Ball milling enables phase-pure synthesis of a temperature sensitive ternary chloride, MgZrCl$_6$

Abstract: Ball milling is a powerful synthetic tool for discovering new inorganic materials. Inspired by the high ionic conductivity in Li$_2$ZrCl$_6$ synthesized via mechanochemistry, we synthesized MgZrCl$_6$ with a similar method. High resolution synchrotron X-ray diffraction shows that MgZrCl$_6$ is poorly crystalline after ball milling, but crystallizes in a layered hexagonal structure ($P31c$) after heat treatment. In situ synchrotron X-ray diffraction reveals a narrow temperature window around 400 {\deg}C in which crystallization occurs. At higher temperatures the phase decomposes. Pair distribution function analysis shows 2D sheets of MgZrCl$_6$ form after milling, with heating driving 3D crystallization. Raman spectroscopy also shows evidence of MgZrCl$_6$ after milling. Electrochemical impedance spectroscopy does not reveal ionic conductivity in MgZrCl$_6$ (limit of detection ca. $1.4 \times 10^{-8}$ S/cm). In addition to supporting existing design rules for Mg-based solid electrolytes, this work shows the power of ball milling to synthesize temperature-sensitive inorganic compounds with high yield.