Proton NMR study of molecular dynamics and phase transitions in methylammonium hexachloro plumbate, [NH3CH3]2PbCl6

Proton spin lattice relaxation time (T₁), measured as a function of temperature in the range 375–77 K, shows slope changes at 333, 221 and 111 K, in addition to a first order phase transition at 150K. The observed T₁ behaviour and second moment (M₂) variation with temperature are explained on the basis of the different possible motions of CH₃ and NH₃ groups.     

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.     

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.     

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 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.     

Dielectric permittivity and AC conductivity investigations of the structural phase transitions in (NH3)2(CH2)7CdCl2Br2

The AC dielectric permittivity as a function of temperature (100–400 K) at different frequencies (between 80 Hz and 20 kHz) for the layered alkylene diammonium insulator containing Cd, namely, [(NH₃)₂(CH2)₇CdCl₂Br₂] has been measured. The formation of the compound was confirmed by microchemical analysis and IR absorption spectrometry. X-ray powder diffraction indicates an orthorhombic unit cell of dimensions: a = 10.219(2) Å, b = 9.168(2) Å and c = 38.694(4) Å. The AC conductivity ([sgrave]) is presented as a function of temperature and frequency. The conductivity and permittivity results indicate the presence of first-order phase transitions at 317 and 345 K. This has been confirmed by thermal analysis techniques. The activation energies were of values ranging between 0.15 and 0.62 eV depending upon the temperature range and the applied frequencies.     

Chlorine-35 nuclear quadrupole resonance and phase transitions in (4ClC6H4NH3)2MnCl4, (4-Cl6H4NH3)2CUCl4, and (4-ClC6H4NH3)2CUBr4

The temperature dependence of the ³⁵Cl NQR frequencies of 4-chloroanilinium cations in (4-ClC₆H₄NH₃)₂MnCl₄, (4-ClC₆H₄NH₃)₂CuCl₄, and (4-ClC₆H₄NH₃)₂CuBr₄ was studied over a wide range of temperature. For the two copper(II) complexes, magnetic susceptibilities were measured below approximately 20 K. The lattice constants of these isomorphous orthorhombic crystals were determined. Magnetic phase transitions to three-dimensionally long-range-ordered states were found at 40, 8, and 13 K for the respective complexes. Additional structural phase transitions were also found at higher temperatures for all of the complexes studied. Magnetic properties in the ordered state and the nature of both types of phase transitions are discussed.     

Study of electrical conductivity changes and phase transitions in Co3O4 doped ZrO2

The electrical conductivity of ZrO₂ doped with Co₃O₄ has been measured at various temperatures for different molar ratios. The conductivity increases due to the migration of vacancies created by doping. The conductivity is also found to increase with rise in temperature up to 120°C, and after attaining a maximum the conductivity decreases due to a collapse of the lattice framework. A second rise in conductivity around 460°C in all the compositions confirms the phase transition in ZrO₂ from monoclinic to tetragonal symmetry. X-ray powder diffraction and DTA studies were carried out for confirming the doping effects and the transition in ZrO₂.     

Critical behavior of quasi-2D organic-inorganic halide perovskite (C6H5CH2CH2NH3)2CuCl4 single crystals

Critical behavior of quasi two-dimensional organic-inorganic halide perovskites (C₆H₅CH₂CH₂NH₃)₂CuCl₄ is investigated using magneto-thermal and isothermal magnetic properties along the easy axis. Banerjee's criterion indicates that phase transition from paramagnetic to ferromagnetic phases below T_C = 9.4 K is of second-order. Scaling analysis reveals that critical exponent β from spontaneous magnetization in the critical region below T_C and δ from critical isotherm at T_C are found to be 0.22(3) and 9.28–9.4, close to 2D finite XY model. Meanwhile, γ from magnetic susceptibility in the critical region above T_C is gradually decreased from 2.4 at high-temperature region. Critical exponents from magneto-thermal properties are found to be consistent with those determined from magnetization isotherms. The reliability of critical values is verified using the scaling hypothesis. Our results evidence that (C₆H₅CH₂CH₂NH₃)₂CuCl₄ crossovers from isotropic 2D Heisenberg to anisotropic 3D models towards T_C and can be promising candidates for magnetocaloric materials for hydrogen reliquefaction.     

X-ray scattering study of the phase transitions in crystalline fullerene-biphenyl C 60 [(C 6 H 5 ) 2 ]

The phase diagram of the newly synthesized mixed crystal C₆₀-biphenyl is investigated as a function of temperature by single-crystal X-ray scattering. Diffuse scattering investigations evidencing complex disorder and local order effects are presented. Two phase transitions leading to two different doublings of the high temperature unit cell are observed, at 212 K and 147 K. The first transition is attributed to the ordering of twisted biphenyls, which couples to the orientational ordering of the C₆₀ molecules as the temperature decreases. Full ordering of the C₆₀ molecules is achieved below 100 K only, in the low temperature phase. The rich phase diagram of C₆₀-biphenyl is due to the interplay between fullerene and biphenyl ordering phenomena. Received 31 August 2001 and Received in final form 4 December 2001     

Kinetics of phase transitions in modulated ferroelectrics: Time-resolved neutron diffraction from Rb2ZnCl4

The kinetics of the ferroelectric phase transition between modulated phases in Rb₂ZnCl₄ at about 190K has been investigated by time-resolved neutron scattering. Observing the time dependence of satellite spectra, it is found that the structural changes associated with the field-induced transformation proceed on a millisecond time scale. Under the influence of fast cycling strong electric fields unusual non-equilibrium states are obtained which are modulated in space as well as in time and characterised by the presence of internal mechanical strains.     

Phase transitions and temperature changes of the optical absorption edge in (NH2(C2H5)2)2CoCl4 layered crystal

This work was devoted to X-ray diffraction study and investigations of temperature changes of the optical absorption edge of (NH₂(C₂H₅)₂)₂CoCl₄ crystals in the region of possible phase transitions. The X-ray powder diffraction data revealed the monoclinic phase at room temperature – space group P2/n. The cobalt atom was found to be square-plane coordinated by four chlorine atoms resulting [CoCl₄]^2– anion, which is surrounded by two DEA⁺ cations. It was shown that the low-energy tail of the absorption edge in these materials possesses an exponential shape. In the temperature range above 255?K it follows the empirical Urbach’s rule. The obtained experimental data confirmed the existence of the ferroelastic phase in (NH₂(C₂H₅)₂)₂CoCl₄ in the temperature range between 255 and 326?K. The anomalous behaviour of the investigated parameters observed at the temperatures below 255?K would be related to earlier unknown phase transitions.     

Microkinetics of the first- and second-order phase transitions

A model of the phase transition in a lattice of interacting nodes, in which each node is a statistical system with internal structure, is introduced. Configuration entropy of microscopic states of the node is defined as a basic parameter of the model. In the frame of the model the first- and second-order phase transitions are considered in details. The distinction between them on the microscopic level is analyzed. Phase diagrams have been calculated in the mean-field approximation. Changes of the phase diagrams and modifications of phase transitions under external pressure and irradiation are investigated in the frame of the microkinetic approach. Results are referred to real systems.