Raman spectra of the double-anion salts M3ZnCl4NO3 (M+ = K+, Rb+, NH4+)

Recently characterized K3ZnCl4NO3 and (NH4)(3)ZnCl4NO3, and newly prepared Rb3ZnCl4NO3 constitute a limited series of isomorphous double-anion salts (space group Pnma, Z = 4). Room-temperature (295 K) Raman spectra from polycrystalline samples of the compounds are reported and interpreted on the basis of the C-s site symmetry of the ZnCl42- - and NO3- ions with reference to the D-2h factor group of the unit cell. The spectra are compared with Raman spectra of the corresponding M2ZnCl4 and MNO3 single-anion salts. Relative positions and frequencies of the ZnCl42- modes vary considerably among the M3ZnCl4NO3 compounds, despite the isomorphism. The NO3- modes are more similar in all three compounds. The NO3- doubly degenerate v(3) and v(4) modes are split into two distinct bands as a result of the decent in symmetry from D-3h for the free ion to C-s at the crystallographic site. The unequal intensities of the v(3), bands observed for K3ZnCl4NO3 and Rb3ZnCl4NO3 and the equal intensities of the v, bands observed for all three compounds suggest the same factor-group assignments as the high-temperature phase NH,NO,(III). The free-ion Raman-inactive planar deformation mode, v,, is evident in all three compounds, but with lesser intensity than its overtone 2v(2). In K3ZnCl4NO3 and Rb3ZnCl4NO3, the symmetric stretching band, in addition to the Very strong component for v,, shows a weak, low-frequency band found in many ionic nitrates, which has been attributed to thermally disordered nitrate ions or hot bands. This feature is not found in the spectrum of (NH4)(3)ZnCl4NO3. The 12 NH4+ ions in the unit cell of (NH4)(3)ZnCl4NO3, which occupy C-1 and C-2 sites in a 2:1 ratio, give rise to extremely broad bands that show no evidence of the individual symmetry distinctions of the cations. The broad band from NH4+ v(4) obscures the region in which NO3- v(3) bands are expected, but the NO3- overtone 2v(2) is evident as a sharp peak above a similarly broad band from NH4+ v(2). (C) 2000 Elsevier Science B.V. All rights reserved.