Thermal structural characterization of the acentric layered perovskite LiHSrTa2O7: X-ray and neutron diffraction, SHG and Raman experiments

Abstract

The present work concerns the thermal structural characterization of the acentric Ruddlesden–Popper LiHSrTa₂O₇. A previous study, performed with powder neutron diffraction data, has revealed that at room temperature, LiHSrTa₂O₇ crystallizes in the Ama2 space group and that the acentric character is mainly due to the unequal distribution of the Li⁺ and H⁺ cations on their sites. In this new paper, the thermal behaviour has been studied by several techniques: powder X-ray and neutron diffraction, SHG experiments and Raman spectroscopy. All of them have revealed that LiHSrTa₂O₇ undergoes a reversible structural transition from an orthorhombic to a tetragonal symmetry around 200 °C. This transition is associated with the progressive vanishing of the TaO₆ octahedra tilting, becoming completely straight in the high temperature form (S.G. I4/mmm), and with a variation of the Li⁺ and H⁺ distribution in the interlayer spacing.