Belik Alexei A, Deyneko Dina V, Baryshnikova Oksana V, Stefanovich Sergey Yu, Lazoryak Bogdan I
Research Center for Functional Materials, National Institute for Materials Science Namiki 1-1 Tsukuba Ibaraki 305-0044 Japan.
Department of Chemistry, Moscow State University Moscow 119992 Russia
RSC Adv. 2020 Mar 17;10(18):10867-10872. doi: 10.1039/c9ra10336h. eCollection 2020 Mar 11.
SrIn(VO) was prepared by a solid-state method at 1270 K in air. This vanadate has the β-Ca(PO)-type structure and crystallizes in polar space group 3. The structural parameters of SrIn(VO) were refined by the Rietveld method from laboratory powder X-ray diffraction data (XRD): the lattice parameters are = 11.18016(9) Å and = 39.6170(3) Å with = 6. In cations occupy the octahedral M5 site, Sr cations occupy the M1, M2, and M3 sites of the β-Ca(PO)-type structure, and the M4 site remains vacant. SrIn(VO) was characterized by differential thermal analysis (DTA), optical second-harmonic generation (SHG), high-temperature XRD, and dielectric measurements. All these methods prove the existence of a ferroelectric-paraelectric phase transition at = 974 K. This transition is compared with a similar transition in CaIn(PO) with lower = 902 K. The polar-to-centrosymmetric phase transition in such compounds has a quite unique mechanism of the order-disorder type. The structural transition involves slight shifts of the M1, M2, M3 cations and the E2O, E3O tetrahedra, while half of the E1O tetrahedra (E = P or V) statistically reverse their orientation along the three-fold axis, so that the centre of symmetry appears in the structure as a whole. To invert the E1O tetrahedron, one oxygen anion should pass a large neighbouring cation (Sr or Ca) that is only possible when intense rotational vibrations of the tetrahedra are excited at high temperatures. The lower Curie temperature in CaIn(PO) corresponds to the smaller rotational vibration amplitude of the P1O tetrahedron required to reverse this tetrahedra at in comparison with V1O in SrIn(VO).
通过固态法在空气中1270K制备了SrIn(VO)。这种钒酸盐具有β-Ca(PO)型结构，结晶于极性空间群3。通过Rietveld方法从实验室粉末X射线衍射数据（XRD）对SrIn(VO)的结构参数进行了精修：晶格参数为a = 11.18016(9) Å和c = 39.6170(3) Å，Z = 6。In阳离子占据八面体M5位点，Sr阳离子占据β-Ca(PO)型结构的M1、M2和M3位点，M4位点保持空位。通过差示热分析（DTA）、光学二次谐波产生（SHG）、高温XRD和介电测量对SrIn(VO)进行了表征。所有这些方法都证明在974K存在铁电-顺电相变。将该转变与CaIn(PO)中较低的902K的类似转变进行了比较。此类化合物中从极性到中心对称的相变具有相当独特的有序-无序型机制。结构转变涉及M1、M2、M3阳离子以及E2O、E3O四面体的轻微位移，而一半的E1O四面体（E = P或V）沿三重轴统计性地反转其取向，从而使整个结构中出现对称中心。要使E1O四面体反转，一个氧阴离子应通过一个大的相邻阳离子（Sr或Ca），这只有在高温下四面体强烈的旋转振动被激发时才可能。与SrIn(VO)中的V1O相比，CaIn(PO)中较低的居里温度对应于在该温度下使P1O四面体反转所需的较小旋转振动幅度。
