| Тойм: | <p>Vacancy-ordered double perovskites A<small><sub>2</sub></small>BX<small><sub>6</sub></small> are attracting attention due to the isolated octahedra making them an ideal playground for studying perovskite structure-property relationships. MA<small><sub>2</sub></small>TeCl<small><sub>6</sub></small> 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 <em>Fm3̄</em><em>m</em> (high temperature, phase I) to trigonal <em>P3̄</em><em>m</em>1 (intermediate temperature, phase II) at 460 K and then to trigonal <em>P3̄</em>1<em>c</em> (low temperature, phase III) at 200 K. Intriguingly, we observe a negative thermal expansion for the <em>c</em>-axis in phase II believed to caused by dynamic tilting of the C–N bond in the MA<small><sup>+</sup></small> ion. We have modified and extended the calculation of the Brown radii ratio in the perovskites to include hybrid A<small><sub>2</sub></small>BX<small><sub>6</sub></small> 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 <em>Fm3̄</em><em>m</em> cubic structure is observed, while for ratios above 1.17, mostly distorted structures are found.</p>
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