs. The shape of the inverse susceptibility data (right axis of Figure 18) 28 reveal a quite different behavior in comparison with usually found Curie-type behavior in bulk Gd2O3

8 Figure 17. (Color online) Room temperature Fourier transforms moduli radial distribution functions EXAFS spectra of LGS systems at (a) La *LIII*-edge with different La2O3 concentrations, (b) Gd *LIII* 9 -

particles size (different calcined temperatures). The first coordination peak located at ~1.8 Å (Figures 17(a), (b)) with the interatomic distance of La3+-O/Gd3+-O looks very much the similar without any perceptible shift at different doping concentrations. However, the interatomic distances of the first coordination peak (~1.8 Å) of LGS4 with different calcination temperatures (Figures 17(c), (d)) are shifted significantly even with very low doping concentration of La2O3/ Gd2O3. It reveals significantly that the average La3+–O/Gd3+–O interatomic distances of LGS4 samples at lower calcination temperature is shorter, suggesting higher oxygen vacancies around La/Gd ions, supported with our previously reported article [16]. Therefore, the dielectric value decreases by annealing the sample at higher temperatures (or, more correctly, with higher NPs size) with identical molar concentration of dopant element. However, identical particle size (magnetic and/or non-magnetic NPs) with concentration dependence does not affect the oxygen vacancies. In other words, oxygen vacancies depend only on the

01234560123456

**Figure 17.** (Color online) Room temperature Fourier transforms moduli radial distribution functions EXAFS spectra of LGS systems at (a) La *LIII*-edge with different La2O3 concentrations, (b) Gd *LIII*-edge with different Gd2O3 concentrations calcined at 700°C, (c) La *LIII*-edge of LGS4 sample at different calcined temperatures, and (d) Gd *LIII*-edge of LGS4 sam‐

The *dc* magnetization (after diamagnetic correction) of LGS5 sample under zero-field-cooled (ZFC) and field-cooled (FC) condition as a function of temperature (2–350 K temperature

(a) LGS6

 LGS5 LGS4 LGS3 LGS2 LGS1

(c) (d)

LGS4-700o

LGS4-800o

LGS4-900o

LGS4-1200o

700 - 1200 o 700 - 1200 C <sup>o</sup>

C

C

C

C

Distance R (Å)

C

ple at different calcined temperatures. Spectra are vertically shifted for clarity.

La *L*III

S5 - S1

La *L*III

700o C 700o C

Gd *L*III

S6 - S2

Distance R (Å)

Gd *L* (b) III

 LGS5 LGS4 LGS3 LGS2

LGS4-700o

LGS4-800o

LGS4-900o

LGS4-1200o

C

C

C

C

 Figure 17 depicts the room temperature experimental EXAFS spectra of LGS systems ((La, Gd)2O3:SiO2 13 NP-glass composite systems) with different doping concentrations (Table II) and particles size (different calcined temperatures). The first coordination peak located at ~1.8 Å (Figures 17(a), (b)) with the interatomic distance of La3+-O/Gd3+ 15 -O looks very much the similar without any perceptible shift at different doping concentrations. However, the interatomic distances of the first coordination peak (~1.8 Å) of LGS4 with different calcination temperatures (Figures 17(c), (d)) are shifted significantly even with very low doping concentration of La2O3/Gd2O3 18 . It reveals significantly that the average La3+–O/Gd3+ 19 –O interatomic distances of LGS4 samples at lower calcination temperature is shorter, suggesting higher oxygen vacancies around La/Gd ions, supported with our previously reported article [16]. Therefore, the dielectric value decreases by annealing the sample at higher temperatures (or, more correctly, with higher NPs size) with identical molar concentration of dopant element. However, identical particle size (magnetic and/or non-magnetic NPs) with concentration dependence does not affect the oxygen vacancies. In other words, oxygen vacancies

Colossal dielectric and MD response of RE2O3 nanoparticles in SiO2 glass matrix

Figure 16. (Color online) Gd *L*III-edge EXAFS spectra of Gd3+-doped SiO2 5 glass samples calcined at different temperatures. Spectra are vertically shifted for clarity. (a) *k*<sup>2</sup> 6 -weighted EXAFS signals. (b) 7 Fourier transforms moduli radial distribution functions. Both experimental data (symbols) and the best-fit theoretical curves (dashed) are also reported. The transformation range is *k* = 2.5–8 Å-1 8 for all

9 the spectra and the range for the first coordination shell fit is *R* = 1–3 Å.

21

13

29 [48].

particle size but not its magnetic phase.

196 Ferroelectric Materials – Synthesis and Characterization


**9. Magnetic measurements**


25 depend only on the particle size but not its magnetic phase.

10 11

**Figure 18.** (Color online) ZFC and FC magnetization versus temperature curves of LGS5 sample calcined at 700°C, right axis shows the temperature dependent inverse susceptibility curve. In the inset, the region close to the superpara‐ magnetic transition is highlighted.

The data obtained for the temperature dependence magnetization of LGS4 samples calcined at 700, 800, 900 and 1200°C are graphically depicted in Figure 19. The superparamagnetic blocking temperature cannot be traced in low accuracy (or resolution) of measurement with very low percentage-doping (~ 0.075 mol%) of magnetic Gd2O3. However, from the observed continuous increase in ZFC and FC curves at low temperature indicating the ferromagnetic nature of the LGS4 sample. Magnetic properties with size dependency are also observed for LGS4 samples calcined at different temperatures, related with the uncompensated surface spins present on the Gd2O3 NPs. It is likely that the Gd2O3 NPs with smaller size (i.e., higher surface-to-volume ratio) contain larger proportion of uncompensated surface spins and

Colossal dielectric and MD response of RE2O3 nanoparticles in SiO2 glass matrix

5 Figure 18. (Color online) ZFC and FC magnetization versus temperature curves of LGS5 sample

23

21 Figure 19. (Color online) ZFC and FC magnetization versus temperature curves of LGS4 sample at 22 different calcination temperatures. Applied *dc* magnetic field 200 Oe. **Figure 19.** (Color online) ZFC and FC magnetization versus temperature curves of LGS4 sample at different calcination temperatures. Applied *dc* magnetic field 200 Oe.

consequently reveal higher ferromagnetic values than larger NPs (higher calcined tempera‐ ture). Temperature dependent inverse susceptibility data for LGS4 samples calcined at different temperatures with respect to bulk Gd2O3 can be fitted by the Curie–Weiss law (Figure 20(a)) having different slopes of straight lines. The intersection points of fitted lines with *x*-axis exhibit the Curie–Weiss temperatures, found to be 32.7, 16.6, 12.1 and 4.8 K for 700, 800, 900 and 1200°C respectively. These positive values for LGS4 sample calcined at different temper‐ atures indicate weak ferromagnetic behavior, whereas, bulk Gd2O3 shows antiferromagnetism with negative Curie–Weiss temperature at -16.3 K. The current tendency of lowering Curie-Weiss temperatures with increasing calcined temperature for LGS4 samples infers that lager sized Gd2O3 NPs possess toward bulk crystalline counterpart. 23 24 25 The data obtained for the temperature dependence magnetization of LGS4 samples calcined 26 at 700, 800, 900 and 1200°C are graphically depicted in Figure 19. The superparamagnetic blocking 27 temperature cannot be traced in low accuracy (or resolution) of measurement with very low percentage-doping (~ 0.075 mol%) of magnetic Gd2O3 28 . However, from the observed continuous 29 increase in ZFC and FC curves at low temperature indicating the ferromagnetic nature of the LGS4 30 sample. Magnetic properties with size dependency are also observed for LGS4 samples calcined at different temperatures, related with the uncompensated surface spins present on the Gd2O3 31 NPs. It is likely that the Gd2O3 32 NPs with smaller size (i.e., higher surface-to-volume ratio) contain larger 33 proportion of uncompensated surface spins and consequently reveal higher ferromagnetic values than 34 larger NPs (higher calcined temperature). Temperature dependent inverse susceptibility data for

Isothermal magnetization-field sweeps were performed to further investigate the nature of the superparamagnetic state and the ferromagnetism below transition temperature. Figure 21 displays the magnetic field dependence of the magnetization (*M-H*) curves for LGS4 sample calcined at 700°C at 300, 200, 100 and 5 K in the *dc* magnetic field range ±60 kOe. Defining the magnetic characteristics, magnetic hysteresis curve obtained at 300 K has zero area, whereas, there is a dramatic change both in magnitude (enhancement of magnetic moment/unit mass) and shape (deviate from linearity) with measurable finite areas at 5 K (lower inset of Figure 21). This constitutes strong evidence that at 5 K the Gd2O3 NPs are going to the magnetically ordered state or in ferromagnetic nature. Moreover, magnetization vs *H/T* curves plotted at different temperatures are linear and collapse to a single curve (upper inset of Figure 21) confirming the existence of superparamagnetic phase of Gd2O3 NPs embedded in SiO2 glass matrix. It is abundantly clear that Gd2O3 NPs grown with high magnetic dilution in glass matrix are best described as an assembly of non-interacting superparamagnetic NPs. The hysteresis curves have no magnetic saturation in the magnetic field range of ±60 kOe, considering large anisotropic fields appears in the Gd2O3 NPs systems [49]. LGS4 samples calcined at different temperatures with respect to bulk Gd2O3 35 can be fitted by the 36 Curie–Weiss law (Figure 20(a)) having different slopes of straight lines. The intersection points of 37 fitted lines with *x*-axis exhibit the Curie–Weiss temperatures, found to be 32.7, 16.6, 12.1 and 4.8 K 38 for 700, 800, 900 and 1200°C respectively. These positive values for LGS4 sample calcined at

17 lowering Curie-Weiss temperatures with increasing calcined temperature for LGS4 samples infers that lager sized Gd2O3 18 NPs possess toward bulk crystalline counterpart. RE2O3 Nanoparticles Embedded in SiO2 Glass Matrix — A Colossal Dielectric and Magnetodielectric Response http://dx.doi.org/10.5772/60677 199

22 **Ferroelectrics**

 The data obtained for the temperature dependence magnetization of LGS4 samples calcined at 700, 800, 900 and 1200°C are graphically depicted in Figure 19. The superparamagnetic blocking temperature cannot be traced in low accuracy (or resolution) of measurement with very low percentage-doping (~ 0.075 mol%) of magnetic Gd2O3 4 . However, from the observed continuous increase in ZFC and FC curves at low temperature indicating the ferromagnetic nature of the LGS4 sample. Magnetic properties with size dependency are also observed for LGS4 samples calcined at different temperatures, related with the uncompensated surface spins present on the Gd2O3 7 NPs. It is likely that the Gd2O3 8 NPs with smaller size (i.e., higher surface-to-volume ratio) contain larger proportion of uncompensated surface spins and consequently reveal higher ferromagnetic values than larger NPs (higher calcined temperature). Temperature dependent inverse susceptibility data for LGS4 samples calcined at different temperatures with respect to bulk Gd2O3 11 can be fitted by the Curie–Weiss law (Figure 20(a)) having different slopes of straight lines. The intersection points of fitted lines with *x*-axis exhibit the Curie–Weiss temperatures, found to be 32.7, 16.6, 12.1 and 4.8 K for 700, 800, 900 and 1200°C respectively. These positive values for LGS4 sample calcined at different temperatures indicate weak ferromagnetic behavior, whereas, bulk Gd2O3 15 shows antiferromagnetism with negative Curie–Weiss temperature at -16.3 K. The current tendency of

Colossal dielectric and MD response of RE2O3 nanoparticles in SiO2 glass matrix 25 31 Figure 20. (Color online) (a) Inverse susceptibility versus temperature curves of LGS4 samples at different calcination temperatures with respect to bulk Gd2O3 32 , (b) the region close to the extrapolated 33 lines intersect with temperature axis is highlighted. **Figure 20.** (Color online) (a) Inverse susceptibility versus temperature curves of LGS4 samples at different calcination temperatures with respect to bulk Gd2O3, (b) the region close to the extrapolated lines intersect with temperature axis is highlighted.

18 Figure 21. (Color online) Hysteresis loop of LGS4 sample calcined at 700°C, lower inset: the region 19 close to the coercive field value is highlighted, upper inset: magnetization vs. *H*/*T* of LGS4 sample. 20 **Figure 21.** (Color online) Hysteresis loop of LGS4 sample calcined at 700°C, lower inset: the region close to the coercive field value is highlighted, upper inset: magnetization vs. H/T of LGS4 sample.

We have synthesized self-organized RE2O3 22 NPs with almost equal size and separation

#### 24 25 Principal findings may be summarized below: **10. Conclusions**

21 **Conclusions:** 

34

consequently reveal higher ferromagnetic values than larger NPs (higher calcined tempera‐ ture). Temperature dependent inverse susceptibility data for LGS4 samples calcined at different temperatures with respect to bulk Gd2O3 can be fitted by the Curie–Weiss law (Figure 20(a)) having different slopes of straight lines. The intersection points of fitted lines with *x*-axis exhibit the Curie–Weiss temperatures, found to be 32.7, 16.6, 12.1 and 4.8 K for 700, 800, 900 and 1200°C respectively. These positive values for LGS4 sample calcined at different temper‐ atures indicate weak ferromagnetic behavior, whereas, bulk Gd2O3 shows antiferromagnetism with negative Curie–Weiss temperature at -16.3 K. The current tendency of lowering Curie-Weiss temperatures with increasing calcined temperature for LGS4 samples infers that lager

**Figure 19.** (Color online) ZFC and FC magnetization versus temperature curves of LGS4 sample at different calcination

 The data obtained for the temperature dependence magnetization of LGS4 samples calcined at 700, 800, 900 and 1200°C are graphically depicted in Figure 19. The superparamagnetic blocking temperature cannot be traced in low accuracy (or resolution) of measurement with very low percentage-doping (~ 0.075 mol%) of magnetic Gd2O3 28 . However, from the observed continuous increase in ZFC and FC curves at low temperature indicating the ferromagnetic nature of the LGS4 sample. Magnetic properties with size dependency are also observed for LGS4 samples calcined at different temperatures, related with the uncompensated surface spins present on the Gd2O3 31 NPs. It is likely that the Gd2O3 32 NPs with smaller size (i.e., higher surface-to-volume ratio) contain larger proportion of uncompensated surface spins and consequently reveal higher ferromagnetic values than larger NPs (higher calcined temperature). Temperature dependent inverse susceptibility data for LGS4 samples calcined at different temperatures with respect to bulk Gd2O3 35 can be fitted by the Curie–Weiss law (Figure 20(a)) having different slopes of straight lines. The intersection points of fitted lines with *x*-axis exhibit the Curie–Weiss temperatures, found to be 32.7, 16.6, 12.1 and 4.8 K for 700, 800, 900 and 1200°C respectively. These positive values for LGS4 sample calcined at

21 Figure 19. (Color online) ZFC and FC magnetization versus temperature curves of LGS4 sample at

T(K)

10 100

Colossal dielectric and MD response of RE2O3 nanoparticles in SiO2 glass matrix

5 Figure 18. (Color online) ZFC and FC magnetization versus temperature curves of LGS5 sample 6 calcined at 700°C, right axis shows the temperature dependent inverse susceptibility curve. In the

LGS4

 700<sup>o</sup> C-ZFC

 700<sup>o</sup> C-FC

 800<sup>o</sup> C-ZFC

 800<sup>o</sup> C-FC

 900<sup>o</sup> C-ZFC

 900<sup>o</sup> C-FC 1200o

 1200o C-FC

C-ZFC

7 inset, the region close to the superparamagnetic transition is highlighted.

23 24 23

Isothermal magnetization-field sweeps were performed to further investigate the nature of the superparamagnetic state and the ferromagnetism below transition temperature. Figure 21 displays the magnetic field dependence of the magnetization (*M-H*) curves for LGS4 sample calcined at 700°C at 300, 200, 100 and 5 K in the *dc* magnetic field range ±60 kOe. Defining the magnetic characteristics, magnetic hysteresis curve obtained at 300 K has zero area, whereas, there is a dramatic change both in magnitude (enhancement of magnetic moment/unit mass) and shape (deviate from linearity) with measurable finite areas at 5 K (lower inset of Figure 21). This constitutes strong evidence that at 5 K the Gd2O3 NPs are going to the magnetically ordered state or in ferromagnetic nature. Moreover, magnetization vs *H/T* curves plotted at different temperatures are linear and collapse to a single curve (upper inset of Figure 21) confirming the existence of superparamagnetic phase of Gd2O3 NPs embedded in SiO2 glass matrix. It is abundantly clear that Gd2O3 NPs grown with high magnetic dilution in glass matrix are best described as an assembly of non-interacting superparamagnetic NPs. The hysteresis curves have no magnetic saturation in the magnetic field range of ±60 kOe, considering large

sized Gd2O3 NPs possess toward bulk crystalline counterpart.

22 different calcination temperatures. Applied *dc* magnetic field 200 Oe.

0

10

Magnetization,

temperatures. Applied *dc* magnetic field 200 Oe.

(10-2emu/g)

198 Ferroelectric Materials – Synthesis and Characterization

20

anisotropic fields appears in the Gd2O3 NPs systems [49].

 samples. (b) Properly annealed sol–gel glass (in which RE ~ Sm, Gd and Er) (Fig. 2) shows an interesting colossal response of dielectric constant along with DPT and MD behavior around room temperature. We have synthesized self-organized RE2O3 NPs with almost equal size and separation embedded in SiO2 glass matrix by the sol–gel method.

> 34 (d) The MDR observed in this glassy composite is considered to be associated with the direct 35 consequence of magnetoresistance changes depending on the calcination temperatures (magnetic

> 37 (e) However, keeping the NPs size constant, the increase in dielectric constant and MDR strongly

38 depends on the magnetic property (superparamagnetism) of the rare earth ions.

26 (a) Presence of superparamagnetic phase occurs in magnetic rare earth oxide NPs doped glass

 (c) The experimental facts strongly suggest that the dielectric anomaly with DPT behavior is related to oxygen vacancy-induced dielectric relaxation in the material without ferroelectric phase transition. Principal findings may be summarized below:

36 NPs size).

embedded in SiO2 23 glass matrix by the sol–gel method.

