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.     

Structural and magnetic phase transitions in (Chloroanilinium)2CuX4 (X = Cl, Br)

Successive structural phase transitions of (4-ClC₆H₄NH₃)₂CuCl₄, which occur in a very narrow temperature range were reinvestigated by Fourier transform nuclear quadrupole resonance (FT NQR) measurements. The phase transitions at 275.5 and 277.0 K were confirmed. The effect of the deuteration of the ammonium end on these transitions was studied. The³⁵Cl NQR frequencies of organic cation were observed to decrease by about 4 kHz and the phase transition temperatures to decrease by about 2 K by the deuteration, suggesting that the ?NH₃ ⁺ … Cl hydrogen bond is weakened by the deuteration. The magnetic phase transition temperature of 8.6 K showed no remarkable change within experimental error by the deuteration. It was found that the magnetically ordered state is broken by the radio-frequency magnetic field of about 15–35 Oe usually employed in pulsed NQR. However, in the deuterated compound (4-ClC₆H₄ND₃)₂CuCl₄, the ordered state was found to be stabler for the usual radiofrequency power. By combining with the NQR data of (4-ClC₆H₄NH₃)₂CuBr₄ and (3,5-Cl₂C₆H₃NH₃)₂CuCl₄, the possibility is discussed of tuning the interlayer interaction between the organic cation layer and the inorganic complex anion layer by the halogen substitution in the organic cation as well as by the halogen replacement in the inorganic complex anion.     

X-ray diffraction investigation of the thermochromic phase transition in an [NH2(C2H5)2]2CuCl4 crystal

The unit cell parameters of an [NH₂(C₂H₅)₂]₂CuCl₄ crystal are determined using x-ray diffraction analysis, and the thermal expansion coefficients along the principal crystallographic directions are calculated in the temperature range 100–330 K. The behavior of the intensities of the diffraction reflections from the (100), (010), and (001) crystallographic planes is studied in the vicinity of the thermochromic phase transition temperature. The occurrence of a first-order phase transition in the [NH₂(C₂H₅)₂]₂CuCl₄ crystal at T ≈ 324 K is confirmed experimentally.     

On the ordering in [(C2H5)4N]2CuCl4 crystal: an X-ray study and theoretical considerations

The crystal structure [(C₂H₅)₄N]₂CuCl₄ has been determined by X-ray diffraction at 243K. The room temperature phase (phase I) belongs to the space group P4₂/nm [1] whereas the low temperature phase (phase II) is orthorhombic and belongs to the space group Pnna. The phase transition at T_c=258K is of improper ferroelastic type and it is associated with the ordering of the CuCl₄ ^2? and a partial ordering of the [(C₂H₅)₄N]⁺ ions which are disordered in the high temperature tetragonal phase. At lower temperature, there occurs another instability which could correspond to a complete ordering in the crystal.     

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.     

Influence of isomorphous cation and anion substitution on optical spectra of cobalt alkylaminehalide crystals

Based on data of spectral investigations of crystals with the group A₂CoCl₄ (A=NH₂(C₂H₅) ₂ ⁺ , N(C₂H₅) ₄ ⁺ , N(CH₃) ₄ ⁺ ) and [N(C₂H₅)₄]₂CoCl₂Br₂, the identification of absorption bands corresponding to internal electron transitions in a Co²⁺ ion has been carried out. The values of the crystalline field force Dq for the above-indicated materials at room temperature have been determined. The regularities of the change in the given parameter with change in the temperature and isomorphous cation-anion substitution have been established. It is shown that substitution of chlorine ions by bromine ions in the [N(C₂H₅)₄]₂CoCl₂Br₂ solid solution leads to the formation of distorted tetrahedral complexes CoCl₂Br₂. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 91–95, January–February, 2000.     

Optical properties of the proton conductor (NH2(CH3)2)2CoCl4

Based on the data from optical-spectral studies of (NH₂(CH₃)₂)₂CoCl₄ crystals, the existence of high-temperature phase transitions at 419, 380, 352, and 313 K is confirmed. It was shown that with the exception of the first transition, they are related to a considerable extent to rearrangement of the network of hydrogen bonds and are not accompanied by, considerable deformation of the lattice. In this connection the character of the electron-phonon interaction remains unchanged in the entire temperature range of the measurements. The (NH₂(CH₃)₂)₂CoCl₄ structure at room temperature is close to that of the ordered low-temperature phases of such isomorphous crystals as β-K₂SO₄. I. Franko State University, L’vov, Ukraine, 8, Kirill and Mefodii St., UA-290005, L’vov. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 412–418, May–June, 1998.     

Valence-delocalization of the mixed-valence oxo-centered trinuclear iron propionates [Fe 2 III FeIIO(C2H5CO2)6(py)3]npy;n=0, 1.5

Mixed-valence trinuclear iron propionates [Fe ₂ ^III Fe^IIO(C₂H₅CO₂)₆(py)₃]npy, wheren=0, 1.5, were synthesized and the structure of the pyridine-solvated complex was determined by single-crystal X-ray diffraction. Mössbauer spectra of the solvated propionate complex showed a temperature-dependent mixed-valence state related to phase transitions, reaching an almost delocalized valence state at room temperature. On the other hand, the non-solvated propionate showed a remarkable change of the spectral shape related to a phase transition, remaining in a localized valence state at higher temperatures up to room temperature.     

Thermochromic Phase Transition in [NH2(C2H5)2]2CuCl4 Crystals

The absorption spectra of [NH₂(C₂H₅)₂]₂CuCl₄ crystals in the visible spectral region in the vicinity of the thermochromic phase transition at T ₁ = 311 K are investigated. It is shown that in these crystalline compounds the phenomenon of thermochromism is primarily associated with the change of the plane-quadratic geometry of the coordination environment of Cu²⁺ to the tetrahedral form. The influence of ionizing irradiation on the phase-transition temperature and on the thermochromic properties of this crystal is studied.     

DFT(B3LYP/LanL2DZ), non-linear optical and electrical studies of a new hybrid compound: [C6H10(NH3)2]CoCl4·H2O

A new organic-inorganic material [C₆H₁₀(NH₃)₂]CoCl₄·H₂O was reported. The title compound was synthesized at room temperature by slow evaporation and then characterized by a single X-ray diffraction, spectroscopic measurements, thermal analysis and dielectric technique. It crystallizes in the non-centrosymmetric space group Pna2₁ with the following unit cell parameters: a=12.5328(1) Å, b=9.0908(1) Å, c=11.7440(1) and α=β=γ=90°. The structure can be described by the alternation of two different cationic-anionic layers. It consists of isolated H₂O, isolated [CoCl₄]²⁻ tetrahedral anions and diammoniumcyclohexane [C₆H₁₀(NH₃)₂]²⁺ cations, which are connected via N–H…Cl, N–H…O and O–H…N hydrogen bonds. The Hirshfeld surface analysis was conducted to investigate intermolecular interactions and associated 2D fingerprint plots, revealing the relative contribution of these interactions in the crystal structure quantitatively. Theoretical calculations were performed using DFT/B3LYP/LanL2DZ method for studying the molecular structure and vibrational spectra and especially to examine the non-linear optical behavior of the compound. Solid state ¹³C NMR spectrum shows three signals correspond to three different carbon environments. Thermal analysis discloses a phase transition at the temperature 315 K and the evaporation of water molecule at 327 K. A detailed dielectric study was reported and shows a good agreement with thermal measurements.     

Raman scattering and X-ray diffraction study of structural phase transitions in the perovskite-type layer compound (C3H7NH3)2CdCl4

The structural phase transitions of (C₃H₇NH₃)₂CdCl₄ (PACC) have been studied by means of Raman scattering. X-ray diffraction and DSC measurements It is shown that the order-disorder phase transition Abma ? Pbca occurs at 156 K and not at 183 K as previously proposed by Chapuis (Acta CrystB34, 1506 (1978)) Apparently, the transition at 183 K does not change the Abma space group; it is suggested that it could be related to the occurrence of an incommensurate phase Another structural transformation of PACC is detected at 114 K, but no conclusion can be given as far as the structure of the low-temperature phase is not determined.     

On the ordering in [(C2H5)4N]2CuCl4 crystal: an X-ray study and theoretical considerations

The crystal structure [(C₂H₅)₄N]₂CuCl₄ has been determined by X-ray diffraction at 243K. The room temperature phase (phase I) belongs to the space group P4₂/nm [1] whereas the low temperature phase (phase II) is orthorhombic and belongs to the space group Pnna. The phase transition at T_c=258K is of improper ferroelastic type and it is associated with the ordering of the CuCl₄ ²⁻ and a partial ordering of the [(C₂H₅)₄N]⁺ ions which are disordered in the high temperature tetragonal phase. At lower temperature, there occurs another instability which could correspond to a complete ordering in the crystal.     

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     

Electronic spectra and nature of transitions in [NH2(C2H5)2]2CuxZn1?xCl4 (x = 0.5, 1) crystals

The absorption spectra of the [NH₂(C₂H₅)₂]₂CuCl₄ crystals grown from a solution in ethyl alcohol and also of solid solutions of [NH₂(C₂H₅)₂]₂Zn_0.5Cu_0.5Cl₄ in the visible spectral region have been investigated in the vicinity of thermochromic phase transitions. It is shown that the phenomenon of thermochromism in these crystalline compounds is primarily related to the replacement of the square-planar geometry of the coordination surrounding of the Cu²⁺ ion by a tetrahedral one.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 798–802, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Dispersion of the birefringence in (CH12H25NH3)2ZnCl4 and (C12H25NH3)2ZnCl4: Co around their isotropic points

The optical dispersion of (C₁₂H₂₅NH₃)₂ZnCl₄ and (C₁₂H₂₅NH₃)₂ZnCl₄: Co is investigated by means of birefringence and optical absorption measurements in the temperature region around their isotropic points. The birefringence dispersion is analyzed in terms of a simple oscillator model based on the absorption band structure of both materials. A fairly good agreement is found between the thermal behaviour of the adjustable parameters of the oscillator model and the temperature dependence of the absorption spectra. The results suggest the existence of a structural phase transition at 315 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.     

The temperature dependence of the angular variation of,and the critical point exponent for the EPR linewidth in the two-dimensional canted antiferromagnetic (C2H5NH3)2MnBr4: Evidence for a structural phase transition

The angular variation of the EPR linewidths in single crystals of (C₂H₅NH₃)₂MnBr₄ has been measured as a function of temperature. The angular dependence is well characterized by δH(θ) = a + b(3 cos²θ ? 1) + c(3 cos²θ ? 1)². The temperature dependence of the expansion coefficients is reported, and the effect of critical point fluctuations near the Néel temperature as well as a linear temperature dependence at high temperature are observed. A sharp decrease in linewidths at 160°K is attributed to a structural phase transition. The Néel temperature is determined to be 46°K (± 1°) from linewidth measurements of a powder sample. The linewidths diverge exponentially near the Néel temperature with a critical point exponent of 1.5.     

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.     

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 Study of Formamidinium Lead Iodide (CH5N2PbI3) from 5 to 357 K

In the present study, the molar heat capacity of solid formamidinium lead iodide (CH₅N₂PbI₃) was measured over the temperature range from 5 to 357 K using a precise automated adiabatic calorimeter. In the above temperature interval, three distinct phase transitions were found in ranges from 49 to 56 K, from 110 to 178 K, and from 264 to 277 K. The standard thermodynamic functions of the studied perovskite, namely the heat capacity C°_p(T), enthalpy [H⁰(T) − H⁰(0)], entropy S⁰(T), and [G°(T) − H°(0)]/T, were calculated for the temperature range from 0 to 345 K based on the experimental data. Herein, the results are discussed and compared with those available in the literature as measured by nonclassical methods.