Intramolecular mobility and phase transitions in ammonium oxofluoroniobates (NH4)2NbOF5 and (NH4)3NbOF6, a NMR and DFT study

Molecular structure, ionic mobility and phase transitions in six- and seven-coordinated ammonium oxofluoroniobates (NH₄)₂NbOF₅ and (NH₄)₃NbOF₆ were studied by ¹⁹F, ¹H NMR and DFT calculations. Equatorial fluorine atoms (F_eq) in [NbOF₅]²⁻ and [NbOF₆]³⁻ are characterized by high ¹⁹F NMR chemical shifts while axial fluorine atoms (F_ax) have those essentially lower. The high-temperature ionic mobility in (NH₄)₂NbOF₅ does not stimulate the ligand exchange F_eq ↔ F_ax, whereas it is observed in (NH₄)₃NbOF₆ as pseudorotation typical for seven-coordinated polyhedra. The transformation of pentagonal bipyramidal structure (BP) of [NbOF₆]³⁻ into capped trigonal prismatic (CTP) one takes place during the phase transition (PT) at 260 K. The PT of order-disorder type in (NH₄)₂NbOF₅ is accompanied by transition of anionic sublattice to a rigid state. The ¹⁹F and ¹H NMR data corroborate the independent motions of NH₄ groups and anionic polyhedra in (NH₄)₂NbOF₅ while they are coordinated in (NH₄)₃NbOF₆.     

Syntheses, crystal structures and optical spectroscopy of Ln2(SO4)3·8H2O (Ln=Ho, Tm) and Pr2(SO4)3·4H2O

The lanthanide sulphate octahydrates Ln₂(SO₄)₃·8H₂O (Ln=Ho, Tm) and the respective tetrahydrate Pr₂(SO₄)₃·4H₂O were obtained by evaporation of aqueous reaction mixtures of trivalent rare earth oxides and sulphuric acid at 300 K. Ln₂(SO₄)₃·8H₂O (Ln=Ho, Tm) crystallise in space group C2/c (Z=4, a_Ho=13.4421(4) Å, b_Ho=6.6745(2) Å, c_Ho=18.1642(5) Å, β_Ho=102.006(1) Å³ and a_Tm=13.4118(14) Å, b_Tm=6.6402(6) Å, c_Tm=18.1040(16) Å, β_Tm=101.980(8) Å³), Pr₂(SO₄)₃·4H₂O adopts space group P2₁/n (a=13.051(3) Å, b=7.2047(14) Å, c=13.316(3) Å, β=92.55(3) Å³). The vibrational and optical spectra of Ho₂(SO₄)₃·8H₂O and Pr₂(SO₄)₃·4H₂O are also reported.     

Synthesis, vibrational and NMR spectroscopic characterization of [N(CH3)4][IO2F2] and X-ray crystal structure of [N(CH3)4]2[IO2F2][HF2]

The salt, [N(CH₃)₄][IO₂F₂], was prepared from [N(CH₃)₄][IO₃] and 49% aqueous HF, and characterized by Raman, infrared, and ¹⁹F NMR spectroscopy. Crystals of [N(CH₃)₄]₂[IO₂F₂][HF₂] were obtained by reduction of [N(CH₃)₄][cis-IO₂F₄] in the presence of [N(CH₃)₄][F] in CH₃CN solvent and were characterized by Raman spectroscopy and single-crystal X-ray diffraction: C2/m, a = 14.6765(2) Å, b = 8.60490(10) Å, c = 13.9572(2) Å, β = 120.2040(10)°, V = 1523.35(3) Å³, Z = 4 and R = 0.0192 at 210 K. The crystal structure consists of two IO₂⁻F₂ anions that are symmetrically bridged by two H⁻F₂ anions, forming a [F₂O₂I(FHF)₂IO₂F₂]⁴⁻ dimer. The symmetric bridging coordination for the H⁻F₂ anion in this structure represents a new bonding modality for the bifluoride anion.     

Reorientation mobility of NH3-group in glycine crystals by means of H NMR spectroscopy

We report results of ¹H NMR measurements of proton mobility in α-, β-, and γ-polymorphs of glycine, including the energy barriers and specific correlation times for the amino group reorientation process. We discuss relations between second-order phase transition in β-glycine observed at 252 K and proton mobility in β-glycine crystals.     

Ionic mobility and structure of fluorozirconates Rb2? x (NH4) x ZrF6 ( x > 1.5) by NMR, x-ray structure analysis, and impedance spectroscopy

The ionic mobility in the temperature interval 180 to 480 K, structure, and electrophysical properties of rubidium-ammonium hexafluorozirconates Rb_2−x (NH₄) _x ZrF₆ (1.5 ≤ x ≤ 2.0) are studied by methods of the ¹⁹F, ¹H NMR spectroscopy, x-ray structure analysis, differential thermal analysis, and impedance spectroscopy. Correlations between the composition of the cationic sublattice, the character of ionic motions, and the phase transition temperature (of the type order-disorder) are established in these compounds. The salient feature of the high-temperature modifications of these fluorozirconates with x ≥ 1.5 is the translation diffusion of ions inside the fluoride and ammonium sublattices and the ¹⁹F NMR spectra are characterized by monoaxial anisotropy of the magnetic shielding tensor of the fluorine nuclei. Fluorozirconates with x > 1.5 are shown to belong with the structural type (NH₄)₂ZrF₆. The rubidium cations isomorphically replace the ammonium cations. The electrophysical characteristics of the compounds are examined in the temperature interval 300 to 480 K. It is established that the electroconductivity of these compounds increases with x. Original Russian Text ? V.Ya. Kavun, A.V. Gerasimenko, A.B. Slobodyuk, N.A. Didenko, N.F. Uvarov, V.I. Sergienko, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 5, pp. 563–570. Based on the paper delivered at the 8th Meeting “Fundamental Problems of Solid-State Ionics”, Chernogolovka (Russia), 2006.     

Three modifications of BaCu(SeO3)2 and the compound SrCu(SeO3)2: Preparation and crystal structure determination

The crystal structures of four hydrothermally synthesized alkaline earth-copper-selenites were determined: BaCu(SeO₃)₂-I [a = 5.247(1), B = 13.353(2), C = 8.981(1) Å, space group Pnm2₁, Z = 4, R_w = 0.024 for 1270 reflections], BaCu(SeO₃)₂-II [a = 5.256(1), B = 13.231(2), C = 8.933(1) Å, β = 90.19(1)°, space group P2₁/c, Z = 4, R_w = 0.046 for 2238 reflections], BaCu(SeO₃)₂-III [a = 8.031(1), B = 5.185(1), C = 15.823(2) Å, β = 90.83(1)°, space group C2/c, Z = 4, R_w = 0.038 for 1866 reflections], and SrCu(SeO₃)₂ [a = 7.929(1), B = 5.132(1), C = 14.997(2) Å, β = 90.53(1)°, space group C2/c, Z = 4, R_w = 0.028 for 1414 reflections; isotypic with BaCu(SeO₃)₂-III].BaCu(SeO₃)₂-I and -II contain Cu(SeO₃)₂ sheets lying parallel to (100) formed by CuO₄ “squares” and selenite groups. These sheets are topologically different: in BaCu(SeO₃)₂-I they are formed by the connection of Cu₂(SeO₃) and Cu₆(SeO₃)₄ rings while in BaCu(SeO₃)₂-II they are formed by Cu₂(SeO₃)₂ and Cu₆(SeO₃)₆ rings. The Cu(SeO₃)₂ sheets are rugged in BaCu(SeO₃)₂-I and they are slightly waved in BaCu(SeO₃)₂-II. In both compounds they are connected to each other by a fifth Cu---O bond and by the Ba atoms. In BaCu(SeO₃)₂-III and in its isotypic Sr analog the CuO₄ “squares” and the selenite groups form parallel chains [010], which are connected by the alkaline earth atoms.     

Phase transitions and molecular motions in [Cd(H2O)6](BF4)2 studied by DSC, H and F NMR and FT-MIR

Two solid phase transitions of [Cd(H₂O)₆](BF₄)₂ occurring on heating at T_C2=183.3 K and T_C1=325.3 K, with 2 K and 5 K hysteresis, respectively, were detected by differential scanning calorimetry (DSC). High value of entropy changes indicated large orientational disorder of the high temperature and intermediate phase. Nuclear magnetic resonance (¹H NMR and ¹⁹F NMR) relaxation measurements revealed that the phase transitions at T_C1 and T_C2 were associated with a drastic and small change, respectively, of the both spin-lattice relaxation times: T₁(¹H) and T₁(¹⁹F). These relaxation processes were connected with the “tumbling” motions of the [Cd(H₂O)₆]²⁺, reorientational motions of the H₂O ligands, and with the iso- and anisotropic reorientation of the BF₄⁻ anions. The cross-relaxation effect was observed in phase III. The line width and the second moment of the ¹H and ¹⁹F NMR line measurements revealed that the H₂O reorientate in all three phases of the title compound. On heating the onset of the reorientation of 3 H₂O in the [Cd(H₂O)₆]⁺², around the three-fold symmetry axis of these octahedron, causes the isotropic reorientation of the whole cation. The BF₄⁻ reorientate isotropically in the phases I and II, but in the phase III they perform slow reorientation only about three- or two-fold axes. A small distortion in the structure of BF₄⁻ as well as of [Cd(H₂O)₆]²⁺ is postulated. The temperature dependence of the bandwidth of the O-H stretching mode measured by Fourier transform middle infrared spectroscopy (FT-MIR) indicated that the activation energy for the reorientation of the H₂O did not change much at the T_C2 phase transition.     

Vibrational study of hydrogen bonding and structural disorder in Na2H(SO4)2, K3H(SO4)2 and (NH4)3H(SO4)2 crystals

Infrared and Raman spectra on Na₃H(SO₄)₂, K₃ H(SO₄)₂ and (NH₄)₃ H(SO₄)₂ crystals have been investigated at 300 and 100 K in the 4000 to 30 cm⁻¹ region. An assignment of bands in terms of OH group frequencies and more or less distorted tetrahedra of ammonium and sulphate ions is given. The crystallographic and spectroscopic symmetry and/or dissymetry of OHO hydrogen bonds linking sulphate ions into dimers is discussed using OH group frequencies and the splitting of the v₁ (SO₄) Raman bands as criteria. In the particular case of (NH₄)₃H(SO₄)₁ compound containing several solid phases it can be shown that the room temperature phase (II) is strongly disordered, principally because of an orientational disorder of ammonium ions, and that a progressive ordering takes place with temperature lowering.     

Investigation of phase equilibria in the systems K2SO4Sc2(SO4)3, Rb2SO4Sc2(SO4)3 and Cs2SO4Sc2(SO4)3

Phase diagrams of the systems K₂SO₄Sc₂(SO₄)₃, Rb₂SO ₄Sc₂(SO₄)₃ and Cs₂SO₄ Sc₂(SO₄)₃ have been investigated by X-ray diffraction phase analysis and differential thermal analysis techniques. A salient feature of all the systems is the formation of M₃Sc(SO₄)₃, which melt incongruently, and MSc(SO₄)₂, which on heating decompose in the solid state.     

One-step synthesis of pentavalent triphenylantimony derivatives Ph3Sb(OSiR3)2, Ph3Sb(OCH2CH2)2NH and Ph3Sb(OCH2CH2NMe2)2: X-ray molecular structure of Ph3Sb(OSiMe3)2

Pentavalent bis(triorganosiloxy)triphenylantimony derivatives, Ph₃Sb(OSiR₃)₂ (R = Me, Ph), were synthesized by reaction of triphenylantimony with trimethyl- or triphenylsilanol in the presence of tert-butylhydroperoxide by the mild reaction conditions (0-5 °C, 2 h). The reaction of triphenylantimony with diethanolamine in the presence of tert-butylhydroperoxide gave the cyclic compound Ph₃Sb(OCH₂CH₂)₂NH. The mixture of Ph₃SbO and Ph₃Sb(OCH₂CH₂NMe₂)₂ was obtained by the reaction of triphenylantimony with 2-(N,N-dimethylamino)ethanol in the presence of tert-butylhydroperoxide.     

Synthesis and crystal structure of gallosilicate- and aluminogermanate tetrahydroborate sodalites Na8[GaSiO4]6(BH4)2 and Na8[AlGeO4]6(BH4)2

Tetrahydroborate enclathrated sodalites with gallosilicate and aluminogermanate host framework were synthesized under mild hydrothermal conditions and characterized by X-ray powder diffraction and IR spectroscopy. Crystal structures were refined in the space group P-43n from X-ray powder data using the Rietveld method. Na₈[GaSiO₄]₆(BH₄)₂: a=895.90(1) pm, V=0.71909(3)×10⁻⁶ nm³, R_P=0.074, R_B=0.022, Na₈[AlGeO₄]₆(BH₄)₂: a=905.89(2) pm, V=0.74340(6)×10⁻⁶ nm³, R_P=0.082, R_B=0.026. The tetrahedral framework T-atoms are completely ordered in each case and the boron atoms are located at the centre of the sodalite cages. The hydrogen atoms of the enclathrated anions were refined on x, x, x positions, restraining them to boron-hydrogen distances of 116.8 pm as found in NaBD₄.The IR-absorption spectra of the novel phases show the typical bands of the tetrahedral group as found in the spectrum of pure sodium boron hydride.The new sodalites are discussed as interesting -containing model compounds which could release pure hydrogen.     

Structure refinement and chemical analysis of Cs3Li(DSO4)4, formerly ‘Cs1.5Li1.5D(SO4)2’

An accurate structure refinement of the deuterated analog of the cesium lithium acid sulfate, formerly identified as ‘Cs_1.5Li_1.5H(SO₄)₂’, has been carried out using neutron diffraction methods. Like the protonated material reported earlier (Merinov et al., Solid State Ionics 69 (1994) 53), the compound is cubic, , however, the correct stoichiometry is Cs₃Li(DSO₄)₄. There are four formula units per unit cell and six atoms in the asymmetric unit. The lattice constant measured in this work is a=11.743(2) Å, comparable to the earlier results. The structure contains one disordered hydrogen bond, formed between O(2) atoms and located on two of the edges of the single LiO₄ tetrahedron. The Li site occupancy is , as is that of the deuterium site. This level of site occupancies is consistent with a structure in which hydrogen bonds are formed only when the lithium site is unoccupied, and explains the otherwise close proximity of the Li and D atoms, 1.394(10) Å. This unusual structural feature furthermore leads to a fixed stoichiometry, as confirmed here by chemical analysis of both the deuterated and protonated materials, despite the partial occupancy of the lithium and deuterium (hydrogen) atom sites.     

Syntheses, structures, and properties of Ag4(Mo2O5)(SeO4)2(SeO3) and Ag2(MoO3)3SeO3

Ag₄(Mo₂O₅)(SeO₄)₂(SeO₃) has been synthesized by reacting AgNO₃, MoO₃, and selenic acid under mild hydrothermal conditions. The structure of this compound consists of cis-MoO₂²⁺ molybdenyl units that are bridged to neighboring molybdenyl moieties by selenate anions and by a bridging oxo anion. These dimeric units are joined by selenite anions to yield zigzag one-dimensional chains that extended down the c-axis. Individual chains are polar with the C₂ distortion of the Mo(VI) octahedra aligning on one side of each chain. However, the overall structure is centrosymmetric because neighboring chains have opposite alignment of the C₂ distortion. Upon heating Ag₄(Mo₂O₅)(SeO₄)₂(SeO₃) looses SeO₂ in two distinct steps to yield Ag₂MoO₄. Crystallographic data: (193 K; MoKα, λ=0.71073 Å): orthorhombic, space group Pbcm, a=5.6557(3), b=15.8904(7), c=15.7938(7) Å, V=1419.41(12), Z=4, R(F)=2.72% for 121 parameters with 1829 reflections with I>2σ(I). Ag₂(MoO₃)₃SeO₃ was synthesized by reacting AgNO₃ with MoO₃, SeO₂, and HF under hydrothermal conditions. The structure of Ag₂(MoO₃)₃SeO₃ consists of three crystallographically unique Mo(VI) centers that are in 2+2+2 coordination environments with two long, two intermediate, and two short bonds. These MoO₆ units are connected to form a molybdenyl ribbon that extends along the c-axis. These ribbons are further connected together through tridentate selenite anions to form two-dimensional layers in the [bc] plane. Crystallographic data: (193 K; MoKα, λ=0.71073 Å): monoclinic, space group P2₁/n, a=7.7034(5), b=11.1485(8), c=12.7500(9) Å, β=105.018(1) V=1002.7(2), Z=4, R(F)=3.45% for 164 parameters with 2454 reflections with I>2σ(I). Ag₂(MoO₃)₃SeO₃ decomposes to Ag₂Mo₃O₁₀ on heating above 550 °C.     

A role of non-hydrogen framework vibrational modes in formation of ferroelectric properties of H-bonded K3H(SO4)2-like materials

An effect of molecular vibrational modes within the dimers [O₃AO–(H/D)OAO₃]³⁻ on behavior of H-bond protons or deuterons is studied theoretically for H-bonded antiferroelectrics and quantum paraelectrics of the M₃(H/D)(AO₄)₂ family (M=alkali metal, A=S, Se). The ‘heavy’ reorganization modes in D-species of this family and both reorganization and promoting modes in corresponding H-analogues are examined. It is shown that the analysis with due regard for the reorganization modes results in insignificant decreasing of the effective tunneling frequency of light nuclei. Hence the reasonable estimations of the critical temperature of the low-temperature phase transition in M₃D(AO₄)₂ materials are available only with additional corrections related to potential profiles of a deuteron. It is noted that in the limits of the proposed model the promoting mode in the case of H-species can favor, in principle, the increasing as well as the decreasing of effective proton level splitting.     

Optical spectroscopy of chromium doped (CH3)2NH2X(SO4)2·6H2O (X=Al, Ga) single crystals

Chromium doped (CH₃)₂NH₂Al(SO₄)₂·6H₂O (DMAAlS) and (CH₃)₂NH₂Ga(SO₄)₂·6H₂O (DMAGaS) single crystals were grown and investigated using the methods of optical spectroscopy. It was found that the Cr³⁺ ions in the two crystals are situated in a strong crystal field in which the ²E state is the lowest. The single narrow R-line associated with the ²E–⁴A₂ phosphorescence of Cr³⁺ in DMAAlS in a ferroelectric phase indicates an undistorted octahedral site, whereas important distortion of O_h symmetry and structural disorder was inferred from spectral data obtained with DMAGaS:Cr³⁺ in a low temperature phase. Results of optical investigation are discussed taking into account the structural data.