C.P. Joshi

Wet-chemical preparation of Ce3+-activated K2LaX5 (X = Cl, Br or I) phosphors.

There has been a renewed interest in Ce(3+) -activated halide phosphors due to applications as scintillation detectors, especially for positron emission tomography. For K(2) LaCl(5), the light yield increases and the energy resolution (FWHM) improves with increasing Ce(3+) doping. K(2) LaX(5) compounds are also important as laser hosts for the mid-IR range. K(2) LaCl(5):Nd crystals show bright mid-IR luminescence, which makes them a candidate for IR laser materials. Efficient emission in the IR range has also been reported in K(2) LaCl(5):U(3+). A one-step, wet chemical process for preparing Ce(3+)-activated K(2) LaCl(5) phosphor is described. Intense luminescence of Ce(3+) can be observed in the as-prepared powders without any heat treatment. The availability of such powders opens up several exciting possibilities, such as growing single crystals without going to the high temperatures required for melting the constituent chlorides, or even obtaining processed, transparent, Ce(3+)-activated materials without taking recourse to crystal growth.

Combustion synthesis of compounds in the Y2O3-Al2O3 system

In the Y2O3-Al2O3 system, compounds Y3Al5O12 (yttrium aluminum garnet, YAG), YAlO3 (yttrium aluminum perovskite, YAP), and Y4Al2O9 (yttrium aluminate monoclinic, YAM) are well known. YAG and YAP are of considerable technological importance. Conventional solid-state reaction techniques require high sintering temperatures (above 1800°C) to prepare phase-pure compounds. Though several soft chemical routes have been explored for synthesis of YAG, YAP and YAM, most of these methods are complex. Moreover, phase-pure materials are not obtained in one step and prolonged annealing at temperatures around 1000°C is necessary. In this paper, one-step combustion synthesis of these compounds is reported using a modified procedure and employing mixed (glycine + urea) fuel. Phosphors based on Ce3+ activation were also prepared and exhibited characteristic photoluminescence.

Preparation of CaF2:U phosphor by solid-state metathesis reaction

CaF2:RE phosphors have received attention of several research workers. CaF2:Sm2+ is well known as a solid-state laser material. CaF2:Dy and CaF2:Tm phosphors are used as phosphors for dosimetry of ionizing radiations through thermoluminescence. A new route to preparation of CaF2 by solid-state metathesis is reported. The method is very fast and CaF2 could be prepared in 10 min. Intense emission attributable to uranate group was observed in synthesized uranium-doped samples.

Luminescence studies of decomposition of ceric sulfate

Literature results on the decomposition products of ceric sulfate are inconsistent. A group of researchers claim that ceric sulfate decomposed to ceric oxide without going through a cerous phase at any stage, while the results of the other group show that cerous sulfate is formed as an intermediate phase. Most of these studies used DTA/TGA, XRD and IR techniques. Cerous compounds can also be detected by the characteristic luminescence of Ce(3+). Using such techniques we show that the thermal decomposition of both monoclinic and βCe(SO(4) )(2) · 4H(2) O in air at 500°C leads to the formation of cerous sulphate. Use of various atmospheres (air/N(2) /vacuum) and temperature profiles for the decomposition by the different researchers may be responsible for the discrepancies between literature results.

Modification of luminescence spectra of CaF2: Eu2+

CaF2:Eu(2+) is a well known phosphor having efficient excitation in the near ultraviolet (NUV) range. Phosphors with NUV excitation are required in newly emerging applications such as photoluminescence liquid crystal displays (PLLCD), solid-state lighting (SSL), and down-conversion for solar cells. However, emission of CaF2:Eu(2+) is around 424 nm. Eye sensitivity drops considerably at these wavelengths. It is thus not useful for display applications for which emission in one of the primary colours (blue - 450 nm, green - 540 nm or red - 610 nm) is required. Efforts were made to modify the Photoluminescence (PL) spectra of CaF2:Eu(2+) to meet these requirements using co-dopants. A Ca0.49 Sr0.50 Eu0.01 F2 phosphor showing better colour coordinates and having an emission maximum around 440 nm was discovered during these studies.