Structure Of Low-temperature Phase Research Articles

Muon-spin relaxation investigation of magnetic bistability in a crystalline organic radical compound

We present the results of a muon-spin relaxation (μ+SR) investigation of the crystalline organic radical compound 4-(2-benzimidazolyl)-1,2,3,5-dithiadiazolyl (HbimDTDA), in which we demonstrate the hysteretic magnetic switching of the system that takes place at T=(274 ± 11)K caused by a structural phase transition. Muon-site analysis using electronic structure calculations suggests a range of candidate muon stopping sites. The sites are numerous and similar in energy but, significantly, differ between the two structural phases of the material. Despite the difference in the sites, the muon remains a faithful probe of the transition, revealing a dynamically-fluctuating magnetically disordered state in the low-temperature structural phase. In contrast, in the high temperature phase the relaxation is caused by static nuclear moments, with rapid electronic dynamics being motionally narrowed from the muon spectra.

Structural Dynamics of Two-Dimensional Ruddlesden-Popper Perovskites: A Computational Study.

Recently two-dimensional (2D) hybrid organic–inorganic perovskites have attracted a lot of interest as more stable analogues of their three-dimensional counterparts for optoelectronic applications. However, a thorough understanding of the effect that this reduced dimensionality has on dynamical and structural behavior of individual parts of the perovskite is currently lacking. We have used molecular dynamics simulations to investigate the structure and dynamics of 2D Ruddlesden–Popper perovskite with the general formula BA2MAn–1PbnI3n+1, where BA is butylammonium, MA is methylammonium, and n is the number of lead–iodide layers. We discuss the dynamic behavior of both the inorganic and the organic part and compare between the different 2D structures. We show that the rigidness of the inorganic layer markedly increases with the number of lead–iodide layers and that low-temperature structural phase changes accompanied by tilting of the octahedra occurs in some but not all structures. Furthermore, the dynamic behavior of the MA ion is significantly affected by the number of inorganic layers, involving changes both in the reorientation times and in the occurrence of specific preferred orientations.

Biaxial strain effect induced electronic structure alternation and trimeron recombination in Fe3O4

The Verwey transition in Fe3O4 is the first metal-insulator transition caused by charge ordering. However, the physical mechanism and influence factors of Verwey transition are still debated. Herewith, the strain effects on the electronic structure of low-temperature phase (LTP) Fe3O4 with P2/c and Cc symmetries are investigated by first-principles calculations. LTP Fe3O4 with each space group has a critical strain. With P2/c, Fe3O4 is sensitive to the compressive strain, but it is sensitive to tensile strain for Cc. In the critical region, the band gap of LTP Fe3O4 with both two symmetries linearly increases with strain. When strain exceeds the critical value, DOS of spin-down t2g electron at Fe(B4) with P2/c and Fe(B42) with Cc changes between dx2-y2 and dxz + dyz. The trimerons appear in Cc can be affected by strain. With a compressive strain, the correlation of trimeron along x and y axes is strengthened, but broken along the face diagonal of FeB4O4, which is opposite at the tensile strains. The results suggest that the electronic structure of Fe3O4 is tunable by strain. The narrower or wider band gap implies a lower or higher transition temperature than its bulk without strains, which also gives a glimpse of the origin of charge-orbital ordering in Fe3O4.

Tunable luminescence and energy transfer in Ca3SiO4Cl2:Ce3+,Li+,Mn2+,Eu2+ phosphors

A series of color-tunable Ca3-2x-y-z SiO4 Cl2 (CSC):xCe3+ ,xLi+ ,yMn2+ ,zEu2+ phosphors with low temperature phase structure was synthesized via the sol-gel method. An energy transfer process from Ce3+ to Mn2+ in CSC:0.01Ce3+ ,0.01Li+ ,yMn2+ (y=0.03-0.09) and the mechanism was verified to be an electric dipole-dipole interaction. The Ce3+ and Mn2+ emission intensities were greatly enhanced by co-doping Eu2+ ions into CSC:0.01Ce3+ ,0.01Li+ ,0.07Mn2+ phosphors due to competitive energy transfers from Eu2+ /Ce3+ to Mn2+ , and Ce3+ to Eu2+ . Under 332nm excitation, CSC:0.01Ce3+ ,0.01Li+,0.07Mn2+ ,zEu2+ (z=0.0005-0.002) exhibited tunable emission colors from green to white with coexisting orange, green and violet-blue emissions. These phosphors could have potential application in white light-emitting diodes.

Phase transition of [2,2]-paracyclophane--an end to an apparently endless story.

In this contribution, the solid-state low-temperature phase structure of [2,2]-paracyclophane is unambiguously characterised by single-crystal X-ray analysis. Additionally, a heat capacity measurement was undertaken, which proves the existence of a λ-type phase transition at 45 K, a transition that is connected with the formation of a secondary Cp/T feature at 60 K. The low-temperature phase (<45 K) crystallises in the lower symmetry space group P4n2, whereas the high-temperature phase (>60 K) crystallises in space group P4(2)/mnm. This proves what has been postulated both by experimental and theoretical chemists but has repeatedly been dismissed as speculation many times.

Well-Defined Crystal Field Splitting Schemes and Non-Kramers Doublet Ground States of f Electrons in PrT2Zn20 (T = Ir, Rh, and Ru)

Inelastic neutron scattering studies of Pr\(T_{2}\)Zn20 (\(T\) = Ir, Rh, and Ru) were carried out in order to reveal \(f\) electron states indispensable for the low-temperature properties of Pr\(T_{2}\)Zn20 associated with multipole degrees of freedom. Sharp excitation peaks, which are located within excitation energies of less than 10 meV, are attributed to crystal field splitting level schemes of \(4f^{2}\) in Pr3+ ions. Data of the Ir-based compound is explained by a non-Kramers doublet ground state in the local symmetry \(T_{d}\) for this system. Results of the Rh-based compound are explained by the scheme for the Pr-site local cubic symmetry \(T\) in the low-temperature structural phase, which is lower than \(T_{d}\). In the Ru-based compound, the doublet is split as a result of the loss of local cubic symmetry through structural transformation. These experimental results confirm the non-Kramers doublet ground state that causes the quadrupole ordering or orbital Kondo effect predicted for this series...

Melting and Low-Temperature Structures of Mixed Ar–Kr Monolayer Films on Graphite

Using Monte Carlo simulation methods in the canonical and grand canonical ensembles, we study the melting and the structure of low-temperature phases of mixed Ar–Kr submonolayer films on graphite. It is shown that such films exhibit a complete mixing in the liquid phase and freeze into a mixed solid phase, independently of the composition. The structure of the solid phase, however, depends upon the film composition, its total density, and the temperature. For submonolayer coverages, when the mole fraction of Kr is lower than about 0.1, the mixture freezes into the incommensurate, argon-like phase. For the higher mole fractions of Kr, the freezing leads to the formation of a mixed commensurate (√3 × √3)R30° phase. The lowering of temperature leads to structural phase transitions in the solid. When the krypton mole fraction is lower than about 0.88, the transition leads to the formation of domain-wall structures, in which the commensurate domains are made of krypton atoms, while the composition of walls depends upon the Kr mole fraction. It is shown that even rather small concentrations of argon atoms can trigger the commensurate–incommensurate transition. For still higher Kr mole fractions, exceeding about 0.88, the commensurate, krypton-like, solid phase is stable at any temperatures below the melting point. At sufficiently low temperatures, the phase separation takes place and argon atoms are removed from the film interior to the peripheries of submonolayer. In the case of films with the total density close to the monolayer completion, the commensurate structure shows much higher stability. It is demonstrated, however, that it is an artifact of the simulation methods used and, in particular, of the periodic boundary conditions applied, rather than a real phenomenon. It is also demonstrated that the phase diagram topology of monolayer films changes with the film composition. In particular, the vapor–liquid critical point appears only when krypton concentration is lower than about 0.45.

Symmetry mode analysis of the phase transitions in Rb2ZnBr4

Abstract Crystal structures of low temperature phases III, IV and V of Rubidium Zinc Bromide have been determined by single crystal X-ray diffraction at 90 K, 85 K, and 30 K, respectively. The analysis of the structures in terms of symmetry-adapted modes of the high temperature prototype phase permits the identification of the modes responsible for each phase and to assess its thermal evolution. The primary mode responsible for the incommensurate and lock-in phases remains dominant throughout the whole transition sequence, maintaining its structure and slightly increasing its amplitude as temperature decreases. The two monoclinic phases IV and V are presumably due to the instability of a second mode associated with a single active irreducible representation. However, the presence with similar weights of several symmetry-adapted distortions of different symmetry indicates a strong hybridization in phase III of the softer normal modes. A symmetry-mode description of the structures of phase IV and V fully consistent with a simple Landau model, can then only be obtained if phase III is taken as effective parent phase. Besides the insight on the microscopic origin of the different phases, the symmetry modes were used as crystallographic coordinates in the refinement of the structure of phase V. The result shows the advantages of using these non-conventional collective structural parameters instead of individual atomic coordinates. The refinable parameters can be reduced to the amplitudes of those symmetry modes with significant weight in the distortions.

Quantum-chemical modeling of ferroelectric solids with hydrogen-bond networks of different dimensionalities and their deuterated analogs

The application of quantum chemistry methods in the microscopic theory of H/D-bonded ferroelectrics and antiferroelectrics is illustrated using a comparative analysis of low-temperature order-disorder structural phase transitions in zero-dimensional materials of the K3H(SO4)2 family, zero-dimensional crystals of 5-halo derivatives of 9-hydroxyphenalenone (5Hal-9HPO), and three-dimensional crystals of potassium dihydrogen phosphate (KDP) and potassium dideuterium phosphate (DKDP) as examples. In the framework of the Ising model with tunneling, it is demonstrated that (in agreement with experimental data) the transition to the low-temperature ordered phase in zero-dimensional materials is possible only in the case of their deuteration, whereas the quantum paraelectric behavior is characteristic of undeuterated samples. This behavior for KDP crystals is impossible due to the sharp increase in the Ising parameters as compared to zero-dimensional materials. The factors responsible for the increase in these parameters are considered.

Optimal packings of superballs

Dense hard-particle packings are intimately related to the structure of low-temperature phases of matter and are useful models of heterogeneous materials and granular media. Most studies of the densest packings in three dimensions have considered spherical shapes, and it is only more recently that nonspherical shapes (e.g., ellipsoids) have been investigated. Superballs (whose shapes are defined by |x1|2p+|x2|2p+|x3|2p<or=1) provide a versatile family of convex particles (p>or=0.5) with both cubic-like and octahedral-like shapes as well as concave particles (0<p<0.5) with octahedral-like shapes. In this paper, we provide analytical constructions for the densest known superball packings for all convex and concave cases. The candidate maximally dense packings are certain families of Bravais lattice packings (in which each particle has 12 contacting neighbors) possessing the global symmetries that are consistent with certain symmetries of a superball. We also provide strong evidence that our packings for convex superballs (p>or=0.5) are most likely the optimal ones. The maximal packing density as a function of p is nonanalytic at the sphere point (p=1) and increases dramatically as p moves away from unity. Two more nontrivial nonanalytic behaviors occur at pc*=1.150 9... and po*=ln 3/ln 4=0.792 4... for "cubic" and "octahedral" superballs, respectively, where different Bravais lattice packings possess the same densities. The packing characteristics determined by the broken rotational symmetry of superballs are similar to but richer than their two-dimensional "superdisk" counterparts [Y. Jiao, Phys. Rev. Lett. 100, 245504 (2008)] and are distinctly different from that of ellipsoid packings. Our candidate optimal superball packings provide a starting point to quantify the equilibrium phase behavior of superball systems, which should deepen our understanding of the statistical thermodynamics of nonspherical-particle systems.

Crystal structure of three Ni-Mn-Ga alloys in powder and bulk materials

In NiMnGa alloy the crystal structure of low temperature phase is strongly related to transformation temperature. In the present study three NiMnGa alloys with different transformation temperatures were studied. Alloy A has the martensitic start temperature (M s ) near ambient temperature and M s of Alloy B is near Curie point (T C ) whereas M s of Alloy C M s is well above T C . The crystal structures of three alloys were studied by X-ray diffraction in powder and bulk materials. The low temperature phase of single crystal in Alloy A was tetragonal phase with c a, and in Alloy C such a phase was tetragonal phase with c > a. The crystal structure of Alloy A in bulk material is in good agreement with powder diffraction, but in Alloys B and C there is a lack of conformity between bulk and powder. Furthermore, the difference of crystal structure in powder and bulk is discussed in more detail.

Study on the in-plane electrical resistivity and thermoelectric power in single crystals of La 2- x Ba x CuO 4

The in-plane electrical resistivity and thermoelectric power have been measured on single crystals of La2-xBaxCuO4 at around x=0.125. The room temperature resistivity and thermopower have their maximum values at x=0.125, indicating that the carrier concentration is the minimum and the carriers are most strongly localized at x=0.125. The observed semiconductor-like behaviour can be well described by the weak-localized quasi-two-dimensional state. The steep rise in electric resistivity of the sample at x=0.125 below 70K is attributed to the formation of static stripe-order of holes and spins, which are pinned by the low-temperature tetragonal (LTT) structure, as discovered in La1.48Nd0.4Sr0.12CuO4. The temperature dependence of electric resistivity below 70K is still well described by the formula ρ∝ lnT. A definite change in the slope of thermopower is observed at the low-temperature orthorhombic-LTT structural phase transition temperature. The origin of the 1/8 anomaly is discussed in the text.

Order-disorder transitions and structural relaxation phenomena in crystals with hydrogen bonds

This work is devoted to the study of structural relaxation in superionic crystals with hydrogen bonds at the recovery of the low temperature phase structure after fast cooling from the superionic phase. The results of experimental investigations of this phenomenon in (NH4)4H2(SeO4)3 crystal by means of Raman spectroscopy are given. It follows that superitonc-ferroelastic transition is connected with the transformation from delocalization of protons to their localization and that the creation of the ordered state of protons with definite space symmetry takes place after their localization. To explain this effect the microscopic approach, based on the proton ordering model and taking into account thermally activated proton hopping and quasi-polaron effect resulting from the interaction of protons with the optical phonons, is proposed.

The Ordered Phase of p-Chloronitrobenzene Studied by Raman Spectroscopy

p-Chloronitrobenzene exhibit an order-disorder phase transition at 282 K. The high temperature disordered phase has P2 1 /c symmetry with two molecules per unit cell. A statistically centrosymmetric array results from the superposition of molecules occupying regular lattice sites but facing randomly opposite directions. The ordered low temperature phase structure could not be resolved yet. In the present work, Raman spectroscopy measurements were carried out as a function of temperature in the range 5 K–300 K in ordered and disordered phases of p-chloronitrobenzene. Symmetry information of the ordered unknown phase was obtained. A comparison of experimental results with Raman spectra of p-dihalobenzenes and p-dinitrobenzene has been carried out and possible molecular arrangements are discussed through lattice dynamics calculations.

Dynamics of phase stability and magnetic order in magnetoresistiveLa0.83Sr0.17Mn0.9857Fe0.02O3

Two samples with the same nominal composition La{sub 0.83}Sr{sub 0.17}Mn{sub 0.98}{sup 57}Fe{sub 0.02}O{sub 3} were prepared in air and argon. Thermograviametric, structural, resistive, and magnetic measurements show that the air-made sample has slightly higher Mn{sup 4+}/Mn{sup 3+} ratio; i.e., it is slightly more highly doped by {approximately}0.15{percent} cation vacancies. Substitution of Fe for Mn depresses the ferromagnetic transition temperature T{sub c} but has very small effect on the rhombohedral/orthorhombic structural phase boundary. Dramatic differences are found in transport properties for small differences in Mn{sup 4+} concentration near a structural and magnetic phase boundary. The dynamics of charge hopping and exchange via the effect of relaxation of the magnetic hyperfine field distribution and its average value are monitored by temperature-dependent Mossbauer effect measurements. Ferromagnetic clustering is observed above T{sub c} at {sup 57}Fe sites. The compound undergoes complex temperature dependent magnetic and phase behaviors that are monitored locally by the {sup 57}Fe Mossbauer effect. The iron probe has a stable 3+ valance configuration whose hyperfine interaction reflects the structural phase changes, the Jahn-Teller distortion and the time-dependent Mn{sup 3+}-Mn{sup 4+} charge fluctuations. It is shown that regions of magnetic order exist over {approximately}100 K range beginning well above T{sub c}. Moreover,more » the low-temperature structural phase, reported to be orthorhombic at this composition, shows dynamic magnetic fluctuation behavior that motionally narrows at low temperature. Any changes in hyperfine interaction due to the structural transition are masked by the fluctuating magnetic behavior, but clearly the two structures coexist in the temperature region near T{sub c}. {copyright} {ital 1998} {ital The American Physical Society}« less

Stability of the bcc phase of Cu-Al-Mn shape-memory alloys

Measurements of the entropy change at the martensitic transition of two composition-related sets of Cu-Al-Mn shape-memory alloys are reported. It is found that most of the entropy change has a vibrational origin, and depends only on the particular close-packed structure of the low-temperature phase. Using data from the literature for other Cu-based alloys, this result is shown to be general. In addition, it is shown that the martensitic structure changes from $18R$ to $2H$ when the ratio of conduction electrons per atom reaches the same value as the eutectoid point in the equilibrium phase diagram. This finding indicates that the structure of the metastable low-temperature phase is reminiscent of the equilibrium structure.

Low critical temperature structural phase transitions induced by magnetic fields

Low critical temperature structural phase transitions induced by magnetic fields

Structure of low temperature phase of isolated hydrogen bond system K3D(SO4)2

Structure of low temperature phase of isolated hydrogen bond system K₃D(SO₄)₂

161Dy Mössbauer spectroscopic study on magnetic properties of pseudo-ternary DyMn 2(Si 1− xGe x) 2 (0 < x < 1) compounds

161Dy Mössbauer effect in DyMn 2(Si 1− x Ge x ) 2 (0 < x < 1) has been measured at temperatures between 4.2 and 45 K. All the compounds which have antiferromagnetic ordering of Mn moments with T N = 430−523 K transform it furthermore into other magnetic structures accompanied by ordering of Dy moments at low temperatures of 25–37 K. Magnitude of the magnetic hyperfine field and its temperature dependence have clarified that the magnetic structure of low-temperature phase is canted-ferrimagnetic in the compounds with 0.1 ≤ x ≤ 0.9, where the Mn moments tilt about 60° from the c-axis. The compounds with x = 0.8 and 0.9 have an intermediate-temperature phase between 26 and 37 K. In this phase, the Dy ions have two kinds of magnetic state, which are observed as a static Mössbauer subspectrum and a relaxation one, respectively. It is concluded that the magnetic structure of the intermediate-temperature phase is a collinear ferrimagnetic one with a long periodic arrangement. This structure can be described as a stacking of Mn moment layers along the c-axis with a + + - + + - sequence.

Low-temperature phase structure of the T ∗-phase compound (La, Tb, Pb) 2CuO 4

The low temperature structure of the T ∗-phase compound (La, Tb, Pb) 2CuO 4 was determined by single crystal x-ray diffraction at 50K. It is characterized by order canting of the CuO 5 pyramids and corrugation of the CuO 2 planes. The existence of two Cu sites with different valences in the CuO 2 planes induces charge ordering which is determental for superconductivity.