Low-temperature Monoclinic Phase Research Articles

New Microscopic Mechanism for Secondary Relaxation in Glasses

The dynamics of simple molecular systems showing glassy properties has been explored by dielectric spectroscopy and nuclear quadrupole resonance (NQR) on the halogenomethanes CBr2Cl2 and CBrCl3 in their low-temperature monoclinic phases. The dielectric spectra display features which correspond to alpha- and beta-relaxation processes, commonly observed in canonical glass formers. NQR experiments, also performed in the ergodic monoclinic phase of CCl4, enable the determination of the microscopic mechanism underlying the beta dynamics in these simple model glasses: Molecules that are nonequivalent with respect to their molecular environment perform reorientational jumps at different time scales. Thus our findings reveal another mechanism that can give rise to typical beta-relaxation behavior, raising some doubt about the existence of a universal explanation of this phenomenon.

Interrelationship between superprotonic conductivity and strain in CsHXO 4

In order to clarify the mechanism of superprotonic conductivity (SPC) for the hydrogen-bonded compound CsHXO 4 ( X = S , Se), we have investigated the elastic properties of CsHXO 4 with various transition temperatures ( T c ), at which CsHXO 4 exhibits a structural phase transition from a low-temperature monoclinic phase with low protonic conductivity (phase II) to a high-temperature SPC phase (phase I). It was found that, in CsHS x Se 1 − x O 4 ( x = 0 – 1 ), the transition temperature T c decreases in proportion to the square of spontaneous strain b in the a II – b II plane of phase II. This result indicates that the spontaneous strain in the a II – b II plane plays an important role in the appearance of SPC in CsHXO 4. Moreover, thermodynamic analyses suggest that proton migration in phase I is induced by the release of the strain energy. From these results, it is deduced that the phase transition temperature in CsHXO 4 is determined by the competition between the kinetic energy of proton and the released-strain energy.

Electron-nuclear double resonance study of [formula omitted] ions in [formula omitted] crystals

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to identify and characterize V 4+ ions in a bulk single crystal of vanadium dioxide (V O 2). These S = 1 / 2 defects are observed in the as-grown crystal because an adjacent nonmagnetic M 4+ impurity, e.g., a Si 4+ ion, has destroyed the normal antiferromagnetic coupling associated with the close pairs of V 4+ ions that occur in the low-temperature monoclinic phase of V O 2. EPR spectra taken near 5 K with the magnetic field along the [110] and [001] axes show resolved hyperfine patterns due to one 51V nucleus. ENDOR spectra taken at 5 K with the magnetic field parallel to the [001] axis have large nuclear electric quadrupole splittings as a result of a significant electric field gradient at the 51V nucleus. Spin-Hamiltonian parameters describing the electron Zeeman, hyperfine, and nuclear electric quadrupole interactions are reported.

Room-Temperature Chemical Synthesis of Silver Telluride Nanowires

We have developed a room-temperature solution-phase route for the preparation of one-dimensional silver telluride nanowires (Ag2Te NWs). The Ag2Te NWs of 600 nm length and 20 nm diameter were synthesized by the reaction of tellurium nanowires (Te NWs) with silver nitrate (AgNO3) in water. We propose that Te NWs act as the template and the reducing agent simultaneously, enabling the formation of polycrystalline Ag2Te NWs. Various microscopic and spectroscopic tools, such as HRTEM, SEM, EDAX, XRD, DSC, XPS, and Raman, were used for the characterization of Ag2Te NWs. The phase transition corresponding to the structural transition from the low-temperature monoclinic phase to the high-temperature face-centered cubic phase occurs at 417.7 K. We studied the electrical resistance and investigated the influence of temperature on the thermal emf. The Seebeck coefficient showed a maximum of ∼142 μV K−1, a value much higher than that of the bulk material and thin films (65−130 μV K−1). A surface-enhanced Raman scattering investigation of dispersed Ag2Te NWs showed them to be sensitive up to 10−7 M crystal violet. The monodispersity and the homogeneity of the NWs through a simple solution-phase protocol would pave the way for further studies and applications.

Structural transformations of K3H(SO4)2 crystals with variations in temperature

Single crystals of the K3H(SO4)2 compound are investigated using X-ray diffraction on Xcalibur S and Bruker diffractometers. The structure of the low-temperature monoclinic phase is refined (space group C2/c, z = 4, a = 14.698(1) A, b = 5.683(1) A, c = 9.783(1) A, β = 103.01(1)°, T = 293 K, Bruker diffractometer), the structural phase transition is revealed, and the structure of the high-temperature trigonal phase is determined (space group R % MathType!MTEF!2!1!+- % feaagaart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabG4mayaara % aaaa!36C2! $$ \bar 3 $$ m, z = 3, a = 5.73(1) A,c = 21.51(1) A,T = 458 K, Xcalibur diffractometer).

Structural changes at the Verwey transition in Fe 3O 4

Investigations of the dynamic process of the appearance of the Verwey transition in magnetite are performed by simultaneous measurements of magnetic susceptibility χ AC and the intensity of the ( 1 1 1 2 2 ) superstructure peak, using the coherent X-ray diffraction (CXD) technique. The results suggest that while the volume of a new low-temperature monoclinic phase increases gradually across the transition, it is initially in the form of grains too small to form the well-developed superlattice reflection. Finally, at the end of the transition, those small grains coalesce into micrometer size structural domains, reflected in ( 1 1 1 2 2 ) superlattice peak.

Changes in Lattice Parameters of VO2 Films Grown on c-Plane Al2O3 Substrates across Metal–Insulator Transition

We demonstrated lattice parameters of vanadium dioxide (VO2) films grown on Al2O3(001) substrates across metal–insulator transition using temperature-controlled X-ray diffraction measurements. Changes in lattice length for both the monoclinic and tetragonal phases were shown against temperature. Films prepared by pulsed laser deposition (PLD) and reactive sputtering were examined to discuss the dependence of the transition properties on lattice parameters. The expansion of cmsin β due to smaller angle β was revealed to be a characteristic in the low-temperature monoclinic phase and to be transformed to a larger in-plane at length in the high-temperature tetragonal phase. We discussed the dependence of transition temperature on ct length in the tetragonal phase in relation to the am length in the monoclinic phase across metal–insulator transition.

Lokale Lithiumfluktuationen mit niedriger Aktivierungsenergie in zwei Festkörperphasen des Lithiumsilylamids [(Me2SiNtBuLi)2O]2

AbstractLocal Lithium Fluctuations with low Activation Energies in two solid Phases of the Lithiumsilylamide [(Me2SiNtBuLi)2O]2The dilithium silylamide [(Me2SiNtBuLi)2O]2 shows in the temperature range between ambient temperature and 150 K three solid state phases with transition points at 253 K and 163 K which were characterised by means of X‐ray diffraction andsolid state NMR spectroscopy. In the high temperature phase the dimeric molecules of [(Me2SiNtBuLi)2O]2 crystallize in the tetragonal space group P42/nmc. It can be deducted from X‐ray diffraction studies that, at room temperature, the four lithium atoms in [(Me2SiNtBuLi)2O]2 are statistically distributed on two split positions at a distance of 0.89(1) Å. By cooling below 253 K the crystal transforms into an intermediate phase with space group P42/n. The symmetry of the molecule ist reduced from point symmetry 2m (D2d) to (S4). The split positions of the lithium atoms are found in different planes and their distance is slightly reduced from0.89 Å to 0.81 Å. Solid state NMR spectroscopic measurements and powder diffraction analysis reveal the existence of a monoclinic low temperature phase below 163 K. From 7Li MAS and 13C CP/MAS NMR spectroscopic measurements follows furthermore that the disorder of the lithium atoms in the two higher temperature phases (above 163 K) is due to a local dynamic exchange of the four lithium atoms over eight possible positions in the molecule. 7Li NMR measurements at variable MAS rates reveal a very small activation energy of 11 KJ/mol for the dynamic process with exchange rates of the lithium atoms in the order of 5 to 15 KHz. With respect to the lithium motions, the dynamics are intramolecular with an intermolecular coupling process due to van‐der‐Waals forces.

Phase transformation and hysteresis behavior in Cs 1 − xRb xH 2PO 4

A new theory on the origin of hysteresis in first order phase transformations was evaluated for its applicability to the phase transformation behavior in the Cs 1 − x Rb x H 2PO 4 solid solution system. Specifically, the correlation between λ 2, the middle eigenvalue of the transformation matrix describing the cubic-to-monoclinic superprotonic transition, and the transformation hysteresis was examined. The value of λ 2 was estimated from a combination of room temperature diffraction data obtained for compositions in the solid solution system and high temperature diffraction data obtained for the CsH 2PO 4 end-member. The transformation hysteresis was determined for Cs 1 − x Rb x H 2PO 4 compositions ( x = 0, 0.25, 0.50 and 0.75) by single-frequency electrical impedance measurements. It was found that the transition temperature increases monotonically with increasing Rb content, from 227.6 ± 0.4 °C for the end-member CsH 2PO 4 to 256.1 ± 0.3 °C for Cs 25Rb 75H 2PO 4, as does the hysteresis in the phase transition, from 13.4 °C to 17.4 °C. Analysis of the transformation matrix reveals that, for this system, λ 2 depends only on the b lattice parameter of the paraelectric phase and the a 0 lattice parameter of the cubic phase. The computed values of λ 2, based on extrapolations accounting for chemical contraction with increasing Rb substitution and thermal expansion on heating, were far from 1, ranging from 0.9318 to 0.9354. The observation of λ 2 increasing with Rb content is attributed to the relatively large thermal expansion in the b-axis of the low temperature monoclinic phase in combination with an increase in transition temperature with increasing x. That the hysteresis does not decrease as λ 2 approaches 1, counter to the theoretical expectations, may reflect uncertainties in the method of estimating λ 2 for Rb substituted compositions, or the discovery of a system in which hysteresis is not dominated by considerations of crystallographic compatibility.

LOW-TEMPERATURE MONOCLINIC PHASE IN EPITAXIAL (001) BARIUM TITANATE ON (001) CUBIC SUBSTRATES

ABSTRACT The possibility of the existence of a low temperature monoclinic phase in epitaxial (001) BaTiO3 films on a (001) compressive substrate is analyzed theoretically and compared to recent experimental data from literature. There is good agreement between the theoretical findings and the experimentally observed behavior. The formation of the monoclinic phase arises from the point group reduction due to the rotation of the polarization vector commensurate with the variations in the in-plane strain state.

Structure, phase transitions, and thermal expansion of ethane C2H6

X-Ray investigations of polycrystalline ethane in the temperature interval 6–90K have been performed. A previous suggestion that the structure of the orientationally ordered low-temperature phase III is monoclinic is confirmed, and the presence of an orthorhombic II and cubic I BCC phases near the melting temperature is also confirmed. It is shown that the intermediate phase II possesses a unit cell with the parameters a=4.289Å, b=5.660Å, and c=5.865Å. The volume jump ΔV at the III–II phase transition is found to be 0.21cm3∕mole (0.5%). It is suggested on the basis of the lack of reproducibility in the heating and cooling regimes that the phase II is metastable. It is determined that the volume change at the monoclinic-BCC phase transition in the interval 89.5–90K reaches ΔV=3.05cm3∕mole or 7.1%. The temperature dependences of the parameters and the volume of the low-temperature monoclinic phase III are studied for the first time. The linear and cubic thermal expansion coefficients are determined. It is found that the linear thermal expansion is anisotropic in the monoclinicity plane ac, increasing substantially as the phase transition temperature is approached. The specific heat of ethane at constant volume CV and the Gruneisen constant are calculated and the difference CP−CV is determined. It is shown that at temperatures above 50K CP−CV increases substantially as a result of an intensification of the rotational motion of the molecules.

Precipitation of Ag 2Te in the thermoelectric material AgSbTe 2

The microstructure of AgSbTe 2, prepared by solidification, is investigated using electron microscopy. During solidification and thermal treatment, the material separates into a two-phase mixture of a rocksalt phase, which is Ag 22Sb 28Te 50, and silver telluride, Ag 2Te. Ag 2Te formation results either from eutectic solidification (large lamellar structures), or by solid-state precipitation (fine-scale particles). The crystal structure of the AgSbTe 2 phase determined by electron diffraction is consistent with a rocksalt structure that has a disordered cation sublattice. A preferred crystallographic orientation relationship at the interface between the matrix and the low-temperature monoclinic Ag 2Te phase is defined and discussed. This orientation relationship is observed for both second-phase morphologies. In both cases, the orientation relationship originates from a topotactic (cube-on-cube) alignment of the Te sublattices in the initially cubic Ag 2Te and the matrix at elevated temperature. This Te sublattice alignment is retained as the Ag 2Te undergoes a cubic-to-monoclinic transformation during cooling. This orientation relationship is observed for both second-phase morphologies.

Phase Transition and Anomalous Low Temperature Ferromagnetic Phase in Pr0.6Sr0.4MnO3 Single Crystals

We report on the magnetic and electrical properties of Pr0.6Sr0.4MnO3 single crystals. This compound undergoes a continuous paramagnetic-ferromagnetic transition with a Curie temperature T C∼301 K and a first-order structural transition at T S∼64 K. At T S, the magnetic susceptibility exhibits an abrupt jump, and a corresponding small hump is seen in the resistivity. The critical behavior of the static magnetization and the temperature dependence of the resistivity are consistent with the behavior expected for a nearly isotropic ferromagnet with short-range exchange belonging to the Heisenberg universality class. The magnetization (M–H) curves below T S are anomalous in that the virgin curve lies outside the subsequent M–H loops. The hysteretic structural transition at T S as well as the irreversible magnetization processes below T S can be explained by phase separation between a high-temperature orthorhombic and a low-temperature monoclinic ferromagnetic phase.

Rb-substitution effect on the metal-insulator transition of hollandite vanadate, K2V8O16

Rb-substitution effect on the metal-insulator transition of hollandite vanadate, K2V8O16 has been investigated. The metal-insulator transition is accompanied by a charge ordering and a structural change from a tetragonal to a monoclinic structure with an increase of volume in the low-temperature monoclinic insulator phase. Rb-substitution for K atoms continuously raises the transition temperature up to 240 K for Rb2V8O16 from 160 K for K2V8O16 and does not make the transition broad in contrast to Ti-substitution. This significant rise of transition temperature can be understood as a negative chemical pressure effect caused by Rb-substitution.

High-pressure properties inferred from normal-pressure properties

From the stable and metastable normal-pressure phase equilibriainvolved in two-component systems sharing compounds of the seriesCCl4−nBrn,n = 0,...,4, several thermodynamic properties concerning non-experimentally available phasetransitions have been determined. To do so, the well-established concept of crossedisodimorphism has been considered to involve the isomorphism relationships betweenthe low-temperature monoclinic phases as well as, for both rhombohedral andface-centred cubic, orientationally disordered phases appearing in the compounds ofthe series. On the basis of such relations, the thermodynamic properties of thetwo-phase equilibria are extrapolated as a function of mole fraction to the purecompounds for which the involved transitions do not exist at normal pressure.The obtained thermodynamic properties are used to build up the topologicalpressure–temperature phase diagrams of the compounds of the series. The results arecompared with the experimental pressure–temperature phase diagrams obtainedby means of density measurements as a function of pressure and temperature.

Magnetic structure and orbital ordering in tetragonal and monoclinic KCrF3 from first-principles calculations

KCrF(3) has been systematically investigated by using the full-potential linearized augmented plane wave plus local orbital method within the generalized gradient approximation and the local spin density approximation plus the on-site Coulomb repulsion approach. The total energies for ferromagnetic and three different antiferromagnetic configurations are calculated in the high-temperature tetragonal and low-temperature monoclinic phases, respectively. It reveals that the ground state is the A-type antiferromagnetic in both phases. Furthermore, the ground states of the two phases are found to be Mott-Hubbard insulators with the G-type orbital ordering pattern. In addition, our calculations show the staggered orbital ordering of the 3d(x(2) ) and 3d(y(2) ) orbitals for the tetragonal phase and the 3d(z(2) ) and 3d(x(2) ) orbitals for the monoclinic phase, which is in agreement with the available data. More importantly, the relationship between magnetic structure and orbital ordering as well as the origin of the orbital ordering are analyzed in detail.

High-temperature phase transitions in the improper ferroelectric KIO3

The ac conductivity (σac) and dielectric permittivity (ϵ) are determined in the temperature range 300 K < T< 520 K at some selected frequencies (0.5–10 kHz) for the polycrystalline samples of KIO3 compound. The results indicated that the compound behaves as an improper ferroelectric and undergoes a ferroelectric phase transition from a high temperature rhombohedral phase I to a low temperature monoclinic phase II at T c = (486 ± 1) K. A second structural phase transition was observed around 345 K. The conductivity varies with temperature range and for T > 428 K intrinsic conduction prevails. Different activation energies in the different temperature regions were calculated. The frequency dependence of σ(ω) was found to follow the universal dynamic response [σ(ω)∝(ω) s(T)]. The thermal behaviour of the frequency exponent s(T) suggests the hopping over the barrier model rather than the quantum mechanical tunneling model for the conduction mechanism.

Domain formation in the structural phase transition of the organic superconductorκL−(DMEDO−TSeF)2[Au(CN)4](THF)

Structural properties of the layered organic superconductor ${\ensuremath{\kappa}}_{\mathrm{L}}\text{\ensuremath{-}}{(\mathrm{DMEDO}\text{\ensuremath{-}}\mathrm{T}\mathrm{Se}\mathrm{F})}_{2}[\mathrm{Au}{(\mathrm{C}\mathrm{N})}_{4}](\mathrm{THF})$ have been investigated, where DMEDO-TSeF is dimethyl(ethylenedithio)tetraselenafulvalene. We have found that a structural phase transition occurs at ${T}_{d}=209\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and that only the $(0\phantom{\rule{0.3em}{0ex}}0\phantom{\rule{0.3em}{0ex}}l)$ Bragg reflections split into two along the ${b}^{*}$ direction below ${T}_{d}$. The low-temperature structure is composed of two monoclinic domains with the space group $P{2}_{1}∕n11$. The low-temperature monoclinic phase has two crystallographically independent dimers in a conducting layer, suggesting that the present compound potentially borders on the checkerboard-type charge ordered state, and has possibly different symmetry of superconductivity from the ordinary $\ensuremath{\kappa}$-type superconductors.

Origin of the Verwey transition in magnetite: Group theory, electronic structure, and lattice dynamics study

The Verwey phase transition in magnetite has been analyzed using the group theory methods. It is found that two order parameters with the symmetries ${X}_{3}$ and ${\ensuremath{\Delta}}_{5}$ induce the structural transformation from the high-temperature cubic to the low-temperature monoclinic phase. The coupling between the order parameters is described by the Landau free energy functional. The electronic and crystal structure for the cubic and monoclinic phases were optimized using the ab initio density functional method. The electronic structure calculations were performed within the generalized gradient approximation including the on-site interactions between $3d$ electrons at iron ions---the Coulomb element $U$ and Hund's exchange $J$. Only when these local interactions are taken into account, the phonon dispersion curves, obtained by the direct method for the cubic phase, reproduce the experimental data. It is shown that the interplay of local electron interactions and the coupling to the lattice drives the phonon order parameters and is responsible for the opening of the gap at the Fermi energy. Thus, it is found that the metal-insulator transition in magnetite is promoted by local electron interactions, which significantly amplify the electron-phonon interaction and stabilize weak charge order coexisting with orbital order of the occupied ${t}_{2g}$ states at Fe ions. This provides a scenario to understand the fundamental problem of the origin of the Verwey transition in magnetite.

Calculation of the electronic structure of the vanadium dioxide VO2 in the monoclinic low-temperature phase M 1 using the generalized transition state method

It is known that the ground state of the vanadium dioxide in the low-temperature monoclinic phase M1 is a nonmagnetic insulator. The calculations in the local-density approximation (LDA) predict the metallic nonmagnetic state, whereas the calculations in terms of the LDA + U approach (local-density approximation with explicit allowance for on-site Coulomb correlations U) predict the insulating antiferromagnetic state. In terms of the method of generalized transition state, the nonmagnetic insulating state of VO2 in the M1 phase with a band gap of 0.3 eV has been reproduced for the first time.