Phase transition and structures of the twinned low-temperature phases of (Et₄N)[ReS₄]

The title compound, tetraethylammonium tetrathiorhenate, [(C₂H₅)₄N][ReS₄], has, at room temperature, a disordered structure in the space group P6₃mc (Z = 2, α-phase). A phase transition to the monoclinic space group P2₁ (Z = 2, γ-phase) at 285 K leads to a pseudo-merohedral twin. The high deviation from the hexagonal metric causes split reflections. However, the different orientations could not be separated, but were integrated using a large integration box. Rapid cooling to 110-170 K produces a metastable β-phase (P6₃, Z = 18) in addition to the γ-phase. All crystals of the β-phase are contaminated with the γ-phase. Additionally, the crystals of the β-phase are merohedrally twinned. In contrast to the α-phase, the β- and γ-phases do not show disorder.