Infrared spectra of the ammonium ion in crystals. Part 11.—Existence of low-temperature transitions in the cubic hexahalogenometallates(IV)(NH4)2MX6

Abstract

Infrared spectra of the NH₃D⁺ probe ion in ten ammonium hexahalogenometallates(IV), (NH₄)₂MX₆(MX = ReCl, OsCl, PtCl; PdBr, OsBr, PtBr, SnBr, TeBr; PtI, TeI) were studied at temperatures between 10 and 293 K. The temperature variation of the isotopically isolated ν₁(NH₃D⁺) and ν_4bc(NH₃D⁺) frequencies has revealed the existence of structural transitions in all the hexabromo and hexaiodo compounds. The hexachloro compounds are cubic over the entire 10–293 K range; the others are cubic above their (upper) transition temperatures with the exception of (NH₄)₂ TeI₆, for which T_tr < 293 K. The probe-ion spectra at 10 K of the compounds undergoing transitions fall into two classes. The class containing the Pd, Os and Pt compounds is characterized by a 1:1:2 ν₁ triplet indicative of an effective symmetry E=C_s of the ammonium ion, corresponding to site symmetries S=C_2h or C_s. The class containing the Sn and Te compounds is characterized by a 1:1:1:1 ν₁ quadruplet indicative of E=C₁ and S=C_i or C₁. The strength of hydrogen bonding increases on cooling through T_tr, but one of the highly-bent hydrogen bonds (which are symmetrically trifurcated in the cubic phases) appears to be retained while the others become normal (i.e. essentially straight) N—H…X hydrogen bonds, as shown by the values of the ν₁ as well as the signs of the temperature coefficients of the ν₁ and ν_4bc frequencies.