Tracking and understanding the first-order structural transition in Er5Si4

Temperature-dependent single crystal x-ray-diffraction studies revealed a reversible first-order phase transition in Er5Si4. The high-temperature phase adopts the orthorhombic Gd5Si4-type structure, and the low-temperature phase has the monoclinic Gd5Si2Ge2-type structure. Unlike the magnetic/martensitic transition in Gd5Si2Ge2, the structural change in Er5Si4 is not coupled with a magnetic transition, and the structural sequence below room temperature is just the reverse. A vibrational mode that breaks half of the interslab silicon dimers and rotates slabs in the monoclinic structure, thus lowering the symmetry from Pnma to P1121/a, has been identified using Landau theory. While the monoclinic phase is electronically stabilized at low temperatures, the orthorhombic phase is entropically preferable at high temperatures.