LATTICE VIBRATION-SPECTRA .86. INFRARED AND RAMAN-SPECTRA OF BARYTE-TYPE TLCLO4, TLBF4, AND NH4BF4 SINGLE-CRYSTALS AND OF B-11-ENRICHED NH4BF4

Polarised infrared reflection spectra of TlClO4, TlBF4, and NH4BF4 sin gle-crystal plates were recorded and analysed for the frequencies of t he transversal and longitudinal optical zone-centre phonon modes and o ther oscillator parameters by using the three-parameter (dielectric su m function) and the four-parameter (factorised form) oscillator-fit me thods. In addition, single crystal Raman spectra of the title compound s and the spectra of polycrystalline samples including those of B-11-e nriched NH4BF4 were monitored. The obtained spectra are assigned and d iscussed with respect to (i) splitting of the respective modes due to all B-10/B-11 isotope substitution, long-range Coulomb forces (TO/LO), distortion of the anionic units (site group), and dynamic interaction (Davydov), (ii) phase transition to the cubic high-temperature polymo rphs, and (iii) change of bonding (including effective charges) on goi ng from both the thallium to the alkali-metal compounds and from perch lorates to tetrafluoroborates.