Abstract Са3(VO4)2-based compositions are considered as promising multifunctional materials combining ferroelectric, optical nonlinear, and Ca2+-ion conductive properties. Their ferroelectric Curie points stretch from minimal for β-Ca3(PO4)2-type compounds temperatures of about 800 K to very high. Investigated in this paper lead substitution for calcium is as a factor controlling ferroelectricity, ionic-conductivity and non-linear optical activity in Ca3(VO4)2-based materials. Polar phase containing powders and ceramics in Ca10.5-xPbx(VO4)7 system are synthesized for 0 ≤ x ≤ 9.5 by the solid state method, and structurally characterized with X-ray powder diffraction and transmission electron microscopy. Dielectric properties, differential thermal analysis and second harmonic generation (SHG) evidence that Ca10.5-xPbx(VO4)7 solid solution (0 ≤ x ≤ 4.5) belongs to whitlockite-type ferroelectrics. SHG activity strongly increases with x up to its maximum at x = 4.5, where it has a record value among all studied before Ca3(XO4)2-related compounds (X = P,V). Ferroelectric Curie temperatures of Ca3(VO4)2 drops from its known value Tc = 1368 K (x = 0) to 770 K (x = 4.5). Crystal symmetry at Tc changes from R3c to R 3 ¯ c . After this, one more phase transition to the symmetry R 3 ¯ m takes place, its temperature bringing down from 1387 K (x = 0) to 804 K (x = 4.5). Ferroelectric and non-ferroelectric phase transitions in the Са10.5-xPbx(VO4)7 are separated by a broad interval Δ T = 20–50 K and both classified as first-order transformations going in the sequence: R3c↔ R 3 ¯ c ↔ R 3 ¯ m . Structures of Са10.5-xPbx(VO4)7 compositions with x = 0.5–4 were refined by the Rietveld method and peculiarities of the Pb2+cations distribution in the M1 - M3 and M4 sites of β-Ca3(PO4)2-type structure are discussed regarding the optical nonlinear, ferroelectric and ion-conductive properties. Manifold increased Ca2+- ion conductivity of Са10.5-xPbx(VO4)7 in vicinity of 1000 K at x = 4–4.5 in combination with their ferroelectric and optical nonlinear properties extends applicability of ion-exchange technologies to new promising materials.

中文翻译：

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Са 10.5-x Pb x (VO 4 ) 7 铁电体中增强的非线性光学活性和Ca 2+ -电导率

摘要 Са3(VO4)2 基组合物被认为是一种很有前途的多功能材料，它结合了铁电、光学非线性和 Ca2+ 离子导电性能。它们的铁电居里点从 β-Ca3(PO4)2 型化合物的最低温度约 800 K 延伸到非常高。本文研究了铅替代钙是控制 Ca3(VO4)2 基材料中铁电性、离子导电性和非线性光学活性的一个因素。采用固相法合成了0≤x≤9.5的Ca10.5-xPbx(VO4)7体系中含有极性相的粉末和陶瓷，并用X射线粉末衍射和透射电子显微镜对其进行了结构表征。介电特性、差热分析和二次谐波产生 (SHG) 证明 Ca10.5-xPbx(VO4)7 固溶体 (0 ≤ x ≤ 4. 5）属于白铁矿型铁电体。SHG 活性随 x 强烈增加，在 x = 4.5 处达到最大值，在与 Ca3(XO4)2 相关的化合物 (X = P,V) 之前的所有研究中，它具有创纪录的值。Ca3(VO4)2 的铁电居里温度从其已知值 Tc = 1368 K (x = 0) 下降到 770 K (x = 4.5)。Tc 处的晶体对称性从 R3c 变为 R 3 ¯ c 。在此之后，再发生一次到对称性 R 3 ¯ m 的相变，其温度从 1387 K (x = 0) 降至 804 K (x = 4.5)。Са10.5-xPbx(VO4)7 中的铁电和非铁电相变被一个很宽的区间 Δ T = 20-50 K 分开，并且都被归类为按以下顺序进行的一阶转换：R3c↔ R 3 ¯ c ↔ R 3 ¯ m 。x = 0 的Са10.5-xPbx(VO4)7 组合物的结构。5-4 是通过 Rietveld 方法精制的，并讨论了 β-Ca3(PO4)2 型结构的 M1-M3 和 M4 位点中 Pb2+阳离子分布的特性，包括光学非线性、铁电和离子导电特性。在 x = 4-4.5 时，Са10.5-xPbx(VO4)7 在 1000 K 附近的多种增加的 Ca2+- 离子电导率与其铁电和光学非线性特性相结合，扩展了离子交换技术对新材料的适用性。