Phase Transitions in the Ruddlesden–Popper Phase Li2CaTa2O7: X-ray and Neutron Powder Thermodiffraction, TEM, Raman, and SHG Experiments

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• 作者：Cyrille Galven ; Denis Mounier ; Boris Bouchevreau ; Emmanuelle Suard ; Alain Bulou ; Marie-Pierre Crosnier-Lopez ; Françoise Le Berre
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：March 7, 2016
• 年：2016
• 卷：55
• 期：5
• 页码：2309-2323
• 全文大小：842K
• ISSN：1520-510X

文摘

The structure of the Ruddlesden–Popper layered perovskite Li₂CaTa₂O₇, known for its high photocatalytic water activity since its discovery in 2008, is reinvestigated. This oxide has been characterized by powder X-ray and neutron thermodiffraction, TEM, second harmonic generation (SHG), and Raman experiments on powders and single crystals. It is shown that it undergoes two structural phase transitions (i) around 220 °C, mainly characterized by the progressive emergence of SHG signal at low temperatures, and (ii) at 660 °C, mainly characterized by changes of the temperature behavior of lattice parameters and by the emergence of Raman signals that linearly increase on decreasing temperature. It is shown by powder neutron diffraction profile refinements at RT, 400, and 800 °C that the space groups of the successive phases of Li₂CaTa₂O₇ are the acentric Pna2₁ (RT ≤ T ≤ 220 °C), Pnma (220 °C ≤ T ≤ 660 °C), and Cmcm (T ≥ 660 °C). A soft mode associated with the transition to the highest symmetry for this structural arrangement (I4/mmm) is also found in the Raman spectra. All these transitions appear continuous: the high temperature ones can be attributed to progressive vanishings of the octahedra tiltings (displacives) while the transition in the vicinity of 220 °C from Pna2₁ to Pnma exhibits order–disorder character.