On the Phase Transition of Ammonium Hexafluoroferrate(III)

Abstract

The heat capacities of (NH4)3[FeF6] crystal have been measured from 15 to 350 K with an adiabatic calorimeter. A heat capacity anomaly was observed at 267.02±0.05 K. The enthalpy and entropy changes of the transition are 6490±500 J mol−1 and 24.8±1.9 J K−1 mol−1, respectively. This value of entropy was discussed in terms of a model involving orientational disorder of the ammonium as well as hexafluoroferrate(III) ions. Based on the assumption of the average fcc symmetry of the crystal and internal rigidity of the ions, the proposed model predicts the transition entropy equal to R In 16=23.05 J K−1 mol−1 which is in close agreement with the experimental value. Temperature dependent splitting of the Raman band due to the internal vibration of the anion was found to be consistent with the model. The excess heat capacity follows the prediction of the Landau theory of phase transitions of the second kind. Properties of the present and the similar crystals including ammonium and alkali salts of hexafluoro-complex ions were discussed in terms of the ionic orientational disorder.

Ferroelastic phase transition in cryolite, Na3AlF6, a mixed fluoride perovskite: High temperature single crystal X-ray diffraction study and symmetry analysis of the transition mechanism

“…Ammonium cryolite, (NH4)3A1F6, shows two phase transitions at 193 and 220.8 K. Calorimetric measurements of ASt .... (23.05 JK-a tool-1) is in agreement with R In 16 indicating orientational disorder of the [A1F6] octahedra and (NH,~) ions (Moriya et al 1979). The ASt .... for the single phase transition in (NH4)3FeF6 at 267 K is also equal to R In 16 (Moriya et al 1977). A 57Fe M6ssbauer resonance study of the phase transition in this compound indicates a zero nuclear quadrupole coupling constant in the cubic phase (Morup and Thrane 1972).…”

“…A 57Fe M6ssbauer resonance study of the phase transition in this compound indicates a zero nuclear quadrupole coupling constant in the cubic phase (Morup and Thrane 1972). Although the iron environment is cubic on the average, the disorder model (Moriya et al 1977) implies that it deviates from the cubic symmetry when observed for a short duration of time. The average octahedral symmetry in the cubic phase is achieved by random jumping of [FeF6] groups over eight configurations.…”

“…Because the dynamic average structure of the fl phase results from the dynamic averaging of four equivalent domains of the e phase the entropy of the e-fl transition (ASt .... ) should be equal to Rln4 (11.53 JK -1 mol-~), where R is the gas constant. The same approach has been used by Moriya et al (1977Moriya et al ( , 1979 to estimate the entropies of phase transitions in (NH4)3A1F6 and (NH4)3FeF6, respectively. From heat capacity measurements of cryolite, Anovitz et al (1987) suggested an average value of 9.9 JK -~ mol -1 for AS t ..... with the transition temperature, T o taken as 836.5 K. Although this value agrees moderately well with the predicted one considering the accuracy of the calorimetric measurements, we could not reproduce the same result from equations given by Anovitz et al (1987).…”

Infrared spectra of the ammonium ion in crystals. Part XIV. Hydrogen bonding and orientation of the ammonium ion in fluorides, with observations on the transition temperatures in cubic cryolite, elpasolite, and perovskite halides

“…22). " The (NH,')" ions, too, would then be of symmetry C I , and four v, quartets and four V4bc octets would be expected, ''The T,, obtained from C,, measurements (24,25) and listed in Table 1 do not always agree with the estimates from X-ray powder diffraction, dta, esr, ir, and other measurements (5,26,27,33,34). A pronounced tendency to undercooling has been reported for the intermediate phase of 59 (24), and similar hysteresis phenomena known to exist in other complex fluorides (e.g.…”

“…"Low-temperature phase is derived from the Fn13m phase by triclinic distortion (5,13). 'A-point, AS,,/R = 3.0 (25).…”