X-ray study with synchrotron radiation of cerium and praseodymium monopnictides with the NaCl-type structure at high pressures

Abstract

By use of synchrotron radiation, powder X-ray diffraction studies for cerium and praseodymium monopnictides [CeX and PrX (X=P, As, Sb and Bi)] with the NaCl-type structure have been carried out up to 40 GPa at room temperature. We have investigated in detail the pressure ( P) vs. cell volume ( V) relationship for CeP, CeAs and CeSb. The phase transition from the NaCl-type (B1) to the CsCl-type (B2) structure is observed at around 25 GPa for CeP and 21 GPa for CeAs. The transition pressures of both compounds are about 6 GPa higher than the results previously reported. Sevan et al. have predicted that a second isostructural B2–B2 transition for CeSb occurs at around 25.2 GPa with a volume collapse of 4%. However, we cannot find the volume collapse for CeSb at least up to 35 GPa. First-order phase transitions with the crystallographic change for PrX (X=P, As and Sb) are found at 26 GPa for PrP, 27 GPa for PrAs, 13 GPa for PrSb. The structure of the high-pressure phase of these compounds is tetragonal (distorted CsCl-type, P4/mmm). For PrBi, anomalous behavior in the X-ray diffraction pattern is observed around 4 GPa. This may be due to a pressure-induced intermediate phase. The structural phase transition in PrBi occurs at about 14 GPa. The high-pressure phases of PrBi consists of coexisting of tetragonal (distorted CsCl-type, P4/mmm) and cubic (CsCl-type, Pm3m) modifications at about 14 GPa. The bulk modulus of CeX and PrX (X=P, As, Sb and Bi) is obtained from the P– V curve fitted by the Birch equation of state. The relationship between bulk modulus and cell volume for these compounds is studied.