Structural characterization of a new acentric Ruddlesden–Popper layered perovskite compound: LiHSrTa2O7
Résumé
A new n = 2 member acentric Ruddlesden–Popper layered perovskite LiHSrTa2O7 (LiDSrTa2O7) has been synthesized and structurally characterized from Rietveld treatment of its powder X-ray and high-resolution neutron diffraction data. It can be synthesized by a partial Li+/H+ exchange from the mother phase Li2SrTa2O7 either in solid state by NH4Cl or in dilute HNO3 by controlling the amount of H+. This compound crystallizes in the orthorhombic acentric space group Ama2 (no. 40) with lattice constants a = √2ap≈ 5.5522(1) Å, b = √2ap≈ 5.5248(1) Å and c≈ 18.7745(4) Å. Classically, Ta5+ ions occupy the octahedral sites of the kinked perovskite blocks and Sr2+ ions completely fill the perovskite cages while Li+ and D+ ions are found in the interlayer spacing. Efficient positive second harmonic generation response, performed at room temperature on a polycrystalline sample, shows unambiguously the acentric character of this new phase. Interestingly, the choice of the acentric Ama2 space group to describe the structure is revealed only by high-resolution neutron diffraction data: in the interlayer spacing, Li+ and D+ cations are unequally distributed on different sites (two 4a sites for Li+ and two 4b sites for D+ ions).