International Journal of Advanced Technology & Engineering Research (IJATER) International Conference on “Recent Advancement in Science & Technology” (ICRAST 2017) © IJATER (ICRAST- 2017) 78 STRUCTURAL PHASE STABILITY AND HALF- METALLIC BEHAVIOUR OF ACTINIDE BISMUTHIDES ( UBi and NpBi) Namrata Yaduvanshi High Pressure Research Lab, Department of Physics, Barkatullah University, Bhopal namrata-yaduvanshi@yahoo.com Abstract The structural phase stability and mechanical properties of two mono-bismuthides (UBi and NpBi) are investigated by using a improved interection potential model(IIPM), employing the minimization technique of Gibbs free energy expressed as a function of pressure. These compound undergoes phase transition from rocksalt type (B1) to tetragonal (distorted CsCl-type, P4/mmm) structure. The calculated values of phase transition pressure and associated volume collapse are found to be well suited with experimental values. We have also calculated the mechanical and thermophysical properties of UBi and NpBi. Keywords: phase transiton; three body interaction; volume collapse; elastic properties. Introduction The uniqueness of rare-earth (RE) has been interesting in theoretical and experimental studies of rare-earth compounds (REX) with NaCl (B1) structure [1]. The study of actinide compounds (AnY) with NaCl (B1) structure are very interesting in theoretical and experimental work. These compounds exhibit a rich variety of electronic and magnetic properties, which can be interpreted in terms of mix valence or valence fluctuations that come from the partially filled 5f electrons in An-An atom of rare earth ions [2]. The phase transition of CuBi and UBi from B1 to BCT transition around 8.5 GPa with volume collapse 12% and isothermal bulk moduli (BT) also observed to be 145 GPa at ambient pressure are reported experimentally by using X-ray diffraction method [3]. The phase transition and volume collapse of PuBi is studied by Petit et. al. [4] using SIC-LSD scheme technique. The structural study of PuBi from NaCl to body centered tetragonal structure using X-ray diffraction technique is reported by Meresse et. al. [5]. Structural, mechanical and thermal properties of NpBi and PuBi is investigated by Ahirwar et. al. [6] using modified inter-ionic potential theory (MIPT). Benedict et.al. [7]. calculated the lattice parameter (a 0 =6.3732Å) of NpBi. Using two body potential model Pagare et. al. [8] determined the phase transformation of four bismuthides (CeBi, PrBi, UBi and PuBi). It is interesting to study effect of pressure on actinide bismuthides. In the present study we have investigated the phase transition in uranium and neptunium bismuthides which crystallize in NaCl to distorted CsCl structure. From the survey of various literatures it is clear that no study is done on UBi and NpBi using the improved interaction potential model (IIPM). Consequently, the main purpose of this work is to provide additional information to the existing data on the mechanical and thermal properties of UBi and NpBi by using improved interaction potential model (IIPM) for the betterment of the results[9]. Computational Methods Application of pressure directly results in compression leading to the increased charge transfer due to the deformation of overlapping electron shell of the adjacent ions (or non rigidity of ions) in solids. These effects have been incorporated in the Gibbs free energy (G = U+PV-TS) as a function of pressure and three body interaction which are the most dominant among the many body interactions. Here, U is the internal energy of the system. At temperature T = 0K and pressure P the Gibbs free energies for NaCl (B1) and BCT structures are given by: G B1 (r) = U B1 (r) + PV B1 (1) G BCT (r′) = U BCT (r′) + PV BCT (2)