Orientation disorder in ammonium elpasolites

Udovenko, A. A.; Laptash, N. M.; Maslennikova, I. I.

doi:10.1016/s0022-1139(03)00166-0

Cited by 48 publications

(23 citation statements)

References 10 publications

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“…The ligand atoms in the Fm3m group are statistically distributed in two positions 24(e) and 96(j). The preference of this 'mixed' (or 'split') position relative to 24(e) has been discussed in our previous paper (Udovenko et al, 2003) concerning the orientational disorder in crystals of ammonium fluoroelpasolites. The location of the ligand atoms in the 24(e) position results in relatively high R values and large isotropic displacement parameters.…”

Section: Crystal Structuresmentioning

confidence: 91%

Orientational disorder in crystals of (NH₄)₃MoO₃F₃ and (NH₄)₃WO₃F₃

Udovenko

Laptash

2008

Acta Crystallogr Sect B

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Triammonium trioxotrifluoromolybdate (NH(4))(3)MoO(3)F(3) (I) and trioxotrifluorotungstate (NH(4))(3)WO(3)F(3) (II) were synthesized in a single-crystal form and their structures were refined by X-ray diffraction. These two isostructural compounds belong to the elpasolite-type structure (cubic system, space group Fm3m, Z = 4). O and F atoms are randomly distributed in two independent positions [24(e) and 96(j)] of the cubic unit cell, and the central atoms and the ammonium cation containing N2 are shifted from the symmetry centers into the 32(f) position. As a consequence, O and F atoms in the equilibrium structure were identified on a local scale by the metal-ligand distance and hydrogen atomic coordinates of the disordered ammonium cation N2H(4) were determined. The slightly compressed MX(6) (M = Mo, W; X = O, F) octahedron has a fac configuration with the central atom shifted toward the face occupied by three O atoms. The true geometry of this polyhedron permits us to explain the observed vibrational spectra of the compounds examined and to eliminate the contradiction in interpretation of the vibrational spectra of [MO(3)F(3)](n-) species reported in the literature. Both complexes reveal a dynamic disordering: the [MX(6)](3-) anions are disordered on 48 equivalent orientations and the N2H(4) groups have eight orientations.

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Section: Crystal Structuresmentioning

confidence: 91%

Orientational disorder in crystals of (NH₄)₃MoO₃F₃ and (NH₄)₃WO₃F₃

Udovenko

Laptash

2008

Acta Crystallogr Sect B

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“…[11,12,13] At ambient pressure, niobate undergoes two structural phase transitions in a very narrow temperature range: T 1 =260 K and T 2 =262 K, whereas titanate shows only one ferroelastic phase transition at T 0 =264.7 K. Entropy of the structural transformations was found very large in both oxyfluorides: in (NH 4 ) 3 NbOF 6 ∆S 1 + ∆S 2 =(8.5+ 30.3) J·(mol·K) −1 and in (NH 4 ) 3 TiOF 5 ∆S 0 =18.1 J·(mol·K) −1 . The dependences of ∆T AD (T, p) and ∆S BCE (T, p) were determined ana- lyzing data on the heat capacity, [11,12] the T − p phase diagrams and the dependences of entropy of the phase transitions on temperature and pressure.…”

Section: Introductionmentioning

confidence: 99%

Conventional and inverse barocaloric effects around triple points in ferroelastics (NH 4 ) 3 NbOF 6 and (NH 4 ) 3 TiOF 5

2017

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We report the conventional and inverse barocaloric effect (BCE) in ferroelastics (NH 4 ) 3 NbOF 6 and (NH 4 ) 3 TiOF 5 . We found a rich set of the structural phase transitions and triple points on the T − p phase diagrams. Extensive and intensive BCE near some transformations and around triple points in both crystals are outstanding (|∆S max BCE |=30-100 J·(kg·K) −1 ; |∆T max AD |=4-22 K) and can be achieved at low pressure 0.01-0.60 GPa.

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“…The compound crystallizes in the cubic symmetry, Fm The crystal structure of (NH 4 ) 3 TiOF 5 was studied in [78], and investigated in detail and refined in [79]. (Fig.…”

Section: Structures Of Oxo-and Hydroxyfluoride Complexes Of Titanium(iv)mentioning

confidence: 99%