Presence Of Structural Phase Transition Research Articles

Investigations on the polymorphism of K4CaSi6O15 at elevated temperatures.

In the present study, single crystals and polycrystalline material of K4CaSi6O15 were prepared from solid-state reactions between stoichiometric mixtures of the corresponding oxides/carbonates. Heat capacity (C p) measurements above room temperature using a differential scanning calorimeter indicated that two thermal effects occurred at approximately T 1=462 K and T 2=667 K, indicating the presence of structural phase transitions. The standard third-law entropy of K4CaSi6O15 was determined from low-temperature C p's measured by relaxation calorimetry using a Physical Properties Measurement System and amounts to S°(298K)=524.3±3.7 J·mol-1·K-1. For the 1st transition, the enthalpy change ΔH tr1=1.48kJ·mol and the entropy change ΔS tr1=3.25 J·mol-1·K-1, whereas ΔH tr2=3.33kJ·mol-1 and ΔS tr2=5.23 J·mol-1·K-1 were determined for the 2nd transition. The compound was further characterized by in-situ single-crystal X-ray diffraction between ambient temperature and 1063 K. At 773 K, the high-temperature phase stable above T 2 has the following basic crystallographic data: monoclinic symmetry, space group P21/c, a=6.9469(4) Å, b=9.2340(5) Å, c=12.2954(6) Å, β=93.639(3)°, V=787.13(7) Å3, Z=2. It belongs to the group of interrupted framework silicates and is based on tertiary (Q3-type) [SiO4]-tetrahedra. Together with the octahedrally coordinated Ca-cations, a three-dimensional mixed polyhedral network structure is formed, in which the remaining K-ions provide charge balance by occupying voids within the net. The intermediate temperature modification stable between T 1 and T 2 shows a (3+2)-dimensional incommensurately modulated structure that is characterized by the following q-vectors: q1=(0.057, 0.172, 0.379), q2=(-0.057, 0.172, -0.379). The crystal structures of the high- and the previously studied ambient temperature polymorph (space group Pc) are topologically equivalent and show a group-subgroup relationship. The index of the low- in the high-symmetry group is six and involves both, losses in translation as well as point group symmetry. The distortion is based on shifts of the different atom species and tilts of the 4- and 6-fold coordination polyhedra. Actually, for some of the oxygen atoms, the displacements exceed 0.5 Å. A more detailed analysis of the distortions relating to both structures has been performed using mode analysis, which revealed that the primary distortion mode transforms according to the Λ1 irreducible representation of P21/c. However, other modes with smaller distortion amplitudes are also involved.

Structural stability, electronic and mechanical properties of transition metal nitrides TMN compounds (TM=Zn, Mn and Tc)

ABSTRACT In this paper attempts to predict the presence of structural phase transition in some nitrides from zinc-blende (B3) to rock-salt (B1) structures when put under pressure using an interaction potential model (IIPM), which includes Coulomb interactions, three-body interaction (TBI) based or charge transfer effect and overlap repulsive interaction operative up to second neighbour. The transition takes places at 3, 6.7 and 36 GPa, accompanied by sudden drops in volumes of 15.31%, 2.35% and 5.11% in ZnN, MnN and TcN, respectively. For investigating electronic properties we have used the full-potential linearized augmented plane wave method based on the density functional theory. The computed electronic band structures confirm the metallic behaviour of these compounds. The second-order elastic constants have also been examined at zero and high pressure and we observed that these values increase monotonically under the influence of pressure. Our outcomes are, in general, in good agreement with the other theoretical results.

Горные породы – сегнетоэластики

Purpose. Identification of the features and the properties of segnetoelastics in rocks. Research Methods. Laboratory experiments on the deformation of rocks in a volume field of compressive stresses, macro- and microscopic studies of the deformations and the orientations of an optical indicatrix, analysis of the data obtained. Research results. As a result of microscopic and optical studies of rocks in thin sections on a polarizing microscope, tests of rocks under the conditions of unequal triaxial compression, analysis of the results obtained, it was found that the rock-forming minerals of the sedimentary sequence at a normal temperature and in the absence of an external load have a domain structure. Under the conditions of a triaxial unequal component compression, the volumetric deformations at the micro and macro levels do not exceed hundredths, while the relative micro deformations fully correspond to the dilatancy volume at the macro level. Shear rotational deformations reach 15–20%, their resonant increment increases several times over three periods before the destruction. During the loading, the direction of the rotation is changed 3-4 or more times, an optical indicatrix changes the direction. All deformations are alternating in sign and, until resonance, the amplitude of their increment does not exceed 0.5%. During the loading, the deformation, elastic, optical, electromagnetic, and other parameters are changed abruptly (structural-phase transitions of the second, and upon the destruction, of the first and third kind). The interconnection between the load and the deformations is nonlinear, a number of characteristic deformations are distinguished at which these jumps occur. All said indicate that rocks are classical distortion segnetoelastics. The following parameters can be taken as parameters of a segnoelastic transition: the value or increase in the value of relative deformation; an elastic modules varying within ± ∞; the sign and the magnitude of the increment of the Lode angle for the deformations; the angle of rotation of the optical indicatrix, the presence of structural-phase transitions in the minerals. Scientific novelty. It has been shown for the first time that all five signs and traits of segnetoelastics are present in sedimentary rocks. Practical significance. The possibility for isolating of segnetoelastics without laborious microscopic and electromagnetic studies.

Disappearance of dielectric anomaly in spite of presence of structural phase transition in reduced BaTiO3: Effect of defect states within the bandgap

We report the structural, optical, ferroelectric, and dielectric properties of reduced BaTiO3 samples. For this purpose, oxygen vacancies in BaTiO3 are created by heating these samples with a Ti metal in a vacuum environment at different temperatures. It is observed that with an increase in oxygen deficiencies, the c/a ratio decreases as compared to that of the oxygen treated sample. The ferroelectric properties of the oxygen deficient samples are visibly different as compared to those of the oxygen treated sample. The disappearance of the P-E loop and the anomaly in the temperature variation of the dielectric constant have been observed; however, the structural phase transition corresponding to ferroelectric phase transitions still persists. Thus, it appears that the anomaly in dielectric data and the presence of the P-E loop are getting masked possibly by the Maxwell-Wagner effect. The presence of Ti+3 states in the prepared samples has been confirmed by X-ray absorption near edge structure measurements. The Kubelka-Munk optical absorption shows the presence of extra states below fundamental transition, indicating the emergence of new electronic states within the bandgap, which might be due to Ti+3 states. These new states appear at different energy positions, and with different intensities for different samples, which are reduced in the presence of Ti. These new states within the bandgap appear to modify the electronic structure, thereby reducing the overall bandgap, and hence, they seem to modify the ferroelectric and dielectric properties of the samples. Our results may be treated as experimental evidence for theoretically proposed defect states in oxygen deficient or reduced BaTiO3.

Phase transition and ductile behavior of IrxOs1-xN alloys from theoretical point of view

Present paper reports theoretical investigation of less explored nitrides of Iridium and Osmium using semi-empirical model with charge transfer effect (CTE). This work indicates the presence of structural phase transition in these nitrides from zinc-blend (B3) to rock-salt (B1) structure on application of pressure on them. Transitions occurred at 71 GPa and 86 GPa, with sudden drops in volume of 9.54% and 8.35% in IrN and OsN. The effect of pressure on elastic properties for B3-IrN and B3-OsN is investigated for the first time. On the basis of mechanical properties, it is observed that both compounds are ductile in nature. The present study is extended to investigate transition metal alloy IrxOs1-xN via Vegard’s law. To widen the applicability of our model and to explore this alloy we report structural, elastic, mechanical and thermophysical properties. The effect of pressure on Debye temperature with different concentration (x = 0, 0.25, 0.5, 0.75, 1) have also been analyzed. The results are in general in good agreement with available theoretical results.

Intermolecular stabilization of 3,3'-diamino-4,4'-azoxyfurazan (DAAF) compressed to 20 GPa.

The room temperature stability of 3,3'-diamino-4,4'-azoxyfurazan (DAAF) has been investigated using synchrotron far-infrared, mid-infrared, Raman spectroscopy, and synchrotron X-ray diffraction (XRD) up to 20 GPa. The as-loaded DAAF samples exhibited subtle pressure-induced ordering phenomena (associated with positional disorder of the azoxy "O" atom) resulting in doubling of the a-axis, to form a superlattice similar to the low-temperature polymorph. Neither high pressure synchrotron XRD, nor high pressure infrared or Raman spectroscopies indicated the presence of structural phase transitions up to 20 GPa. Compression was accommodated in the unit cell by a reduction of the c-axis between the planar DAAF layers, distortion of the β-angle of the monoclinic lattice, and an increase in intermolecular hydrogen bonding. Changes in the ring and -NH2 deformation modes and increased intermolecular hydrogen bonding interactions with compression suggest molecular reorganizations and electronic transitions at ∼ 5 GPa and ∼ 10 GPa that are accompanied by a shifting of the absorption band edge into the visible. A fourth-order Birch-Murnaghan fit to the room temperature isotherm afforded an estimate of the zero-pressure isothermal bulk modulus, K0 = 12.4 ± 0.6 GPa and its pressure derivative K0' = 7.7 ± 0.3.

Study of the optically diffused reflectance and thermal-microstructure for the phase transformation of AgNO3

Optical, microstructural, and thermal properties of the investigated silver nitrate samples were characterized by various techniques, such as X-ray analysis, scanning electron microscopy, UV–Vis–NIR absorption and differential scanning calorimetry (DSC). The presence of structural phase transition [orthorhombic structure (phase II) to rhombohedral structure (phase I)] was checked by DSC and X-ray analysis measurements. The thermal energy required for such transformation is found to be 11.6 J/g. The optical band gaps of AgNO3 are 1.4 and 2.02 eV for phase II and phase I, respectively, at the low-energy region. But at high-energy region, the optical band gaps are 3.41 and 3.43 eV for phase II and phase I, respectively. Characteristic peaks for AgNO3 corresponding to (2 1 1), (0 0 4) and (3 5 1) for phase II and (0 0 4), (3 1 1) and (0 2 4) for phase I have been observed. The average crystalline size for AgNO3 samples and the values of dislocation density δ and the strain e for the planes of two phases II and I are calculated and also the texture coefficient is determined. Such information can considerably aid in understanding the process of phase transformations in AgNO3.

First-order structural transition in the multiferroic perovskite-like formate [(CH3)2NH2][Mn(HCOO)3

In this work we explore the overall structural behaviour of the [(CH3)2NH2][Mn(HCOO)3] multiferroic compound across the temperature range where its ferroelectric transition takes place by means of calorimetry, thermal expansion measurements and variable temperature powder and single crystal X-ray diffraction. The results clearly proof the presence of structural phase transition at Tt ~187 K (temperature at which the dielectric transition occurs) that involves a symmetry change from R-3c to Cc, twinning of the crystals, a discontinuous variation of the unit cell parameters and unit cell volume, and a sharp first-order-like anomaly in the thermal expansion. In addition, the calorimetric results show a 3-fold order-disorder transition. The calculated pressure dependence of the transition temperature is rather large (dTt/dP = 4.6 $\pm$ 0.1 K/kbar), so that it should be feasible to shift it to room temperature using adequate thermodynamic conditions, for instance by application of external pressure.

Synchrotron based XRD study on nano crystalline SnO2 under pressure

Results of in-situ high pressure x ray diffraction measurements on 5 nm SnO2 nanoparticles are reported. The ambient rutile phase remains stable upto the pressure of 18.1 GPa. However, beyond 19 GPa, the diffraction pattern shows the new diffraction peaks indicating the presence of structural phase transition in the material. These new peaks could be assigned to cubic structure. The coexistence of both the phases could be seen over a large pressure region indicating the sluggishness of the transition.

Study of cerium phase transitions in shock wave experiments

Cerium has a complex phase diagram that is explained by the presence of structural phase transitions. Experiments to measure the sound velocities in cerium by two methods were carried out to determine the onset of cerium melting on the Hugoniot. In the pressure range 4–37 GPa, the sound velocity in cerium samples was measured by the counter release method using manganin-based piezoresistive gauges. In the pressure range 35–140 GPa, the sound velocity in cerium was measured by the overtaking release method using carbogal and tetrachloromethane indicator liquids. The samples were loaded with plane shock wave generators using powerful explosive charges. The onset of cerium melting on the Hugoniot at a pressure of about 13 GPa has been ascertained from the measured elastic longitudinal and bulk sound velocities.

Proton Dynamics and Structural Modifications in the Protonic Conductor Perovskites

Quasi-elastic neutron scattering, neutron diffraction as well as in situ and ex situ Raman studies were performed on the proton conducting (Ba/Sr)Zr 1- x Ln x O 3-δ dense ceramics. The results confirm the homogeneous distribution of the bulk protonic species, their variable content as a function of Ln content and sample processing as well as successive re-orientation local motion between 400 and 600 °C. The analysis of the elastic incoherent structure factor suggests the complex motion of proton between different sites and with different length jumps. The proton dynamics are accompanied by the presence of structural phase transitions near 350/450 and 550 °C as well as by the changes of conductivity activation energy.

“Freezing” of domain structure in Rb2ZnCl4

Abstract A comprehensive study of dielectric and polarization properties of the ferroelectric Rb2ZnCl4 crystal in the vicinity of 150 K has been performed. The investigation results were compared with the temperature dependences of threshold field of domain boundary unpinning from stoppers and relaxation time of soliton structure near this temperature. The peculiarities of dielectric and polarization properties of Rb2ZnCl4 near 150 K are discussed in terms of the ideas of the presence of structural phase transition as well as of the change in the domain wall structure that may lead to the “freezing” of the domain structure.

Impurity states in Sb-doped amorphous semiconductors

A systematic investigation of the effects of antimony dopant on the electronic transport properties of amorphous (GeSe3.5)100−xSbx under high pressure (up to 120 kbar) has been carried out down to liquid-nitrogen temperature for the first time. Differential thermal analysis and x-ray diffraction methods were used for the characterization of freshly prepared and pressure-quenched materials which indicated the presence of structural phase transition in both GeSe3.5 and (GeSe3.5)100−xSbx around 105 kbar pressure. Electrical transport data revealed the strong compositional dependence of the electronic conduction process. A distinct kink in the conductivity temperature plot at pressures>15 kbar was observed in the Sb-doped compositions indicating the presence of different conduction processes. An attempt has been made to interpret the pressure-induced effect in the transport properties of these glasses considering the possible presence of both thermally activated conduction in the extended states and hopping process in the localized tail states. However, the interpretation of the transport data is not straightforward and the pressure dependence of the thermoelectric power will be needed to complete the picture.

Phase Transitions in Tetramethylammonium Hexachloroplatinate(IV), -tellurate(IV), and -stannate(IV) as Studied by 35Cl NQR and Powder X-Ray Diffraction

Abstract The 35Cl NQR spectra of [(CH3)4N]2PtCl6, [(CH3)4N]2TeCl6, and [(CH3)4N]2SnCl6 were recorded in a temperature range between 77 and ca. 350 K. Below room temperature, the temperature dependence measurements of NQR frequencies for all of these complexes confirmed the presence of structural phase transitions reported very recently. From the experiments of DTA carried out above room temperature, structural phase transitions were located at 385, 351, and 365 K in the same order of the above complexes. The X-ray powder patterns taken in the highest-temperature phase of these complexes (at ca. 390 K) were well interpreted as arising from the K2PtCl6-type cubic structure with the lattice constant a0=12.828, 13.053, and 12.975 Å for the Pt, Te, and Sn complexes, respectively. In this phase of each complex, however, no NQR spectra were observed. The mechanism of the phase transitions detected is discussed in connection with the structure of these complexes.

Conductivity in allotropic phase transitions

A method is given for calculating the resistivity as a function of temperature in the presence of structural phase transitions in solids.

Raman spectroscopy of structural phase transitions in Ag26I18W4O16

Raman spectra of single crystals of silver iodide tetratungstate (Ag26I18W4O16) have been obtained as a function of temperature from 77 to 350 K. These data indicate the presence of structural phase transitions near 199 and 285 K. Dielectric and conductivity measurements perpendicular to the unique twofold axis confirm the presence of these two transition temperatures. Plots of log (σT) versus 1/T reveal three distinct regions of linearity with activation energy 2.4 eV below 199 K; 0.07 eV between 199 and 285 K; and 0.42 eV above 285 K.