Phase transitions and optical properties of the trigonal perovskite (CH3NH3)2TeCl6

Vacancy-ordered double perovskites A(2)BX(6) are attracting attention due to the isolated octahedra making them an ideal playground for studying perovskite structure-property relationships. MA(2)TeCl(6) single crystals have been successfully grown from solution. X-ray structure experiments as a function of temperature have demonstrated two structural phase transitions: from cubic Fm3m (high temperature, phase I) to trigonal P3m1 (intermediate temperature, phase II) at 460 K and then to trigonal P31c (low temperature, phase III) at 200 K. Intriguingly, we observe a negative thermal expansion for the c-axis in phase II believed to caused by dynamic tilting of the C-N bond in the MA(+) ion. We have modified and extended the calculation of the Brown radii ratio in the perovskites to include hybrid A(2)BX(6) compounds by selecting the shortest A-X distance for the ratio equation. We find that for ratios between 0.89 and 1.17, typically the Fm3m cubic structure is observed, while for ratios above 1.17, mostly distorted structures are found.

Automated summary

Vacancy-ordered double perovskites A(2)BX(6) with isolated octahedra are ideal for studying perovskite structure-property relationships. We grew MA(2)TeCl(6) single crystals from solution and found two structural phase transitions as a function of temperature. Interestingly, we observed negative thermal expansion for the c-axis in phase II.