High pressure structural investigation on LaGa

Abstract

High pressure X-ray diffraction studies on rare-earth gallide LaGa was carried up to a pressure of ∼28 GPa using synchrotron, as well as rotating anode X-ray source in an angle dispersive mode. LaGa exhibits CrB (B33)-type orthorhombic structure (space group Cmcm) at ambient pressure. It undergoes a reversible structural phase transition from orthorhombic to tetragonal structure (Space. Group P4/mmm) at ∼5 GPa. Both the phases coexist up to the highest pressure studied. The transition to tetragonal phase has implications in predicting the new intermediate phase in the generally observed B1 to B2 transition sequence in AB type of rare-earth intermetallics. This investigation reveals that the LnX-type compounds may exhibit the structural sequence B1→ B33→ P4/mmm→ B2 under pressure. Considering the variation of lattice parameters a, b, c and also various bond lengths with pressure, the B33 to P4/mmm transition in LaGa may be displacive in nature. The zero pressure bulk modulus and its derivative for parent phase have been estimated to be B0 = 60 ± 3 GPa and = 4.6 ± 1.5. The bulk modulus for the high pressure tetragonal phase at ∼8 GPa also has been estimated to be B8 = 77 ± 8 GPa. We have also done the non-spin-polarized total energy calculations using the full potential linearized augmented plane wave method as implemented in the WIEN2 K code which confirms the experimental results.