Distortions from the Tetragonal HfCuSi2-Type Structures to the Orthorhombic GdCuAs1.15P0.85-Type Structures in REMX 2 (RE = Rare Earth; M = Cu, Ag; X = P, As, Sb): Mismatch between the Sizes of the M and X Atoms

The RECuAs2 - x P x phases undergo symmetry-breaking transitions from tetragonal HfCuSi2-type structures (P4/nmm space group) to orthorhombic GdCuAs1.15P0.85-type structures (Pmmn space group) upon substitution of arsenic by phosphorus. According to Landau theory, the distortions correspond to the B 1 g irreducible representation and can be continuous. The P4/nmm → Pmmn transitions are not caused by a Peierls-like instability of the square As/P …