Synthesis,TGA and EPR studies of phase transitions in Cu2+ doped (NH4)4ThF8 and (NH4)3ThF7

The paper presents the EPR evidence for the occurrence of phase transitions in (NH₄)₄ThF₈ and (NH₄)₃ThF₇. In both cases the Cu²⁺ probe occupies a NH⁴⁺ site and predominantly experiences a dynamic coordination, either due to dynamic Jahn-Teller effect, or due to fluxional behaviour of surrounding (ThF₈)^4- units in (NH₄)₄ThF₈. It is proposed that the structural phase transition in this compound at 214 K is associated with the transition of the dynamic coordination of Cu^2- into a static one, probably due to freezing of motion of (ThF₈)^4- units below this temperature. In (NH₄)₃ThF₇ the dynamic features of the Cu²⁺ EPR spectrum are absent and characteristics of a local orthorhombic symmetry are seen down to 77 K. However, in the high temperature range a change from orthorhombic to axial symmetry is observed at 524 K, possibly due to a phase transition.     

Temperature‐dependent vibrational studies of [N(C2H5)4]2MnCl4

Room‐temperature polarized Raman spectra of a single crystal and IR spectra of a polycrystalline sample were measured for [N(C₂H₅)₄]₂MnCl₄ and the assignment of the observed bands to the respective modes has been proposed. Temperature‐dependent Raman and far‐IR studies were also performed for the polycrystalline sample in order to obtain information on changes occurring in this material as a result of phase transitions at T₁ = 227 K and at T₂ = 199 K. These studies revealed that the higher‐temperature ferroelastic phase transition is associated with significant modification of vibrational properties due to ordering of tetraethylammonium groups. The lower‐temperature phase transition does not lead to any clear changes in the spectra. However, our results suggest that disorder of MnCl₄²⁻ ions decreases with decreasing temperature. Copyright © 2007 John Wiley & Sons, Ltd.     

Phase transitions in [NH(CH3)3]2MeCl4 (Me = Cd,Zn,Cu) crystals

Dielectric properties of the new [NH(CH₃)₃]₂ZnCl₄ and [NH(CH₃)₃]₂CdCl₄ crystals from the [(CH₃) _n NH_4-n ]₂MeCl₄ group have been investigated in a wide temperature range (4.2–320 K). A series of phase transitions has been discovered at T₃ = 325 K,T₄ = 251 K,T₅ = 193 K, for [NH(CH₃)₃]₂CdCl₄ and at T₃ = 309 K, T₄ = 282 K, T₅ = 269 K for [NH(CH₃)₃]₂ZnCl₄. A ferroelectric phase has been discovered in the temperature interval T₄—T₅ from the temperature and frequency dependence of the dielectric permittivity ε(T, v). According to optical investigations the existence of ferroelastic phases in the temperature interval T₁ = 349 K–T₂ = 391 K and below T₅ for [NH(CH₃)₃]₂CdCl₄ and both above T₃ and below T₅ for [NH(CH₃)₃]₂ZnCl₄ has been ascertained.     

1H NMR study of [N(CH3)4]2ZnCI4 at high pressures and low temperatures

Wide-line proton NMR studies on polycrystalline tetramethylammonium tetrachlorozincate have been carried out at high hydrostatic pressures up to 15 kbar in the temperature range 77-300 K and at ambient pressure down to 4.2 K. A second-moment transition is observed to occur starting around 161 K, the temperature for the V-VI phase transition. This transition temperature is seen to have a negative pressure coefficient up to 2 kbar, beyond which it changes sign. At 77 K the second moment decreases to 4 kbar and then increases again as a function of pressure. The results are explained in terms of the dynamics of the N(CH₃)₄ groups.     

The thermoelastic properties of [N(CH3)4]2ZnBr4 and [N(CH3)4]2MnBr4

The ferrodistortive phase transition in the bis-tetramethylammonium tetrabromide crystals below room temperature is studied within the framework of the Landau theory. The specific heats of [N(CH₃)₄]₂MnBr₄ and [N(CH₃)₄]₂ZnBr₄ are correctly described down to 40°C below the transition temperature. The phenomenological parameters are determined from calorimetric results, elastic constants and thermal expansion data. Using these coefficients, the monoclinic angle in the ferrodistortive phases is obtained. The anharmonic quantities, such as the isothermal compressibility, calculated from the specific heat data, are in good agreement with the values derived from the elastic measurements.     

Calculation of the specific heat for the first order,tricritical and second order phase transitions in NH4C1

Abstract

This work presents our calculation for the specific heat C _v under an Ising model that uses our Raman frequencies of the _1/7TA (93cm ¹) and _1/5TO (144cm^?1) modes for NH₄C1. The specific heat calculation has been performed for first-order (P = 0kbar), tricritical (P=1.6kbar) and second-order (P = 2.8kbar) phase transitions in the NH₄C1 crystal. Our calculated C _v values are in good agreement with the experimentally observed Cp data from the literature for NH₄C1. This indicates that the NH₄C1 crystal can be adequately described in the lattice region by means of the Ising model studied here.     

NQR Study of Phase Transition and Cationic Motion in 4-NH2C5H4NHBiBr4·H2O

Four ⁸¹Br NQR lines in 4-NH₂C₅H₄NHBiBr₄·H₂O were observed in the temperature range between 77 and ca. 380 K; with increasing temperatures the respective sets of higher and lower two resonance lines coalesced into single lines discontinuously at 274 K, showing the occurrence of a first-order type phase transition of this crystal. The transition was confirmed with heat anomaly on a DTA curve. Each higher and lower line of high-temperature phase is assignable to the terminal Br atoms and the bridging ones of one-dimensional poly anions (BiBr₄ ⁻) _n in the crystal structure (C2/c), which was investigated by a X-ray structure analysis at room temperature. The 1/T ₁ temperature dependence of ⁸¹Br NQR follows the usual T ² law in the temperature range between 77 and ca. 140 K, being explained by fluctuation of the EFG at Br nucleus due to lattice vibrations. The T ₁ vs. 1/T curve in the temperature range between about 160 and 190 K was describable by the exponential curves, allowing us the estimation of activation energies. These exponential behaviors of T ₁ of ⁸¹Br NQR are attributable to the fluctuations caused by the thermal motion of 4-NH₂C₅H₄H⁺ ions. Echo signals of the ⁸¹Br NQR could not be detected above 190 K owing to poor S/N with very short T ₂.     

PMR studies of molecular motions,phase transitions and quantum tunnelling in NH4SnCl3 and N(CH3)4SnCl3

Molecular reorientation and low temperature relaxation effects of NH⁺ ₄ ion and the effect of CH₃ substitution (in place of H) are investigated by proton spin lattice relaxation time (T₁) measurements at 10 MHz in NH₄SnCl₃ and N(CH₃)₄SnCl₃ in the temperature range 4.2 K upto the melting points of the compounds (? 440 K). Phase transitions around 360 K in NH₄SnCl₃ and around 361 and 116K in N(CH₃)₄SnCl₃ have been observed. In NH₄SnCl₃, the high temperature minimum at 330.5 K is attributed to the translational diffusion of the NH⁺ ₄ ions, while the other T₁, minima at 103.5, 60 and 50 K are ascribed to the reorientations of the NH⁺ ₄ ion about the C₂ and C₃ axes. The low temperature minimum at 13.5 K is attributed to rotational tunnelling of the NH⁺ ₄ ions. In N(CH₃)₄SnCl₃, in addition to the high temperature minima at 212.2 and 182.6 K due to N(CH₃)₄ tumbling and CH₃ reorientation, a temperature independent T₁ behaviour between 83 and 31 K is observed, below which T₁ decreases and tends to go through a minimum around 5 K. This low temperature minimum is attributed to rotational tunnelling of the CH₃ groups. The motional parameters and tunnel frequencies are estimated.     

Temperature dependence of soliton density in the incommensurate phase of [N(CH3)4]2ZnCl4 crystals

The successive phase transitions of [N(CH₃)₄]₂ZnCl₄ have been investigated by EPR in a Mn²⁺ doped-sample. The temperature change in the resonance line shape near the commensurate-incommensurate transition temperature was analysed by a one-dimensional phase soliton model, so that the soliton density, modulation amplitude and initial phase were determined. The discommensuration phenomenon was also observed and discussed in terms of the domain wall resulting from the soliton.     

Elastic studies of phase transitions

We report some results on acoustic studies of phase transitions in which the order parameter is coupled to the elastic wave strain, when direct behaviour observation is not possible. First by Brillouin scattering it was possible to observe the softening of C₅₅ of ammonium oxalate hemihydrate and it was concluded that the order parameter is not the deformation. In the case of members of A₂MX₄ family a softening of C₆₆ related to a shear wave near the lock-in transition was observed for [N(CH₃)₄]₂ZnCl₄, as previously found for C ₅₅ of K₂SeO₄. Such a behaviour did not occur for (NH₄)₂BeF₄ in which a strong hysteresis effect appeared.     

Incommensurate cooperative tilting modes of MnF6 octahedra relatedto the structural phasetransitions in BaMnF4

The structural phase transitions of the layer compound BaMnF₄ were studied by high-resolution X-ray diffraction using synchrotron radiation. The intensities and profiles of two kinds of superlattice reflections having incommensurate reduced wave vectors q ₁= (～ ± 1/5,0,0) _p and q ₂ =(～ ± 2/5,1/2,1/2)_p, respectively, were measured as a function of temperature from 25 K to 280 K. These temperature dependencies show that incommensurate structural phase transitions of second order occur at 234 K and 244 K. These structural phase transitions are interpreted as successive condensations of a folding-screen-like incommensurate plane-distortion mode and a commensurate anti-ferro-distortive tilting mode of the MnF₆ octahedra around the primitive a₀ - and b₀ -axes, when cooled down. It is also found that there is another structural phase transition at about 45 K related to a precursor structural distortion for the antiferromagnetic transition occurring at about 26 K.     

Determination of the low energy values of CH4 transitions in the 2ν3 region near 1.66 μm from absorption spectra at 296 and 81 K

The high resolution absorption spectra of ¹³CH₄ were recorded at 81 K by differential absorption spectroscopy using a cryogenic cell and a series of distributed feed back (DFB) diode lasers and at room temperature by Fourier transform spectroscopy. The investigated spectral region corresponds to the high energy part of the ¹³CH₄ tetradecad dominated by the 2ν₃ overtone near 5988 cm⁻¹. Empirical line lists were constructed containing, respectively, 1629 ¹³CH₄ transitions detected at 81 K (5852-6124 cm⁻¹) and 3481 features (including 85 lines of ¹²CH₄) measured at room temperature (5850-6150 cm⁻¹); the smallest measured intensities are about 3 × 10⁻²⁶ and 4 × 10⁻²⁵ cm/molecule at 81 and 296 K, respectively. The lower state energy values were derived for 1196 ¹³CH₄ transitions from the variation of the line intensities between 81 and 296 K. These transitions represent 99.2% and 84.6% of the total absorbance in the region, at 81 and 296 K, respectively. Over 400 additional weak features were measured at 81 K and could not be matched to lines observed at room temperature. The quality of the resulting empirical low energy values is demonstrated by the excellent agreement with the already-assigned transitions and the clear propensity of the empirical low J values to be close to integers. The two line lists at 81 and at 296 K provided as Supplementary material will enable future theoretical analyses of the upper ¹³CH₄ tetradecad.     

Phase transitions in PbZr0.72Sn0.28O3 single crystals studied by Raman spectroscopy

ABSTRACT

Raman light scattering measurements in the temperature range from 80 to 830 K were performed on a PbZr_0.72Sn_0.28O₃ single crystal. The frequencies of the Raman lines were analyzed and discussed in terms of the sequence of structural phase transitions. It was found that Raman spectrum displays important changes near 440, 480 and 493 K. The incorporation of more than 25 mol% of Sn⁴⁺ ions into the structure of PbZrO₃ enhances polar fluctuations above T_C as compared to the less Sn-doped crystals. These fluctuations lead to appearance of a ferroelastic intermediate phase below T_C. It is demonstrated that the structural phase transformation in PbZr_0.72Sn_0.28O₃ can be considered as the result of softening of a number of modes.     

Vibrational spectra of (NH4)3ZnCl5

IR and Raman spectra of (NH₄)₃ZnCl₅ have been recorded. The observed spectra have been analysed on the basis of the vibrations of ZnCl ₄ ²⁻ and NH ₄ ⁺ ions. The appearance of multiple Raman bands indicates the presence of two different types of ammonium ions. The effect of anisotropic crystalline field over the ZnCl₄ and NH₄ tetrahedra is also discussed. The assignment of internal modes has been verified by the potential energy distribution calculations.     

Temperature evolution of the intra-ion absorption spectra of (NH2(C2H5)2)2CoCl4 crystals in the region of their phase transitions

On the basis of spectroscopic studies of (NH₂(C₂H₅)₂)₂CoCl₄ crystals, the absorption bands corresponding to the internal electronic transitions in the Co²⁺ ion were identified. The values of the crystal field and Racah parameters were calculated. The temperature evolution of the absorption spectra of (NH₂(C₂H₅)₂)₂CoCl₄ crystals reveals the anomalies of their parameters at the points of phase transitions. The corresponding changes of the absorption spectra were discussed in terms of distortion of the metal-halogen complex. The temperature dependences of the absorption spectra of (NH₂(C₂H₅)₂)₂CoCl₄ crystals confirm the presence of the thermochromic phase transitions at 255 and 330?K.     

On the structural phase transitions in the incommensurate Cs2CdBr4

Abstract

In the series of incommensurate A₂BX₄ halides with the β-K₂SO₄ type structure, Cs₂CdBr₄ exhibits an unusual behaviour since the “lock-in” phase transition occurs at the centre of the Brillouin zone. The observed phase sequence is the following: Pnma (Z = 4)?INC(k₀ ≈ 1/6a*)?P2_1/n(Z = 4)?P1 (Z = 4). These phase transitions have been studied by means of Raman scattering and ultrasonic measurements. It is shown that the Pnma?INC?P2_1/n sequence is governed by order-disorder processes due to CdB2-₄ tetrahedra reorientations coupled with translations of the Cs⁺ cations, and that the low-temperature P2_1/n?P1 transition is of a displacive nature, governed by a soft optical mode. The “pseudo-proper” ferroelastic character of these transformations is clearly established. A model potential developed in the framework of Landau theory is proposed; this model is able to reproduce the general trends observed in the temperature dependence of the soft-modes and of the elastic constants in the different phases.     

Analysis of incommensurate structure in [N(CH3)4]2ZnCL4 crystal

We analyzed the temperature dependence of the amplitude of modulated structures of [N(CH₃)₄]₂ZnCl₄ crystal in the incommensurate (INC) phase by measuring the EPR spectra for Mn²⁺ doped single crystal. The amplitude of the vibrational modulation linearly increased with decreasing temperature. Applying Frank and Van der Merwe model, we simulated the EPR spectra in the INC phase. The simulated spectra agreed with the observed one very well. We, therefore, revealed that the vibrational modulation in this INC phase is due to the interaction between the harmonic chain of inter-particles and the modulation due to underlying potential which comes from the commensurate structure.     

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.     

Temperature dependence of the indirect band gap, steepness parameter and related optical constants of [Kx(NH4)1−x]2ZnCl4 mixed crystals

Optical transmittance measurements near the absorption edge of [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals, where x=0.00, 0.232, 0.522, 0.644, 0.859 and 1.00, are reported over 276–350 K range. Analysis reveals that the type of transition is the indirect allowed one. The absorption edge shifted towards lower energy with increasing temperature. It is shown that [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals with x0.644 reveal a phase transition at 319 K, this phase disappeared at high concentrations of K⁺ ions. The steepness parameter is given, its value is used to estimate the temperature dependence of the indirect energy gap. In the region of the absorption edge, the absorption coefficient obeys Urbach's rule. Urbach parameters are investigated as a function of temperature.