Structural order–disorder plays a decisive role in the physical properties of materials, such as magnetism, second-order harmonic generation, and ionic conductivity, and it is thus widely utilized to manipulate the crystal structure and understand structure–property correlations. Herein, we report the structural polymorphism, complex crystal structure and temperature-driven irreversible order–disorder phase transition of the polar oxides (Sr_1−xCa_x)SrZn₂Ga₂O₇. The low-temperature (LT) structure crystallizes in Pna2₁ with partial Zn/Ga ordering. Upon heating, (Sr_1−xCa_x)SrZn₂Ga₂O₇ undergoes an irreversible phase transition from orthorhombic Pna2₁ to hexagonal P6₃. Interestingly, the high-temperature (HT) P6₃ structure possesses an unexpected 3/2-fold superstructure rather than a substructure of the low-temperature (LT) Pna2₁ structure, which is a rare structural phenomenon in solid-state chemistry. This new HT superstructure is the most complex one in this series of oxides with 21 crystallographically independent sites determined accurately by a combination of the maximum entropy method and Rietveld refinement against high-resolution neutron powder diffraction data. In terms of the mechanism, this is a temperature-driven order-to-disorder transition in the oxygen sublattice. A careful structural analysis revealed that the oxygen disordering mainly occurs in the [SrO₃] layers of the HT structure and it can be understood as respective clockwise and anticlockwise rotations of distinct GaO₄-tetrahedra along the c-axis. Alternating current electrochemical impedance spectroscopic analysis revealed that the oxygen disordering in the HT structure is incapable of giving rise to oxide ionic conductivity but does lead to increased electronic conduction compared to the LT structure. The optical properties of the CaSrZn₂Ga₂O₇ and Sr₂Zn₂Ga₂O₇ representatives are also investigated in-depth via diffuse reflectance spectroscopy and theoretic calculations.

中文翻译：

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氧亚晶格中温度驱动的有序-无序结构转变和 CaSrZn2Ga2O7 高温多晶型物的复杂超结构

结构有序-无序对材料的物理性质起着决定性的作用，例如磁性、二次谐波产生和离子电导率，因此被广泛用于操纵晶体结构和理解结构-性质的相关性。_{在此，我们报告了极性氧化物 (Sr 1− x} Ca _x )SrZn ₂ Ga ₂ O ₇的结构多晶型、复杂晶体结构和温度驱动的不可逆有序-无序相变。低温 (LT) 结构在具有部分 Zn/Ga 排序的Pna 2 _{1中结晶。}加热后，(Sr _{1− x} Ca _x )SrZn₂ Ga ₂ O ₇经历了从正交晶系Pna 2 ₁到六方晶系P 6 ₃的不可逆相变。有趣的是，高温 (HT) P 6 ₃结构具有意想不到的 3/2 倍超结构，而不是低温 (LT) Pna 2 _{1的子结构}结构，这是固态化学中罕见的结构现象。这种新的 HT 超结构是该系列氧化物中最复杂的一种，具有 21 个晶体学独立位点，通过结合最大熵法和 Rietveld 精修高分辨率中子粉末衍射数据准确确定。就机制而言，这是氧亚晶格中温度驱动的有序到无序转变。仔细的结构分析表明，氧无序化主要发生在 HT 结构的 [SrO ₃ ] 层中，可以理解为不同的GaO ₄四面体沿c分别顺时针和逆时针旋转-轴。交流电化学阻抗谱分析表明，与 LT 结构相比，HT 结构中的氧无序化不能产生氧化物离子电导率，但会导致电子传导增加。还通过漫反射光谱和理论计算深入研究了CaSrZn ₂ Ga ₂ O ₇和Sr ₂ Zn ₂ Ga ₂ O ₇代表物的光学性质。