Condensed phonon modes and the recently discovered displacive superlattice of nickel arsenide

Abstract

Recently a weak, orthorhombic, displacive superlattice was observed in both mineral and synthetic samples of NiAs. The symmetry-adapted displacement eigenvectors and corresponding atomic displacement patterns of the normal modes of the unmodulated sub-cell associated with the observed modulation wavevector (q= 1 / 3 (1120)) are derived. Comparison with an experimental convergent-beam electron diffraction study strongly suggests that the deviation of the modulated structure from the unmodulated parent structure can largely be ascribed to the condensation, or freezing-in, of a q= 1 / 3 (1120), Sigma 6 phonon mode.