Galven Cyrille, Mounier Denis, Bouchevreau Boris, Suard Emmanuelle, Bulou Alain, Crosnier-Lopez Marie-Pierre, Le Berre Françoise
LUNAM Université du Maine, Institut des Molécules et Matériaux du Mans (IMMM), UMR CNRS 6283 , Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.
Ecole Nationale Supérieure d'Ingénieurs du Mans (ENSIM), Université du Maine , Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.
Inorg Chem. 2016 Mar 7;55(5):2309-23. doi: 10.1021/acs.inorgchem.5b02659. Epub 2016 Feb 22.
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.
对Ruddlesden-Popper层状钙钛矿Li2CaTa2O7的结构进行了重新研究，该化合物自2008年发现以来就因其高光催化水活性而闻名。通过粉末X射线和中子热衍射、透射电子显微镜（TEM）、二次谐波产生（SHG）以及对粉末和单晶的拉曼实验对这种氧化物进行了表征。结果表明，它经历了两个结构相变：（i）在220°C左右，主要特征是低温下SHG信号逐渐出现；（ii）在660°C，主要特征是晶格参数的温度行为发生变化以及拉曼信号的出现，这些信号在温度降低时线性增加。通过在室温、400°C和800°C下的粉末中子衍射谱精修表明，Li2CaTa2O7连续相的空间群分别为非中心对称的Pna21（室温≤T≤220°C）、Pnma（220°C≤T≤660°C）和Cmcm（T≥660°C）。在拉曼光谱中还发现了与这种结构排列向最高对称性（I4/mmm）转变相关的软模。所有这些转变似乎都是连续的：高温转变可归因于八面体倾斜（位移型）的逐渐消失，而在220°C附近从Pna21到Pnma的转变表现出有序-无序特征。
