Structural phase transitions in $f$-electron materials have attracted sustained attention both for practical and basic science reasons, including that they offer an environment to directly investigate relationships between structure and the $f$-state. Here we present results for UCr$_2$Si$_2$, where structural (tetragonal $\rightarrow$ monoclinic) and antiferromagnetic phase transitions are seen at $T_{ \rm{ S}}$ $=$ 205 K and $T_{ \rm{ N}}$ $=$ 25 K, respectively. We also provide evidence for an additional second order phase transition at $T_{ \rm{ X}}$ = 280 K. We show that $T_{ \rm{ X}}$, $T_{ \rm{ S}}$, and $T_{ \rm{ N}}$ respond in distinct ways to the application of hydrostatic pressure and Cr $\rightarrow$ Ru chemical substitution. In particular, hydrostatic compression increases the structural ordering temperature, eventually causes it to merge with $T_{ \rm{ X}}$ and destroys the antiferromagnetism. In contrast, chemical substitution in the series UCr$_{ 2-x}$Ru$_x$Si$_2$ suppresses both $T_{ \rm{ S}}$ and $T_{ \rm{ N}}$, causing them to approach zero temperature near $x$ $\approx$ 0.16 and 0.08, respectively. The distinct $T-P$ and $T-x$ phase diagrams are related to the evolution of the rigid Cr-Si and Si-Si substructures, where applied pressure semi-uniformly compresses the unit cell and Cr $\rightarrow$ Ru substitution results in uniaxial lattice compression along the tetragonal $c$-axis and an expansion in the $ab$-plane. These results provide insights into an interesting class of strongly correlated quantum materials where degrees of freedom associated with $f$-electron magnetism, strong electronic correlations, and structural instabilities are readily controlled.
Categories: cond-mat.str-el cond-mat.mtrl-sci