Indian Journal of Pure & Applied Physics Vol. 45, May 2007, pp. 459-464 High-pressure behaviour of rare earth mono bismuthides Gitanjali Pagare 1,2 , Sankar P Sanyal 2 & P K Jha 3 1 Government M L B Girls P G College, Bhopal 462 011 2 Condensed Matter Physics Laboratory, Department of Physics, Barkatullah University, Bhopal 462 026 3 Department of Physics, M S University of Baroda, Vadodra 390 002 2 E-mail: spsanyal@sancharnet.in Received 1 March 2006; revised 19 December 2006; accepted 4 January 2007 Pressure induced structural phase transition of mono-bismuthides of cerium, praseodymium, uranium and plutonium (REBi, RE= Ce, Pr, U and Pu) has been studied theoretically by using suitable two body inter-ionic potential which parametrically includes the effect of Coulomb screening by the delocalized f electrons of rare earth (RE) ion. The anomalous structural properties of these bismuthides, where Bi ion has the largest ionic radius, have been studied to investigate the role of f electrons on the structural phase transition. Except UBi, all the three compounds transform to body -centered tetragonal (BCT) structure at high pressure. UBi prefers to transform to cubic CsCl phase at high pressure. A static simulation technique has been used to calculate the equation of state, change in anion-anion and anion-cation distances with increasing pressure, for these compounds. The important finding is that, from the total energy point of view except UBi, all the compounds prefer to transform to BCT structure than CsCl structure at high pressure. Keywords: Phase transition, Tetragonal phase, Coulomb screening, Pnictides IPC Code: G12B 1 Introduction In recent years, the mono-pnictides of rare earths (RE) with NaCl structure have drawn considerable interest of material scientists, because of their diverse structural, transport, magnetic and vibrational properties 1-4 . The partially filled f-electron orbitals of RE atom are in general found to be responsible for the anomalous properties in these compounds. X-ray diffraction technique has been used recently by Shirotani et al 5 . to investigate the high-pressure structural properties in these compounds 5-7 , and it was observed that many of these compounds undergo a structural phase transition either to CsCl or body centered tetragonal (BCT) structures. High-pressure structural phase transformation of four rare earth pnictides i.e. CeBi, PrBi, UBi and PuBi has been studied theoretically. The mono-bismuthides contain the pnictogen ion with the highest Z and largest atomic radius that can be studied to investigate the role of p-f mixing on the structural properties in REBi compounds. These also have the p orbitals with the highest quantum numbers of the pnictogen. Therefore, the knowledge of the pressure behaviour of bismuthides is important in developing, comparing and understanding the systematics of the lanthanide and actinide mono pnictides. Also for a better description, it is necessary to take into account the hybridization of the f and conduction band orbitals, which can be related to the hybridization of the f and p orbitals of the metalloids. The pnictides of heavy lanthanide like GdY (Y=As, Sb, Bi) are magnetic semiconductors and have been studied experimentally by using magnetic neutron diffraction 8 . Amongst the actinides, the high-pressure experimental results are reported for the bismuthides of curium and uranium by Gensini et al 9 . Both the compounds show B 1 B 2 transition at high pressure and show hysteresis behaviour upon releasing the pressure. In the actinide series, Pu is considered to be the last member where 5f electrons are involved in the metallic bonding at normal temperature and pressure. Thus, there is a special interest in compounds of Pu to determine what effect pressure may have on the 5f electrons in compounds of Pu. The high-pressure X-ray diffraction studies of PuBi has been carried out by Meresse et al 10 . The PuBi is reported to transform to BCT structure at high pressure. The electronic structure and structural properties of LaSb and LaBi have been studied by means of self-consistent tight binding linear muffin tin orbital method 11 , and were predicted to have BCT phase at high pressure. X-ray diffraction of PrY and CeY (Y=P, As, Sb and Bi) compounds with NaCl