LOW-TEMPERATURE HEAT-CAPACITY AND MAGNETIC-PROPERTIES OF THE RNIX2 COMPOUNDS (R = LA, CE, X = SI, GE, SN)

Heat-capacity (1.4-80 K) and magnetic-susceptibility (1.5-300 K) measurements were carried out for the series of six ternary intermetallic compounds with RNiX2 composition (R = La, Ce and X = Si, Ge, and Sn). All of the LaNiX2 compounds exhibit normal metallic behavior with an electronic specific-heat coefficient in the range 5.2-5.6 mJ/mol K2 and Debye temperature decreasing from 451 K (LaNiSi2) to 258 K (LaNiSn2). The CeNiX2 compounds display anomalous behaviors. CeNiSi2 is an intermediate-valence compound at high temperatures with the cerium valence varying from 3.35+ at room temperature to 3.65+ at T = 50 K, and becomes a spin fluctuator at low (around 3.3 K) temperature. The spin fluctuations can be quenched by an external magnetic field of about 5.3 T. Both CeNiGe2 and CeNiSn2 undergo two-step antiferromagnetic phase transitions at T(N)II = 3.9 K and T(N)I = 3.2 K, which are accompanied by sharp anomalies for all properties studied. The electronic specific constants (in units of mJ/mol Ce K2) of the three Ce compounds were 45.3 +/- 0.6 for CeNiSi2, 97.6 +/- 0.9 for CeNiGe, and 60.8 +/- 0.7 for CeNiSn2.