Dynamic model and tunneling splittings in LMH4 non-rigid hydrides

A non-rigid model for the molecules LiBH₄. NaBH₄⁺. LiCH₄⁺ is suggested on the basis of ab initio data. The model is used in numerical calculations of the levels of large-amplitude deformation-migration motion of the cation in these molecules. It is shown that the lowest states can be regarded as highly anharmonic bending vibrations, while levels above the barrier should be considered as hindered rotation of the MH₄ group. T_d symmetry is used to classify the states. Tunneling splittings of the vibrational states are 0.01–1 cm^?1.     

Non-condon effects on radiationless triplet decay in benzene and naphthalene

Model calculations that avoid the Condon approximation are reported for T₁$\text{}\text{?}$ar S₀ intersystem crossing in normal and perdeuterated benzene and naphthalene. In both benzenes and in naphthalene-d₈, skeletal bending vibrations coupling T₁ to T₂ are found to be more efficient as accepting modes than CH-stretching vibrations.     

NMR study of molecular reorientations in NH4H2AsO4

Zeeman (T_1Z) and dipolar (T_1D) spin-lattice relaxation times of protons in NH₄H₂AsO₄ were measured as a function of temperature. The existence of a slow motion (τ ≈ 10^?3 see) is established, which is most probably a low frequency hindered reorientation of H₂AsO₄ groups. This motion is slowed down below the Curie point T_c. A sharp increase of the dipolar relaxation rate above T = 314°K indicates the possibility of a new high temperature phase transition in this compound.     

Vibrational Spectra of the Cyclic SN-Compounds S4N4, S4N4H4 and S3N3Cl3

The IR and Raman spectra of solid and dissolved S₄N₄, S₄N₄H₄, S₄N₄D₄ and S₃N₃Cl₃ have been recorded and are assigned according to D_2d, C_4v and C_3v symmetry respectively. In the solid state, many forbidden bands and splittings of degenerate vibrations are observed because of the symmetry lowering in the crystals. Due to the different size and shape of the rings and to strong coupling of the normal modes there is no clear correlation between the SN ring stretching vibrations and the strength of the SN bonds, except for the one of the E modes. However, the stretching force constant show the trend expected from changes in interatomic distances.     

Thermodynamics of G-3(D4) and G-6(D4) carbosilanecyclosiloxane dendrimers

Temperature dependences of the heat capacity of G-3(D₄) and G-6(D₄) carbosilanecyclosiloxane dendrimers are studied for the first time by precision adiabatic vacuum and differential scanning calorimetry in the range of 6 to 350–450 K. Physical transformations in the investigated temperature range are observed and their standard thermodynamic characteristics are determined and discussed. Standard thermodynamic functions for a mole unit are calculated from the experimental data: C _p ^○ (T), H ^○(T), ? H ^○(0), S ^○(T) ? S ^○(0), and G ^○(T) ? H ^○(0) in the range of T → 0 to (350–449) K and standard entropies of formation at 298.15 K. Low-temperature (T ≤ 50 K) heat capacity is analyzed using the Debye theory of heat capacity of solids and the multifractal model. The values of fractal dimensionality D are determined and some conclusions on the topology of the investigated structures are drawn. The corresponding thermodynamic properties of the investigated carbosilanecyclosiloxane dendrimers under study are compared.     

Self-assembly and high anisotropy thermal expansion of compounds consisting of <Emphasis Type="Italic">T</Emphasis>O<Subscript>3</Subscript> triangular radicals

Thermal motion is the inherent driving force of crystal substances formation. In borates, carbonates and nitrates such an organizing force are the strong anisotropy of thermal vibrations of atoms in the TO₃ triangles (T = B, C, N), flat triborate and other B–O rigid groups containing TO₃ triangles. These triangles and rigid groups, due to the sharp anisotropy of thermal vibrations, tend to be arranged in the parallel (or preferable) orientation. In this case, chemical compound has usually the least energy. The examples of the self-organization of atoms, TO₃ triangles and rigid groups during crystallization process are given.     

Quantum vibration dynamics and deformation of skeleton of polymer molecules

The results of investigation on the temperature dependences of strain of the regularly constructed fragments of the carbon skeleton in linear polymers are reviewed. Stretching of the skeleton is induced under the action of twisting and bending vibrations when both ends of an oscillating chain fragment are fixed. In this case, the anharmonic character of vibrations plays no key role. At temperatures T _t and T _b, the statistics of atomic vibrations changes from quantum to classical, and the slope of the corresponding temperature dependences changes as well. At T < T _t, vibrations are not excited, and the skeleton is extended owing to zero-point vibrations. At T _t, twisting vibrations are induced, and a skeleton of the molecules is extended with temperature linearly. At T _b, bending vibrations are activated, and the slope of the temperature dependences of the strain of the skeleton increases. Additional stretching and an increased amplitude of vibrations of the skeleton carbon atoms are provided owing to the scattering of stretching vibrations at the boundaries of crystals.     

Thermal,dielectric and barocaloric properties of NH4HSO4 crystallized from an aqueous solution and the melt

A study of heat capacity, thermal dilatation, permittivity, dielectric loops and susceptibility to hydrostatic pressure was carried out on quasi-ceramic samples of NH₄HSO₄ obtained from an aqueous solution as well as the melt. The main parameters of the successive P2₁/c (T₁) ↔ Pc (T₂) ↔ P1 phase transitions did not depend on the method of preparation of the samples, and were close to those determined in previous studies of single crystal and powder, except for the sign and magnitude of the baric coefficient for T₂. Direct measurements of the pressure effect on the permittivity and thermal properties showed dT₂/dp = −123 K·GPa⁻¹, which is consistent in terms of magnitude and sign with the baric coefficient evaluated using dilatometric and calorimetric data in the framework of the Clausius-Clapeyron equation. Thus, the temperature region of the ferroelectric Pc phase existence is extended under pressure. A strong decrease in the entropy jump at the Pc ↔ P1 transformation with an increase in pressure, and the linear dependence of T₂ on pressure, indicate that an increase in pressure shifts this phase transition towards the tricritical point on the T–p phase diagram. A significant barocaloric effect was found in the region of the Pc ↔ P1 phase transition.     

Phase formation in the Ag3VO4-ScVO4 quasi-binary system

Subsolidus phase formation in the Ag₃VO₄-ScVO₄ quasi-binary system was studied using X-ray powder diffraction and DTA in air under atmospheric pressure over the entire range of component concentrations in 5 mol % steps between 20 and 800°C. Two compounds were found to form: Ag₃Sc(VO₄)₂ and Ag₃Sc₂(VO₄)₃, both melting incongruently at 750 ± 5 and 960 ± 5°C, respectively. A T-x diagram of the Ag₃VO₄-ScVO₄ quasi-binary system was constructed. A eutectic (T _m = 450 ± 5°C) is between the compounds Ag₃VO₄ and Ag₃Sc(VO₄)₂; Ag₃VO₄ concentration is ～5 mol %.     

Conductivity of molten LiF-ZrF<Subscript>4</Subscript>, NaF-ZrF<Subscript>4</Subscript>, KF-ZrF<Subscript>4</Subscript>, RbF-ZrF<Subscript>4</Subscript>, and CsF-ZrF<Subscript>4</Subscript> systems

Specific conductivity of molten salt mixtures of the LiF-ZrF₄, NaF-ZrF₄, KF-ZrF₄, RbF-ZrF₄, and CsF-ZrF₄ systems is measured in the whole concentration range using the reference capillary technique. The results are presented in the form of equations of the χ = a + bT + cT ² [S m^?1] type. The concentration dependences of molar conductivity are calculated on the basis of the density data. The obtained regularities are explained in the terms of the complex model of ion melt structures.     

Proton NMR study of molecular dynamics in ammonium tribromo stannate (NH4SnBr3)

The proton second moment M₂ and spin-lattice relaxation time T₁ have been measured in ammonium tribromo stannate (NH₄SnBr₃) in the temperature range 77–300 K, to determine the ammonium dynamics. The continuous wave signal is strong and narrow at 77 and 300 K but has revealed an interesting intensity anomaly between 210 and 125 K. T₁ shows a maximum (13 s) around 220 K. No minimum in the T₁ vs 1000/T plot was observed down to 77 K. M₂ and T₁ results are interpreted in terms of NH⁺₄ ion dynamics. The activation energy E_a for NH⁺₄ ion reorientation is estimated to be 1.4 kcal mol⁻¹.     

Vacuum ultraviolet mcd and absorption spectra of P4 isolated in an ar matrix using synchrotron radiation

The VUV MCD and absorption spectra of P₄ isolated in an argon matrix were obtained using synchrotron radiation. Four well resolved ¹A₁ → ¹T₂^J transitions are observed (J = A, B, C, D from low to high energy). A Jahn-Teller effect is clearly evident in the ¹T₂^C excited state. The ¹T₂^D excited state contains substantial 3d character.     

Vibronic coupling to T1 and T2 vibrations and the Jahn-Teller effect in transition-metal exchange tetraclusters

A study has been made on the four-center Jahn-Teller effect in tetrahedral transition-metal tetraclusters of M₄X₄L₄ and M₄OX₆L₄ types (M metal, X and L ligands) for twofold orbital degeneracy at each of the four centers and strong coupling between the electronic states there and the cluster vibrations. An analytic expression is derived for the adiabatic potential in the space of all active vibrations. The features of the adiabatic-potential sheets in the T₁ and T₂ vibration space are examined, and the spectrum is derived for the lowest vibronic states. There is a qualitative discussion of the system dynamics.Translated from Teoreti-cheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 6, pp. 657–665, November–December, 1987.     

Thermodynamic properties and phase transitions of Tutton salt (NH4)2Fe(SO4)2·6H2O from MAS NMR and single-crystal NMR

The thermodynamic properties and phase transitions of Tutton salt (NH₄)₂Fe(SO₄)₂·6H₂O were investigated using thermogravimetric analysis, differential scanning calorimetry, and nuclear magnetic resonance. The first mass loss occurs around 330 K (T _d), which is interpreted as the onset of partial thermal decomposition. Phase transitions were found at 387 K (=T _C1) and 500 K (=T _C2). The temperature dependences of the spin–lattice relaxation time in the rotating frame, T _1ρ, and that in the laboratory frame, T ₁, for the H nuclei change abruptly near T _C1. These changes are associated with changes in the geometry of the arrangement of octahedral water molecules and ammonium protons.     

Thermal properties of TbB4

This paper presents the results of an experimental study of the temperature dependencies of the heat capacity and the lattice parameters of terbium tetraboride (TbB₄) measured in a temperature range of 2–300 K. This study reveals the anomalies of the above characteristics of TbB₄ at the temperatures of the corresponding magnetic phase transitions (Neel points) T _N1 = 44 K and T _N2 = 24 K. The transitions of TbB₄ from the tetragonal crystalline structure into the orthorhombic one at temperatures below T _N1 were identified. Within our experimental accuracy, no structural transition into the orthorhombic symmetry at temperatures above 80 K was observed. The individual temperature variations of lattice-related and magnetically induced components of the measured properties of TbB₄ are extracted using the data for the diamagnetic LuB₄. As a result, separate contributions of lattice and magnetic subsystems of TbB₄ are determined.     

High-temperature heat capacity of YbAl3(BO3)4

The isobaric heat capacity C _p (T) of YbAl₃(BO₃)₄ grown by spontaneous crystallization from solution (100 ? n) wt % (Bi₂Mo₃O₁₂ + 2.5% B₂O₃ + 0.75% Li₂MoO₄) + n wt % YbAl₃(BO₃)₄ is studied experimentally in the region of 344–1016 K. It is established that there are no extrema on the C _p (T) dependence, and the obtained data can be described using the Berman-Brown polynomial. The temperature variations of enthalpy and entropy are calculated from the C _p (T) dependence.     

Some new observations on the luminescence of PbMoO4 and PbWO4

We report on the luminescence of doped and undoped PbMoO₄ and PbWO₄ single crystals; a red emission is found for the first time for PbMoO₄. The degree of polarization of both the emissions of PbWO₄ and of the shorter-wavelength emission of PbMoO₄ is measured. The shorter-wavelength emissions are assigned to the ³T₁¹A₁ transition in the tetrahedral MoO₄^2?(WO₄^2?) group. It is assumed that the ³T₁ level is split due to spin-orbit coupling. The longer-wavelength emissions are assigned to a transition in a molybdate (tungstate) group lacking an oxygen ion, i.e., a MoO₃(WO₃) group.     

Broken orbital symmetry and the description of valence hole states in the tetrahedral [CrO4]2− anion

The localization of ligand-based valence holes in the tetrahedral complex ion [CrO₄]^2? in a crystalline environment is studied by SCF calculations on the hole states, with progressively less restrictions on the spatial symmetry of the molecular orbitals. The final wavefunctions are obtained by constructing, from the symmetry broken SCF solutions, wavefunctions that exhibit again the proper transformation properties under the operations of T _d . The crystal environment of the [CrO₄]^2? anion is represented by a point charge model. In contrast with the situation for core hole states, the projection afterwards into T _d symmetry is important. The final ionization energies, which are obtained from projected C _3v adapted SCF solutions, are reduced considerably (?3 eV) with respect to the T _d ΔSCF results, but the ordering of the states has not changed essentially. The calculated ionization energies compare favourably with results of XPS experiments on Na₂CrO₄. The evaluation of the energies of projected symmetry broken SCF solutions requires the calculation of hamiltonian matrix elements between determinantal wavefunctions built from mutually non-orthogonal orbital sets. An efficient method for the calculation of such matrix elements is presented.     

Thermodynamics of Mg–Al Order-Disorder Reaction in MgAl2O4-Spinel: Constrained by Prolonged Annealing Experiments at 773–1123 K

MgAl₂O₄-spinel has wide industrial and geological applications due to its special structural and physical–chemical features. It is presumably the most important endmember of complex natural spinel solid solutions, and therefore provides a structural model for a large group of minerals with the spinel structure. There exists a well known but still inadequately understood phenomenon in the structure of MgAl₂O₄-spinel, the Mg–Al cations readily exchanging their positions in response to variations of temperature, pressure, and composition. A large number of experiments were performed to investigate the Mg–Al cation order-disorder process usually quantified by the inversion parameter x (representing either the molar fraction of Al on the tetrahedral T-sites or the molar fraction of Mg on the octahedral M-sites in the spinel structure), and some thermodynamic models were thereby constructed to describe the x-T relation. However, experimental data at some key T were absent, so that the different performance of these thermodynamic models could not be carefully evaluated. This limited the interpolation and extrapolation of the thermodynamic models. By performing some prolonged annealing experiments with some almost pure natural MgAl₂O₄-spinel plates and quantifying the x values with single-crystal X-ray diffraction technique, we obtained some critical equilibrium x values at T down to 773 K. These new x-T data, along with those relatively reliable x values at relatively high T from early studies, clearly indicate that the CS94 Model (a model constructed by Carpenter and Salje in 1994) better describes the Mg–Al cation order-disorder reaction in MgAl₂O₄-spinel for a wide range of T. On the basis of the CS94 Model, a geothermometer was established, and its form is T_-closure = 21362 × x³ − 12143 × x² + 6401 × x − 10 (T_-closure standing for the closure temperature of the Mg–Al cation exchange reaction). This geothermometer can be used to constrain the thermal history of the geological bodies containing MgAl₂O₄-spinel.     

Structure de SnPb2O4 à quatre températures: relation entre dilatation et agitation thermiques

The structural study of SnPb₂O₄ oxide, an isomorphic compound belonging to the general family “MeX₂O₄” like Pb₃O₄, is made from accurate X-ray and neutron diffraction techniques on powdered samples. The structural evolution of SnPb₂O₄ is analyzed from 300 to 5 K: no phase transition is observed, contrary to Pb₃O₄, which exhibits a tetragonal → orthorhombic transition at 170 K. The thermal expansion tensor is practically isotropic in this temperature range: the α_a, α_c and α_V coefficients are neighboring those observed in the Pb₃O₄ tetragonal phase at the same temperature. On the other hand, the thermal vibrations are strongly anisotropic, with large amplitudes in the (a, b) plane. In this study the thermal vibrations are connected to the thermal expansion. $\text{B}\text{ab}$ and $\text{B}\text{c}$ temperature factors are considered as functions of the a and c cell parameters. The relation established by Grüneisen between the mean-square amplitudes of vibrations and the thermal volume expansion is discussed. The interatomic distances found show that the bindings are similar to that of Pb₃O₄: only the [Sn⁴⁺O₆] octahedrons are smaller than [Pb⁴⁺O₆] octahedrons.