Heat capacity of molecular chromium sandwich compounds: chromocene Cr(C5H5)2 and dibenzene chromium Cr(C6H6)2

Heat-capacity measurements have been made in the temperature range 110 to 300 K. In the range 160 to 240 K the chromocene curve shows an anomaly related to an order-disorder phase transition in the solid. On the other hand the dibenzene chromium curve indicates no phase transition and is in accordance with an ordered structure at room temperature.     

Phase transitions and molecular motions in [Ni(ND3)6](ClO4)2

[Ni(ND₃)₆](ClO₄)₂ has three solid phases between 100 and 300 K. The phase transitions temperatures at heating (T_C1^h=164.1 K and T_C2^h=145.1 K) are shifted, as compared to the non-deuterated compound, towards the lower temperature of ca. 8 and 5 K, respectively. The ClO₄⁻ anions perform fast, picosecond, isotropic reorientation with the activation energy of 6.6 kJ mol⁻¹, which abruptly slow down at T_C1^c phase transition, during sample cooling. The ND₃ ligands perform fast uniaxial reorientation around the Ni-N bond in all three detected phases, with the effective activation energy of 2.9 kJ mol⁻¹. The reorientational motion of ND₃ is only slightly distorted at the T_C1 phase transition due to the dynamical orientational order-disorder process of anions. The low value of the activation energy for the ND₃ reorientation suggests that this reorientation undergoes the translation-rotation coupling, which makes the barrier to the rotation of the ammonia ligands not constant but fluctuating. The phase polymorphism and the dynamics of the molecular reorientations of the title compound are similar but not quite identical with these of the [Ni(NH₃)₆](ClO₄)₂.     

35Cl NQR and 1H NMR studies of [C(NH2)3]AuCl4, [C(NH2)3]2PtCl4, and [C(NH2)3]2PdCl4

The temperature dependence measurements of ³⁵Cl NQR frequencies and ¹H NMR spin-lattice relaxation time T₁ were carried out for guanidinium tetrachloro-aurate(III), -platinate(II), and -palladate(II). The gold(III) complex showed four NQR lines at various temperatures between 77 and 344 K, while the platinum-(II) and palladium(II) complexes gave two NQR lines in the temperature ranges 77–169 K and 77–220 K, respectively. An unusual phase transition was located at 363 K for the gold(III) complex. The high-temperature phase was easily supercooled. All the complexes studied yielded a T₁ minimum attributable to the reorientation of the planar cation about its C₃ axis. The motional parameters were evaluated. The Zeeman-quadrupole cross relaxation between protons and chlorine nuclei was observed for the platinum(II) and palladium(II) complexes at various temperatures below room temperature, while it was also detected for the high-temperature phase of the gold(III) complex.     

Calorimetric and spectroscopic studies of the critical phase transition in (CH3NH3)2[SnCl6]

(CH₃NH₃)₂[SnCl₆] undergoes a critical phase transition at 154.32 ± 0.06 K with the critical exponents α = 0.36 ± 0.04, α' = 0.19 ± 0.02, the critical entropy ratio 0.062 and the total transition entropy (1.60 ± 0.18) R. Infrared spectral change favors the order-disorder mechanism of the transition proposed from the thermodynamic data.     

Vibrational spectra of [(CH3)3NH]3Sb2Cl9, [(CH3)3NH]3Bi2Cl9 and their mixed crystals

The IR and Raman spectra of [(CH₃)₃NH]₃Sb₂Cl₉ (A), [(CH₃)₃NH]₃Bi₂Cl₉ (B) and two of their mixed crystals containing respectively 33% (AB.33) and 42% Bi (AB.42) are analyzed and compared. A and AB.33 show ferroelectric–paraelectric phase transition at 364 K and 344 K, respectively. AB.42 and B are paraelectric in the temperature range between 90 and 365 K. Most of the vibrational modes show continuous changes, with the temperature, in the IR frequencies or intensities with no soft mode behavior. However, characteristic ν(NHCl) and δ(NHCl) vibrations of weakly hydrogen-bonded species are only observed in A and AB.33 below the temperature of the phase transition and are related to the ferroelectricity. The evolution of the IR spectra with the temperature suggests that the ferroelectric properties are connected with the reorientation of the cations which needs a breaking of the weak NHCl hydrogen bonds in the paraelectric phase.     

Evolution des tenseurs de dilatation thermique en fonction de la temperature. II. Etude calorimetrique et cristallographique de la transition de phase ordre-desordre du thiophene chrome tricarbonyle C4H4SCr(CO)3

Benzenechromium tricarbonyl C₆H₆Cr(CO)₃ and thiophenechromium tricarbonyl C₄H₄SCr(CO)₃ are isomorphous at room temperature. We have measured, in the range 77–295°K, the variations of specific heat and of the tensor of thermal expansion for these two molecular compounds. The first one exhibits quite normal behavior; on the other hand, the second one exhibits a first order phase transition, monoclinic ? triclinic at T = 185°K, associated with an order-disorder phase transition. Structural evolution of thiophene-chromium tricarbonyl, during phase transition, has been explained from the variation of its tensor of thermal expansion.     

Metal-dependent thermal behaviour Ln (n-CnH2n+1NH3)2MCl4

Compounds of the type (n-C_nH_2n+1NH₃)₂MCl₄ with n = 12 or n = 16 and with M = Mn, Cu, Co, Fe, Hg, Zn show order-disorder solid-solid phase transitions in the temperature range 300–380 K. Three general types of thermal behaviour can be observed for compounds with different metals, but with the same n. The different thermal behaviour appears to be related to different coordination geometries at the metal atoms.     

Evaluation of the valency of Fe,Cr, Cu,Sb doping ions in Pb(Zr0.95Ti0.05)O3 ceramics

The valency of the ions Fe, Cr, Cu, Sb, incorporated in PZT 95/5 ceramics, is evaluated from the viewpoint of a correlation between the mean radius of the ions at the octahedral site and the two parameters: the temperature of the antiferroelectric-ferroelectric phase transition, and the volume of the unit cubic cell. It is found that Fe and Cr enter as Fe²⁺ and Cr²⁺ at the B site. Experiments suggest that Cu is incorporated at the B site as Cu⁺ and Cu²⁺ and that Sb enters partly at the A site as Sb³⁺ and mainly at the B site as Sb⁵⁺.     

Crystal structure, Fermi surface calculations and Shubnikov-de Haas oscillation spectrum of the organic metal θ-(BETS)4HgBr4(C6H5Cl) at low temperature

The organic metal θ-(BETS)₄HgBr₄(C₆H₅Cl) is known to undergo a phase transition as the temperature is lowered down to about 240 K. X-ray data obtained at 200 K indicate a corresponding modification of the crystal structure, the symmetry of which is lowered from quadratic to monoclinic. In addition, two different types of cation layers are observed in the unit cell. The Fermi surface (FS), which can be regarded as a network of compensated electron and hole orbits according to band structure calculations at room temperature, turns to a set of two alternating linear chains of orbits at low temperature. The field and temperature dependence of the Shubnikov-de Haas oscillation spectrum have been studied up to 54 T. Eight frequencies are observed which, in any case, point to an FS much more complex than predicted by band structure calculations at room temperature, even though some of the observed Fourier components might be ascribed to magnetic breakdown or frequency mixing. The obtained spectrum could result from either an interaction between the FSs linked to each of the two cation layers or to an eventual additional phase transition in the temperature range below 200 K.     

Raman spectroscopic study of K3H(SO4)2 (phases I and II) single crystals

A Raman study of K₃H(SO₄)₂ as a function of temperature reveals that this compound undergoes a phase transition at T_c = 483 K prior to the decomposition at 508 K.     

On structural phase transitions in (i-C4H9NH3)3Bi2Br9: differential scanning calorimetry, dielectric and infrared studies

Two structural phase transitions at 263 and 252 K are detected in a new isobutylammonium crystal (i-C₄H₉NH₃)₃Bi₂Br₉ by means of differential scanning calorimetry (DSC) and dielectric studies. Internal vibrations modes of (i-C₄H₉NH₃)₃Bi₂Br₉ are studied through their phase transitions using the infrared spectrscopy. The infrared studies show that the vibrational state of isobutylammonium cations changes weakly during the phase transition at 252 K. The 263 K phase transition is not reflected in the infrared spectra. The lower temperature phase transition (252 K) is believed to be governed by the reorientational motion of the isobutylammonium cations and may be classified as an ‘order–disorder' type.     

Temperature behavior of the protonic conductor K4LiH3(SO4)4

The results of the X-ray structural study for the K₄LiH₃(SO₄)₄ single crystal are presented at a wide temperature range. The thermal expansion of the crystal using the X-ray dilatometry and the capacitance dilatometry from 8 to 500 K was carried out. The crystal structures data collection, solution and refinement at 125, 295, 443 and 480 K were performed. The K₄LiH₃(SO₄)₄ crystal has tetragonal symmetry with the P4₁ space group (Z=4) at room temperature as well as at the considered temperature range. The existence of a low-temperature, para-ferroelastic phase transition at about 120 K is excluded. The layered structure of the crystal reflects a cleavage plane parallel to (001) and an anisotropy of the protonic conductivity. The superionic high-temperature phase transition at T_S=425 K is isostructural. Nevertheless, taking into account an increase of the SO₄ tetrahedra libration above T_S, a mechanism of the Grotthus type could be applied for the proton transport explanation.     

Crystal structure and phase transitions in perovskite-like C(NH2)3SnCl3

X-ray single-crystal diffraction, high-temperature powder diffraction and differential thermal analysis at ambient and high pressure have been employed to study the crystal structure and phase transitions of guanidinium trichlorostannate, C(NH₂)₃SnCl₃. At 295 K the crystal structure is orthorhombic, space group Pbca, Z=8, a=7.7506(2) Å, b=12.0958(4) Å and c=17.8049(6) Å, solved from single-crystal data. It is perovskite-like with distorted corner-linked SnCl₆ octahedra and with ordered guanidinium cations in the distorted cuboctahedral voids. At 400 K the structure shows a first-order order-disorder phase transition. The space group is changed to Pnma with Z=4, a=12.1552(2) Å, b=8.8590(2) Å and c=8.0175(1) Å, solved from powder diffraction data and showing disordering of the guanidinium cations. At 419 K, the structure shows yet another first-order order-disorder transformation with disordering of the SnCl₃⁻ part. The space group symmetry is maintained as Pnma, with a=12.1786(2) Å, b=8.8642(2) Å and c=8.0821(2) Å. The thermodynamic parameters of these transitions and the p-T phase diagram have been determined and described.     

In and F MAS NMR study of (NH4)3InF6 phases

This study presents for the first time an NMR spectroscopic characterization of the room and high temperature phases of (NH₄)₃InF₆ using ¹⁹F and ¹¹⁵In as probe nuclei. The reversible phase transition to the cubic phase at 353 K was followed by MAS NMR in situ. Static NMR experiments of the room temperature phase and MAS NMR experiments of the high temperature phase allowed the determination of the NMR parameters of both nuclei. Finally, the scalar In-F coupling, rarely observed in solid state NMR, is evidenced in both room and high temperature phases of (NH₄)₃InF₆, and measured in the high temperature phase.     

Low temperature phase transition studies on Pb(Mg0.5W0.5)O3 ceramic

We present here the results of X-ray diffraction (XRD), dielectric and calorimetric studies on lead magnesium tungustate, Pb(Mg_0.5W_0.5)O₃ (PMW) ceramic. It is shown that the low temperature antiferroelectric phase of PMW having orthorhombic structure (space group Pmcn) transforms to paraelectric cubic (space group Fm3m) phase at 281 K. The phase transition is of first order character as confirmed by coexistence of Pmcn and Fm3m phases over wide temperature range ∼50 K. The first order character of phase transition is also revealed by the observation of thermal hysteresis in the real part of dielectric permittivity and calorimetric studies. We do not find any evidence for the additional intermediate phase between antiferroelectric (Pmcn) and paraelectric (Fm3m) phases as reported in the literature. Anomalies in the heat flow and dielectric measurements support the finding of our XRD results and reveals that the phase transition temperature (T_c) is 281 K instead of 312 K reported in the literature.     

Semiconductor-metal transition in Ti3O5

Ti₃O₅ shows a first-order phase transition from the monoclinic structure to the pseudobrookite structure at 448°K, at which temperature a magnetic susceptibility anomaly has been reported earlier in the literature. There is an electrical conductivity discontinuity accompanying the phase transition. Incorporation of Fe stabilizes the high-temperature phase of Ti₃O₅; while with 2% Fe the transition temperature and enthalpy change are lowered, with 5% Fe there is no transition. Mössbauer spectra of 2% Fe-doped Ti₃O₅ are similar below and above the transition temperature and show no evidence for magnetic ordering in the low-temperature phase. These results are compared to the VO₂ transition.     

Etude calorimetrique et dilatometrique sous pression des transitions de phase de Pb(Zr0,965Ti0,035)O3 dope

The pressure—temperature phase diagram of the solid solution is drawn by calorimetric measurements. With temperature increasing the observed phase transitions are, according to the pressure: antiferroelectric, ferroelectric, paraelectric, or antiferroelectric, antiferroelectric, paraelectric. The heats of transition are measured and compared with those calculated using the Clausius—Clapeyron relationships for first-order transitions; the deviations observed at some phase transitions are discussed.     

Pressure-induced metastable phase transition in orthoenstatite (MgSiO3) at room temperature: a Raman spectroscopic study

Phase behavior of a synthetic orthoenstatite in a diamond-anvil cell has been studied up to ∼22 GPa by using Raman spectroscopy at room temperature. Under quasi-hydrostatic conditions, orthoenstatite undergoes a reversible phase transformation at an apparent transition pressure of ∼10 GPa for compression and ∼9.5 GPa for decompression. The 3d transition-metal cations, e.g., Fe²⁺ and Ni²⁺, show only a minor effect on the transition pressure within 10 wt% of addition. All the Raman frequencies in both orthoenstatite and its high-pressure phase increase monotonically with increasing pressure. The amount of forward or backward transition is fixed at a given pressure and forms a hysteresis loop in the transition %-pressure plan. The type for the present metastable phase transition is inferred to be of first order and the high-pressure polymorph may be the intermediate between orthoenstatite and the high-pressure clinoenstatite (i.e., the high-P C2/c phase). A mechanism based on Mnyukh's edgewise model of interface motion has been suggested to account for the observed phenomena.     

Low-temperature heat capacity and thermodynamic properties of crystalline [Re2(Ala)4(H2O)8](ClO4)6 (Re = Eu, Er; Ala = alanine)

[Re₂(Ala)₄(H₂O)₈](ClO₄)₆ (Re=Eu, Er; Ala=alanine) were synthesized, and the low-temperature heat capacities of the two complexes were measured with a high-precision adiabatic calorimeter over the temperature range from 80 to 370 K. For [Eu₂(Ala)₄(H₂O)₈](ClO₄)₆, two solid–solid phase transitions were found, one in the temperature range from 234.403 to 249.960 K, with peak temperature 243.050 K, the other in the range from 249.960 to 278.881 K, with peak temperature 270.155 K. For [Er₂(Ala)₄(H₂O)₈](ClO₄)₆, one solid–solid phase transition was observed in the range from 270.696 to 282.156 K, with peak temperature 278.970 K. The molar enthalpy increments, ΔH_m, and entropy increments,ΔS_m, of these phase transitions, were determined to be 455.6 J mol⁻¹, 1.87 J K⁻¹ mol⁻¹ at 243.050 K; 2277 J mol⁻¹, 8.43 J K⁻¹ mol⁻¹ at 270.155 K for [Eu₂(Ala)₄(H₂O)₈](ClO₄)₆; and 4442 J mol⁻¹, 15.92 J K⁻¹ mol⁻¹ at 278.970 K for [Er₂(Ala)₄(H₂O)₈](ClO₄)₆. Thermal decompositions of the two complexes were investigated by use of the thermogravimetric (TG) analysis. A possible mechanism for the thermal decomposition is suggested.     

Ultrasonic Study on Jahn-Teller Distortions in La1=3Sr2=3CoO3

The longitudinal ultrasonic velocity (Vl), attenuation (ffl), magnetization and resistivity of single phase polycrystalline La1=3Sr2=3CoO3 were measured as a function of temperature from 20 K to 300 K. The resistivity shows metallic behavior in the whole temperature range and a kink at 235 K was observed, which coincides with the ferromagnetic transition temperature (Tc). As the temperature cools down from Tc, the Vl softens conspicuously at beginning and reaches a minimum at 120 K. After that the Vl dramatically stiffens below 120 K accompanied by a wide attenuation peak. The analysis of the results suggests that these ultrasonic anomalies may correspond to local lattice distortions via the Jahn-Teller effect of intermediate spin Co3+.