On the structural polymorphism of CePt2Sn2: experiment and theory

Samples of the heavy fermion compound CePt2Sn2 with varying compositions ha ve been synthesised and examined from room temperature up to the melting po int. Two crystallographic structures of the intermetallic alloy CePt2Sn2 we re observed and they were systematically investigated using X-ray powder di ffraction. The structural phase transition between the two variants has bee n studied using an in situ high temperature X-ray powder diffraction camera . The effect of varying the stoichiometry on the structural properties has been investigated. The primitive tetragonal phase with the CaBe2Ge2-type st ructure (space group P4/nmm, a=4.5877(2) Angstrom, c=10.3984(6) A) is stabl e as an equilibrium phase over the entire temperature range and is favoured by lower Pt:Ce ratios, while the monoclinic LaPt2Ge2-type structure (space group P2(1), a=4.5935(3) Angstrom, b=4.5890(3) Angstrom, c=10.4128(7) Angs trom, beta=91.425(6)degrees) appears as a metastable phase during the quenc hing process, and is favoured by higher Pt:Ce ratios. The crystal structure s of both modifications have been refined by the Rietveld method and their structural relationships are discussed. Theory shows that the 4f states of Ce are essentially localised and that a phase transition from tetragonal to monoclinic should occur as a function of applied pressure. (C) 2000 Elsevi er Science S.A. All rights reserved.