Phase transitions in orthorhombic oxyfluoride (NH4)2MoO2F4

(NH₄)₂MoO₂F₄ single crystals were grown and studied using polarization-optical methods, and the birefringence was measured in the temperature range 90–350 K. The following sequence of phase transitions is revealed: G ₀ ? G ₁ ? G ₂. It is established that the phase transition at T ₀₁ ≈ 267 K is of the first order and exhibits thermal hysteresis δT ₀₁ ≈ 0.6 K. A weak anomaly is found in Δn(T) at T ₀₂ ≈ 180 K. The crystals are shown to retain the orthorhombic symmetry during the phase transitions.     

Thermodynamic properties of (NH4)2KGaF6 elpasolite

The heat capacity of (NH₄)₂KGaF₆ elpasolite is measured in the temperature range from 80 to 350 K. A sequence of three phase transitions at T ₁=288.5 K, T ₂=250 K, and T ₃=244.5 K is revealed, and the thermodynamic characteristics of these transitions are determined. The influence of hydrostatic pressure on the phase transition temperature is investigated. The results obtained are discussed within the model of orientational ordering of NH ₄ ⁺ and GaF ₆ ^3? ionic groups.     

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.     

The heat capacity of the layer compounds (CH3CH2CH2NH3)2MnCl4 and (CH3CH2CH2NH3)2CdCl4 from 10 K TO 300 K

The heat capacity of the layer compounds tetrachlorobis (n-propylammonium) manganese II and tetrachlorobis (n-propylammonium) cadmium II, (CH₃CH₂CH₂NH₃)₂MnCl₄ and (CH₃CH₂CH₂NH₃)₂CdCl₄ respectively, has been measured over the temperature range 10 K ?T ? 300 K.Two known structural phase transitions were observed for the Mn compound in this temperature region: at T = 112.8 ± 0.1 K (ΔH_t= 586 ± 2 J mol^?1; ΔS_t = 5.47 ± 0.02 J K^?1mol^?1) and at T =164.3 ± (ΔH_t = 496 ± 7 J mol^?1; ΔS_t =3.29 ± 0.05 J K^?1mol^?1). The lower transition is known to be from a monoclinic structure to a tetragonal structure, while the upper is from the tetragonal phase to an orthorhombic one. From comparison with the results for the corresponding methyl Mn compound it is deduced that the lower transition primarily involves changes in H-bonding while the upper transition involves motion in the propyl chain.A new structural phase transition was observed in the Cd compound at T= 105.5 ± 0.1 K (ΔH_t= 1472.3 ± 0.1 J mol^?1; ΔS_t = 13.956 ± 0.001 J K^?1mol^?1), in addition to two transitions that have been observed previously by other techniques. The higher of these transitions(T = 178.7 ± 0.3 K; ΔH_t = 982 ± 4 J mol^?1 ΔS_t = 6.16 ± 0.02 J K^? mol^?1) is known to be between two orthorhombic structures, while the structural changes at the lower transition (T= 156.8 ± 0.2 K; ΔH_t = 598 ± 5 J mol^?1, ΔS_t = 3.85 ± 0.03 J K^?1 mol^?1) and at the new transition are not known. It is proposed that these two transitions correspond respectively to the tetragonal to orthorhombic and monoclinic to tetragonal transitions in the propyl Mn compounds.In addition to the structural phase transitions (CH₃CH₂CH₂NH₃)₂MnCl₄ magnetically orders at t? 130 K. The magnetic contribution to the heat capacity is deduced from the heat capacity of the corresponding diamagnetic Cd compound and is of the form expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Ferroelastic phase transitions in (NH4)2TaF7

The heat capacity, unit cell parameters, permittivity, optical properties, and thermal expansion of the (NH₄)₂TaF₇ compound with a seven-coordinated anion polyhedron have been measured. It has been found that the compound undergoes two successive phase transitions with the symmetry change: tetragonal → (T ₁ = 174 K) orthorhombic → (T ₂ = 156 K) tetragonal. The ferroelastic nature of structural transformations has been established, and their entropy and susceptibility to hydrostatic pressure have been determined.     

Structural Phase Transition and the Dielectric Permittivity of the Model Lipid Bilayer [(CH2)12(NH3)2]CuCl4

Structural phase transitions in the lipid-like bilayer material [(CH₂)₁₂(NH₃)₂]CuCl₄ have been observed using differential thermal scanning. The compound shows an irreversible thermochromic transition at ? 465 K and three reversible transitions at T ₁ = 433 ± 4 K and T ₂ = 411 ± 2 K and T ₃ = 358 K. The transition at 350 K is ascribed to chain melting. The other two correspond to crystalline phase transformation.

Phase (IV) T₃ = 358 ± 2K Phase (III) T₂ = 411 ± 2K Phase (II) T₁ = 433 ± 4K Phase (I)

Dielectric permittivity is studied as a function of temperature in the range 300-440 K and frequency, range (60 Hz-100 kHz). It confirms the observed transitions. The dielectric permittivity reflects rotational and conformational transitions for the compound. The variation of the real part of the conductivity with temperature is thermally activated in the temperature range above 350 K, with frequency-dependent activation energy, the values of activation energy lie in the range of ionic hopping. The dependence of the conductivity on frequency follows the universal power law σ = σ₀ + A(T) ω ^{s ( T )} with 0<s<1. Comparison of this material with other members of the series is discussed     

Critical behavior of heat capacity in the vicinity of phase transitions in [NH2(CH3)2]5Cd3Cl11

The heat capacity of a [NH₂(CH₃)₂]₅Cd₃Cl₁₁ crystal was studied calorimetrically in the temperature interval 100–300 K. The C _p (T) dependence indicates that, as the temperature is lowered, phase transitions occur at temperatures T ₁ = 176.5 K and T ₂ = 123.5 K. The thermodynamic characteristics of this crystal were determined. It is shown that the transition at T ₂ = 123.5 K is an incommensurate-commensurate phase transformation and that the transition at T ₁ = 176.5 K is a normal-incommensurate phase transition.     

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.     

Inelastic neutron scattering study of the specific features of the phase transitions in (NH4)2WO2F4

Oxyfluoride (NH₄)₂WO₂F₄ has been studied by the inelastic neutron scattering method over a wide temperature range 10–300 K at two initial neutron energies of 15 and 60 meV. The role of tetrahedral ammonium groups in the mechanism of sequential phase transitions at T ₁ = 201 K and T ₂ = 160 K has been discussed.     

Mössbauer study of (NH4)2FeCl5·H2O

Mössbauer spectra of the compound (NH₄)₂FeCl₅·H₂O have been studied as a function of temperature. Two phase transitions are observed in the temperature range between 7 K and 9 K. The transition at 9 K is structural and presents an unusually high thermal hysteresis. AroundT=8 K the substance orders magnetically and different Fe³⁺ contributions are present.     

14N NMR study and Landau theory of phase transitions in [N(CH3)4]2ZnI4

The¹⁴N NMR spectra and spin-lattice relaxation timeT ₁ of [N(CH₃)₄]₂ZnI₄ have been studied between room temperature and 200 K. Two phase transitions atT _c 1=255 K and atT _c 1=217 K are observed. The¹⁴N NMR lineshape andT ₁ data suggest that the intermediate phase is commensurate rather than incommensurate in spite of the presence of a Lifshitz invariant in the expansion of the free energy density in powers of the order parameter. We also discuss the phenomenological theory of structural phase transitions in [N(CH₃)₄]₂ZnI₄.     

The heat capacity of the layer compound tetrachlorobis (methylammonium) manganese—II (CH3NH3)2MnCl4, from 10 to 300k

The heat capacity of the layer compound, tetrachlorobis (methylammonium) manganese II, (CH₃NH₃)₂MnCl₄, has been measured over the range 10K <T<300K. In this region, two structural phase transitions have been observed previously by other techniques: one transition is from a monoclinic low temperature (MLT) phase to a tetragonal low temperature (TLT) phase, and the other is from TLT to an orthorhombic room temperature (ORT) phase. The present experiments have shown that the lower transition (MLT→TLT) occurs at T = 94.37±0.05K with ΔH_t = 727±5 J mol^?1 and ΔS_t = 7.76±0.05 J K^?1 mol^?1, and the upper transition (TLT→ORT) takes place at T = 257.02±0.07K with ΔH_t = 116±1J mol^?1 and ΔS_t = 0.451±0.004 J K^?1mol^?1. These results are discussed in the light of recent measurements on (CH₃NH₃)₂CdCl₄, and also with regard to a recent theoretical model of the structural phase transitions in compounds of this type.In addition to the structural phase transitions, (CH₃NH₃)₂MnCl₄ also undergoes magnetic ordering at T < 150K. The magnetic component to the heat capacity, as deduced from a corresponding states comparison of the heat capacity of the present compound with that of the Cd compound, is shown to be consistent with the behaviour expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Investigation of thermal expansion,phase diagrams,and barocaloric effect in the (NH4)2WO2F4 and (NH4)2MoO2F4 oxyfluorides

The thermal expansion along the principal crystallographic axes of the (NH₄)₂WO₂F₄ and (NH₄)₂MoO₂F₄ oxyfluorides has been studied. The anomalous behavior of α _i (T) due to the phase transitions has been revealed at T ₁ = 271.4 K and T ₂ ≈ 180 K for the molybdate and at T ₁ = 201.5 K and T ₂ ≈ 161 K for the tungstate. The quantities dT/dp and dT/dσ _i , which characterize the dependence of the phase transition temperatures on the hydrostatic and uniaxial pressures, have been determined from analyzing the results of studies of the thermal expansion and heat capacity with the use of the Pippard relations. The p-T and α _i -T phase diagrams reflect different characters of the influence of the pressure on the stability of the initial and distorted phases of the oxyfluorides. The magnitudes of the extensive and intensive barocaloric effects determined in the vicinity of the structural phase transitions are as follows: ΔS _BCE varies from approximately −10 to −17 J/mol K and ΔT _AD ≈ 8−17 K for the molybdate and ΔS _BCE varies from approximately −10 to −17 J/mol K and ΔT _AD ≈ 8−13 K for the tungstate.     

Detailed infrared spectroscopic study of thermal transitions in new hybrid [(CH3)2CHNH3]4Cd3Cl10 crystal

Phase transitions of tetra(isopropylammonium)decachlorotricadmate(II) [(CH₃)₂CHNH₃]₄Cd₃Cl₁₀ crystal have been studied by infrared, far infrared and Raman measurements in wide temperature range, between 11 K and 388 K. The temperature changes of wavenumber, center of gravity, width and intensity of the bands were analyzed to clarify cationic and anionic contributions to the phase transitions mechanism. The results of investigation showed earlier by differential scanning calorimetry (DSC), thermal expansion and dielectric measurements clearly confirmed the sequence of phase transitions at T₁=353 K, T₂=294 K and T₃=260 K. The current results derived from DSC and infrared measurements revealed additional phase transition at T₄=120 K.     

An ESR study of phase transitions in crystals of M(BF4)2·6H2O (M = Fe,Co, Ni,Zn)

From a temperature dependent ESR study of Mn²⁺-doped crystals of M(BF₄)₂·6H₂O, M Zn, Co and Ni, new structural phase transitions have been detected and studied. First order structural phase transitions occur in Co(BF₄)₂·6H₂O at T₁ ~ 281K, T₂~189 K and T₃~172K (during cooling), in Zn(BF₄)₂·6H₂O at T₁ ~ 286 K and in Ni(BF₄)₂·6H₂O at T₁ ~ 301 K. A continuous phase transition occurs in Co(BF₄)₂·6H₂O at T_p ~ 257 K, in Zn(BF₄)₂·6H₂O at T_p ~ 277 K and in Ni(BF₄)₂·6H₂O at T_p ~ 294 K. The ESR spectral characteristics suggest similarities in the structures of these fluoroborate compounds in the phase above T₁ with the room temperature structure of Mg(ClO₄)₂·6H₂O. All these compounds are found to have a tendency to crystallise in a triply-twinned pseudo-hexagonal form, although the unit cell above T₁ is found to be orthorhombic. The structural changes related to the water octahedron around the metal at T₁ were found to be very small and basically the same for these three compounds. Although the unit cell structure of Fe(BF₄)₂·6H₂O above the first order phase transition temperature T₁ was found to be similar to that of the other fluoroborate compounds, the structural changes occurring at T₁ appeared to be quite different. The low temperature thermal behaviour differs considerably in the Co, Fe and Zn compounds.     

Thermoreflectance of single-crystal HgCr2Se4 near the ferromagnetic-to-paramagnetic phase transition

The thermoreflectance of the ferromagnetic semiconductor HgCr₂Se₄ (T_c = 106 K) has been measured between 45 and 340 K in the photon energy range from 1.7 to 2.8 eV. The spectral line shape and the signal intensity behave critically near the magnetic phase transition. The optical transitions and the effect of magnetic ordering are discussed.     

Heat capacity,structure, and <Emphasis Type="Italic">p-T</Emphasis> phase diagram of elpasolite (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>KMoO<Subscript>3</Subscript>F<Subscript>3</Subscript>

Thermophysical and structural studies of an (NH₄)₂KMoO₃F₃ crystal show that this crystal belongs to the family of elpasolites (space group \(Fm\bar 3m\)) and undergoes an order-disorder phase transition at T ₀ = 241.5 K. Under hydrostatic pressure, this phase transition splits into two consecutive transitions at the tricritical point with parameters T _tr = 232.5 K and p _tr=0.21 GPa. It was found that anomalous hysteresis and relaxation phenomena accompany the transitions from the cubic to both distorted phases. The results are analyzed taking into account the data on the phase transition in the related elpasolite (NH₄)₂KWO₃F₃.     

Structural and magnetic phase transitions of kagome-like compounds REBaCo4O7 (RE=Dy,Ho, Er,Tm, Yb,Lu)

In a temperature range 5–300 K the specific heat C(T) on a new mixed valence cobalt oxides REBaCo₄O₇ (RE=Dy, Ho, Er, Tm, Yb, Lu) was investigated. The first-order structural phase transitions from hexagonal P6₃mc to orthorhombic Cmc₂1 phase was indicated by a peak-like anomaly in C(T) curves at T_S～160, 178, 224, and 280 K for RE=Lu, Yb, Tm, and Er correspondingly. The magnetic phase transitions was indicated as the changes of slope on the C(T) curves were found at corresponding temperatures: T_N～50, 74, 98, and 98 K for RE=Lu, Yb, Tm, and Er, correspondingly.     

Anomalous states of the structure of (NH4)2SO4 crystals in the temperature range 4.2–300 K

Crystals of the (NH₄)₂SO₄ ammonium sulfate are studied using x-ray diffractometry. It is revealed that the temperature dependence of the lattice parameters exhibits an anomalous behavior, namely, a global hysteresis, and an anomalous increase in the lattice parameter a and the unit cell volume at temperatures below the ferroelectric phase transition point (T _c=223 K). The series of superstructure reflections observed corresponds to an incommensurate composite structure. Analysis of the temperature behavior of the mismatch parameters for the matrix (host) and superstructure (guest) lattices demonstrates that the (NH₄)₂SO₄ compound undergoes a number of phase transitions, including a transition to a three-dimensionally incommensurate composite phase and transitions to commensurate (along one of the crystallographic directions) composite phases.     

Raman studies of phase transitions in ferroelectric [C2H5NH3]2ZnCl4

The present paper accounted for the synthesis, differential scanning calorimetric and vibrational spectroscopy of [C₂H₅NH₃]₂ZnCl₄grown at room temperature. Differential scanning calorimetric (DSC) disclosed five phase transitions at T₁=231 K, T₂=234 K, T₃=237 K, T₄=247 K and T₅=312 K. The temperature dependence of the dielectric constant at different temperatures proved that this compound is ferroelectric below 238 K. Raman spectra as function temperature have been used to characterize these transitions and their nature, which indicates a change of the some peak near the transitions phase. The analysis of the wavenumber and the line width based on the order–disorder model allowed to obtain information relative to the thermal coefficient and the activation energy near the transitions phase.