Free NH3 quantum rotations in Hofmann clathrates: structure factors and line widths studied by inelastic neutron scattering

Quantum rotations of NH₃ groups in Hofmann clathrates Ni–Ni–C₆H₆ and Ni–Ni–C₁₂H₁₀ have been studied using inelastic neutron scattering. Calculations of the dynamical structure factor for a free uniaxial quantum rotor reproduce the neutron scattering data with respect to their Q- and T-dependence as well as the relative intensities for the 0 → 1, 0 → 2 and 1 → 2 transitions. Though the effective NH₃ rotation constant is different from the gas phase value, the effective radius of rotation (i.e., the average distance of protons from the rotation axis) is equal or very close to the geometrical value r = 0.94 Å for a NH₃ group. Comparing the experimental data with the calculated dynamical structure factor for the 0 → 3 transition it could be shown, that the corresponding transition line, in contrast to transitions between j = 0,1,2 levels measured so far, has a finite width at T = 0 K.     

Jahn–Teller phase transition in Cu0.50TiO(PO4): Powder structural characterization of the β-variety and thermal study

The thermal study of Cu_0.50TiO(PO₄), by X-ray diffraction and DSC, shows a phase transition α → β with a hysteresis (∼600 °C during heating; ∼300 °C during cooling). Single crystals have been obtained for the α-phase but the β-phase can only be stabilised at room temperature as a powder mixture with α. Structural characterization of the β-variety has been done with diffraction data (X-ray Cu Kα₁ and neutrons) using a powder rich in β-phase (α(20%) + β(80%)). A monoclinic cell (a = 7.1134(7) Å; b = 7.7282(7) Å; c = 7.3028(7) Å; β=119.30(1)°; V = 350.1(1) Å³) has been found for β-phase, space group P2₁/c. An “ab initio” structure determination has been done, and the Rietveld refinement leads to cR_wp = 0.150 and R_B = 0.041. The results from the X-ray data were confirmed by refinements from neutron data.Similarly to the α-phase, the structure of β-Cu_0.50TiO(PO₄) can be described as a TiOPO₄ framework constituted of chains of tilted corner-sharing [TiO₆] octahedra running parallel to the c axis and cross linked by [PO₄] tetrahedra. Ti atoms are displaced from the centres of the octahedral units, leading to long (2.27 Å) and short (1.73 Å) Ti–O(1) bonds. The [CuO₆] octahedra exhibit a typical Jahn–Teller distorted coordination with four short equatorial Cu–O bonds (2 × 1.93 Å and 2 × 2.06 Å), and two longer apical Cu–O bonds (2 × 2.33 Å). The two longer Cu–O bonds are almost parallel to the b axis.The transition from the α to the β-phase is characterized by a “rocking” of the Jahn–Teller elongation from the (a,c) plane to the b direction accompanied by a relatively strong expansion of the cell volume.     

Raman spectroscopy investigation on (Pb1−xLax)(Zr0.90Ti0.10)1−x/4O3 ceramic system

A Raman spectroscopy study at room temperature was carried out on (Pb_1−xLa_x)(Zr_0.90Ti_0.10)_1−x/4O₃ ceramics (x = 2, 3, 4 at%). The results were analyzed considering the x-ray patterns at room temperature showing a mixture of two phases: a rombohedral-ferroelectric phase and an orthorhombic-antiferroelectric, increasing the% of the second one with the lanthanum concentration. For x = 3 at%, the analysis was also carried out in a wide temperature range. Two anomalies were evaluated, one around 363 K, which has been associated to a ferroelectric-antiferroelectric phase transition; the second one around 430 K, which has been associated to a transition from an incommensurable state to a ferroelectric phase.     

The crystal structure of Rb2SeO4 at high temperature

The crystal structure of Rb₂SeO₄ in its high-temperature phase is reported for the first time. Powder diffraction data collected at T = 898 K show that it is hexagonal (a = 6.3428(1) Å, c = 8.5445(1) Å, V = 297.71(1) Å³, space group P6₃/mmc (194), Z = 2) and is isostructural to Tl₂SeO₄, thus belonging to the α-K₂SO₄ structure type. DSC measurements indicate that the phase transition occurs at T = 818 K.     

Low-temperature thermal properties of 2,2-dimethyl-1,3-propanediol and its deuterated analogues

Heat capacities of 2,2-dimethyl-1,3-propanediol(CH₃)₂C(CH₂OH)₂ were measured in the temperature range between T =  13 K and T =  350 K using an adiabatic calorimeter. The compound underwent a first-order phase transition at T =  (314.5  ±  0.1) K. The enthalpy and the entropy of transition were (12.52  ±  0.02)kJ · mol ^− 1and (39.81  ±  0.08)J · K ^− 1· mol ^− 1, respectively. Measurement of the fusion peak by d.s.c. showed that the purity of the sample was 0.9999 mass fraction and the entropy of fusion was 9.9 J · K ^− 1· mol ^− 1. Another first-order phase transition was observed at T =  (60.4  ±  0.1) K with the associated entropy change of (2.93  ±  0.05)J · K ^− 1· mol ^− 1. Heat capacities of two deuterated samples,(CH₃)₂C(CH₂OD)₂ and(CD₃)₂C(CD₂OD)₂ , were also measured and the results were compared with those on the natural compound. Possible mechanisms of the transition have been discussed from the isotope effects on the thermodynamic quantities associated with the transition. Standard thermodynamic functions of the compounds are tabulated.     

Characterizing phase transitions in liquid cesium by a soft-core and large-attraction equation of state

This paper investigates to identify phase transitions in condensed liquid cesium metal by considering the variation of intermolecular potential parameters ɛ and r_m in the whole liquid range, with ɛ being the potential well-depth and r_m the position of minimum potential. These parameters were obtained from the parameters of a new equation of state that was derived recently by using the characteristic potential function. By this method, transitions at about 575, 800, 1000, 1350, and 1650 K were identified. Transitions at 575, 800, and 1000 K are weak but, the one at 1350 K is very significant and has been explored experimentally and theoretically as the metal non-metal transition (MNMT), which is a phase transition before the critical condition dominates the thermodynamics. Also variations of the linear correlation coefficient of the isotherms generate a spot point pattern of these transitions. Our observations at 575 K for ɛ and r_m are in accord with the anomalies in adiabatic thermal coefficient of pressure, density, viscosity, electrical conductivity, and structure factor.     

Phase transitions and thermodynamic properties of (NH4)3VO2F4 cryolite

Calorimetric measurements performed in a wide temperature range on (NH₄)₃VO₂F₄ have shown the presence of four heat capacity anomalies at T₁ = 438 K, T₂ = 244 K, T₃ = 210.2 K, T₄ = 205.1 K associated with the first order phase transitions. In accordance with the permittivity behavior, the structural transformations are of nonferroelectric nature. Pressure dependence of the phase transition temperatures has been studied by DTA under pressure. The entropy of phase transitions is analyzed mainly in the framework of the orientational disordering of NH₄⁺ and VO₂F₄^3? ions in a cubic phase.     

The p–T phase diagram for ferroelectric bis-thiourea pyridinium nitrate

The effect of temperature and pressure on physical properties of the ferroelectric bis-thiourea pyridinium nitrate inclusion compound has been studied by dielectric spectroscopy and nuclear magnetic resonance (NMR). At ambient pressure the ferroparaelectric phase transition observed at T₂ = 216 K is continuous in contrast to the nonferroelectric phase transition observed at T₁ = 273 K. Under small pressures, the temperatures of the phase transitions T₁ and T₂ increase with increasing pressure. Starting from about 250 MPa, T₁ temperature decreases with increasing pressure, while T₂ temperature increases with increasing pressure. At 450 MPa and 245 K a triple point is observed. Bis-thiourea pyridinium nitrate undergoes a continuous phase transition from the ferroelectric to paraelectric phase under 450 MPa, while above this pressure the phase transition from the ferroelectric to paraelectric phase is discontinuous. The change in the phase transition character is related to the crystallographic change in the group–subgroup relation between the ferro- and paraelectric phases taking place with increasing pressure.     

A Ca substitution study of NaV2O4: High-pressure synthesis of the Na1−xCaxV2O4 solid solution

Ambient pressure CaV₂O₄ and high-pressure NaV₂O₄ crystallize in the CaFe₂O₄ structure type containing double chains of edge-sharing VO₆ octahedra. Recent measurements on NaV₂O₄ reveal low-dimensional metallicity and evidence of half-metallic ferromagnetism. In contrast, CaV₂O₄ is an antiferromagnetic insulator. To explore the evolution of these ground-state behaviors, we have prepared a series of Ca-doped NaV₂O₄ compounds with the formula Na_1?xCa_xV₂O₄ (x = 0–1) using high-pressure synthesis. Samples at the Na end (x = 0–0.07) show a broad antiferromagnetic transition in the 120–160 K range in accordance with earlier reports. Transport measurements show an insulator–metal transition at x ～ 0.2. Samples with higher Ca concentrations (x = 0.4–0.7) exhibit a metal–insulator transition around 150 K. The results for the Na_1?xCa_xV₂O₄ solid solution is discussed in comparison to existing studies at the Ca- and Na-rich ends.     

Cooperative Jahn–Teller distortion leading to the spin-1/2 uniform antiferromagnetic chains in triclinic perovskites AgCuF3 and NaCuF3

Polycrystalline samples of AgCuF₃, isostructural with NaCuF₃, were synthesized by solid state reaction and characterized by powder X-ray diffraction. The magnetic properties of AgCuF₃ and NaCuF₃ were examined by measuring their magnetic susceptibilities and evaluating their spin exchange interactions. The three-dimensional CuF₃ network of corner-sharing CuF₆ octahedra present in AgCuF₃ and NaCuF₃ shows a cooperative Jahn–Teller distortion such that their magnetic susceptibilities above 50 K are well described by an S = 1/2 Heisenberg uniform antiferromagnetic chain model with average spin exchange of J/k_B ≈ ?300 and ?180 K, respectively. The relative strengths of these interactions are well reproduced by spin dimer analysis based on tight-binding calculations, but not by mapping analysis based on first principles density functional calculations.     

Structural characterization,molecular dynamics,dielectric and spectroscopic properties of tetrakis(pyrazolium) bis(μ2-bromo-tetrabromobismuthate(III)) dihydrate, [C3N2H5]4[Bi2Br10]·2H2O

The structure of [C₃N₂H₅]₄[Bi₂Br₁₀]·2H₂O, (PBB) was determined by single crystal X-ray diffraction at 100 K. It crystallizes in the monoclinic space group C2/m, with a = 12.992(4) Å, b = 16.326(5) Å, c = 8.255(3) Å, β = 108.56°(3), V = 1659.9(9) Å³ and Z = 2. The structure consists of discrete binuclear [Bi₂Br₁₀]⁴⁻ anions, ordered pyrazolium cations and water molecules. The crystal packing is governed by strong N–H⋯O and weak O–H⋯Br hydrogen bonds. A sequence of structural phase transitions in PBB was established on the basis of differential scanning calorimetry and dilatometric studies. Two reversible first-order phase transitions were found: (I → II) at 381/371 K (on heating/cooling) and (II → III) at 348/338 K. Dielectric response near both phase transitions is characteristic of crystals with the “plastic-like” phases. Over the phase III a low frequency dielectric relaxator is disclosed. The possible molecular motions in the PBB compound are characterized by the ¹H NMR studies. The infrared spectra of polycrystalline compound in the temperature range 300–380 K are reported for the region 4000–400 cm⁻¹. The observed spectral changes through the structural phase transition III → II are attributed to an onset of motion both of the pyrazolium cations and water molecules.     

Mössbauer spectroscopic study of photo-sensitive organic–inorganic hybrid system, (SP)[Fe(II)Fe(III)(dto)3](dto = C2O2S2, SP = spiropyran)

A photo-sensitive organic–inorganic hybrid system (SP)[Fe^IIFe^III(dto)₃] (SP = spiropyran, dto = C₂O₂S₂), has recently been developed, where the photo-isomerization of the intercalated spiropyran in solid state triggers the change of the magnetic properties, including the ferromagnetic transition temperature from 5 to 22 K. We performed ⁵⁷Fe Mössbauer measurement in order to probe the microscopic states of iron ions in (SP)[Fe^IIFe^III(dto)₃] and have investigated the photo-induced effect on them. The sample without UV-irradiation shows the charge transfer phase transition between 200 and 70 K and the higher and lower temperature phases coexist below 70 K, whereas the UV-irradiated sample does not undergo the charge transfer phase transition and the higher temperature phase is stable between 200 and 6 K.     

Low temperature structural phase transition of Ba3NaIr2O9

Single crystal X-ray and synchrotron X-ray powder diffraction have been used to probe the structure of Ba₃NaIr₂O₉ from 300 K down to 20 K. Ba₃NaIr₂O₉ is found to undergo a structural transition from hexagonal symmetry, P6₃/mmc, at ambient temperature to monoclinic symmetry, C2/c, at low temperature. The evolution of the unit cell volume upon cooling is indicative of a higher order structural transition, and the symmetry breaking becomes apparent as the temperature is decreased. The low temperature monoclinic structure of Ba₃NaIr₂O₉ contains strongly distorted [NaO₆] and [IrO₆] octahedra in comparison to the room temperature hexagonal structure.     

Mechanism of reversible spin transition with a thermal hysteresis loop in [FeIII(qsal)2][Ni(dmise)2] · 2CH3CN: Selenium analogue of the precursor of an Fe(III) spin-crossover molecular conducting system

A novel Fe(III) spin-crossover complex, [Fe(qsal)₂][Ni(dmise)₂] · 2CH₃CN 1 [qsalH = N-(8-quinolyl)-salicylaldimine, dmise = 4,5-dithiolato-1,3-dithiole-2-selone] was prepared. The magnetic susceptibility measurements revealed 1 exhibited a cooperative spin transition with a thermal hysteresis loop of 15 K. The high and the low temperature structures of 1 indicated three-dimensional intermolecular π?π interactions play a key role in the cooperative spin transition, accompanying a reversible molecular slipping of π-dimer of Ni(dmise)₂ along the molecular long axis. The transfer integral calculation for 1 suggested the π-dimer of Ni(dmise)₂ is in the spin singlet state.     

Magnetic dilution effect on the charge transfer phase transition and the ferromagnetic transition for an iron mixed-valence complex, (n-C3H7)4N[FeII1−xZnIIxFeIII(dto)3] (dto = C2O2S2)

Iron mixed-valence complex, (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto = C₂O₂S₂) shows a new-type of phase transition coupled with spin and charge around 120 K, where the charge transfer between the Fe^II and Fe^III sites occurs reversibly, and shows the ferromagnetic transition at 7 K. To investigate the magnetic structure and its dimensionality of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃], we have synthesized a mixed crystal system, (n-C₃H₇)₄N[Fe^II_1?xZn^II_xFe^III(dto)₃], and measured its magnetic properties. In this system, the magnetic moment is reduced with increasing of Zn ratio. Moreover, the ferromagnetic interaction changes to the antiferromagnetic one and the remnant magnetization disappears between x = 0.48 and 0.96, while the charge transfer between the Fe^II and Fe^III sites disappears above x = 0.26. In this paper, we present the magnetic dilution effect on the charge transfer phase transition and the ferromagnetic transition by means of magnetic susceptibility measurement and ⁵⁷Fe Mössbauer spectroscopy.     

Crystal structure of a new small-pore zirconosilicate Na2ZrSi2O7·H2O and its relation to stoichiometrically and topologically similar compounds

The small-pore zirconosilicate Na₂ZrSi₂O₇·H₂O crystallizes in monoclinic system with space group C2/c, a = 5.4715(4); b = 9.4111(6); c = 13.0969(8) Å, β = 92.851(7)°. Its framework consists of layers built of ZrO₆ octahedra and SiO₄ tetrahedra and forming condensed [Si₂O₇] pyrogroups by connection. The sodium ions and water molecules are placed in channels set up between the layers. The stoichiometric and structural similarities of the studied phase with anhydrous compounds having general chemical formula A₂₍₃₎MT₂O₇ (A = Na,K; M = Zr,Lu,Sc; T = Si,Ge) are discussed. The topological relationship of their structures is interpreted in the light of spatial combination of silicon and zirconium polyhedra as basic building units into larger composite building units and their three-dimensional arrangement.     

Investigation of anharmonicity in rotational phonon mode for BaWO4 crystal

Spontaneous Raman spectra in the BaWO₄ were measured in the temperature range from 4 K to 280 K, and the temperature dependence of the linewidth of the A_g (191 cm⁻¹) Raman mode was analyzed using the lattice dynamical perturbative approach and one-phonon density of states (PDOS). The linewidth slope for the 191 cm⁻¹ peak for an external mode is 7.2 times larger than that for the 926 cm⁻¹ peak for a breathing mode. The different behaviors of these two modes in the case of temperature broadening could be attributed to the large energy band gap in the one-phonon density of states (PDOS) resulting in different anharmonic interactions. The origin may be that the ratio of up-conversion TDOS to down-conversion TDOS for Eg mode (191 cm⁻¹) is more than that for A_g (926 cm⁻¹). The peak of the Eg mode (191 cm⁻¹) is attributed to the coupling mode both a rotation of the Barium and an out-of-phase rotation of the oxygen in x–y plane as a librational mode.     

Low-dimensional quantum magnetic systems: Synthesis,structure and magnetic behavior of (2,5-dimethylpyrazine)copper(II) chloride and synthesis and magnetic behavior of bis(2,6-dimethylpyrazine)copper(II) chloride

The synthesis, X-ray structure and magnetic susceptibility of (2,5-dimethylpyrazine)copper(II) chloride (1), and the synthesis and magnetic susceptibility of (2,6-dimethylpyrazine)₂copper(II) chloride (2), are reported. Compound 1 crystallizes in the space group P2₁/c as a coordination polymer of Cu(II) ions bridged by 2,5-methylpyrazine. The resulting chains are magnetically linked via short chloride–chloride contacts. The magnetic susceptibility responds as a uniform Heisenberg chain (2J/k = −20(5) K) with a phase transition to three dimensional order near 5 K. Susceptibility data for compound 2 show that the compound is a linear chain coordination polymer with the copper ions linked by bihalide bridges. A fit to the model for a uniform Heisenberg chain yields 2J = −22.7(2) K.     

X-ray emission from LiTaO3 induced by thermal changes and structure analysis

X-ray emission from LiTaO₃ induced by the thermal changes from about 300–480 K has been investigated in a vacuum system. The single-crystal X-ray structure analysis of LiTaO₃ was carried out at 269, 286, 303, 319, 337, 358, 381, 413, and 436 K, respectively, far below the phase transition (∼930 K). The relationship between the observed X-ray emission and the pyroelectric coefficient of LiTaO₃ is discussed.     

Measurements of (p, ρ, T) properties for isobutane in the temperature range from 280 K to 440 K at pressures up to 200 MPa

Measurements of (p, ρ, T) properties for isobutane in the compressed liquid phase have been obtained by means of a metal-bellows variable volumometer in the temperature range from 280 K to 440 K at pressures up to 200 MPa. The volume-fraction purity of isobutane used was 0.9999. The expanded uncertainties (k = 2) of temperature, pressure, and density measurements have been estimated to be less than 3 mK, 1.5 kPa (p ⩽ 7 MPa), 0.06% (7 MPa < p ⩽ 50 MPa), 0.1% (50 MPa < p ⩽ 150 MPa), and 0.2% (p > 150 MPa), and 0.11%, respectively. In region more than 100 MPa at 280 K and 440 K, the uncertainty in density measurements rise up to 0.15% and 0.23%, respectively. The differences of the present density values at the same temperature between two series of measurements, in which the sample fillings are different, are within the maximum deviation of 0.09% in density, which is enough lower than the expanded uncertainty in density. Eight (p, ρ, T) measurements at the same temperatures and pressures as the literature values have been conducted for comparison. In addition, vapour pressures were measured at T = (280, 300) K. Moreover, the comparisons of the available equations of state with the present measurements are reported.