Incommensurate Phases, Phase Transitions and Structural Chemistry of (PN(METHYL)(4))(2)MX(4) Salts.

The phase transistions and structures of seventeen ((CH_3)_4 Pn) _2 MX_4 (Pn = P, As, Sb; M = Co, Cu, Zn; X = Cl, Br, I) compounds have been studied. ((CH _3)_4P) _2CuCl_4, ((CH _3)_4P) _2 CoBr_4, ((CH_3 )_4P) _2CoI _4, ((CH_3) _4P) _2ZnBr _4, ((CH_3) _4P) _2ZnI _4, ((CH_3) _4As) _2CuBr _4, ((CH_3) _4As) _2CoI _4, and ((CH_3) _4As) _2ZnI _4 have distorted beta -K_2SO_4 type structures. Their room temperature monoclinic space groups are the result of continuous transitions from a determined or suspected common high temperature Pmcn phase. The associated distortion representations are compatible with X_2 and Z_2 of Pmcn. Previously reported ((CH_3) _4 Pn] _2 MX_4 compounds are also shown to have this compatibility feature for their sub-Pmcn phases. The set of compounds, ((CH _3)_4P) _2CoCl_4, ((CH _3)_4P) _2 ZnCl_4, ((CH_3 )_4As) _2 CuCl_4, ((CH_3 )_4Sb) _2 CoCl_4, ((CH_3 )_4Sb) _2 CuCl_4, ((CH_3 )_4Sb) _2 ZnCl_4, and ((CH _3)_4Sb) _2CoBr_4, ((CH _3)_4Sb) _2CuBr_4, ((CH _3)_4Sb) _2ZnBr_4, which have larger ratios of cation to anion radii, are grouped into a separate ((CH_3)_4 As) _2CoCl_4 structure type. The room temperature structures of ((CH_3)_4P) _2CoI_4 and ((CH_3)_4P) _2CoBr_4 plus the commensurately modulated structures of ((CH _3)_4P) _2 CoCl_4, ((CH_3 )_4Sb) _2 CuCl_4 and ((CH_3 )_4Sb) _2 ZnBr_4 are reported. The latter three structures are correlated by an ascent of symmetry diagram with a fictional, pseudo-antifluorite Fm| 3m aristotype. (P(CH_3)_4 ) _2CuCl_4 transforms from a Pmcn basic phase, with the beta-K_2SO _4 structure type, to an incommensurate phase with superspace group symmetry P(Pmcn):(ss| 1) at T_{i} = 108^circC, and then locks -in to a P2_1/c commensurately modulated phase at T_{c} = 73 ^circC. The associated modulation vector increases monotonically from 0.179c^ * at T_{i} to 0.258c^* at T_ {c}, then jumps discontinuously to the lock-in phase value of c^*/3. DSC and single crystal X-ray lattice parameter data verify the continuous and weakly first order nature of the respective transitions. The critical exponent, 2beta , for the continuous transition is 0.54. The basic structure at 111^circC has been conventionally determined. The incommensurate structure at both 100^circC and 81 ^circC has been determined by superspace group techniques. The lock-in structure at 21 ^circC has been equivalently determined both by conventional and superspace methods. The incommensurate and lock-in results show large first harmonic Fourier coefficients corresponding to the Sigma_3(k) primary order parameter. Of the secondary order parameters, the zero harmonic A_{1g} component has significant magnitude in the incommensurate phase, whereas the B_{1g} and Sigma_1(2k) modes also become significant in the lock-in phase.