J.W. Brightwell

Preparation, crystal growth and luminescence in calcium sulphide

Abstract The preparation of microcrystalline CaS of high purity, as determined from X-ray fluorescence and atomic absorption spectrophotometry, is reported. Systematic doping of CaS with between 2.0×10-5 and 1.5×10-3 mole parts of Mn has been carried out. Single crystal CaS, taking an octahedral habit with side dimensions up to 1 mm, has been prepared by chemical vapour transport using iodine at a concentration of 1 μg mm-3 for growth over a temperature differential 1200 to 800°C. A model supported by thermodynamic data is advanced for crystal growth of CaS with HI rather than I2 being postulated as the principal transporting agent. Fluorescent emission spectra have indicated substantial emission in the blue-green for undoped single crystal CaS and for microcrystalline CaS with low Mn levels, whilst for 7.5×10-4 and 1.5×10-3 mole parts of Mn significant orange emission at 575 nm

Composition effects on excitation and flourescence emission spectra in CaxSr1-xS

Samples of CaxSr1-xS with 0 <=x <1, in increments of 0.1, were prepared by reduction of mixed sulphates with H2/H2S at 1150°C, and their photoluminescence properties were determined. At 300 K the dominant emissions from CaS and SrS are at 360 and 389 nm respectively. For intermediate compositions the peak moves to 390 nm, strengthens and is supplemented by a peak at 530 nm which becomes dominant for the slightly Ca-rich mid-range compositions. At 77 K the dominant emissions are at 385 nm for CaS and 370 and 460 nm for SrS with enhanced intensities relative to those at 300 K. Intermediate compositions behave in a complex manner with a range of emission energies at, e.g., 395, 425, and 445 nm which become dominant in some samples. Additional excitation energies are observed for the mid-range samples at significantly less than the band edge. It is proposed that these emissions and excitation energies are related to electronic transitions to an from anion vacancies in a variety of environments within the lattice.

Electrical and phase behaviour of the system AgI-PbI2

Abstract Mixtures of AgI and PbI2 form a phase stable at temperatures above 125°C with a substantial solid solubility about the composition 67% AgI, this phase exhibits a high electrical conductivity (∼0.1S m-1). A proposal is made for the basic form of the AgI-PbI2 equilibrium diagram below 200°C, and comparison made to the systems AgI-ZnI2 and AgI-CdI2.

Emission spectra in Ca1-xCdxS solid solution at 77 and 300 K

Solid solutions of Ca1-xCdxS have been prepared for 0.12 <=x<= 0 from mixed sulphates in flowing H2/H2S at 1050°C. Compositions have been determined by atomic absorption spectrophotometry recognising the difficulties in retaining cadmium. A monotonic linear variation of lattice parameter with composition is observed with the rock salt structure being sustained throughout. Both emission and excitation spectra have been determined at 77 and 300 K throughout the composition range. Excitation at wavelengths shorter than the band edge gave rise at 300 K to increased emission in the ultraviolet-violet region between 320 and 420 nm for x between 0.008 and 0.04, whilst at 77 K the emission was restricted to a 390 nm peak with excitation by longer wavelengths at 280 to 285 nm. Cooling to liquid nitrogen temperatures generally sharpened the emission peaks and increased intensities by up to two orders of magnitude. The absorption edge is observed to decrease linearly with increasing cadmium content. An explanation for the excitation and emission behaviour with change in temperature and composition is discussed.

Hot stage X-ray diffractometer studies on Ag2PbI4 and Ag4PbI6

Abstract Mixtures of AgI and PbI 2 cooled from the melt result in the peritectic formation of a fast ion conducting phase centred about Ag 4 PbI 6 , which is face centred cubic with a = 6.33(5) A ; this phase exhibits high electrical conductivity. On cooling to about 125°C, dissociation occurs to γAgI and PbI 2 , accompanied by the transient formation of another phase, centred about Ag 2 PbI 4 . A modified form of the T-x section of the equilibrium phase diagram at AgI concentrations greater than 60 mole % and below 300°C is proposed.

Impact of phase concentrations on structure and electroluminescence of ZnS:Cu

Abstract A systematic study has been undertaken of the influence of processing ambients and time at 1100°C on the fluorescence and AC electroluminescence of luminescent grade zinc sulphide doped with between 0.1 and 0.4 mol% of copper. Short processing times of the order of an hour in flowing nitrogen for concentrations of copper up to 0.1 mol% have given rise to the brightest ac EL; this has been linked to the predominance of the wurtzite structure which may also be induced by treatment in an oxidising atmosphere at lower temperatures. Extended heat treatment depresses the electroluminescence intensity and this is related to growth in particle size.

Structural studies and electrical conductivity versus temperature measurements in mixed silver-lead iodide phases

Abstract A phase diagram is proposed for the pseudo-binary AgI-PbI2 system. Two intermediate phases, centred around 67 mole % and 80 – 90 mole % AgI, form at 130 and 280°C, respectively. The formation of the Ag2PbI4 phase is associated with a sharp increase in electrical conductivity to a value which, at 150°C, is only one to two orders less than that for α-AgI. Inclusion of low levels of PbI2 into the AgI structure promotes the formation of the γ-(zinc blende) phase rather than the β-(wurtzite) phase.

Excitation and fluorescence spectra in Ca1-xCdxS solid solutions

Abstract Ca 1− x Cd x S solid solutions have been prepared at a slight excess sulphur pressure in a nitrogen atmosphere at 1025°C. So solutions up to x = 0.40 have been observed in samples treated under these conditions for two hours. The lattice parameter is found to vary linearly with composition in line with predicted unit cell dimensions. Excitation spectra indicate that for x = 0.01 and 0.10 at 77 K very intense emissions in the blue/violet are observed for excitation at 300 nm and for x = 0.01 at 300 K in the blue. The fluorescence emission intensity in samples with x = 0.31 and 0.40 is enhanced by an increasing presence of a CdS phase emitting in the green at 500 nm.

Effect of preparation method on thermoluminescence in CaS: Ce

Abstract The work reported here is part of a systematic study of host lattice defects in CaS. CaS phosphors doped with cerium have been prepared by the reduction of sulphates in H 2 /H 2 S at 1020°C for 2 h. These samples were refired at 1020°C with Na 2 CO 3 /S, CaF 2 or NH 4 Cl added as fluxing agents. Phase purity was monitord by X-ray diffractometry. Thermoluminescence spectra have been measured on both fluxed and unfluxed samples in the range 80 to 450 K. Five glow peaks were observed. One may be related to sulphur vacancies, another to calcium vacancies and a third to the presence of transition metal impurities in the lattice. With increasing cerium concentration the TL emission increase and also broaden.

Analysis of transitions to the fast ion conducting state in Ag1−xCuxI

Abstract Electrical conductivity versus temperature studies of Ag 1−x Cu x I for x ⩽0.25 have shown several steps in the transition to the fast ion conducting state observed with the higher temperature stable body centred cubic structure. Although there is not a total correspondence between such studies and those involving differential scanning calorimetry, there is a strong support for the existence of two hitherto undeterminedsolid state reactions at 160 and 180°C. The electrical conductivity data also suggests that there may be another reaction at between 130 and 150°C linked to the wurtzite-zinc blende transition at low values of x.