Role of metal fluoride octahedra in the mechanism of phase transitions in A2BMF6 elpasolites

115In and 19F MAS NMR study of (NH4)3InF6 phases

“…At least one additional phase transition to an intermediate phase is observed at 318 K, however, no change in the X-ray power diffraction diagram was observed at that temperature. The overall entropy difference between the high-temperature and room-temperature is around R ln 16, hence the room-temperature phase must be ordered [5]. This result is inconsistent with the assumed tetragonal symmetry of this phase, requiring at least one twofold disordered ammonium ion.…”

“…Among ammonium elpasolites, the ones with small trivalent cations (M = Al, Ga, Cr, Fe) are best known. Phase transitions have been studied using X-ray diffraction and differential scanning calorimetry [4,5]. These compounds exhibit a cubic high-temperature phase (Fm3m), which is stable at room temperature, a monoclinic low-temperature phase (P2 1 /n).…”

Solvothermal indium fluoride chemistry: Syntheses and crystal structures of K5In3F14, β-(NH4)3InF6 and [NH4]3[C6H21N4]2[In4F21]

“…It can be seen that the octahedral tilt system is much more complex, with tilts allowed along all three pseudo-cubic axes; the principal contributions the additional lowering of symmetry arise from displacements of the fluorine atoms, a fact that was verified by a displacement mode analysis using the program ISODISPLACE [26]. It has been suggested [27] that the phase transitions in NH þ 4 -based cryolites are order-disorder transitions driven by hydrogenbonding requirements (as opposed to alkali metal based systems, which have phase transitions of the displacive type). In the present case, ordering of the NH þ 4 groups does indeed occur, with well-defined H-bonding contacts from each of the three crystallographically distinct NH þ 4 sites (Table 3, Fig.…”

Prediction of new halo-elpasolites

“…Since the unit cell of elpasolite can be regarded as a distorted unit cell of perovskite with doubled parameters [9], attempts have been made [9][10][11][12][13][14] to use the well-known Goldschmidt's tolerance factor and its modification that includes functions of ionic radii of the components, to predict new compounds with this crystal structure. Our attempt to use the modification [10][11][12][13][14] of tolerance factor t = √2(r A + r Hal )/(r B + r C + 2r Hal ), where r are the Shannon ionic radii, demonstrated that there is significant overlapping between the stability regions of cubic elpasolites and the stability regions of compounds with crystal structures of other types (Cs 2 NaCrF 6 , cryolite, etc.).…”