Solid-solid phase-transition temperatures, entropies of transition, and room-temperature lattice parameters of sixteen [(${\mathrm{CH}}_3$${)}_4$Z${]}_2$${\mathit{MX}}_4$ (Z=P,As,Sb; M=Co,Cu,Zn; X=Cl,Br,I) compounds are reported. [(${\mathrm{CH}}_3$${)}_4$P${]}_2$${\mathrm{CoBr}}_4$, [(${\mathrm{CH}}_3$${)}_4$P${]}_2$${\mathrm{CoI}}_4$, [(${\mathrm{CH}}_3$${)}_4$P${]}_2$${\mathrm{ZnBr}}_4$, [(${\mathrm{CH}}_3$${)}_4$P${]}_2$${\mathrm{ZnI}}_4$, [(${\mathrm{CH}}_3$${)}_4$As${]}_2$${\mathrm{CuBr}}_4$, [(${\mathrm{CH}}_3$${)}_4$As${]}_2$${\mathrm{CoI}}_4$, and [(${\mathrm{CH}}_3$${)}_4$As${]}_2$${\mathrm{ZnI}}_4$ have β-${\mathrm{K}}_2$${\mathrm{SO}}_4$-type structures, analogous to known [(${\mathrm{CH}}_3$${)}_4$N${]}_2$${\mathit{MX}}_4$ compounds. Their room-temperature monoclinic space groups correspond to Landau-allowed continuous transitions from a suspected common high-temperature Pmcn phase. The associated distortion representations are compatible with ${\mathit{X}}_2$ and ${\mathit{Z}}_2$ of Pmcn.

Previously reported [(${\mathrm{CH}}_3$${)}_4$Z${]}_2$${\mathit{MX}}_4$ (Z=N,P; M=Mn,Fe,Co,Ni,Cu,Zn; X=Cl,Br,I) compounds are also shown to have this compatibility feature for their sub-Pmcn phases. The set of compounds [(${\mathrm{CH}}_3$${)}_4$P${]}_2$${\mathrm{CoCl}}_4$, [(${\mathrm{CH}}_3$${)}_4$P${]}_2$${\mathrm{ZnCl}}_4$, [(${\mathrm{CH}}_3$${)}_4$As${]}_2$${\mathrm{CuCl}}_4$, [(${\mathrm{CH}}_3$${)}_4$Sb${]}_2$${\mathrm{CoCl}}_4$, [(${\mathrm{CH}}_3$${)}_4$Sb${]}_2$${\mathrm{CuCl}}_4$, [(${\mathrm{CH}}_3$${)}_4$Sb${]}_2$${\mathrm{ZnCl}}_4$ and [(${\mathrm{CH}}_3$${)}_4$Sb${]}_2$${\mathrm{CoBr}}_4$, [(${\mathrm{CH}}_3$${)}_4$Sb${]}_2$${\mathrm{CuBr}}_4$, [(${\mathrm{CH}}_3$${)}_4$Sb${]}_2$${\mathrm{ZnBr}}_4$, which have larger ratios of cation to anion radii, are grouped into a separate [(${\mathrm{CH}}_3$${)}_4$As${]}_2$${\mathrm{CoCl}}_4$ structure type, which is characterized by an apparent association with a cubic unit cell having ≊12.7 Å axes.

• Received 12 July 1990

DOI:https://doi.org/10.1103/PhysRevB.43.13549