Raman spectrum of (NH4)2TiF6 and trigonal Cs2TiF6

The polarized Raman spectra of (NH4)2TiF6 and trigonal Cs2TiF6 are studied at 85 and at 300 K. All seven Raman active fundamental vibrations of Cs2TiF6 and 13 of the 14 Raman active fundamental vibrations of (NH4)2TiF6 are identified. The missing band of the (NH4)2TiF6 spectrum is assigned to the NH4 + ion rotary normal vibration. The bandwidths of the NH4 + internal and the lattice vibrations of (NH4)2TiF6 increase strongly between 85 and 300 K. It is surmised that the unobserved NH4 + ion rotary band has a large bandwidth at these temperatures, caused by reorientational motions of the ammonium ions. The crystal structure of (NH4)2TiF6 suggests that orientational disorder of the NH4 + ions may exist; however, an order–disorder phase transition involving this disorder cannot take place. The undamped mode frequency of the A1g species lattice antitranslational vibration of (NH4)2TiF6 decreases by 11 cm−1 between the temperatures studied. In the Raman spectra of both crystals the intensity of the Eg (octahedral) species TiF6 2− internal band is 2 to 3 orders of magnitude weaker than the intensity of the other two TiF6 2− ion internal Raman bands. The Jahn–Teller effect, suggested to explain the low intensity of the Eg vibration of several other XY6 complex ions, cannot occur in Cs2TiF6 and (NH4)2TiF6. Two previously published criteria for detecting hydrogen bonding effects in the infrared absorption spectra of complex ammonium halide crystals are manifest in the Raman spectrum of (NH4)2TiF6 also.