Sequence Of Structural Phase Transitions Research Articles

Thermal Expansion and Dielectric Properties of Triangular Lattice Antiferromagnet RbCoBr3

Thermal expansion and dielectric measurements of RbCoBr3 were performed to examine the lattice distortion accompanied by structural phase transitions. It was confirmed that RbCoBr3 undergoes structural phase transitions at 90, 37, and 10 K because its linear thermal expansion and the temperature dependence of its linear thermal expansion coefficients showed clear anomalies at these temperatures. The pyroelectric charge measurement showed that a spontaneous electric polarization appears between 37 and 31 K and below 10 K. We propose a successive structural phase transition sequence and crystal symmetries at each phase.

Microscopic origin of the structural phase transitions at the Cr2O3(0001) surface

The surface of a Cr2O3 (0001) film epitaxially grown on Cr undergoes an unusual reentrant sequence of structural phase transitions. In order to understand the underlying microscopic mechanisms, the structural and magnetic properties of the Cr2O3 (0001) surface are here studied using first-principles electronic structure calculations. Two competing surface Cr sites are identified. The energetics of the surface is described by a configurational Hamiltonian with parameters determined using total energy calculations for several surface supercells. Effects of epitaxial strain and magnetic ordering on configurational interaction are also included. The thermodynamics of the system is studied using Monte Carlo simulations. At zero strain the surface undergoes an ordering phase transition at 165 K. Tensile epitaxial strain together with antiferromagnetic ordering drive the system toward strong configurational frustration, suggesting the mechanism for the disordering phase transition at lower temperatures.

How Nonequivalency of the Flexibility of the Ligand Bridges Leads to Anisotropy of Perturbation Transmission in a 3D Spin‐Crossover Coordination Network

AbstractThe role of the covalent connections of iron(II) spin‐crossover centres in the transmission of perturbation produced by the HS ↔ LS transition has been investigated for the new 3D coordination polymer {[Fe(qbtr)3](ClO4)2}∞ [qbtr = 1,5‐di(1,2,3‐triazol‐1‐yl)pentane]. In this complex, a sequence of structural phase transitions occurs. A Ph1 (HS,HS) → Ph2 (HS,HS,HS) transition precedes spin crossover and is associated with an increased number of crystallographically unique iron(II) ions, whereas the change of spin state Ph2 (HS,HS,HS) → Ph3 (HS,LS) triggers another structural phase transition accompanied by a reduction in the number of the unique iron(II) sites. Iron(II) ions occupying two crystallographically unique Fe1 and Fe2 sites form the nodes of a coordination network in the Ph1 phase. The characteristic feature of the polymeric skeleton is the presence of three kinds of ligand bridges tethering the Fe1···Fe1, Fe1···Fe2 and Fe2···Fe2 sites. The ligand linkages differ from each other in rigidity, and the diversification of their properties arises from the susceptibility of the ligand molecules to undergo conformational changes. The high rigidity of the bridge established in the Ph2 phase between iron(II) ions occupying Fe2 sites allows for the propagation of the perturbation produced by the HS (Fe2) → LS (Fe2) transition through the crystal lattice. A strong compression along the Fe2···Fe2 bridging direction occurs. In contrast, the ligand molecules that form the two remaining types of bridge are disordered in the Ph2 phase and the change of spin state for the iron(II) ions occupying Fe2 sites induces their ordering, which leads to an increase in the separation between donor atoms. In effect, perturbation cannot be effectively transmitted along the Fe1···Fe1 and Fe1···Fe2 linkage directions because the “elongation” of conformationally labile ligand molecules compensates for the shortening of the Fe2–N bond lengths.

Floating zone growth of large single crystals of [formula omitted

The Fe4+-containing cubic perovskite phase SrFeO3−δ is of interest both for its high-temperature, oxygen-conducting properties as a solid oxide fuel cell component, and at low temperatures, where it exhibits a plethora of helical magnetic phases and a candidate Skyrmion lattice. However, a sequence of structural phase transitions encountered on cooling to room temperature has limited the size of single crystals. We report the floating-zone growth and oxygen-annealing of multiple-cubic-centimetre-sized single crystals of SrFeO3−δ, suitable for inelastic neutron scattering and other measurement techniques requiring large sample volumes.

Phase stability and superconductivity of strontium under pressure

We have used the ab initio random structure searching method together with density functional theory calculations to find stable structures of strontium under pressures up to 50 GPa. We predict a sequence of structural phase transitions and the stability of an orthorhombic structure of Cmcm symmetry above 25 GPa. Our energy, lattice dynamics, and molecular dynamics calculations confirm the stability of the Cmcm structure. The electron-phonon coupling calculations show that superconductivity arises in the bcc structure of compressed Sr and that it continues to exist in the Cmcm structure. The calculated superconducting transition temperatures are in good agreement with experiment. Our study gives an excellent account of the experimental observations.

Temperature-dependent Raman spectra of Bi2Sn2O7 ceramics

The Bi2Sn2O7 pyrochlore is known to undergo a sequence of structural phase transitions with an increase in temperature. Raman spectroscopy was employed in the investigation of the temperature dependence of the active phonons in the Raman spectrum. We observed 19 broad modes at room temperature, reflecting the low symmetry of the α-phase of Bi2Sn2O7. The modes were discussed in relation to the Raman spectra of other pyrochlore-based oxides. The temperature dependence of the phonons evidences the α→β structural phase transition observed near 127°C.

Synchrotron x-ray spectroscopy studies of valence and magnetic state in europium metal to extreme pressures

In order to probe the changes in the valence state and magnetic properties of Eu metal under extreme pressure, x-ray absorption near-edge spectroscopy, x-ray magnetic circular dichroism, and synchrotron M\"ossbauer spectroscopy experiments were carried out. The M\"ossbauer isomer shift exhibits anomalous pressure dependence, passing through a maximum near 20 GPa. Density functional theory has been applied to give insight into the pressure-induced changes in both Eu's electronic structure and M\"ossbauer isomer shift. Contrary to previous reports, Eu is found to remain nearly divalent to the highest pressures reached (87 GPa) with magnetic order persisting to at least 50 GPa. These results should lead to a better understanding of the nature of the superconducting state found above 75 GPa and of the sequence of structural phase transitions observed to 92 GPa.

Structural and thermoelastic study of the protonic conducting perovskite SrCe0.95Yb0.05Oξ (ξ ∼ 3) between 373 K and 1273 K

The temperature dependence of the crystal structure of the protonic conducting perovskite SrCe0.95Yb0.05Oξ (ξ ∼ 3) has been determined from Rietveld refinement of neutron time-of-flight powder diffraction data in 25 K steps from 373 K to 1273 K. In contrast to most SrBIVO3 perovskites which show a sequence of structural phase transitions from Pmcn – Incn – I4/mcm - \( Pm\overline 3 m \) with increasing temperature, SrCe0.95Yb0.05Oξ remains orthorhombic with space group Pmcn from 4.2 K to the highest temperature measured. Crystallographic results are presented graphically in the form of the magnitude of the seven unique symmetry adapted basis vectors of a hypothetical aristotype perovskite phase. Thermoelastic properties of SrCe0.95Yb0.05Oξ deduced from the temperature variation of the unit cell volume and Debye-Waller factors are compared with literature values derived from independent calorimetric, Raman and elastic measurements. Thermodynamically, SrCe0.95Yb0.05Oξ behaves as a Debye-like solid with an average Gruneisen parameter in the range 1.4–1.5, but with two characteristic temperatures, ∼200 K and ∼600 K, the lower temperature being more related to the cation vibrations, the higher to the anion vibrations.

Phase separation and frustrated square lattice magnetism of Na1.5VOPO4F0.5

Crystal structure, electronic structure, and magnetic behavior of the spin-$\frac{1}{2}$ quantum magnet Na${}_{1.5}$VOPO${}_{4}$F${}_{0.5}$ are reported. The disorder of Na atoms leads to a sequence of structural phase transitions revealed by synchrotron x-ray powder diffraction and electron diffraction. The high-temperature second-order $\ensuremath{\alpha}\ensuremath{\leftrightarrow}\ensuremath{\beta}$ transition at 500 K is of the order-disorder type, whereas the low-temperature $\ensuremath{\beta}\ensuremath{\leftrightarrow}\ensuremath{\gamma}+{\ensuremath{\gamma}}^{\ensuremath{'}}$ transition around 250 K is of the first order and leads to a phase separation toward the polymorphs with long-range ($\ensuremath{\gamma}$) and short-range (${\ensuremath{\gamma}}^{\ensuremath{'}}$) order of Na. Despite the complex structural changes, the magnetic behavior of Na${}_{1.5}$VOPO${}_{4}$F${}_{0.5}$ probed by magnetic susceptibility, heat capacity, and electron spin resonance measurements is well described by the regular frustrated square lattice model of the high-temperature $\ensuremath{\alpha}$-polymorph. The averaged nearest-neighbor and next-nearest-neighbor couplings are ${\overline{J}}_{1}\ensuremath{\simeq}\ensuremath{-}3.7$ K and ${\overline{J}}_{2}\ensuremath{\simeq}6.6$ K, respectively. Nuclear magnetic resonance further reveals the long-range ordering at ${T}_{N}=2.6$ K in low magnetic fields. Although the experimental data are consistent with the simplified square-lattice description, band structure calculations suggest that the ordering of Na atoms introduces a large number of inequivalent exchange couplings that split the square lattice into plaquettes. Additionally, the direct connection between the vanadium polyhedra induces an unusually strong interlayer coupling having effect on the transition entropy and the transition anomaly in the specific heat. Peculiar features of the low-temperature crystal structure and the relation to isostructural materials suggest Na${}_{1.5}$VOPO${}_{4}$F${}_{0.5}$ as a parent compound for the experimental study of tetramerized square lattices as well as frustrated square lattices with different values of spin.

Phase stability and pressure-induced structural transitions at zero temperature inZnSiO3 andZn2SiO4

Using density functional total energy calculations the structural phasestability and pressure-induced structural transition in different polymorphs ofZnSiO3 andZn2SiO4 have been studied. Among the considered monoclinic phase with space groups(P 21/c) and(C 2/c), rhombohedral and orthorhombic (Pbca) modifications the monoclinic phase(P 21/c) ofZnSiO3 is found to be the most stable one. At high pressure monoclinicZnSiO3 (C 2/c) can co-exist with orthorhombic (Pbca) modification. Differences in equilibrium volumeand total energy of these two polymorphs are very small, which indicates thatit is relatively easier to transform between these two phases by temperature,pressure or chemical composition. It can also explain the experimentally establishedresult of metastability of the orthorhombic phase under all conditions. Thefollowing sequence of pressure-induced structural phase transitions is found forZnSiO3 polymorphs: monoclinic monoclinic rhombohedral . Among the rhombohedral (), tetragonal , orthorhombic (Pbca), orthorhombic (Imma), cubic and orthorhombic (Pbnm) modifications ofZn2SiO4, the rhombohedral phase is found to be the ground state. For this chemical composition ofzinc silicate the following sequence of structural phase transitions is found: rhombohedral tetragonal orthorhombic orthorhombic (Imma) cubic orthorhombic (Pbnm). Based on the analogy of crystal structures ofmagnesium and zinc silicates and using the lattice and positional parameters ofMg2SiO4 as input, structuralproperties of spinel Zn2SiO4 have also been studied.

Phase evolution and magnetic property of Bi 1 − xDy xFeO 3 ceramics

Rare-earth ion Dy was substituted at the bismuth site in BiFeO 3 to produce Bi 1 − x Dy x FeO 3 ( x = 0, 0.1, 0.2 and 0.3) polycrystalline ceramics. A two-stage solid-state reaction method was adopted in synthesizing the materials. The effects of varying the Dy doping concentration on the crystalline structures, morphologies and magnetic properties of the final products have been investigated. It is found that Dy doping resulted in a sequence of structural phase transitions and led to small grain sizes in the materials. As a result, the magnetic property of the doped samples was much enhanced. Therefore, it seems a promising way to improve the weak ferromagnetic property of BiFeO 3 based materials by Dy doping.

Phase transition and ferroelectric properties of epitaxially strained KNbO3/NaNbO3 superlattice

The influence of epitaxial strain on the crystal structure and polarization of KNbO3/NaNbO3 (KNO/NNO) superlattice has been quantified using density functional theory based on pseudopotential and plane-wave basis. A sequence of structural phase transitions with changing the in-plane misfit strain has been identified. If the compressive strain is more than −0.8%, tetragonal phase with polarization along [001] is stable. For the misfit strain between −0.8% and 0.36%, the stable phase is monoclinic while the stable phase becomes orthorhombic with polarization along [110] when the tensile strain is more than 0.36%. The spontaneous polarization in the orthorhombic phase reaches more than 70 μC/cm2.

Neutron Diffraction Study of Structural Intermediate Phase IV in TlCoCl3

The structural changes of TlCoCl 3 around phase IV (68 < T < 75 K) were studied by single-crystal neutron diffraction measurement. The present investigation revealed that a small but definite Bragg peak at (1/6, 1/6, 2) appeared only in phase IV. A precursor phenomenon of the phase V ( T < 68 K) structure in the phase IV temperatures was also identified as the coexistence of a slightly broadened (1/4, 1/4, 2) Bragg peak. We confirmed that intermediate phase IV is a stable, nontransient, single phase with \(2\sqrt{3}a \times 2\sqrt{3}a \times c\) unit cells. A possible sequence of structural phase transitions was proposed.

Phonon properties of protonic conductor SrZrO3 in cubic phase

Strontium zirconate has an orthorhombic structure at room temperature and undergoes a sequence of structural phase transition. SrZrO3 has a rather high melting temperature of about 2647 °C and consequently it is cubic in a wide range of temperature where most of its applications take place. In this work, we are reporting the results of our theoretical investigation on the phonon properties of SrZrO3 in its cubic phase by using lattice dynamical simulation method based on de Launey angular force (DAF) constant model to understand the role of phonon in this system. The calculated zone center frequencies agree well with available results. The phonon dispersion curves of SrZrO3 in cubic phase are also drawn. The present calculation gives rise to softening of acoustical mode at zone boundary along [qqq] direction. The calculated results are compared and analyzed with other results.

Piezoelectric properties and structural phase transitions of naturally polarized (Na0.75K0.25Bi)0.5TiO3 crystal

Electrical properties of a naturally polarized (Na0.75K0.25Bi)0.5TiO3 crystal have been investigated over a broad range of temperatures. Noticeable piezoelectric resonance signals have been observed up to 540°C. Based on the piezoelectric measurement, we suggest a R3c-P4bm-Pm3¯m sequence of structural phase transitions in this crystal. Moreover, at temperatures above 520°C, mobile sodium ions in the crystal are proposed to contribute to the electrical conduction; they have an activation energy of 2.06eV. This migration of Na+ at high temperatures can explain the naturally generated piezoelectricity in (Na0.75K0.25Bi)0.5TiO3 crystal.

Multiple rings in a 3D anisotropic Wigner crystal: Structural and dynamical properties

The structural and dynamical properties of small three-dimensional (3D) anisotropically confined Wigner crystals of particles interacting via a Coulombic interparticle potential are investigated. Varying the degree of anisotropy of the confinement potential drives the system from a 3D to a one-dimensional configuration intermediated by a sequence of first and second order structural phase transitions. We classified the ground state configurations with respect to their symmetry resulting in three different groups of configurations: multiple ring, degenerate multiple ring, and nonsymmetric structures. The results are summarized in a phase diagram where the different configurations are identified for systems ranging from $N=4$ to 25 particles. The behavior of the ground state symmetry of large systems was investigated for the cases of $N=50$, 60, and 70 particles. A normal mode analysis reveals that multiple ring structures exhibit inter-ring and∕or vortex∕antivortex excitation modes of oscillations.

Structural characterization, molecular dynamics, dielectric and spectroscopic properties of tetrakis(pyrazolium) bis(μ 2-bromo-tetrabromobismuthate(III)) dihydrate, [C 3N 2H 5] 4[Bi 2Br 10]·2H 2O

The structure of [C 3N 2H 5] 4[Bi 2Br 10]·2H 2O, (PBB) was determined by single crystal X-ray diffraction at 100 K. It crystallizes in the monoclinic space group C2/ m, with a = 12.992(4) Å, b = 16.326(5) Å, c = 8.255(3) Å, β = 108.56°(3), V = 1659.9(9) Å 3 and Z = 2. The structure consists of discrete binuclear [Bi 2Br 10] 4− anions, ordered pyrazolium cations and water molecules. The crystal packing is governed by strong N–H⋯O and weak O–H⋯Br hydrogen bonds. A sequence of structural phase transitions in PBB was established on the basis of differential scanning calorimetry and dilatometric studies. Two reversible first-order phase transitions were found: (I → II) at 381/371 K (on heating/cooling) and (II → III) at 348/338 K. Dielectric response near both phase transitions is characteristic of crystals with the “plastic-like” phases. Over the phase III a low frequency dielectric relaxator is disclosed. The possible molecular motions in the PBB compound are characterized by the 1H NMR studies. The infrared spectra of polycrystalline compound in the temperature range 300–380 K are reported for the region 4000–400 cm −1. The observed spectral changes through the structural phase transition III → II are attributed to an onset of motion both of the pyrazolium cations and water molecules.

Phase transitions in the oxyfluoride (NH4)3NbOF6

(NH4)3NbOF6 single crystals were grown, polarization-optical studies were performed, and birefringence was measured over the temperature range 90–500 K. A sequence of first-order structural phase transitions was found at temperatures T1↓ = 259.7 K and T2↓ = 257.7 K with temperature hysteresis δT1 = 0.9 K and δT2 = 1.9 K. The transitions are accompanied by twinning and the following change in the crystal symmetry: cubic ↔ tetragonal ↔ monoclinic. Optical second harmonic generation is found to occur at room temperature, which indicates that the cubic phase is not centrosymmetric. It is assumed that the phase transitions are ferroelastic and ferroelectric in nature.

Metal-insulator transition in [formula omitted] studied by X-ray scattering

The precise crystal structure of Y 1 - x Ca x TiO 3 ( x = 0.38 ) has been investigated by the single crystal X-ray scattering. We have demonstrated the sequence of structural phase transitions, i.e. the high-temperature orthorhombic structure to monoclinic one (Mono) at T mono and the Mono to the low-temperature orthorhombic structure (LTO) at the lower temperature. It provides a strong evidence that the structural phase transition, LTO-Mono, is responsible for the metal-insulator (MI) transition, though the phase separation of LTO and Mono was reported to be the origin of the MI transition. The pressure–temperature phase diagram of the crystal structure was also determined to elucidate the relation between the MI transition and the LTO-Mono phase transition.

Structural, dielectric and Raman scattering studies of (1− x )PbMg1/3Nb2/3O3– x PbTiO3 single crystals (0 ≤ x ≤ 0.38)

The X-ray powder diffraction, dielectric and Raman studies of the (1 − x) PbMg1/3Nb2/3O3–xPbTiO3 (0 ≤ x ≤ 0.38) single crystals were performed in a wide temperature range. The results of these complementary studies revealed that with the increase in PT content, the relaxor behaviour transforms into the ferroelectric one and a sequence of different structural phase transitions occur. On the base of the structural and dielectric results the phase diagram was determined. In addition, these results were used as a base for the group theory analysis and interpretation of the observed Raman spectra. The Raman spectra were deconvoluted using the independent damped oscillator model. From this model the temperature dependencies of the wavenumbers, reduced intensities and line widths were determined.