MAGNETIC-SUSCEPTIBILITY AND HEAT-CAPACITY STUDIES OF NIS2-XSEX SINGLE-CRYSTALS - A STUDY OF TRANSITIONS AT NONZERO TEMPERATURE

Heat-capacity and magnetic-susceptibility studies have been carried ou t on NiS2-xSex single crystals for 0.38 less than or equal to x less t han or equal to 0.58 and 0 less than or equal to x less than or equal to 0.71, respectively. These and earlier physical measurements documen t the gradual evolution, with rising x, of the alloys from good insula tors to poor metals at low temperatures. The transitions between Vario us magnetically ordered or disordered phases are marked by anomalies i n these physical measurements. The trend of magnetic susceptibility wi th temperature indicates that alloys near the crossover to the highly correlated metallic state exhibit increasing charge-carrier localizati on with rising temperature; this is ascribed to the dominance of entro pic contributions to the free energy. It is stressed that these variat ions in properties are achieved by isoelectronic substitutions in the anion sublattice that leave the cation sublattice undisturbed.