Temperature-driven order–disorder structural transition in the oxygen sub-lattice and the complex superstructure of the high-temperature polymorph of CaSrZn2Ga2O7

• Published in: Dalton transactions : an international journal of inorganic chemistry 2022-12, Vol.51 (48), p.18549-18561
• Main Authors: Qin, Jie, Jiang, Pengfei, Lu, Guangxiang, Wang, Rong, Yang, Tao
• Format: Article
• Language: English
• Subjects: Crystal structure; Crystallography; Electrochemical impedance spectroscopy; Harmonic generations; High temperature; Ion currents; Low temperature; Magnetic properties; Material properties; Maximum entropy method; Neutron diffraction; Optical properties; Oxygen; Phase transitions; Physical properties; Polymorphism; Structural analysis; Superstructures; Tetrahedra
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d2dt03145k

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 (Sr1−xCax)SrZn2Ga2O7. The low-temperature (LT) structure crystallizes in Pna21 with partial Zn/Ga ordering. Upon heating, (Sr1−xCax)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.