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.     

Phase transitions in [Ni(NH3)6](NO3)2

Electric permittivity ^* = ′ − i″ of nickel-hexammino nitrate (NHN) has been measured within the range of temperature from 9 to 300 K at a frequency of 8.8 GHz (X-band). It has been found that the phase transitions at T_k1 = 247 K and T_k2 = 90 K are discontinuous structural transitions between centrosymmetric phases, whereas the transition at T_c = 63 K is a continuous phase transition (glass?).     

metamagnetic behavior of linear chain pyridine compounds: Co(Pyridine)2Cl2, Fe(Pyridine)2Cl2, Fe(Pyridine)2(NCS)2 and Ni(Pyridine)2Cl2

Magnetic phase transitions in the pyridine (pyr) compounds Co(pyr)₂Cl₂, Fe(pyr)₂Cl₂, Fe(pyr)₂(NCS)₂ and Ni(pyr)₂Cl₂ have been observed at applied magnetic fields of ～0.7, 0.7, 1.1 and 2.7 kG respectively. These low field phase transitions are observed in the Fe and Ni compounds at T = 4.2 K, and in the Co compound at T < 3K, and are consistent with metamagnetic behavior. Magnetic saturation is not achieved in any of these compounds for fields of 60 kG, reflecting high anisotropy.     

Structure and phase transitions in chloroantimonate(V) crystals: [(C2H5)3NH]SbCl6 and [(C2H5)3NH]SbCl6·1/2[(C2H5)3NH]Cl

New triethylammonium salts: [(C₂H₅)₃NH]SbCl₆ (TCA) and [(C₂H₅)₃NH]SbCl₆·1/2[(C₂H₅)₃NH]Cl (TCAT) have been synthesized. The compounds crystallise in monoclinic symmetry: space groups P2₁/n and P2₁/c, for TCA at 293 K and TCAT at 100 K, respectively. The crystal structure of [(C₂H₅)₃NH]SbCl₆ consists of discrete ionic pairs—triethylammonium cations and hexachloroantimonate anions—linked via the bifurcated N-H?Cl hydrogen bonds. The crystal structure of [(C₂H₅)₃NH]SbCl₆·1/2[(C₂H₅)₃NH]Cl is composed of three symmetrically independent triethylammonium cations, chlorine anion and two symmetrically independent hexachloroantimonate anions. TCA undergoes a structural phase transition at 336 K (on heating) into the orthorhombic C222 space group, whereas TCAT reveals a structural phase transition at 332 K. The phase transitions are of the first order type. TCA shows a ferroelastic domain structure below 336 K. Differential scanning calorimetry, dilatometric, dielectric dispersion and Raman scattering measurements have been used to study the phase transition mechanisms in these triethylammonium salts.     

Magnetic ordering and instability investigation of multiferroic Pb(Fe2/3W1/3)O3 ceramics by microwave dielectric spectroscopy

The microwave dielectric and magnetic properties of Pb(Fe_2/3W_1/3)O₃ multiferroic ceramics were investigated. A dielectric dispersion occurring in the frequency range 100 MHz-3 GHz and in a broad temperature range showed itself to be a powerful tool to detect magnetostrictive effects. The experimental results revealed the following remarkable features: the temperature dependence of f_R (characteristic frequency) and the dielectric strength Δε (characteristic of the dispersion) enabled us to identify not only the para-ferroelectric (T_C≈180 K) but also the para-antiferromagnetic (T_N≈340 K) phase transitions, while magnetic measurements revealed the para-antiferromagnetic ordering and a weak superexchange interaction (T_N2∼15 K). Additionally, both characterizations confirmed the existence of structural or magnetic instabilities around 250 K.     

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.     

Phase diagram of the relaxor (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 investigated by dielectric and Raman spectroscopies

Lead zinc niobate-lead titanate[(1−x)Pb(Zn_1/3Nb_2/3)O₃-xPbTiO₃] (PZN-PT) crystals with x=4.5% and x=12% have been investigated using dielectric and Raman measurements over a range of temperatures. Above room temperature, dielectric measurements show that both compositions exhibit structural phase transitions according to the phase diagram proposed by Kuwata et al. [Ferroelectrics 387 (1981) 579]. Below room temperature, an anomaly at around 180 K for the x=12% sample is observed, suggesting another phase transition. Raman measurements are used to study all phase transitions.     

Electrical investigations in the normal and incommensurate phases of [N(CH3)4]2ZnCl4 single crystals

The dielectric permittivity (?) of TMA-ZC single crystals was measured along the mean crystallographic axes a, b and c, in a temperature range from 273 to 340 K. The ?-T relationship exhibited peak values at T_i=296 K and T_c1=279 K for the three axes. These peaks are attributed to the contribution of discommensurations. The d.c. and a.c. electrical conductivity showed anomalous variation at the same transition temperatures, with a remarkable change in the value of the activation energy around the transition temperatures. The j-E characteristic indicates different types of electrical conduction. The mechanism of the phase transition and the electrical process were discussed on the basis of Shottky and Frenkel conduction mechanisms.     

Structure, phase transitions and molecular dynamics in 4-methylpyridinium tetrachloroantimonate(III), [4-CH3C5H4NH][SbCl4]

Crystal structure of the 4-methylpyridinium tetrachloroantimonate(III), [4-CH₃C₅H₄NH][SbCl₄], has been determined at 240 K by X-ray diffraction as monoclinic, space group, P2₁/n, Z=8. Differential scanning calorimetry and dilatometric studies indicate the presence of two reversible phase transitions of first order type, at 335/339 and 233/289 K (cooling/heating) with ΔS=0.68 and 2.2 J mol⁻¹ K⁻¹, respectively. Crystal dynamics is discussed on the basis of the temperature dependence of the ¹H NMR spin-lattice relaxation time T₁ and infrared spectroscopic studies. The low temperature phase transition at 233 K of an order-disorder type is interpreted in terms of a change in the motional state of the 4-methylpyridinium cations. The phase transition at 335 K, probably of a displacive type, is characterised by a complex mechanism involving the dynamics of both the cationic and anionic sublattice. The ¹H NMR studies show that the low temperature phase III is characterised only by the dynamics of the CH₃ groups.     

Thermal and optical properties of the ferroelectric (C3N2H5)5Bi2Cl11 crystal

Thermal (specific heat) and optical (linear birefringence) studies were performed for a new ferroelectric crystal (C₃N₂H₅)₅Bi₂Cl₁₁. Two phase transitions were confirmed and described. The first-order paraelastic-ferroelastic phase transition at 360 K was studied with a polarizing microscope. The continuous second-order phase transition at 165 K to the ferroelectric phase is described by the Landau model using specific heat and linear birefringence data. The Landau expansion coefficients B and C are of an order of magnitude higher than the closely related ferroelectric crystal; MAPCB — (CH₃NH₃)₅Bi₂Cl₁₁. Thermal parameters (such as the excess enthalpy and the excess entropy ) of the continuous transition were estimated and discussed. The ‘two-site’ model describing the motion of three of the five imidazolium cations, which is proposed from the structural studies, is fully confirmed by the data from the ac-calorimetric measurements.     

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.     

Optical studies of phase transitions in the (NH4)3VO2F4 crystal

(NH₄)₃VO₂F₄ crystals were grown, and polarization-optical studies and measurements of birefringence were conducted on crystal plates of various cuts over a wide temperature range. Phase transitions were detected at temperatures T _1↑ = 417 K, T _3↑ = 211 K, and T _4↑ = 205 K (on heating) and at T _1↓ = 413 K, T _3↓ = 210 K, and T _4↓ = 200 K (on cooling). The transitions are accompanied by anomalies of the birefringence and by twinning. The sequence of changes in the phase symmetry is assumed to be as follows: cubic Fm m ↔ orthorhombic Immm (I222₁) ↔ monoclinic 112/m) ↔ triclinic P . Near temperatures T ₂ ≈ 240–250 K, an additional anomaly of the birefringence is observed, with the crystal retaining the orthorhombic symmetry. Original Russian Text ? S.V. Mel’nikova, A G. Kocharova, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 562–564.     

Electric-field dependence of dielectric properties of sol-gel derived Ba(Zr0.2Ti0.8)O3 ceramics

The effect of a dc bias field on the diffuse phase transition and nonlinear dielectric properties of sol-gel derived Ba(Zr_0.2Ti_0.8)O₃ (BZT) ceramics are investigated. Diffuse phase transitions were observed in BZT ceramics and the Curie–Weiss exponent (CWE) was γ∼2.0. The dielectric constant versus temperature characteristics and the γ in the modified Curie–Weiss law, ε ⁻¹=ε _m ⁻¹[1+(T−T _m ) ^γ /C₁](1≤γ≤2), as a function of the dc bias field was obtained for BZT ceramics. The results indicated that γ is a function of dc bias field, and the γ value decreased from 2.04 to 1.73 with dc bias field increasing from 0 to 20 kV/cm. The dielectric constant decreases with increasing dc bias field, indicating a field-induced phase transition. The dc bias field has a strong effect on the position of the dielectric peak and affects the magnitude of the dielectric properties over a rather wide temperature range. The peak temperature of the dielectric loss does not coincide with the dielectric peak and an obvious minimum value for the dielectric loss at the temperature of the dielectric peaks is observed. At room temperature, 300 K, the high tunability (K=80%), the low loss tangent (≈0.01) and the large FOM (74), clearly imply that these ceramics are promising materials for tunable capacitor-device applications.     

Synthesis, crystal structure, thermal and dielectric properties of bis(p-phenylenediammonium) chloride hexachlorobismuthate(III) monohydrate [C6H4(NH3)2]2ClBiCl6.H2O

Synthesis, crystal structure, and dielectric properties of [C₆H₄(NH₃)₂]₂ClBiCl₆.H₂O are reported. The compound crystallizes in the monoclinic system with space group P2₁/n. The unit cell dimensions are a = 9.836(5), b = 19.582(5), c = 13.082(5) ?, β = 104.731(5)° with Z = 4. The atomic arrangement can be described by an alternation of organic and inorganic layers. The anionic layer is built up of octahedral of [BiCl₆]^3- arranged in sandwich between the organic layers. The crystal packing is governed by means of the ionic N–H···Cl hydrogen bonds, forming a three-dimensional network. The dielectric properties have been investigated at temperature range from 297 to 410 K at various frequencies (10 Hz–100 kHz). Dielectric studies were performed to confirm results obtained with thermal analysis. The evolution of dielectric constant as a function of temperature and frequency of single crystal has been investigated in order to determine some related parameters.     

Phase transition of [(C6H5)3PCH3]+(TCNQ)-2 and electrical transport properties

The electrical conductivity at 10GHz, the dielectric constant, and the thermoelectric power (TEP) of [(C₆H₅)₃PCH₃]⁺(TCNQ)^-₂, from 230 up to 400 K, have been measured. This organic quasi-one-dimensional solid undergoes a first order phase transition at 314 K. At the transition the conductivity increases by a factor of 2.2 and the activation energy drops to 0.26 from 0.31 eV. At 314 K TEP decreases abruptly from -75 to -60μVK^-1 and remains almost constant for T > 314 K. The dielectric permeability ?₀ is constant and equal to 5 in the low temperature phase, increases abruptly by 7% at the transition, and then depends strongly on temperature in the high temperature phase. Results of the high temperature phase are interpreted in terms of a strongly correlated salt.     

Two-dimensional critical behaviour in antiferrodistortive Al ur6(CIO4)3 and Ga ur6(ClO4)3

Critical behaviour with dimensionality d = 2 has been observed for the 300 K antiferrodistortive phase transition in Al ur₆(ClO₄)₃ and Ga ur₆(ClO₄)₃ by means of the temperature dependence of the ESR parameter D. The systems exhibited d = 2 behaviour in the static critical behaviour for T<T_c?40 K for T>T_c + 40 K. From the ESR data including line width measurements the local order parameter relaxation rate ω₁ has been obtained for various temperatures above T_c, with a lowest value of ω₁ = 150 MHz at T_c + 15 K     

Structural and dielectric properties in the (Ba1−xCax)(Ti0.95Zr0.05)O3 ceramics

Lead-free (Ba_1−xCa_x)(Ti_0.95Zr_0.05)O₃ (x = 0.05-0.40) (BCZT) ceramics were prepared by solid-state reaction technique. XRD results show that the samples in the composition range of 0.05 ≤ x ≤ 0.25 exhibit pure perovskite structures and undergo a polymorphic phase transitions from orthorhombic to tetragonal phase around room temperature. The biphasic structures are detected at x ≥ 0.30 and the dielectric peaks become broad and dielectric constants decrease with increasing Ca content. Ca replacement at Ba site leads to diffuseness, whereas Ca occupancy at Ti site leads to decrease of the T_c.     

Crystal growth, spectroscopic and crystal field studies of [N(CH3)4]2MnCl4 and [N(CH3)4]2CoCl4 single crystals in the paraelectric phase

Single crystals [N(CH₃)₄]₂MnCl₄ and [N(CH₃)₄]₂CoCl₄ were grown by the slow evaporation technique from the super-saturated solutions. The samples obtained were undergone the X-ray and spectroscopic studies. Absorption spectra in the paraelectric phase at T=303 K have been recorded using the Shimadzu 160A double beam automatic scanning spectrophotometers. On the basis of the exchange charge model and Racah theory the crystal field parameters and Racah parameters have been calculated; all absorption bands for both crystals were given an assignment.     

Influence of hydrogenation on the electronic structure and the itinerant-electron metamagnetic transition in strong magnetocaloric compound La(Fe0.88Si0.12)13

The influence of interstitial hydrogen on the electronic structure and the itinerant-electron metamagnetic (IEM) transition in strong magnetocaloric compound La(Fe_0.88Si_0.12)₁₃H_1.6 has been investigated by Mössbauer spectroscopy. A slight change in the average hyperfine field at 4.2 K was observed after hydrogen absorption. In contrast, the thermally induced first-order transition related to the IEM transition for y=1.6 appears at the Curie temperature T_C=330 K, much higher than T_C=195 K for y=0.0. The increase of isomer shift δ_IS at 4.2 K indicates that the valence electron transfer from hydrogen to Fe is negligibly small, hence the change in the magnetic state is closely associated with a volume expansion after hydrogen absorption. No change in shape by hydrogenation for the Mössbauer spectra in the paramagnetic state has been observed except for a difference in only δ_IS, indicating the volume expansion by hydrogenation is isotropic. Accordingly, the significant increase of T_C by hydrogen absorption is attributed to the magnetovolume effect associated with characteristic feature in IEM compounds. A discontinuous change of ferromagnetic moment, ΔM, around T_C has been observed by Mössbauer spectra, as expected from the magnetization measurement. The value of ΔM is slightly decreased by increase of T_C after hydrogenation but its magnitude is almost the same due to the stabilization of ferromagnetic moment. As a result, strong magnetocaloric effect is maintained up to room temperature after hydrogenation.