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首页> 外文期刊>RSC Advances >Sr9In(VO4)(7) as a model ferroelectric in the structural family of beta-Ca-3(PO4)(2)-type phosphates and vanadates
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Sr9In(VO4)(7) as a model ferroelectric in the structural family of beta-Ca-3(PO4)(2)-type phosphates and vanadates

机译：SR9（VO4）（7）作为β-CA-3结构家族的模型铁电，（2）型磷酸盐和钒酸盐

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Sr9In(VO4)(7) was prepared by a solid-state method at 1270 K in air. This vanadate has the beta-Ca-3(PO4)(2)-type structure and crystallizes in polar space group R3c. The structural parameters of Sr9In(VO4)(7) were refined by the Rietveld method from laboratory powder X-ray diffraction data (XRD): the lattice parameters are a = 11.18016(9) angstrom and c = 39.6170(3) angstrom with Z = 6. In3+ cations occupy the octahedral M5 site, Sr2+ cations occupy the M1, M2, and M3 sites of the beta-Ca-3(PO4)(2)-type structure, and the M4 site remains vacant. Sr9In(VO4)(7) 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 T-c = 974 K. This transition is compared with a similar transition in Ca9In(PO4)(7) with lower T-c = 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(4), E3O(4) tetrahedra, while half of the E1O(4) 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(4) tetrahedron, one oxygen anion should pass a large neighbouring cation (Sr2+ or Ca2+) that is only possible when intense rotational vibrations of the tetrahedra are excited at high temperatures. The lower Curie temperature in Ca9In(PO4)(7) corresponds to the smaller rotational vibration amplitude of the P1O(4) tetrahedron required to reverse this tetrahedra at T-c in comparison with V1O(4) in Sr9In(VO4)(7).
机译：Sr9In（VO4）（7）在空气中1270ķ制备的固态方法。此钒酸盐具有β-CA-3（PO 4）（2）型结构和R3C极性空间群结晶。 Sr9In（VO4）（7）的结构参数，通过从实验室粉末X射线衍射数据（XRD）的Rietveld法精制：晶格参数为a = 11.18016（9）埃和c = 39.6170（3）埃其中Z = 6 IN3 +阳离子占据八面体M5站点，SR2 +阳离子占据所述β-的Ca-3（PO 4）（2）型结构的M1，M2和M3的网站，和M4站点保持空缺。 Sr9In（VO4）（7）的特征在于通过差热分析（​​DTA），光学二次谐波产生（SHG），高温XRD，和介电测量。所有这些方法证明在Tc = 974 K.一个铁电 - 顺电相变的存在这种转变与在Ca9In（PO 4）（7）类似的过渡相比具有较低=锝902 K.极性到中心对称相变这些化合物中具有有序 - 无序型的相当独特的机构。结构转变涉及M1，M2，M3阳离子和E2O（4），E3O的轻微位移（4）四面体，而E1O的一半（4）四面体（E = P或V）统计学反转它们的取向沿三个倍轴，使对称的中心出现在结构作为一个整体。反转E1O（4）四面体，一个氧阴离子应该通过一个大的相邻阳离子（SR2 +或Ca2 +）时的四面体的强烈旋转振动在高温下激发时才是可能的。在Ca9In（PO4）较低居里温度（7）对应于P1O的小旋转振动振幅所需的（4）四面体扭转这种四面体在T-C在比较V1O（4）在Sr9In（VO4）（7）。

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