Anti-isostructural phase transition and twinning in CrAs at low temperatures and high pressures

Chromium arsenide CrAs (Pnma, Z = 4) is studied with synchrotron single-crystal diffraction in diamond anvil cells at high-pressures and low temperatures to examine its structural evolution across the boundary (TN) between the antiferromagnetic (AF) and paramagnetic (PM) states. Compressing CrAs across TN at low temperatures is equivalent to warming up the material from the AF to PM phases at atmospheric pressure. The phase transition at different conditions is determined from the abrupt changes of the lattice parameters, unit-cell volumes, axial ratios, and interatomic distances. Although, the space group symmetry does not change at TN, the transition is associated with the formation of twin domains. All experimental observations are rationalized with the concept of an anti-isostructural phase transition, in which both orthorhombic phases have the same space group symmetry, but different distortions of the parent hexagonal structure of the NiAs type (P63/mmc, Z = 2). The magneto-structural phase transformation in CrAs is the first example of the anti-isostructural phase transition, in which twinning, as a signature of lost higher rotational symmetry, has been detected.