Compression of HgCr$_2$S$_4$ and HgCr$_2$Se$_4$ spinels

The family of ACr$_2$X$_4$ spinels constitutes a prototype system for studying magnetism in solids [1]. More recently, members of this series were found to exhibit multiferroicity [2]. The origin of the ferroic properties is unknown; the role of the structure, however, appears to be important [3]. Given the strong interplay between structural and ferroic properties in these systems, structural tuning by pressure can provide valuable hints for multiferroicity. We have performed high-pressure structural investigations on the multiferroic HgCr$_2$S$_4$ and the HgCr$_2$Se$_4$ compounds. HgCr$_2$S$_4$ exhibits three structural transitions: the starting cubic phase adopts a tetragonal structure at 20 GPa, at 27 GPa an orthorhombic distortion occurs, and a third transition takes place above 37 GPa. As for HgCr$_2$Se$_4$, our studies detect a structural transition at 14 GPa, near the theoretically predicted band gap closure [4]. We discuss the possible mechanisms for the observed phase transitions for both Cr-spinels.\\[4pt] [1] T. Rudolf \textit{et al}., N. J. Phys. 9, 27 (2007) and refs. therein\\[0pt] [2] S. Weber \textit{et al}., PRL 96, 157202 (2006)\\[0pt] [3] V. Gnezdilov \textit{et al}., PRB 84, 045106 (2011)\\[0pt] [4] S. Guo \textit{et al}., JPCM 24, 045502 (2012)