Phase transitions in the (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>NbOF<Subscript>5</Subscript> oxyfluoride

The thermal and dielectric properties of the (NH₄)₂NbOF₅ oxyfluoride have been investigated. It has been established that the structural phase transitions Cmc2₁ → C2 → Ia observed at the temperatures T ₁ = 258.0 K and T ₂ = 218.9 K exhibit a nonferroelectric nature. The hydrostatic pressure, which stabilizes the initial phase and destabilizes the low-temperature phase, hardly affects the temperature range of stability of the intermediate phase. The model of sequential ordering of the structural elements due to phase transitions has been analyzed using experimental data on the entropies of the phase transitions.     

Thermophysical studies of the phase transitions in (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>NbOF<Subscript>6</Subscript> crystals

The thermophysical properties of oxyfluoride (NH₄)₃NbOF₆ were studied in detail over wide ranges of temperatures and pressures. At atmospheric pressure, a sequence of four structural phase transitions was established with the following changes in entropy: ΔS ₁ = Rln 2.7, δS ₂ = Rln38.3, ΔS ₃ = 0.08R, and ΔS ₄ = 0.17R. An external hydrostatic pressure was found to narrow the region of existence of the initial cubic phase. A triple point was detected in the p-T diagram; at a pressure above 0.07 GPa, the transition between the tetragonal and monoclinic phases occurs through a distorted high-pressure phase.     

Vibrational spectra of disordered oxyfluoride (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>WO<Subscript>3</Subscript>F<Subscript>3</Subscript>

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.     

Optical studies of phase transitions in the (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>Ti(O<Subscript>2</Subscript>)F<Subscript>5</Subscript> crystal

Polarization-optical study of twinning and measurements of the Raman spectra and birefringence in oxyfluoride (NH₄)₃Ti(O₂)F₅ were carried out over the temperature range 90–350 K. Phase transitions were detected at temperatures T ₀₁ = 266 K (second-order transition) and T ₀₂ = 225 K (first order). It is assumed that the crystal symmetry is changed as follows: Fm3m ? I4/mmm ? I4/m. Anomalies of the spectral parameters are established in the frequency range of internal vibrations of ammonium ions and Ti(O₂)F₅ complexes. An analysis of the results shows that the transition at T ₀₁ is likely due to small shifts of the tetrahedral groups from their position on the triad axis and that the transition at T ₀₂ is due to fluorine-oxygen ordering of Ti(O₂)F₅ complexes.     

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 transitions of SC(NH<Subscript>2</Subscript>)<Subscript>2</Subscript> ferroelectrics in Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-based nanoporous matrices

Temperature dependences of the linear permittivity ε' and the third harmonic amplitude γ_3ω of composites prepared by introducing ferroelectrics SC(NH₂)₂ into matrices of porous aluminum oxide Al₂O₃ with pore sizes 60 and 100 nm are determined. It is found that temperature T _c of the ferroelectric phase transition and the temperature T _i of the phase transition from incommensurable phase to the paraphrase increase significantly. The transition shifts increase as pore diameters decrease.     

Anomalous behavior of the velocity of hypersonic acoustic phonons in (NH<Subscript>4</Subscript>)<Subscript>3</Subscript> H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals

The Gross-Pitaevski equation modified through the inclusion of a term accounting for the nonlocality of interatomic interaction was used to demonstrate the occurrence of extremely narrow two-and three-dimensional solitonic states in atomic Bose-Einstein condensates. The estimates of the sizes of these states gave a value of ～ 20–60 nm (atomic “needles” and “bullets”) for lithium atoms. The soliton lifetimes caused by two-and three-particle collisions were estimated. The limiting possibilities of the formation of nanostructures using needles and bullets were compared with the possibilities of other nanolithographic methods.     

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.     

Effect of deuteration on the thermal properties and structural parameters of the (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>WO<Subscript>2</Subscript>F<Subscript>4</Subscript> oxyfluoride

The thermal properties and structure of (ND₄)₂WO₂F₄ crystals are investigated. It is established that deuteration does not lead to a change in the symmetry of the initial phase Cmcm but considerably decreases the extent of its disordering, which, in turn, brings about a substantial decrease in the phase transition entropy. Apart from the anomalies associated with phase transitions characteristic of the protonic compound, the heat capacity exhibits two additional anomalies. Analysis of the phase diagram of the deuterated crystal reveals a triple point at a pressure p = 0.18 GPa, which is predicted for (NH₄)₂WO₂F₄ at about 0.7 GPa.     

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.     

Refractometry of mechanically compressed (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals

The effect of uniaxial mechanical pressure σ _m ≤ 150 bar on the spectral (300–800 nm) dependence of the birefringerence Δn _i and refractive indices n _i of (NH₄)₂SO₄ crystals has been investigated. It is shown that the dispersion of n _i (λ) and Δn _i (λ) is normal and sharply increases with approach to the absorption edge. It is established that uniaxial pressure does not change the character of the dispersions dn _i /dλ and dΔn _i /dλ and only affects their magnitudes. It is shown that the increase in the refractive indices under uniaxial stress is mainly due to the increase in the refraction caused by the increase in the band gap and long-wavelength shift of the UV absorption band maximum.     

Cascade of phase transitions in GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Cascade of phase transitions in GdFe₃(BO₃)₄ at 156, 37, and 9 K has been detected by specific heat measurements and further studied by Raman scattering and Nd³⁺ spectroscopic probe method. A weakly first-order structural phase transition at 156 K is followed by a second-order antiferromagnetic ordering phase transition at 37 K and a first-order spin-reorientational phase transition at 9 K.     

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.     

Effect of uniaxial pressure on the infrared spectra of (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals

The infrared reflectance spectra of a mechanically free or uniaxial-pressure-confined (NH₄)₂SO₄ crystal were studied for the first time in the spectral range 800–1700 cm^?1 in three crystallographic directions. Using the Kramers-Kronig relations, the dispersion and pressure dependences of the following quantities are obtained: the index of refraction n, the real (?₁) and imaginary (?₂) parts of the permittivity, the frequencies of longitudinal (ω_LO) and transverse (ω_TO) optical vibrations, the damping constant γ, and the oscillator strength f of the mechanically free or clamped (NH₄)₂SO₄ crystal. A considerable change in the main reflection bands with pressure was observed, which is due to the effect of uniaxial pressure on the NH₄ and SO₄ tetrahedral frames.     

Optical absorption spectra and magnetic phase transitions in TbFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Optical absorption spectra of the trigonal crystal of TbFe₃(BO₃)₄ in the vicinity of the ⁷F₆ → ⁵D₄ transition in a Tb³⁺ ion were studied as a function of temperature (2–70 K) and magnetic field strength (0–60 kOe) at 2 K. The splitting of the excited states of Tb³⁺ due to both the magnetic ordering of iron and an external magnetic field was determined. Abrupt splitting of the absorption lines of Tb³⁺ at temperature T_N of the magnetic ordering of the subsystem of iron was revealed, suggesting that the nature of such splitting is not entirely magnetic.     

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 and x-ray diffraction studies of the symmetry of distorted phases of the (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>ZrF<Subscript>7</Subscript> crystal

(NH₄)₃ZrF₇ single crystals were grown, and polarization-optical and x-ray diffraction studies were performed on powders and crystalline plates of various cuts over a wide temperature range. Phase transitions are revealed at temperatures T _1↑ = 280 K, T _2↑ = 279.6 K, T _3↑ = 260–265 K, and T _4↑ = 238 K on heating and at T _1↓ = 280 K, T _2↓ = 269–270 K, T _3↓ = 246 K, and T _4↓ = 235 K on cooling. The sequence of changes in symmetry is established to be as follows: O _h ⁵ (Z = 4) ? D _2h ²⁵ (Z = 2) ? C _2h ³ (Z = 2) ? C _i ¹ (Z = 108) ? monoclinic²(Z = 216).     

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.     

Features of the pressure-induced phase transitions in Eu<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The structural changes induced by a 9-GPa pressure in Eu₂(MoO₄)₃ single crystals at room temperature have been studied using x-ray diffraction. It is established that a structural phase transition from the initial tetragonal phase to the new high-pressure tetragonal phase occurs rather than solid-phase amorphization that was observed previously in polycrystalline samples. The samples in the observed transition remain single-crystalline despite a significant difference (ΔV ～ 18%) between the specific volumes of the initial and final phases. It is shown that the transition from the initial state to the high-pressure phase occurs via the formation of broad transition zones featuring a continuous and smooth change of the crystal lattice parameters.     

The Composition of Intermediate Products of the Thermal Decomposition of (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>ZrF<Subscript>6</Subscript> to ZrO<Subscript>2</Subscript> from Vibrational-Spectroscopy Data

Thermal decomposition of (NH₄)₂ZrF₆ resulting in ZrO₂ formation within the temperature range of 20°–750°С has been investigated by means of thermal and X-ray diffraction analysis and IR and Raman spectroscopy. It has been established that thermolysis proceeds in six stages. The vibrational-spectroscopy data for the intermediate products of thermal decomposition have been obtained, systematized, and summarized.