High-Pressure Synthesis of Silicon-Rich Chromium Silicides: MoSi₂-Type CrSi₂ and PdGa₅-Type CrSi₅ with a Si Network

Two novel chromium silicides, MoSi₂-type CrSi₂ and PdGa₅-type CrSi₅, were successfully synthesized under high-pressure and high-temperature (HPHT) conditions at 12 GPa and 600 °C. The "missing" MoSi₂-type CrSi₂, which was theoretically predicted to be a stable phase of chromium disilicide, was experimentally obtained for the first time, and adopted a tetragonal I4/mmm structure with a = 3.09422(2) Å and c = 7.54051(5) Å. The crystal structure of MoSi₂-type CrSi₂ differs from that of ambient-pressure phase CrSi₂-type CrSi₂ in the stacking of close-packed layers. HPHT in situ XRD experiments suggest that the stacking transition in CrSi₂ involves a diffusion-promoting liquid phase. PdGa₅-type CrSi₅, crystallizing in the I4/mcm structure with a = 5.99998(8) Å and c = 9.2363(2) Å, represents the most Si-rich compound among the reported transition-metal silicides. Its formation was confirmed by HPHT in situ XRD, and DFT calculations revealed that its enthalpy was lower than those of the other phases under high pressure. The structure contains covalently bonded Si-Si networks forming tunnel-like frameworks, reminiscent of clathrate architectures. These findings suggest the potential of exploring a novel class of silicon-rich transition-metal silicides and demonstrate that high-pressure synthesis is a powerful strategy for expanding the structural diversity of transition-metal silicides.