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Single crystals of hexahydrates of divalent fluoridotitanates MTiF6·6H2O (M = Mn, Co, Ni, Zn) are synthesized. Their crystal structures are determined by X-ray diffraction at room temperature (RT-structure) and below the phase transition (LT structure). The phase transition is accompanied by a transformation of the disordered trigonal lattice into the ordered monoclinic one. The initial RT-structures differ in the set and strength of O–H⋯F hydrogen bonds linking isolated [M(H2O)6]2+ and [TiF6]2– octahedra into a 3D framework. There is a correlation between the strength of hydrogen bonds and the thermal behavior of the complexes. [ABSTRACT FROM AUTHOR]

The role of chemical pressure as an effective tool in the processes of formation of initial and distorted (as a result of structural transformations) phases, thermodynamic properties, and direct and inverse barocaloric effects in some complex oxyfluorides and fluorides with octahedral, tetrahedral, and spherical anion and cation groups in the structure has been studied. It is found that, due to the small temperature hysteresis and high baric sensitivity of materials, the maximum values of absolute and integral barocaloric characteristics can be implemented at low pressures. Correspondingly, the temperature range of reversibility of thermodynamic cycles based on fluorides/oxyfluorides as solid-state coolants can be expanded. [ABSTRACT FROM AUTHOR]

The heat capacity and thermal expansion of potassium heptafluorotantalate were studied. The room temperature phase P 2 1 / c is stable at least to 4 K. The strong first-order phase transition P 2 1 / c - P n m a at T 0 = 486.2 K is accompanied by giant changes in the entropy, Δ S 0 = 22.3 J (mol K) - 1 , and volume strain, δ V 0 / V = - 3.6 % . A rather high sensitivity of K 2 TaF 7 to pressure was found, d T 0 / d p = - 220 K G Pa - 1 . Significant extensive and intensive barocaloric effects are found at low pressure. The possibility of improving the barocaloric properties is discussed. [ABSTRACT FROM AUTHOR]

Abstract: Raman spectra of (NH4)3TiF7 within spectral range 10–3,400 cm−1 at temperatures 8–370 K are recorded, which revealed 2 phase transitions at temperatures T1 = 355 K and T2 = 283 K. First principles calculations of lattice dynamics and assignment of the observed lines to specific vibrational types are performed. The first phase transition is classified as an “order–disorder” one. Manifestation of 97 cm−1 vibration (mixed translational [NH4 + F7]) evidences the creation of the hydrogen bond between ammonium group and free fluorine atom, indicating to reconstructive type of phase transition an T2 = 283 K. The emergence of a large number of vibrational lines in Raman spectra at low temperatures proves symmetry lowering of NH4 tetrahedra with the simultaneous ordering of TiF6 octahedra. [ABSTRACT FROM AUTHOR]

Single crystals of silicon double salt (NH4)3SiF7 = (NH4)2SiF6·NH4F = (NH4)3[SiF6]F were grown and studied by the methods of polarization optics, X-ray diffraction and calorimetry. A sequence of symmetry transformations with the temperature change was established: P4/mbm (Z = 2) (G1) ↑Pbam (Z = 4) (G2) ↑P21/c (Z = 4) (G3) ↑P1̅ (Z = 4) (G4) ↑P21/c (Z = 8) (G5). Crystal structures of different phases were determined. The experimental data were also interpreted by a group-theoretical analysis of the complete condensate of order parameters taking into account critical and noncritical atomic displacements. Strengthening of the N–H…F hydrogen bonds can be a driving force of the observed phase transitions. [ABSTRACT FROM AUTHOR]

The heat capacity, thermal expansion, and T-p phase diagrams of (NH4)3TiF7and (NH4)3SnF7were studied in wide temperature and pressure ranges. The total excess entropies at successive Pa-3 ↔ P4/mnc ↔ 4/m (Ti) and single Pa-3 ↔ Pm-3m (Sn) phase transitions are close to each other and characteristic for the order–disorder processes. The Pm-3m cubic phase and direct Pa-3 ↔ Pm-3m transformation were found at high pressure in (NH4)3TiF7. The different sign of baric coefficients for phase transition between two cubic phases in (NH4)3TiF7and (NH4)3SnF7was supposed due to nonlinear phase boundary with pressure in the latter fluoride. [ABSTRACT FROM PUBLISHER]

Using the data about the temperature - pressure phase diagram and the phase transition entropy at ambient and high pressure, intensive and extensive barocaloric effects were analyzed in the region of single and successive order-disorder phase transitions in fluorides and oxyfluorides with the elpasolites-cryolite structure. It was found that hydrostatic as well as chemical pressure are the effective tools to change the entropy of compounds with disordered ionic groups or atoms in structure and to realize the significant intensive and extensive barocaloric effects comparable with the values of baro- and magnetocaloric effects, characteristic for materials, considered as promising solid refrigerants. [ABSTRACT FROM AUTHOR]

Single crystals of germanium double salt (NH4)3GeF7 = (NH4)2GeF6·NH4F = (NH4)3[GeF6]F were grown and studied by the methods of polarization optics and X-ray diffraction. The birefringence Δn = (no−ne), the rotation angle of the optical indicatrix Φ(T) and unit cell parameters were measured in the temperature range 100–400 K. Three structural phase transitions were found at the temperatures: T1↓ = 279.2 K (T1↑ = 279.4 K), T2↑ = 270 K (T2↓ = 268.9 K), T3↓ = 218 K (T3↑ = 227 K). An unusual sequence of symmetry transformations with temperature change was established: P4/mbm (Z = 2) (G1) ↑Pbam (Z = 4) (G2) ↑P21/c (Z = 4) (G3) ↑Pa3̅ (Z = 8) (G4). The crystal structures of different phases were determined. The experimental data were additionally interpreted by a group-theoretical analysis of the complete condensate of order parameters taking into account the critical and noncritical atomic displacements. Strengthening of the N–H…F hydrogen bonds can be a driving force of the observed phase transitions. [ABSTRACT FROM AUTHOR]

It is considered practically impossible to differentiate between oxygen and fluorine atoms by X-ray diffraction in disordered structures of oxyfluoride compounds due to the similarity of their ionic radii and diffusion factors. Indeed, many transition metal oxyfluoride compounds containing polar pseudo-octahedral MOF ( x = 1-3) anions form crystal structures without any fluorine-oxygen (F/O) ordering owing to a large number of local anion configurations. Because of this static disorder, it is impossible to determine the positions of O and F atoms and find the real geometry of the polyhedron. However, this becomes possible in the case of dynamic disorder of oxyfluoride anions when the central atom is displaced from the center of the octahedron toward a vertex, edge, or face (depending on the number of oxygen atoms in the polyhedron), which enables the identification of O and F atoms owing to inherent differences between M-O and M-F bonding. On cooling, such compounds undergo phase transitions of the order-disorder type with substantial changes in the entropy. The examples of static and dynamic orientational disorder in oxyfluoride compounds of d transition metals are given. [ABSTRACT FROM AUTHOR]

An unusual phase transition P4/ mnc→ has been detected after cooling the (NH4)3TiF7 compound. Some TiF6 octahedra, which are disordered in the room-temperature tetragonal structure, become ordered in the low-temperature cubic phase due to the disappearance of the fourfold axis. Other TiF6 octahedra undergo large rotations resulting in huge displacements of the F atoms by 1.5-1.8 Å that implies a reconstructive phase transition. It was supposed that phases P4/ mbm and could be a high-temperature phase and a parent phase, respectively, in (NH4)3TiF7. Therefore, the sequence of phase transitions can be written as → P4/ mbm→ P4/ mnc→. The interrelation between (NH4)3TiF7, (NH4)3GeF7 and (NH4)3PbF7 is found, which allows us to suppose phase transitions in relative compounds. [ABSTRACT FROM AUTHOR]

The (NH)WOF compound undergoes a series of phase transitions: G → 201, K → G → 160, and K → G, with a significant change in entropy (Δ S ∼ Rln10 at the G → G transition), which indicates significant orientational disordering in the G phase and the order-disorder type of the phase transition. X-ray diffraction is used to identify the crystal structure of the G phase as rhombohedral (sp. gr. Cmcm, Z = 4), determine the lattice parameters and the positions of all atoms (except hydrogen), and show that [WOF] ions can form a superposition of dynamic and static orientational disorders in the anionic sublattice. A determination of the orientational position of [NH] ions calls for the combined method of elastic and inelastic neutron scattering. Inelastic neutron scattering is used to determine the state of hindered rotation for ammonium ions in the G phase. Powder neutron diffraction shows that the orientational disorder of NH ions can adequately be described within the free-rotation approximation. [ABSTRACT FROM AUTHOR]

X-ray crystallography is used to analyze the concomitant polymorphism of KNaNbOF crystals. The second β-modification of the compound, crystallizing in the tetragonal crystal system is found for the first time: space group P4/ nmm, a = 5.9352(2) Å, c =8.5487(5) Å, V = 301.14(2) Å, Z = 2, R1 = 0.0095. Parameters of the orthorhombic noncentrosymmetric structure of the α-phase, previously described by Poeppelmeier et al. ( J. Am. Chem. Soc., 129, 13963-13969 (2007)), are refined and a comparative analysis of both structures is performed. The structures are characterized by the complete ordering of oxygen and fluorine atoms, the Nb-O distance in the α-phase (1.738(1) Å) being noticeably longer than that in the β-phase (1.709(2) Å). In β-KNaNbOF, alternating NbOF and NaOF octahedra share vertices, while in α-KNaNbOF, they share both vertices and edges. The existence of the non-polar centrosymmetric β-modification of KNaNbOF cancels the assumption of the substantial contribution of potassium cations to the polar structure of α-KNaNbOF. [ABSTRACT FROM AUTHOR]

Raman scattering spectra of (NH4)3MoO3F3 crystal have been studied in temperature range from 77 to 377 K. Parameters of Raman lines have been quantitatively analyzed. Transitional anomalies were found in the internal modes octahedral groups MоO3F3 and ammonium groups at 297 K. The ferroelectric phase transition (297 K) in (NH4)3MoO3F3 is associated with the partly ordering of octahedral MоO3F3 and the ammonium cations. The ferroelastic phase transition (205 K) is associated with the further ordering of octahedral MоO3F3. [ABSTRACT FROM PUBLISHER]

n analysis of the effect that chemical and hydrostatic pressures have on the thermodynamic properties of perovskite-like fluorine-oxygen compounds A A′ MeOF has revealed that materials under-going order-disorder transitions and having significant external-pressure compliance have the highest caloric efficiency. Some of the fluorides and oxyfluorides under study can be considered promising solid coolants. [ABSTRACT FROM AUTHOR]

In order to understand the role of central atom in the nature of phase transitions in two series of ammonium oxyfluorides, A2MeO2F4 and A2A'MeOxF6-x, the permittivity was studied in wide temperature range. In most cases structural distortions are associated with the ferroelasticity. The ferro- and antiferroelectric properties were revealed only in molybdates. [ABSTRACT FROM AUTHOR]

Using the data about the temperature-pressure phase diagram and the phase transition entropy at ambient pressure, intensive ΔTAD and extensive ΔSBCE barocaloric effects were estimated near structural order-disorder phase transitions in oxyfluorides Rb2KTiOF5 and (NH4)2NbOF5. It was found that hydrostatic pressure is an effective tool to change the entropy of compounds with ordering ionic groups in structure and to realize the significant caloric effects comparable with the values of electro- and magnetocaloric effects, characteristic for a number of materials, considered as promising solid refrigerants. [ABSTRACT FROM AUTHOR]

Comparative analysis of the IR absorption and Raman scattering spectra of a polycrystalline sample of perovskite-type oxyfluoride (NH4)3WO3F3 has been performed in the frequency range 370–4000 cm−1 at temperatures from 92 to 303 K, including the transition between the orientationally disordered cubic and low-symmetry phases. The conformation of WO3F3 octahedral groups is established and transitional anomalies of the internal modes of these groups and ammonium ions are revealed. Comparative analysis of the IR and Raman spectra suggests that the phase transition under study is mainly related to the ordering of octahedral groups and formation of W-O...H-N hydrogen bonds. [ABSTRACT FROM AUTHOR]

Crystal structures of (NH4)3ZrF7 (I) and (NH4)3NbOF6 (II) are refined by X-ray diffraction at room temperature. The compounds are isostructural and belong to the structural type of elpasolite: space group F23; a(I) = 9.4185(3) Å, a(II) = 9.3371(5) Å; V(I) = 835.50(5) Å3, V(II) = 814.02(8) Å3; Z = 4; R(I) = 0.0145, and R(II) = 0.0138. The refinement of the structures in the space group Fm3 m yields abnormally short X-X distances in the pentagonal bipyramid MX7 (X = F, O). The oxygen atom in II is identified by Nb-X distances and occupies one of the axial vertices of the bipyramid. The Nb atom in II is statistically distributed over the position 24 f, while Zr in I resides in the symmetry center. The pentagonal bipyramid MX7 has six independent orientations in I and twelve in II. One of three crystallographically independent ammonium groups of the structures is disordered over six or twelve equivalent orientations. [ABSTRACT FROM AUTHOR]

Vibrational and 17O NMR spectroscopy in combination with quantum chemical calculations are used to investigate the hydrolysis of antimony(III) fluoride complexes. A hydrolytic decomposition of SbF3 and [SbF4]− is accompanied by oligomerization with the formation of edge-and corner-connected dimers ([Sb2O2F4]2−, [Sb2OF8]4−) and trimers ([Sb3O3F6]3−, [Sb3OF9]2−) with bridging oxygen atoms. The hydrolysis of [SbF4]− is also characterized by the presence in the solution of a discrete cation of [SbF5]2− which is least hydrolized. Only a partial isomorphic substitution of fluoride ion by hydroxide one is possible, which is reflected in the composition of K2Sb(OH)xF5− x ( x = 0.3) crystals isolated from the fluoride aqueous solution. [ABSTRACT FROM AUTHOR]

Thermal properties and structure of initial and distorted phases of crystals K2KWO3F3, (NH4)2KWO3F3, (NH4)3WO3F3, Cs2NH4WO3F3 and Cs2KWO3F3 were studied to reveal the effect of cationic substitution on disordering and stability of cubic Fm3m phase. It was found that the nature of phase transitions depends on the shape of cation in 8c position. [ABSTRACT FROM AUTHOR]

Raman spectra of perovskite-like (NH4)3WO3F3 (assigned below as A3), (NH4)2KWO3F3 (A2K), Cs2(NH4)WO3F3 (C2A) oxyfluorides are obtained in a wide temperature range including transition points for A3 and A2 K crystals. Transition anomalies are found and analyzed. Transformation in A3 crystal is shown to be bound with octahedron groups ordering and H-bond formation, that is accompanied by cell volume multiplication, hardening and formation of complex discrete low frequency lattice spectrum. Same cell doubling was found in A2 K crystal, but without lattice ordering. C2A lattice stays disordered down to 15 K. [ABSTRACT FROM AUTHOR]

Ammonium oxofluorotungstates, (NH4)2WO2F4 and (NH4)3WO3F3, are characterized by vibration spectroscopy and quantum chemistry methods with the use of NMR 19F and 1H data. It is shown in the approximation of the density-functional theory that in isolated octahedrons [WO2F4]2− and [WO3F3]3− the mutual arrangement of oxygen atoms in cis-position corresponds to the energy minimum. The presence of intraspheric disorder in [WO3F3]3− (unlike [WO2F4]2−) explains the complex character of vibrational spectra of this anion and eliminates existent in the literature differences in their interpretation (between C 2 v and C 3 v structure variants). Models of intraspheric dynamics of [WO3F3]3− are discussed. [ABSTRACT FROM AUTHOR]

Hydrolysis of NH4Ti$\{\backslash rm\; O\}\_x\; \{\backslash rm\; F\}\_\{5-2x\}$ ($x\backslash leqslant\; 0.4$) (I) was investigated by 19F, 17O, and 49Ti NMR. The interaction of complex I with water is accompanied by the formation of [TiF6$]^\{2-\}$ and multinuclear titanium forms. The composition of the main forms resulting from hydrolysis of I has been established. The bonding of titanium octahedra into dimers and other oligomers occurs by formation of hydroxyl bridges, considerably lowering the pH of the solution. Close analogy has been found between hydrolysis of the title complex and that of titanium tetrafluoride. [ABSTRACT FROM AUTHOR]

The crystal structure of NH4TiF5 (I) was determined (monoclinic crystals, a = 14.683(1), b = 6.392(1), c = 20.821(2) Å, β = 110.538(2); space group P21/n, Z = 4). Structure I is built from infinite zigzag chains of TiF6 octahedra linked by their cis-vertices in the [101] direction; the chains are connected by the ammonium ions forming hydrogen bonds. The chains of octahedra with the surrounding ammonium ions are staggered in the crystal lattice of I. 19F and 1H NMR spectroscopy was used to study the dynamics of the complex ions in NH4TiF5 in the temperature range 270-530 K. Types of ionic motion in the fluoride and proton subsystems were determined, and activation energies evaluated. [ABSTRACT FROM AUTHOR]

The thermal behaviour of ammonium oxofluorotitanates (NH4)3TiOF5, (NH4)2TiOF4 and NH4TiOF3 was investigated by thermoanalytical, X-ray and IR spectroscopic methods. The first decomposition stages under quasi-isobaric conditions are characterized by the formation of (NH4)2TiF6 and ammonium oxofluorotitanate with the less content of ammonium and fluorine than in the initial compound. The decomposition process is accompanied by the Ti(IV) reducing due to ammonia evolved. The new ammonium oxofluorotitanate of high volatility was isolated and characterized. Ammonium-containing non-stoichiometric titanium oxyfluorides are the final products of thermal decomposition of ammonium oxofluorotitanates. [ABSTRACT FROM AUTHOR]