PHASE TRANSITIONS IN [Me_4-nEt_n]₂MX₄ SALTS. R. D. Willett, Y. Fujii, R. Nicholson, and M. Wei, Department of Chemistry, Washington State University, Pullman, WA 99164 USA

We have initiated a study of the crystal chemistry of a new variation of the A₂MX₄ system based on A = Me_nEt_3-nN⁺ cations. Previously we have shown that the structures of the (Me₄Pc) ₂MX₄ series (Pc = group 6A element) assume the incommensurate phase bearing (-K₂SO₄ structure for small cation:anion radius ratios, while phases with a cubic aristotype are formed for large radius ratios. The same rule holds for this new series, with the n = 1 and n = 2 salts generally forming the (-K₂SO₄ structure type. Among the significant results to date are:

1. The (Me₂Et₂N)₂CuX₄ salts (X = Cl^-, Br^-) undergo second order phase transitions at T_c ~ 35(C from P112₁/n to Pmcn. We have measured the lattice constants of the bromide salt as a function of temperature through T_c, verifying the apparent second order nature of the transition. The volume is continuous. However, broadening of the diffraction peaks occurs near T_c. A major feature of the phase transition is a reorientation of the ethyl groups of the ammonium cations.

2. The (Me₃EtN)₂CuX₄ salts have an acentric version of the parent structure with distortion at the zone center P2₁cn structure at room temperature. Both salts undergo two phase transitions below room temperature, with ~ -45(C and ~ 15(C. The shape of the DSC curves strongly suggests that the intermediate phase is incommensurate.

3. The (Me₃EtN)₂MnCl₄ salt crystallizes in the space group P12₁/n1, with a commensurate a*/2 modulation of the parent (-K₂SO₄ structure in the a direction. This is the first example of a modulation in this direction of the lattice and corresponds to a previously unknown Y₂ distortion of the parent structure.