Opposite pressure effects in the orbitally-induced Peierls phase transition systems CuIr₂S₄ and MgTi₂O₄

The iso-spinel structural systems CuIr₂S₄ and MgTi₂O₄ exhibit phase transitions of a similar nature at ∼230 K and ∼260 K respectively, which are explained as an orbitally-induced Peierls phase transition. However, in this work, we uncover that the applied pressure has opposite pressure effects on the phase transitions in CuIr₂S₄ and MgTi₂O₄. As the pressure increases, the phase transition temperature (T_MI) for CuIr₂S₄ increases while that for MgTi₂O₄ decreases. In addition, the phase transition intensity becomes weaker for CuIr₂S₄ but gets stronger for MgTi₂O₄ under pressure. Our results indicate that the applied pressure suppresses the metallic phase in CuIr₂S₄, while enhances that in MgTi₂O₄. Combining the experimental observations with first-principles electronic structure calculations, we suggest that the opposite pressure effects in CuIr₂S₄ and MgTi₂O₄ originate from the different orbital ordering configurations (d_xy, d_yz/d_xz) caused by different lattice distortions in these two systems. Our findings indicate directly that the interplay between the orbital and lattice degrees of freedom plays an important role in the orbitally-induced Peierls phase transition.