Pressure-induced phase transitions of theLiAlD4system

Compression of the $\mathrm{Li}\mathrm{Al}{\mathrm{D}}_{4}$ system has been studied using in situ time-of-flight neutron diffraction up to $7.15\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $60\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. At ambient temperature $({T}_{a})$, the structure remains as monoclinic $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Li}\mathrm{Al}{\mathrm{D}}_{4}$ up to $7.05\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, displaying a 26.9% reduction in unit cell volume to $200.9\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{3}$, yielding a 28.7% increase in volumetric efficiency to $136.5\phantom{\rule{0.3em}{0ex}}\mathrm{kg}\phantom{\rule{0.3em}{0ex}}{\mathrm{D}}_{2}∕{\mathrm{m}}^{3}$. Decompression to a small $3\phantom{\rule{0.3em}{0ex}}\text{ton}$ load at ${T}_{a}$ and holding for $24\phantom{\rule{0.3em}{0ex}}\text{hours}$ did not induce any phase transformation, and the starting $0\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Li}\mathrm{Al}{\mathrm{D}}_{4}$ structure was eventually recovered. Heating from ${T}_{a}$ to $60\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ while under compression to $7.15\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ produces new and intense reflections in the diffraction pattern, while also retaining $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Li}\mathrm{Al}{\mathrm{D}}_{4}$ reflections, indicating a two-phase $\ensuremath{\alpha}∕\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Li}\mathrm{Al}{\mathrm{D}}_{4}$ mixture. The $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Li}\mathrm{Al}{\mathrm{D}}_{4}$ structure indexed from this data is in the monoclinic space group $I2∕b$, with unit cell dimensions $a=4.099(3)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $b=4.321(4)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $c=10.006(7)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $\ensuremath{\gamma}=88.43(2)\ifmmode^\circ\else\textdegree\fi{}$, yielding a volumetric efficiency of $151.2\phantom{\rule{0.3em}{0ex}}\mathrm{kg}\phantom{\rule{0.3em}{0ex}}{\mathrm{D}}_{2}∕{\mathrm{m}}^{3}$. The atomic arrangement in $I2∕b$ is identical to that in the theoretically predicted $I{4}_{1}∕a$ super group, with isolated symmetric $\mathrm{Al}{\mathrm{D}}_{4}$ tetrahedra displaying $\mathrm{Al}\mathrm{D}$ distances of $1.545(2)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Once again, decompression and cooling recovers the ambient $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Li}\mathrm{Al}{\mathrm{D}}_{4}$ structure.