Thermal Expansion and Crystal Distortion of NiS₂

Abstract

Thermal expansion of NiS₂ single crystal was measured by strain gauge method with the interest in the crystal distortion below T_N1 and/or T_c. The lattice contracts abruptly below T_c. The contractions between T_c and 15 K along [100], [110] and [111] are Δ₁₀₀=12×10⁻⁵, Δ₁₁₀=20×10⁻⁵ and Δ₁₁₁=24×10⁻⁵, respectively. It is inferred that NiS₂ undergoes a slight distortion from cubic symmetry below T_c. On the assumption that NiS₂ had been cooled in a stress along the direction of strain gauge, it is concluded that NiS₂ undergoes a tetragonal distortion. The tetragonality (c−a)⁄a is estimated to be 1.6×10⁻⁴.