Structure Of High-temperature Phase Research Articles

Formation of new crystalline qtz-[Zn(mIm)2] polymorph from amorphous ZIF-8.

The structure of a new ZIF-8 polymorph with quartz topology (qtz) is reported. This qtz-[Zn(mIm)2] phase was obtained by mechanically amorphising crystalline ZIF-8, before heating the resultant amorphous phase to between 282 and 316 °C. The high-temperature phase structure was obtained from powder X-ray diffraction, and its thermal behaviour, CO2 gas sorption properties and dye adsorption ability were investigated.

Unraveling the electronic structure, mechanical, and dielectric properties of ZnPurBr-MOF: Ab initio calculations

A novel nanocrystalline metal–organic framework (MOF), ZnPurBr, was recently discovered to undergo an unusual thermal solid–solid phase transition, resulting in a stable high-temperature phase structure, ZnPurBr-HT. In order to fully elucidate this fascinating phase transition, the structural, electronic, optical, and mechanical properties of the ZnPurBr high and low temperature phases are thoroughly investigated through first-principles calculations. This study confirms the retention of the metal tetrahedral bonding and the integrity of the framework during phase transition. Calculations on the electronic and optical properties show that ZnPurBr is a semiconductor and has low refractive indexes at both phases; however, increases in the dielectric constants and the plasmon frequency upon going through the phase transition can be clearly captured. Tensorial analysis of the elastic constants for both phases shows that the high temperature phase is stiffer than its low temperature counterpart, but less anisotropic. Importantly, the techniques used for the modeling and physical properties calculations can be easily be applied to other MOFs, which can certainly reveal many of the interesting features hitherto undiscovered.

A Nonlinear Optical Switchable Sulfate of Ultrawide Bandgap

Nonlinear optical (NLO) switchable materials have attracted intense attention because of their promising applications in optoelectronic devices. However, previous studies are mainly limited to mole...

Complex water dynamics in crystalline [Ca(H2O)2](ReO4)2, studied by the vibrational spectroscopy and proton magnetic resonance

A 1H nuclear magnetic resonance study of [Ca(H2O)2](ReO4)2 has confirmed the existence of one phase transition at TCh=261.2 K (on heating) and TCc=231.2 K (on cooling), detected earlier by the DSC method. These transitions were reflected by changes in the temperature dependences of second moments of NMR line M2H. The study revealed anisotropic reorientations of whole [Ca(H2O)2]2+ cations, and the reorientations by 180° jumps of H2O ligands. It was found that the phase transition at TC is associated with the reorientation of H2O signed by oxygen O10 in crystal structure in high temperature phase. The oscillations of the cation as a whole unit around the axis crossing Ca and the middle of O9–O10 distance take place in both phases. From the temperature dependence of the full-width at half-maximum values (FWHM) of the bands connected with ρr (H2O) mode in IR spectra, we can conclude that the reorientational motions of H2O ligands contribute to the phase transition mechanism. The ligands perform fast (τR ≈ 10−11 to 10−13 s) stochastic reorientational motions in the high temperature phase. The estimated mean value of activation energy for the reorientation of the H2O ligands is ca. 11.1 kJ mol from Raman spectroscopy (RS) and 11.6 kJ mol from infrared spectroscopy, which is consistent with NMR results.The proton-weighted phonon density functions G(ν) obtained at 5 K show some separate peaks characteristic for ordered phase. Additionally, IR, RS and IINS spectra were calculated by the DFT method and an excellent agreement with the experimental data was obtained using CASTEP code.

An analysis of the high-temperature phase structure of multiferroic solid solutions of the PFW-PT

An analysis of the high-temperature phase structure of multiferroic solid solutions of the PFW-PT

An analysis of the high-temperature phase structure of multiferroic solid solutions of the PFW–PT

The temperature evolution of multiferroic solid solutions of the PFW–PT system, namely a (1–x)Pb(Fe2/3W1/3O3)–(x)PbTiO3 crystal structure where x = 0, 0.2, 0.3, has been studied by neutron powder diffraction in the region of the morphotropic phase boundary. The coexistence of cubic and tetragonal phases in the solutions with x = 0.2, 0.3 was found below T = 259 and 285K, respectively. As a result of the data treatment, the atom coordinates, the occupation factors and the temperature dependences of cell parameters were determined in the cubic perovskite phase. The refinement of the crystal structure in terms of ideal perovskite exhibited anomalously large Debye–Waller factors for the lead cations, indicating the appearance of random static displacements of these cations from the ideal perovskite (000) position. Using the split-ion model we estimated the value of Pb static shifts (∼0.1Å) from their high-symmetry positions along the [110] direction. It was shown that these shifts decrease with increasing the PbTiO3 concentration.

Research on phase transition behavior of lutetium orthoborate LuBO3

Cerium-doped lutetium orthoborate LuBO3 : Ce crystal is a promising scintillator with excellent properties, but the single crystal is difficult to grow because of the phase transition. In this article, the reported high-temperature (HT) vaterite phase was proved to exist at a low temperature (below 1200°C). Moreover, the real HT phase of LuBO3 was identified for the first time. The observed HT phase structure of LuBO3 was isostructural with the hexagonal structure of YbBO3 with space group P6322(182). The temperature dependence of the lattice constant and unit cell of the above phase structures were also investigated. The relative stability of calcite phase of LuBO3 could associate with fitness of the radius of Lu3+ ion for the six-coordination structure.

Possible Phonon Density of States of High-Temperature Phase Structure of the Negative Thermal Expansion Compound ZrW2O8

To study the phonon properties of β-ZrW 2 O 8 showing the negative thermal expansion (NTE), the heat capacities of Zr 1- x M x W 2 O 8- y (M = Sc, Lu; x = 0.02, 0.04) at low temperatures were measured and analyzed. The analysis presents the effective phonon density of states (DOS) of β-ZrW 2 O 8 , showing a rounded form around 5 meV. The rounded phonon DOS of β-ZrW 2 O 8 is in marked contrast to that of the low-temperature phase of ZrW 2 O 8 , and their distinction is consistent with the difference in NTE nature between two structures.

Crystal Structure of High Temperature Phase and Ionic Conductivity Mechanism of CuHgSX (X: Cl, Br).

AbstractFor Abstract see ChemInform Abstract in Full Text.

Crystal Structure of High Temperature Phase and Ionic Conductivity Mechanism of CuHgSX (X = Cl, Br)

Abstract We found a new ionic conduction phase in CuHgSCl above 373 K and in CuHgSBr above 346 K. The crystal structures of these novel phases have been determined by Rietveld refinement of powder X-ray diffraction patterns. The electric conductivity at 500 K measured by AC impedance method was 1.4 × 10−5 S cm−1 for CuHgSCl and 4.0 × 10−6 S cm−1 for CuHgSBr. The activation enthalpy was determined to be 53 kJ mol−1 for CuHgSCl and 67 kJ mol−1 for CuHgSBr. The ionic transport number measurements indicated that Cu+ ions constitute the majority charge carriers in these samples. The electronic contribution to the conduction process is small in comparison with the Cu+ ionic contribution. The charge density analysis by the maximum entropy method (MEM) combined with Rietveld analysis clearly showed that the Cu+ ionic conduction path was along the crystallographic (100) direction.

Structural Study of Si(111)(2√3×2√3)R30°–Sn Surfaces

In order to clarify the structure of high-temperature phase of Si(111)(2√3×2√3)R30°–Sn surfaces, in situ scanning tunneling microscopic observations were performed. The study led to important findings that the high-temperature phase is not a (1×1) structure but a disordered one and that the (2√3×2√3)R30° structure changes to the disordered one through a high-temperature (2√3×2√3)R30° structure newly found. A model for the (2√3×2√3)R30° structure at room temperature was proposed through an ab initio structure relaxation calculation, which can explain well the features of diffraction intensity, Paterson function and scanning tunneling images of the (2√3×2√3)R30° structure.

Magnetic Structures of High Temperature Phases of TbBaCo2O5.5

Neutron diffraction studies have been carried out on a single crystal of oxygen-deficient perovskite TbBaCo2O5.5 in the temperature range of 7-370 K. There have been observed several magnetic or structural transitions. Among these, the existence of the transitions to the insulating phase from the metallic one at ~340 K, to the one with the ferromagnetic moment at ~280 K and possibly to the antiferromagnetic one at ~260 K, with decreasing temperature T correspond to those reported in former works. We have studied the magnetic structures at 270 K and 250 K and found that all Co3+ ions of the CoO6 octahedra are in the low spin state and those of the CoO5 pyramids carry spins which are possibly in the intermediate spin state. Non-collinear magnetic structures are proposed at these temperatures. Two other transitions have also been observed at the temperatures, ~100 K and ~250 K.

Thermal and Mechanochemical Initiated Phase Transformations in Silver Carboxylates

Small-angle X-ray scattering, electron and optical microscopy, and thermal analytical investigation of the structure and morphology changes in silver laurate, myristate, palmitate, and stearate show that these fatty acid silver salts undergo an irreversible Martensitic phase change in the 110 – 120°C temperature range. This phase change is characterized by the formation of a more symmetrical phase, compared to the initial crystalline silver carboxylate. The formation of the high temperature phase proceeds according to a Martensitic transition mechanism, which is a diffusionless transformation by a large number of atoms in a cooperative movement. An ordered orientation is observed that separates Martensitic layers in the matrix of the initial crystal-line silver carboxylate. Along with this first phase transition, a micro-twin structure on the basal plane of the crystal is formed. Increasing the temperature above 120°C leads to a decrease in the distance between silver layers in the high temperature phase structure, which is a result of the disordering of the carbon chains of the molecule.

Structural Study of Semiconductive CdTe-ZnTe Alloy by High-Energy X-ray Diffraction

Semiconductive cadmium telluride and zinc telluride mixture (Cd 1 m x Zn x Te) is observed to have ferroelectric behaviors such as the D-E hysteresis measurements, dielectric measurements and thermal measurements that suggest the existent of ferroelectric phase transition. The paraelectric structure in high-temperature phase is zinc-blende structure. An order-disorder type phase transition occurs and the structure of Cd 1 m x Zn x Te crystal changes to ferroelectric pseudo cubic structure. However, the detail of the mechanism of the phase transition and the structure of ferroelectric phase have not been revealed. We performed X-ray diffraction measurements with synchrotron radiation and obtained a clear evidence of short range order or intermediate range order structure in ferroelectric phase. The role of glassy structure in the mechanism of ferroelectric behavior is discussed.

X-ray Structure Analysis of K 2 Cd 2 x Mn 2(1− x ) (SO 4 ) 3 Mixed Crystal in the High-Temperature Phase

We determined the high-temperature phase structure and the phase transition temperature of langbeinite K 2 Cd 2 x Mn 2(1 m x ) (SO 4 ) 3 (nominally x =0.5) annealed under several conditions. It turns out that the steric hindrance between B ions (mixture of Cd and Mn) and SO 4 ions is relaxed by annealing. We discuss that the steric hindrance plays an important role on the phase transition.

Comment on `X-ray structural characterization, Raman and thermalanalysis of LiNH4SO4 above room temperature'

In a recent paper (1999 J. Phys.: Condens. Matter 11 8995) Solans et al observed a newphase transition and solved the structure of high-temperature phases. Webelieve that the results commented on can be explained by the mixture of twodifferent components (a and ß modifications or ferroic domains)in the sample and not by introducing the two new intermediate phases supposed bySolans et al.

Electronic Structure in High Temperature Phase of Fe3O4

Band structure of the ferrimagnetic Fe 3 O 4 was calculated in the high temperature cubic phase by means of the full-potential linearized augmented-plane-wave method with the local spin density approximation. In the majority spin bands, the Fermi level stays in the band gap and 3d states of A site Fe are unoccupied while 3d states of B site Fe are completely occupied. In the minority spin bands, 3d states of A site are occupied and the Fermi level stays in dε bands of B site Fe. The obtained Fermi surfaces well explain the mechanism of the structural phase transition in Fe 3 O 4 . The calculated charge density has good correspondence to the recent experimental results.

Structural transformations in ferroelastic TINO3

Abstract The structures of high-temperature phases (phase II (79°C < T < 144°C) and phase I (144°C < T < 206°C)) of TINO3 are refined by the Rietveld technique using neutron powder diffraction data. The structure of phase II (trigonal; P31; Z = 9) is ordered and that of phase I (cubic; Pm3m; Z = 1) is disordered. The transition from room-temperature (RT) phase III to phase II involves the flipping of one of the eight nitrate ions in the RT unit cell about an in-plane axis and small but significant displacements of the other atoms from their phase III locations. Phase I is characterized by twelvefold orientational disorder of the nitrate ions. A new glassy-type phase is observed on quenching the sample from phase I to RT.

Theory of displacive phase transitions in minerals

A lattice-dynamical treatment of displacive phase transitions leads naturally to the softmode model, in which the phase-transition mechanism involves a phonon frequency that falls to zero at the transition temperature. The basic ideas of this approach are reviewed in relation to displacive phase transitions in silicates. A simple free-energy model is used to demonstrate that Landau theory gives a good approximation to the free energy of the transition, provided that the entropy is primarily produced by the phonons rather than any configurational disorder. The rigid unit mode model provides a physical link between the theory and the chemical bonds in silicates and this allows us to understand the origin of the transition temperature and also validates the application of the soft-mode model. The model is also used to reappraise the nature of the structures of high-temperature phases. Several issues that remain open, such as the origin of first-order phase transitions and the thermodynamics of pressure-induced phase transitions, are discussed.

Distortions of framework structures

Abstract We review the Rigid Unit Mode model that was developed to explain a number of features of framework silicate crystals composed of linked tetrahedral units, but which has implications also for the stability of perovskite structures if the octahedra have some degree of rigidity. The discussion includes the constraints on possible modes of deformation, experimental validation of the model, implications for the local structure of high-temperature phases, the local structures of defects, and the relationship to the soft-mode theory of displacive phase transitions.