Large-amplitude molecular and inversion centre displacements in ferroelastic 9-hydroxy-1-phenalenone at 298 K

The a and c axes of a single-crystal of 9-hydroxy-1-phenalenone with lattice constants at 298 K of a = 8.8941, b = 28.8159, c = 7.2117 Å and β = 97.937° are ferroelastically transformed under the application of compressive stress on the order of 10⁵ N  m⁻². The spontaneous strain is 0.18, assuming tetragonal 4/m prototype symmetry in accordance with the observed domain patterns. The domains are separated by temperature-dependent walls having Miller indices (1.86_′0_′1) and (0_′1.86) at 298 K. The ferroelastic transformation is accompanied by translation of the inversion centers through distances of about 7.68 Å. In the process, crystallographically independent molecules (I) and (II) exchange identity as each undergoes an additional translation of about 1.1 Å combined with rotation.