Influence of the substitution of the central Me atom in ammonium oxyfluorides on phase transitions: IR spectroscopy study

Comparative investigations of infrared absorption spectra of ion-molecular crystals of the ammonium-containing oxyfluorides (NH₄)₃WO₃F₃ and (NH₄)₃MoO₃F₃ have been performed. It has been found that the parameters of the vibrational spectra exhibit anomalies in the ranges of internal vibrations of the MeO₃F₃ octahedral groups and ammonium ions near the phase transition temperatures. A comparative analysis of the IR spectra allows us to affirm that the studied phase transition in (NH₄)₃WO₃F₃ is primarily associated with the ordering of octahedral groups, whereas in (NH₄)₃MoO₃F₃, it is mainly due to the ordering of ammonium groups and the distortion of ammonium ions by the crystal environment.     

Raman scattering study of temperature and hydrostatic pressure phase transitions in Rb2KTiOF5 crystal

Raman spectra of Rb₂KTiOF₅ crystal were obtained and analyzed in the temperature range from 77 to 297 K and under hydrostatic pressure up to 4.2 GPa (at T = 295 K). The experimental results were compared with quantum‐chemical simulation of TiOF₅ pseudo‐octahedron. To interpret effects of lattice ordering, phonon spectra of several ordered phases of Rb₂KTiOF₅ were calculated within ab initio generalized Gordon–Kim model, and ordering of TiOF₅ molecular groups were simulated within Monte Carlo approach. The spectra exhibited orientation disordering in the cubic phase under ambient conditions. Cooling below the phase transition temperature (215 K) leads to partial ordering of the structure. The isotropic perovskite‐like phase was found to undergo first‐order transition into a low‐symmetry anisotropic phase at about 1 GPa. Further compression up to 4.1 GPa did not show any effects associated with phase transitions. Copyright © 2011 John Wiley & Sons, Ltd.     

In situ Raman spectroscopic study of interaction between phengite and water under <Emphasis Type="Italic">P</Emphasis>–<Emphasis Type="Italic">T</Emphasis> conditions typical of subduction zones

In situ Raman spectroscopy is used to study the layered mineral phengite K(Al, Mg)₂(Si, Al)₄O₁₀(OH)₂ in water under the high temperature and pressure conditions typical of cold subduction zones (T up to 373°C and P = 12.5 GPa). High pressure and temperature were created in a resistively heated diamond anvil cell. Raman spectra show the high P–T stability of phengite. No reversible polymorphic transitions (overhydration or notable amorphization) are observed.     

Vibrational spectroscopy of alkaline tungsten oxyfluoride crystals: structure,lattice dynamics,ordering processes,and phase transitions

Raman scattering (RS) and infrared absorption (IR) of ammonium oxyfluoride crystals (NH₄)₃WO₃F₃, (NH₄)₂KWO₃F₃, and Cs₂(NH₄)WO₃F₃are compared. Conformation of the WO₃F₃ octahedral groups has been established; anomalies have been found close to the transition temperatures in the internal vibrational regions of ammonium and WO₃F₃ groups. The phase transition in (NH₄)₃WO₃F₃ is associated mostly with the ordering of octahedral groups and formation of W O···H N hydrogen bonds. In (NH₄)₂KWO₃F₃ crystal, the transition is not related to the ordering processes; Cs₂(NH₄)WO₃F₃ retains its disordered structure down to 10 K. Copyright © 2010 John Wiley & Sons, Ltd.