Sphalerite-wurtzite phase transformation in CdS

An analytical study of the solid-solid phase transformation from the cubic zinc blende or sphalerite (S) metastable modification of CdS (beta -CdS) to the wurtzite (W) hexagonal stable phase (alpha -CdS) is presented. Polycrystalline CdS layers in the cubic phase were prepared on glass substrates by means of chemical bath deposition. Films were heated in Ar + S-2 in the temperature (T) range 100-550 degreesC. X-ray diffraction data of annealed samples allowed us to observe a process of gradual change from beta- to alpha -CdS. Optical absorption spectra let us obtain the band-gap energy (E-g) of samples (both as grown and annealed). The unit-cell volume (V) vs T line shape was explained as an interface movement of the S-W interdomain wall driven by T. The E-g vs T plot exhibits a minimum as a sharp peak at 300 degreesC, which is the T value assigned by us to the critical point (T-c) of the experimental structural transformation. The E-g vs V curve indicates an E-g = AV(2) + BV+ C functional behavior. This E-g vs V dependence was explained through a relation between (partial derivativeE(g)/partial derivativeT)(P) and C-P. where C-P is the specific heat at constant pressure. The C-P against T function was also calculated in the interval 300-800 K.