Infrared spectroscopic studies on spinel-type chromium indium sulfide solid solutions

Far infrared reflection spectra (near normal incidence) of metastable, quenched spinel-type cobalt chromium indium sulfide solid solutions CoCr2-xIn2xS4 were recorded and analysed in order to elucidate the behaviour of the four IR allowed phonon modes on going from (Co) [Cr2]S4 with normal metal atom distribution to (In)[CoIn]S4 with an inverse one. The energies of the transversal and longitudinal optic phonon modes were determined from dielectric dispersion relations epsilonʹʹ and -Im(1/epsilon), respectively, via Kramers Kronig analyses. The highest-energy phonon mode exhibits one-mode behaviour whereas the three low-energy phonons reveal two-mode behaviour in contrast to M1-xMʹxCoCr2S4 spinel-type solid solutions (with M and Mʹ bivalent metal atoms at the tetrahedral 8a site), which mainly display one-mode behaviour. This different behaviour of CoCr2-xIn2xS4 solid solutions is owing to the change of the metal atom distribution mentioned above in addition to the different masses of the atoms involved. The one-mode behaviour of the highest-energy phonon mode at 389 and 401 cm-1 (CoCr2S4) and at 323 and 350 cm-1 (CoIn2S4) is due to the fact that in the eigenvectors of this phonon almost only motions of the non-metal atoms are involved. The spectra of the solid solutions under investigation, which decompose on annealing at elevated temperatures to chromium and indium rich specimens, reveal further that there is a random distribution of the atoms at the octahedral 16d sites not only in the X-ray but also in the vibration spectroscopic size scale.