Pressure-induced phase transition of SmBi with a NaCl-type structure

By use of synchrotron radiation, powder x-ray diffraction of SmBi with a NaCl-type structure has been studied up to 30 GPa at room temperature. A first-order phase transition began to occur at around 16.2 GPa. SmBi transformed from the NaCl-type (B1) to the tetragonal (distorted CsCl-type) structure with the volume collapse of about 8.2 % at around 18.3 GPa. Crystal data of the single phase are a = 3.775(5) A, c = 3.25 (2) A, c/a = 0.86 and V = 46.4 (3) A3 at 20.3 GPa. The high-pressure form of SmBi is isostructural with that of SmAs. A bulk modulus of SmBi was estimated from the volume vs. pressure curve fitted by a Birch equation of state. The bulk modulus (B0) and its pressure derivative (B0') are 68 ± 3 GPa and 3.9 ± 0.3, respectively. The transition pressure and the bulk modulus of SmBi are larger than those of CeBi and PrBi. The structural high-pressure behaviour for SmX (X = P, As, Sb and Bi) is discussed. The structural phase transition of LaBi and NdBi occurred at around 11.5 GPa and 16.8 GPa, respectively. The transition pressures of LnBi (Ln = La, Ce, Pr, Nd and Sm) decreased with increasing lattice constant.