**10.6. Phosphors**

conductivity (in logarithmic form) in the oxyapatite systems. Doping a cation with large ionic

**Fig. 10.** Structural defect position and possible conduction mechanism along the c-axis representation of two adjacent

The c-axis-oriented apatite-type lanthanum silicate (La10Si6O27) ceramics was prepared by NAKAYAMA et al [85] via the sintering process under high magnetic field. The degree of orientation in the La10Si6O27 ceramics sintered at 1700°C was 48.1%. The conductivity of the caxis-oriented ceramics is about 0.5 orders of magnitude higher than that of non-oriented ceramics. Higher conductivity is caused by the orientation of oxide ions in the grains com‐ posing the ceramics, which are located along the c-axis and are responsible for the ionic

Compacted sinters of Ln9.33+x/3Si6−xAlxO26 (0 ≤ *x* ≤ 2.0, Ln = La, Nd and Sm) are composed of an apatite-like phase with a hexagonal structure. Compacted sinters were used as potentiomet‐ ric oxygen sensors (**Fig. 11**). The concentration dependence of EMF was well expressed by the

obs

( ) ( ) <sup>2</sup> 2 O

nF p II <sup>=</sup> (11)

O

Furthermore, the electron number *n* is comparable to the theoretical value of 4. The sensing characteristics of the sinters are comparable to those of the sensors with 3 and 8 mol.% YSZ [86].

RT p I E ln

radius and low electronegativity tends to increase the ionic conductivity of oxyapatite.

472 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

conduction.

unit-cells [77].

**10.5. Sensors**

Nernst equation:

Fluorescent lamps typically have transparent glass envelope enclosing sealed discharge space containing an inert gas and mercury vapor. When subjected to a current provided by electro‐

des, mercury ionizes to produce the radiation having the primary wavelengths of 185 nm and 254 nm. This ultraviolet radiation, in turn, excites phosphors on the inside surface of the envelope to produce visible light that is emitted through the glass. Generally, the fluorescent lamp for illumination uses a phosphor that absorbs the 254 nm Hg-resonance wave; the phosphor is activated so as to convert the ultraviolet light into the visible light. In order to improve the color-rendering properties and the emission output of fluorescent lamps, efficient illumination of a white color has been recently provided using a three-band-type fluorescent lamp, which employs a mixture ofred, green and blue-emitting phosphors. In such three-bandtype phosphorlamp, the emitting colors of the respective phosphors are considerably different from one another. Therefore, if the emitting intensity of any of the three corresponding phosphors is decreased, the color deviation occurs, degrading the color-rendering properties of the lamp [90]. The literature dedicated to the preparation of apatite-type light-emitting phosphors is really abundant.

A series of orange-red-emitting Ba2Y3(SiO4)3F:xSm3+ (0.003 ≤ *x* ≤ 0.08) fluorosilicate apatite phosphors were synthesized via the conventional solid-state reaction by YU et al [91]. The emission spectra of the Ba2Y3(SiO4)3F:Sm3+ phosphors contained some sharp emission peaks of Sm3+ ions centered at 564, 601, 648 and 710 nm. The strongest one is located at 601 nm. The optimum dopant concentration of Sm3+ ions in Ba2Y3(SiO4)3F:xSm3+ is around 3 mol.% and the critical transfer distance of Sm3+ was calculated to be 26 Å. The quenching temperature is above 500 K.

Red-emitting phosphors Ba2Gd8(SiO4)6O2:Eu3+ (BGS:Eu3+) with silicate apatite structure were prepared by LIU et al [92] via the conventional high-temperature solid-state reaction method. There are two different sites (4*f* and 6*h* [93]) for Eu3+ occupying the host. It was found that the phosphors BGS:Eu3+ exhibit red emission with high quenching concentration at ~70.75 at.%, and the critical transfer distance of Eu3+ in BGS:Eu3+ was calculated to be ~12.3 Å. More importantly, it has better CIE chromaticity coordinate for white light-emitting diode (w-LED) application in comparison with commercial phosphor (Y,Gd)BO3:Eu3+ (YGB:Eu3+) under nearultraviolet (n-UV) 393 nm excitation [92],[94]. White Tb3+/Sm3+ ions co-doped Ca2La8(GeO4)6O2 (CLGO) phosphors prepared by JEON et al [95] show observable emission spectra under 374 nm excitation.

A novel blue-emitting phosphor Sr8La2(PO4)6O2:Eu2+ was synthesized by LIU et al [96] via conventional high-temperature solid-state method and its photoluminescence (PL) proper‐ ties were investigated for the applications in white light-emitting diodes. The phosphor exhibited strong broad absorption band in the near-ultraviolet (n-UV) range and generated bright-blue emission centered at 442 nm upon 365 nm excitation light. The critical Eu2+ quenching concentration (QC) mechanism was verified to be the dipole-dipole interaction.

A green-emitting phosphor of Eu2+-doped Ca5(PO4)2SiO4 was prepared via a solid-state reaction by ROH et al [97]. The phosphor was excited at the wavelengths of 220 – 450 nm, which was suitable for the emission band of near-ultraviolet (n-UV) light-emitting diode (LED) (350 – 430 nm). In Ca5(PO4)2SiO4:Eu2+ phosphor, there were three distinguishable Eu2+ sites, which resulted in a strong green emission peaking at 530 nm and broad bands up to 700 nm.
