First-principles comparative study of multiferroic compound PbVO3

We report a comprehensive first-principles investigation of the structural, electronic, magnetic and phase transition properties in multiferroic compound PbVO₃ with systematic comparisons of various exchange-correlation (XC) functionals. The antiferromagnetic (AFM) insulating ground state of tetragonal phase has been obtained in the framework of the band theory, which is characterized by C-type two-dimensional AFM magnetic ordering in the ab plane. A first-order structural transformation from tetragonal phase to idea cubic perovskite structure takes place at 1.75 GPa, corresponding to the ferroelectric to paraelectric phase transition. Electronic structure calculations suggest that the ground state of the cubic paraelectric phase is a nonmagnetic orbital-disorder metal.     

The domination of ionic conductivity in tetragonal phase of the organometal halide perovskite CH3NH3PbI3-xClx

Organometal trihalide perovskites have recently gained extreme attention due to their high solar energy conversion in photovoltaic cells. Here, we investigate the contribution of iodide ions to a total conductivity of the mixed lead halide perovskite CH₃NH₃PbI_3−xCl_x with a use of the modified DC Hebb–Wagner polarization method. It has been identified that an ionic conductivity dominates in tetragonal phase which is associated with room temperature. The obtained activation energy for this type of hopping mechanism is equal to (0.87 ± 0.02) eV, which is in a good agreement with previous literature reports. The high contribution of ionic conductivity at room temperature might be a reason of the observed hysteresis in halide perovskite solar cells.     

Jahn–Teller distortion in perovskite KCuF3 under high pressure

Single crystals of KCuF₃ have been grown by the solution method. We report neutron diffraction under high pressure up to 8 GPa on a powder sample pulverized from KCuF₃ crystals. The type-A spin ordering structure found in KCuF₃ can be well-explained based on the orbital ordering resolved from this structural study. In comparison with perovskite oxides, the fluoride exhibits a much reduced bulk modulus. Our results also reveal the change of local structural distortion as a function of pressure; the magnitude of Jahn–Teller distortion at ambient condition is dramatically suppressed under high pressure. Extensive comparison of the pressure dependence of the Jahn–Teller perovskite fluoride with that of analogous perovskite oxides has also been made.     

Cooperative Jahn-Teller distortion, phase transitions, and weak ferromagnetism in the KCrF3 perovskite

At ambient conditions, the ternary fluoride with formula KCrF3 adopts a perovskite-type structure and incorporates the Jahn-Teller active Cr2+ (d4) whose electronic configuration and magnetic response are analogous to those of Mn3+ (d4) and Cu2+ (d9). These ingredients make it an attractive system to study owing to the striking similarities with LaMnO3 and the expected strong interplay between spin, orbital, and structural ordering phenomena. Indeed, probing the properties of KCrF3 as a function of temperature (5 < T < 300 K) has revealed a structurally and magnetically far richer phase diagram than hitherto supposed. We found that KCrF3 exhibits large cooperative Jahn-Teller distortions which are driven by orbital ordering, a series of temperature induced complex structural transitions, and weak ferromagnetism which is reminiscent of what is observed in LaMnO3.     

Magnetic interactions in ternary cobalt oxides with cubic perovskite structure

Magnetic properties were measured on the cubic perovskite systems SrCoO_3?δ, (La_1?xSr_x)CoO₃ (0.5 ≦ x ≦ 1.0), and Sr(Co_1?xMn_x)O₃ (0 ≦ x ≦ 1.0). It is found that S²⁺ and La³⁺ ions strongly affect the spin state of the Co²⁺ ion and that the Mn⁴⁺ ion located at the octahedral site affects the spin state of Co⁴⁺ ion. The magnetic properties (T_c, T_θ, and σ) are explained by the magnetic interaction Co³⁺OCo³⁺, Co³⁺OCo⁴⁺, Co⁴⁺OCo⁴⁺, Mn⁴⁺OMn⁴⁺, and Mn⁴⁺OCo⁴⁺ in these systems.     

GdAlO3 perovskite

Flux‐grown gadolinium aluminate perovskite, GdAlO₃, was examined using single‐crystal 0.7 Å‐wavelength synchrotron X‐ray diffraction. In the context of other well categorized rare earth aluminate (RAlO₃) perovskite phases, the orthorhombic Pnma symmetry determined for the current compound is unsurprising. Corner‐linked AlO₆ octahedra form the structural backbone of RAlO₃ perovskites and distort to accommodate the various rare earth ions in the structural voids. For GdAlO₃, the octahedral distortion, characterized by tilting of the octahedra about the shortest R—Al—R vectors, and octahedral deformation, characterized by strain of the octahedra along those axes, are in accordance with trends in the RAlO₃ series.     

Magnetic structure and orbital ordering in tetragonal and monoclinic KCrF(3) 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.     

Synthesis and spectroscopic characterization of copper(II) tetraazaiminooxime macrocyclic complexes--a tetragonal distortion analysis

Herein we describe the synthesis and spectroscopic (infrared and UV-vis) analysis of [Cu(II)(dohpn)(L)](n+) (dohpn=imineoximic tetraazamacrocyclic ligand 2,3,9,10-tetramethyl-1,4,8,11-tetraazaundecane-1,3,8,10-tetraen-11-ol-1-olate) and L=SCN(-), I(-), Cl(-) (n=0) and 4-aminopyridine (ampy), 4,4'-bipyridine (bipy), imidazole (im), 2-aminopyrazine (ampz) and water (n=1+). The following order of the Jahn-Teller stabilization energy (cm(-1)) was observed: I(-)(6452)相似文献   

Molecular nanoclusters as magnetic refrigerants: The case of Fe14 with very large spin ground-state

We report on the magnetic and thermal properties of the Fe₁₄ molecular nanocluster. We find a huge magnetocaloric response in the temperature range below 10 K. This is to large extent caused by its very large spin ground-state combined with an excess of entropy arising from the admixture of low-lying excited S states. We also show that the high degree of symmetry of the Fe₁₄ cluster core, resulting in a very small cluster magnetic anisotropy, enables the occurrence of long-range antiferromagnetic order below T_N = 1.87 K.     

Magnetic properties of linear trimers in fluoride analogs of tetragonal tungsten bronze

The compounds KZnTiF₆, KZnVF₆, KVScF₆, KCrScF₆, and KMnScF₆ are fluoride analogs of Tetragonal Tungsten Bronze. M²⁺-M³⁺ ionic ordering in these fluorides provided systems which contained linear trinuclear complexes of their respective paramagnetic ions. Magnetic coupling within these linear trimers occurred below 100 K in each of the five systems. Derived magnetic susceptibility equations were fitted to observed magnetic susceptibilities for each of the possible spin systems: KZnTiF₆ (S=1/2), J/k=−114 K; KZnVF₆ (S=1), J/k=−39 K; KVScF₆ (S=3/2), J/k=−16 K; KCrScF₆ (S=2), J/k=−4 K; and KMnScF₆ (S=5/2), J/k=−7.5 K.     

Generalized energy-based fragmentation approach for computing the ground-state energies and properties of large molecules

We present a generalized energy-based fragmentation (GEBF) approach for approximately predicting the ground-state energies and molecular properties of large molecules, especially those charged and polar molecules. In this approach, the total energy (or properties) of a large molecule can be approximately obtained from energy (or properties) calculations on various small subsystems, each of which is constructed to contain a certain fragment and its local surroundings within a given distance. In the quantum chemistry calculation of a given subsystem, those distant atoms (outside this subsystem) are modeled as background point charges at the corresponding nuclear centers. This treatment allows long-range electrostatic interaction and polarization effects between distant fragments to be taken into account approximately, which are very important for polar and charged molecules. We also propose a new fragmentation scheme for constructing subsystems. Our test calculations at the Hartree-Fock and second-order M?ller-Plesser perturbation theory levels demonstrate that the approach could yield satisfactory ground-state energies, the dipole moments, and static polarizabilities for polar and charged molecules such as water clusters and proteins.     

An efficient fragment-based approach for predicting the ground-state energies and structures of large molecules

An efficient fragment-based approach for predicting the ground-state energies and structures of large molecules at the Hartree-Fock (HF) and post-HF levels is described. The physical foundation of this approach is attributed to the "quantum locality" of the electron correlation energy and the HF total energy, which is revealed by a new energy decomposition analysis of the HF total energy proposed in this work. This approach is based on the molecular fractionation with conjugated caps (MFCC) scheme (Zhang, D. W.; Zhang, J. Z. H. J. Chem. Phys. 2003, 119, 3599), by which a macromolecule is partitioned into various capped fragments and conjugated caps formed by two adjacent caps. We find that the MFCC scheme, if corrected by the interaction between non-neighboring fragments, can be used to predict the total energy of large molecules only from energy calculations on a series of small subsystems. The approach, named as energy-corrected MFCC (EC-MFCC), computationally achieves linear scaling with the molecular size. Our test calculations on a broad range of medium- and large molecules demonstrate that this approach is able to reproduce the conventional HF and second-order Moller-Plesset perturbation theory (MP2) energies within a few millihartree in most cases. With the EC-MFCC optimization algorithm described in this work, we have obtained the optimized structures of long oligomers of trans-polyacetylene and BN nanotubes with up to about 400 atoms, which are beyond the reach of traditional computational methods. In addition, the EC-MFCC approach is also applied to estimate the heats of formation for a series of organic compounds. This approach provides an appealing approach alternative to the traditional additivity rules based on either bond or group contributions for the estimation of thermochemical properties.     

g factors and the tetragonal distortion of ligand octahedron for Co2 + in Rb2MgF4 crystal

The calculation formulas of g-factors g(parallel) and g(perpendicular) for 3d7 ion in tetragonal octahedral crystals are established from a cluster approach. In these formulas, the parameters related to covalency effect, configuration interaction and low-symmetry crystal field can be determined from the optical spectra and the structural data of the studied system. Based on these formulas, the structural parameters of ligand octahedra of Co2+ in Rb2MgF4 crystal are obtained by fitting the calculated g(parallel) and g(perpendicular) to the observed values. The result suggests that the CoF6 (and hence MgF6) octahedra in Rb2MgF4:Co2+ are tetragonal compressed. The relationship between the sign of deltag( = g(perpendicular) - g(parallel)) and the sign of distortion (elongated or compressed) of ligand octahedron and the causes of the mistakes of octahedron distortion for Rb2MgF4:Co2+ in the previous papers are discussed.     

Antiferromagnetism of perovskite EuZrO3

Polycrystalline EuZrO₃ has been synthesized by the solid-state reaction between EuO and ZrO₂, and its structural and magnetic properties have been investigated. Rietveld analysis of the X-ray diffraction pattern indicates that EuZrO₃ crystallizes in an orthorhombic perovskite structure. ¹⁵¹Eu Mössbauer effect measurement reveals that almost all the europium ions are present as the divalent state and occupy distorted sites with non-axial electric field gradients, in agreement with the orthorhombic structure. In contrast to previous reports, an antiferromagnetic transition was observed around 4.1 K. The magnetic structure below the Néel temperature has been discussed.     

Phonon behavior of CaSnO3 perovskite under pressure

Raman scattering and x-ray diffraction studies of CaSnO(3) perovskite were performed under high-pressure conditions. This high-pressure study was motivated by a recent theoretical study predicting a phase transition in CaSnO(3) from GdFeO(3)-type perovskite to CaIrO(3)-type structure occurred at 12 GPa. Despite no obvious structure change up to a pressure of 26 GPa based on the x-ray diffraction data, high pressure Raman measurements revealed that some Raman modes disappeared upon compression; either merging into neighboring bands or vanishing. The signals for these Raman peaks were recovered during decompression. The measured pressure derivative of Raman shift (?ν∕?P) of CaSnO(3) ranged from ~1.29 to ~4.35, up to 20 GPa. Due to the lack of lattice dynamic study for CaSnO(3) perovskite, the mode symmetry for CaSnO(3) was tentatively assigned based on the empirical relation among Ca-bearing perovskites. The pressure derivative of the Raman shifts was found to be related to their mode vibrations: modes related to Ca and O shifts had a strong pressure dependence compared with those associated with oxygen octahedral rotation.