Tuning the structural and antiferromagnetic phase transitions in UCr2Si2: Hydrostatic pressure and chemical substitution

Structural phase transitions in f-electron materials have attracted sustained attention both for practical and basic science reasons, including the fact that they offer an environment to directly investigate relationships between structure and the f-state. Here we present results for UCr2Si2, where structural (tetragonal -> monoclinic) and antiferromagnetic phase transitions are seen at T-S = 205 K and T-N = 25 K, respectively. We also provide evidence for an additional second-order phase transition at T-X = 280 K. We show that T-X, T-S, and T-N respond in distinct ways to the application of hydrostatic pressure and Cr -> Ru chemical substitution. In particular, hydrostatic compression increases the structural ordering temperature, eventually causes it to merge with T-X, and destroys the antiferromagnetism. In contrast, chemical substitution in the series UCr2-xRuxSi2 suppresses both T-S and T-N, causing them to approach zero temperature near x approximate to 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 semiuniformly compresses the unit cell, and Cr -> 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 in which degrees of freedom associated with f-electron magnetism, strong electronic correlations, and structural instabilities are readily controlled.