High-Temperature Phase Transition in N(CH3)4CdCl3 Studied Using Static NMR and MAS NMR

To clarify the nature of microscopic structural changes of N(CH₃)₄CdCl₃ at high temperatures, the nuclear magnetic resonance (NMR) spectra of the protons and carbons in N(CH₃)₄CdCl₃ were measured. NMR studies of the ¹H and ¹³C spin–lattice relaxation time, T _1ρ, in the rotating frame were also performed. No changes in the T _1ρ of ¹H and ¹³C associated with the N(CH₃)₄ groups were observed at the high-temperature transition from phase I to phase I′. However, the ¹⁴N NMR spectra reflected changes in the structural geometry during the transition to phase I′, indicating that this transition is driven by N(CH₃)₄.     

Manifestations of Phase Transitions and the Thermooptical-Memory Effect in the Absorption Spectra of (N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>)<Subscript>2</Subscript>CuCl<Subscript>4</Subscript> Crystals

The manifestations of phase transitions and the spatial modulation of the structure of the (N(CH₃)₄)₂CuCl₄ crystal in its absorption spectra have been investigated. It has been established that the thermooptical-memory effect manifesting itself in the absorption spectra of this crystal entirely corresponds to the model of ordering of defects and impurities in a sample with a modulated structure. It is shown that, as a result of stabilization of the crystal studied in the incommensurate phase, its symmetry and the symmetry of the metal-halogen complex in it lower depending on the defect wave. This manifests itself as a marked shift of the intraionic-absorption bands and an increase in their intensity. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 386–390, May–June, 2005.     

Electrical characterization of the [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>][N(C<Subscript>2</Subscript>H<Subscript>5</Subscript>)<Subscript>4</Subscript>]ZnCl<Subscript>4</Subscript> compound

The [N(CH₃)₄][N(C₂H₅)₄]ZnCl₄ compound has been synthesized by a solution-based chemical method. The X-ray diffraction study at room temperature revealed an orthorhombic system with P2₁2₁2 space group. The complex impedance has been investigated in the temperature and frequency ranges 420–520 K and 200 Hz–5 MHz, respectively. The grain interior and grain boundary contribution to the electrical response in the material have been identified. Dielectric data were analyzed using the complex electrical modulus M ^* for the sample at various temperature. The modulus plots can be characterized by full width at half height or in terms of a non-exponential decay function ϕ(t) = exp[(−t/τ) ^β ]. The detailed conductivity study indicated that the electrical conduction in the material is a thermally activated process. The variation of the AC conductivity with frequency at different temperatures obeys the Almond and West universal law.     

X-ray study of [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> at low temperatures

The unit cell parameters a, b, and c of [N(CH₃)₄]₂ZnCl₄ have been measured by x-ray diffraction in the temperature range 80–293 K. Temperature dependences of the thermal expansion coefficients α_a, α_b, and α_c along the principal crystallographic axes and of the unit cell thermal expansion coefficient α_V were determined. It is shown that the a=f(T), b=f(T), and c=f(T) curves exhibit anomalies in the form of jumps at phase transition temperatures T₁=161 K and T₂=181 K and that the phase transition occurring at T₃=276 K manifests itself in the a=f(T) and b=f(T) curves as a break. A slight anisotropy in the coefficient of thermal expansion of the crystal was revealed. The phase transitions occurring at T₁=161 K and T₂=181 K in [N(CH₃)₄]₂ZnCl₄ were established to be first-order.     

Electronic spectra and phase transitions in thin [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>CuCl<Subscript>4</Subscript> microcrystals

Optical absorption spectra in thin [N(CH₃)₄]₂CuCl₄ crystals in the thickness range 10 μm ≤ d < 100 μm have been studied. Strengthening of the crystal field has been found with a decrease in the [N(CH₃)₄]₂CuCl₄ crystal size. The reasons for absorption band shifts in the visible region depending on the [N(CH₃)₄]₂CuCl₄ crystal thickness and the manifestation of a size effect in crystals with an incommensurate superstructure are discussed.     

Optical transmission coefficient of crystalline [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>MeCl<Subscript>4</Subscript> in the incommensurate phase

This a study of the optical transmission of [N(CH₃)₄]₂ZnCl₄ and [N(CH₃)₄]₂CuCl₄ crystals in the incommensurate phase when a defect density wave is present. It is found that an anomalous reduction in the transmission coefficient is caused by scattering of light owing to a realignment of the superstructure during transitions between metastable states. When a defect density wave is present, the anomalous optical transmission of the crystal is related to the scattering of light on superstructure inhomogeneities produced by a superposition of existing modulation waves.     

Magnetic properties of the quasi-two-dimensional crystal (CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>)<Subscript>2</Subscript>CuBr<Subscript>4</Subscript>

The magnetic properties of (CH₃NH₃)₂CuBr₄ quasi-two-dimensional crystals were studied experimentally. The magnetic-field and temperature dependences of magnetization were measured for various magnetic field orientations relative to the crystallographic axes. Possible reasons for features in the behavior of the magnetization are discussed.     

Nonlinear magnetic resonance in the (CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>)<Subscript>2</Subscript>CuBr<Subscript>4</Subscript> crystal

The nonlinear microwave absorption in the (CH₃NH₃)₂CuBr₄ antiferromagnetic crystal is investigated experimentally. The temperature and angular dependences of the parameters of nonlinear resonance and the dependences of these parameters on the microwave pump power are analyzed. It is found that the nonlinear properties deteriorate with decreasing temperature and the linear and nonlinear contributions are competitive in character.     

Electric field-induced formation of a new commensurate phase of the [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>CuCl<Subscript>4</Subscript> crystal

The formation of a new commensurate phase with a dimensionless wave number q = 1/3 in an [N(CH₃)₄]₂CuCl₄ crystal under the action of an external electric field is analyzed theoretically. The phase diagram is constructed on a plane specified by two coefficients of the thermodynamic potential in the presence and in the absence of an external electric field.     

A new commensurate phase on the theoretical phase diagram of the [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>CuCl<Subscript>4</Subscript> crystal

A theoretical phase diagram of the [N(CH₃)₄]₂CuCl₄ crystal with a new commensurate phase characterized by a dimensionless wavenumber q = 2/5 is constructed on a plane specified by two coefficients of the thermodynamic potential. This diagram is used as the basis for the construction of a theoretical pressure-temperature (P-T) phase diagram. The theoretical P-T phase diagram thus obtained is compared with the experimental P-T phase diagram.     

Signs of phase transitions and a thermooptic memory effect in absorption spectra of (N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>)<Subscript>2</Subscript>Zn<Subscript>0.8</Subscript>Ni<Subscript>0.2</Subscript>Cl<Subscript>4</Subscript> solid solutions

We have studied the signs of phase transitions and spatial modulation of the structure in the absorption spectra of an (NCH₃)₄)₂Zn_0.8Ni_0.2Cl₄ crystal. We have observed the existence of phase transitions in the given solid solution at temperatures of 155 K, 168 K, 275 K, 280 K, and 296 K. We have established that the thermooptic memory effect observed in the absorption spectra is completely consistent with a model of defect ordering in the sample in the field of the modulated structure. According to this model, stabilization of the sample in an incommensurable phase leads to fixing of a certain symmetry in the crystal (usually a lower symmetry than the average symmetry of the incommensurable phase) and a metal-halogen complex corresponding to the defect wave. As a result, we observe an appreciable shift of the intra-ionic absorption bands and an increase in their intensity. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 5, pp. 717–723, September–October, 2008.     

A Phenomenological Model of Structural Phase Transitions in RbDy(Wo<Subscript>4</Subscript>)<Subscript>2</Subscript>

A phenomenological model of structural phase transitions in double rubidium–dysprosium tungstate is proposed. The model is constructed by equivariant catastrophe theory methods. The temperature dependence of the heat capacity near the phase transitions at temperatures T₁ = 9 K and T₂ = 4.9 K was calculated. Comparison with the experimental data shows a satisfactory qualitative agreement. In terms of the proposed model, the low-temperature phase transition can be interpreted as isomorphic.     

Phase transitions in the oxyfluoride (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>NbOF<Subscript>6</Subscript>

(NH₄)₃NbOF₆ single crystals were grown, polarization-optical studies were performed, and birefringence was measured over the temperature range 90–500 K. A sequence of first-order structural phase transitions was found at temperatures T _1↓ = 259.7 K and T _2↓ = 257.7 K with temperature hysteresis δT ₁ = 0.9 K and δT ₂ = 1.9 K. The transitions are accompanied by twinning and the following change in the crystal symmetry: cubic ? tetragonal ? monoclinic. Optical second harmonic generation is found to occur at room temperature, which indicates that the cubic phase is not centrosymmetric. It is assumed that the phase transitions are ferroelastic and ferroelectric in nature.     

Crystallographic characteristics and phase transitions in the [N(C<Subscript>2</Subscript>H<Subscript>5</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>CdBr<Subscript>4</Subscript> crystal in the low-temperature range

The results of x-ray structural studies of the [N(C₂H₅)₄]₂CdBr₄ crystal at low temperatures are presented. The unit cell parameters and the thermal expansion coefficients along the main crystallographic directions are measured at temperatures in the range from 90 to 320 K. The integrated intensities of the diffraction reflections are investigated as a function of the temperature. It is shown that the curves a = f(T), c = f(T), I ₅₀₀ = f(T), and I ₀₀₆ = f(T) at temperatures T ₁ ≈ 174 K and T ₂ ≈ 226 K exhibit anomalies in the form of abrupt changes in the lattice parameters and the diffraction reflection intensities. This indicates that the [N(C₂H₅)₄]₂CdBr₄ crystal undergo phase transitions at these temperatures. Moreover, there is an anomaly in the form of a small maximum at the temperature T ₃ = 293 K.     

Structural and dehydriding properties of Ca(BH<Subscript>4</Subscript>)<Subscript>2</Subscript>

We have investigated the structural and dehydriding properties of Ca(BH₄)₂. It was found that Ca(BH₄)₂ undergoes a structural phase transformation from an orthorhombic low-temperature (LT) modification into a tetragonal high-temperature (HT) modification between 433 K and 523 K. The amount of hydrogen desorbed from Ca(BH₄)₂ during the pressure–composition (p–c) isotherm measurement was 5.9 mass%. This hydrogen desorption is caused by the partial dehydrogenation of Ca(BH₄)₂ accompanied by the formation of CaH₂ and orthorhombic intermediate phases.     

Structural changes during phase transitions and the critical and noncritical order parameters in the (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>Nb(O<Subscript>2</Subscript>)<Subscript>2</Subscript>F<Subscript>4</Subscript> crystal

The structures of two phases of the (NH₄)₃Nb(O₂)₂F₄ crystal, namely, the parent cubic phase and the most distorted low-temperature phase, have been determined from data of an X-ray diffraction experiment performed for a powder sample. The profile and structural parameters have been refined according to the procedure implemented in the DDM program. The results obtained have been discussed with invoking the group-theoretical analysis of the complete order parameter condensate, which takes into account the critical and noncritical atomic displacements and allows the interpretation of the obtained experimental data. It has been found that the most probable sequence of structural transformations occurring in the crystal can be schematically represented in the following form:

Phase behaviour and superionic phase transition in K<Subscript>3</Subscript>H(SeO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The phase behaviour of K₃H(SeO₄)₂ (TKHSe) above room temperature has been studied by differential scanning calorimetric (DSC), thermogravimetric analysis (TGA), simultaneous thermogravimetric and mass spectroscopy analysis (TG-MS), impedance spectroscopy (IS) and X-ray powder diffraction (XRD). Our results show that the previously claimed superionic phase transition in TKHSe at around 388 K (114.85 °C) is also the onset temperature of a slow thermal dehydration that occurs at reaction sites distributed over the surface of the crystal. That is, we propose that the TKHSe undergoes simultaneously a superionic phase transition and a decomposition process with a very slow reaction rate that is evident when the sample is pulverized to fine powder, both starting at the same temperature. As a matter of fact, we observe a decrease of the magnitude of the dc-conductivity on successive thermal runs in powdered sample attributed to sample decomposition that starts at the surface of the TKHSe grains, but the jump in conductivity is only a consequence of the order–disorder transition in the TKHSe phase that remains inside the grains.     

Comparative Study of the Magnetoelectric Effect in HoAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> and HoGa<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> Single Crystals

The comparative study of the magnetoelectric properties and magnetostriction of HoGa₃(BO₃)₄ and HoAl₃(BO₃)₄ single crystals has been carried out. The investigated compounds exhibit qualitatively similar magnetodielectric and inverse magnetoelectric ME _E effects with the close absolute values, which is indicative of the weak effect of a nonmagnetic metal ion. On the contrary, the magnetostriction of the galloborate has been found to be threefold higher than that of the alumoborate. In addition, the difference between the qualitative behaviors of magnetostriction has been established: the magnetic-field dependence of magnetostriction for the alumoborate has the maximum near 70 kOe at T = 4.2 K, while the galloborate magnetostriction has no maximum and does not saturate in a field of 140 kOe.