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首页> 外文期刊>RSC Advances >Sr9In(VO4)7 as a model ferroelectric in the structural family of β-Ca3(PO4)2-type phosphates and vanadates
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Sr9In(VO4)7 as a model ferroelectric in the structural family of β-Ca3(PO4)2-type phosphates and vanadates

机译：SR9（VO4）7作为β-CA3（PO4）2型磷酸盐和钒酸盐结构家族的模型铁电

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Sr _(9) In(VO _(4) ) _(7) was prepared by a solid-state method at 1270 K in air. This vanadate has the β-Ca _(3) (PO _(4) ) _(2) -type structure and crystallizes in polar space group R 3 c . The structural parameters of Sr _(9) In(VO _(4) ) _(7) were refined by the Rietveld method from laboratory powder X-ray diffraction data (XRD): the lattice parameters are a = 11.18016(9) ? and c = 39.6170(3) ? with Z = 6. In ~(3+) cations occupy the octahedral M5 site, Sr ~(2+) cations occupy the M1, M2, and M3 sites of the β-Ca _(3) (PO _(4) ) _(2) -type structure, and the M4 site remains vacant. Sr _(9) In(VO _(4) ) _(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 Ca _(9) In(PO _(4) ) _(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 (Sr ~(2+) or Ca ~(2+) ) that is only possible when intense rotational vibrations of the tetrahedra are excited at high temperatures. The lower Curie temperature in Ca _(9) In(PO _(4) ) _(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 Sr _(9) In(VO _(4) ) _(7) .
机译：SR _（9）IN（VO _（4））_（7）通过在空气中的1270 k处通过固态方法制备。该钒酸盐具有β-Ca_（3）（PO _（4））_（2）型结构，并在极性空间组R 3 C中结晶。 SR _（9）中的结构参数（VO _（4））_（7）由实验室粉末X射线衍射数据（XRD）改进：晶格参数是A = 11.18016（9）？和c = 39.6170（3）？ Z = 6.在〜（3+）阳离子中占据八面体M5位点，SR〜（2+）阳离子占据β-CA _（3）的M1，M2和M3位点（PO _（4）） _（2） - 型结构，M4位点仍然空置。 SR _（9）IN（VO _（4））_（7）的特征在于差分热分析（DTA），光学第二谐波产生（SHG），高温XRD和电介质测量。所有这些方法都证明了在T _（c）= 974k处的铁电 - 琼脂相转变的存在。将该转变与Ca _（9）中的类似转变进行比较（PO _（4））_（7）降低T _（c）= 902 K.此类化合物中的偏光偏差相转变具有相当独特的秩序障碍型机制。结构转变涉及M1，M2，M3阳离子和E2O_（4），E3O _（4）四面体的轻微偏移，而E1O _（4）四（E = P或V）的一半统计上逆转其取向沿三折轴，使对称中心整体出现在结构中。倒置E1O _（4）四面体，一个氧气阴离子应通过大型相邻阳离子（Sr〜（2+）或Ca〜（2+）），只有在高温下振奋的强烈旋转振动时才可能。 CA _（9）中的较低居里温度（PO _（4））_（7）对应于P1O _（4）四面体所需的较小旋转振动幅度，以便在T _（c）中逆转该Tetrahedra（c）。使用V1O _（4）在SR _（9）中（VO _（4））_（7）。

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