Magnetic-field switching of crystal structure in an orbital-spin-coupled system: MnV2O4

We studied the magnetic and structural properties of spinel MnV2O4, which has S=5/2 spin with no orbital degrees of freedom on the Mn2+ site and S=1 spin and three orbital degrees of freedom on the V3+ site. We found that the ferrimagnetic ordering at TN=56.5K and the structural phase transition at Ts=53.5K are closely correlated in this compound and found a switching of crystal structure between cubic and tetragonal phases by the magnetic field. This phenomenon can be explained by the coupling between orbital and spin degrees of freedom in the t2g states of the V site.     

KCrF3中的轨道有序及其成因

强关联体系中的轨道有序及其成因一直是凝聚态物理研究的热点问题.轨道有序对于巨磁阻和 超导材料的研究有非常重要的地位.利用第一性原理计算研究了KCrF₃的四方相和立方相中的轨道有序 及其成因.在四方相中, GGA和GGA+U两种方法计算结果都表明其基态是A型反铁磁和G型轨道有序. 对于立方结构, GGA方法得出铁磁半金属态是基态,而GGA+U(U_eff = 3.0 eV)得到的基态是A型 反铁磁绝缘体. 光电导测量是少数能从实验上观察到轨道有序的方法之一,因此计算了其光电导,并结合投影态密度讨论 了KCrF₃中的轨道有序.最后找到了其轨道有序的成因:电子强关联效应,而非电-声子相互作用是其 轨道 有序的物理根源.     

Evidence for electronic phase separation between orbital orderings in SmVO3

We report evidence for phase coexistence of orbital orderings of different symmetry in SmVO3 by high resolution x-ray powder diffraction. The phase coexistence is triggered by an antiferromagnetic ordering of the vanadium spins near 130 K, below an initial orbital ordering near 200 K. The phase coexistence is the result of the intermediate ionic size of samarium coupled to exchange striction at the vanadium spin ordering.     

Raman Spectroscopy Studies of Nanocrystalline Lead Zirconate Titanate as Functions of Particle Size and Pressure

The Raman studies of lead zirconate titanate with varied grain size from 27.5 nm to 983 nm were performed under pressure up to 32 GPa to elucidate the scenario of phase transition pressures of lead zirconate titanate, which lies in the morphotropic phase boundary. The coexistence of ferroelectric rhombohedral and tetragonal phases at ambient condition changed to the coexistence of tetragonal and cubic phases at intermediate pressure and finally, transited to paraelectric cubic phase at elevated pressure. The pressure evolution of Raman spectra results indicated that the phase transition pressure increased with the reduction of the particle size.     

91Zr NMR characterisation of phases in transformation toughened zirconia.

A detailed 91Zr NMR investigation is presented of the five component (cubic, monoclinic, tetragonal, orthorhombic and delta) phase mixture in the transformation toughened engineering ceramic, magnesia-partially-stabilised zirconia (MgPSZ). The phases are distinguished on the basis of the quadrupole interaction displayed in the powder pattern of the 91Zr(1/2, -1/2) transition. This paper reports: (i) the first characterisation of the magnesia-fully-stabilised cubic phase at the eutectic composition (13.5 mol% MgO); (ii) the observation of a poorly ordered tetragonal phase on fast cooling ZrO2 (9.3 mol% MgO) from the cubic phase field at 1720 degrees C, and the subsequent growth and ordering of the tetragonal phase precipitates due to further annealing; (iii) the observation of the (partial) transformation of the cubic phase to the ordered delta-phase (Mg2Zr5O12) on annealing MgPSZ at 1100 degrees C for 8 h; and (iv) the observation of the transformation of the tetragonal phase into the orthorhombic phase after cooling in liquid nitrogen, and the reverse transformation after heating to 600 degrees C.     

Novel orbital ordering induced by anisotropic stress in a manganite thin film

A novel structure of orbital ordering is found in a Nd0.5Sr0.5MnO3 thin film, which exhibits a clear first-order transition, by synchrotron x-ray diffraction measurements. Lattice parameters vary drastically at the metal-insulator transition at 170 K (= T(MI)), and superlattice reflections appear below 140 K (= T(CO)). The electronic structure between T(MI) and T(CO) is identified as A-type antiferromagnetic with a d(x2-y2) ferro-orbital ordering. The new type of antiferro-orbital ordering characterized by the wave vector (1/4 1/4 1/2) in cubic notation emerges below T(CO). The accommodation of the large lattice distortion at the first-order phase transition and the appearance of the novel orbital ordering are brought about by the anisotropy in the substrate, a new parameter for the phase control.     

Structural and magnetic phase transitions in rare-earth-cadmium equiatomic compounds

The tendency to structural instability and the nature of the magnetic ordering are investigated in all the cubic rare-earth-cadmium equiatomic compounds from measurements of resistivity and magnetic susceptibility. The CsCl-type structure is stable at room temperature in all the compounds. However, LaCd exhibits a lattice change at 61 K, while CeCd and PrCd undergo two transitions at 107 and 216 K, and 125 and 190 K, respectively. The low-temperature phases are unknown, but seem to have a symmetry lower than tetragonal. Other compounds are cubic at least in their paramagnetic phase. In connection with the change in the lattice symmetry, a change of the magnetic ordering is observed from ferromagnetism towards antiferromagnetism. Among the heavy rare-earth compounds, cubic thus ferromagnetic, DyCd plays a peculiar role since it undergoes a structural transition in its ordered range, the magnetoelastically stressed lattice becoming unstable again. The strength of bilinear interactions and the occurrence of quadrupolar pair coupling are then discussed.     

Anomalous Optical and Electronic Properties of CaTiO3 Perovskites

With the help of the first-principles full potential linearized augmented plane wave method, absorption coefficients, reflectivity, dielectric behavior and electronic properties, including electronic energy bands, density of states and charge density distributions, are studied for the tetragonal and cubic CaTiO3. By considering the thermal expansion effects, an approximate method is proposed for the study of the stability of ground state and a tendency of phase transition, based on the minimum free energy principle. Subsequently, numerical calculations are carried out by using the first-principles perturbation method. We demonstrate that the high-temperature phase is cubic. It is shown that optical spectra in tetragonal phase exhibit single-peak feature and differ from multi-peak character in cubic. We find that strong orbital hybridization results in the co-valent bonds between Ti 3d and O 2p electrons and forms two-type dipoles (Ti-O1 and Ti-O2) in tetragonal, while the Ti-O dipoles are identical in cubic. It is argued that crystal structure determines the dipole distributions and leads to some electron states among which the dipole-dipole transition forbidden is a key, causing such anomalous optical phenomena with the insulator characteristics. The predicted charge density distribution and the tendency of phase transition from tetragonal to cubic are in good agreement with experimental observations.     

Anomalous Optical and Electronic Properties of CaTiO3 Perovskites

With the help of the first-prlnciples full potential linearized augmented plane wave method, absorption coefficients, reflect/vity, dielectric behavior and electronic properties, including electronic energy bands, density of states and charge density distributions, are studied for the tetragonal and cubic CaTiO3. By considering the thermal expansion effects, an approximate method is proposed for the study of the stability of ground state and a tendency of phase transition, based on the minimum free energy principle. Subsequently, numerical calculations are carried out by using the first-principles perturbation method. We demonstrate that the high-temperature phase is cubic. It is shown that optical spectra in tetragonal phase exhibit single-peak feature and differ from multi-peak character in cubic. We find that strong orbital hybridization results in the co-valent bonds between Ti 3d and O 2p electrons and forms two-type dipoles （Ti-Ol and Ti-02） in tetragonal, while the Ti-O dipoles are identical in cubic. It is argued that crystal structure determines the dipole distributions and leads to some electron states among which the dipole-dipole transit/on forbidden is a key, causing such anomalous optical phenomena with the insulator characteristics. The predicted charge density distribution and the tendency of phase transition from tetragonal to cubic are in good agreement with experimental observations.     

First-principles study of martensitic phase transformation of TiRh alloy

Geometric structures and atomic positions were studied with plane wave pseudo-potential method based on density functional theory for cubic, tetragonal, and monoclinic phases of TiRh alloy. Their phonon dispersion curves were obtained with frozen phonon method at harmonic approximation using density-functional perturbation theory. Our calculations revealed that both B2 and L1₀ phases are thermodynamically unstable. Jahn-Teller effect triggers the occurrence of Bain transformation from cubic to tetragonal phase, and then soft-mode phonon further leads to the transition from tetragonal to monoclinic phase on cooling. The monoclinic phase was predicted to be P2/m space group through atomic vibrational movement along [001] direction of virtual frequency modes of L1₀ phase. The temperature from B2 to L1₀ and then to P2/m were predicted to be about T=1100.53 K and T=324.48 K through free energy calculations with the electronic plus vibrational energy of formation, respectively, which is in good agreement with experimental observations.     

Phase coexistence in annealed CaMn7O12

The structural phase transition in annealed CaMn₇O₁₂ has been investigated by using high resolution synchrotron radiation powder diffraction. There is a phase coexistence phenomenon: two different crystallographic phases coexist in the material between 410 and 458 K. The first one is trigonal and it has a charge ordering (CO) of the Mn³⁺ and Mn⁴⁺ ions, while the second one is cubic and charge delocalized (CD). The volume fraction of the CD phase increases with temperature from 22% at 418 K up to 100% at 468 K. Both phases have domains of at least 150 nm at each temperature. The annealing of CaMn₇O₁₂ relaxed a part of the strains in the lattice, but did not influence the phase coexistence phenomenon.     

Orbital and Spin Chains in ZnV2O4

Our powder inelastic neutron scattering data indicate that ZnV2O4 is a system of spin chains that are three-dimensionally tangled in the cubic phase above 50 K due to randomly occupied t(2g) orbitals of V3+ (3d(2)) ions. Below 50 K in the tetragonal phase, the chains become straight due to antiferro-orbital ordering. This is evidenced by the characteristic wave vector dependence of the magnetic structure factor that changes from symmetric to asymmetric at the cubic-to-tetragonal transition.     

Structural study of ND4I at high pressures and low temperatures

Abstract

Structure, positional, and thermal parameters of ND₄I were studied at high pressures up to 90 kbar and low temperatures down to 10 K using time-of-flight neutron diffraction. The phase transition from a disordered CsCI-type cubic phase ND₄I(II) into a recently discovered high pressure phase ND₄I(V) was observed at P = 80(5) kbar. Surprisingly, the structure of the high pressure phase V was found to bear a strong resemblance to that of the ambient pressure, low-temperature phase III - tetragonal structure with an antiparallel ordering of ammonium ions, space group P4/nmm. The critical value of the deuterium positional parameter corresponding to the II-V transition is close to the one for the phase transition between the disordered and ordered CsCl-type cubic phases II and IV in other ammonium halides.     

Orbital ordering and magnetic field effect in MnV2O4

We studied the structural properties of an orbital-spin-coupled spinel oxide, MnV2O4, mainly by single-crystal x-ray diffraction measurement. It was found that a structural phase transition from cubic to tetragonal and ferrimagnetic ordering occur at the same temperature (Ts,TN=57 K). The structural phase transition was induced also by magnetic field above Ts. In addition, magnetic-field-induced alignment of tetragonal domains results in large magnetostriction below Ts. We also found that the structural phase transition is caused by the antiferro-type ordering of the V t2g orbitals.     

Magnetic and quadrupolar properties of PrPb3

The cubic rare earth intermetallic compound PrPb₃ (AuCu₃-type structure) undergoes a transition at 0.35 K: as it is a Van Vleck paramagnet, the low temperature phase was assumed to be quadrupolarly ordered. In order to specify this possibility, we first discuss the cubic level scheme using specific heat and first-order magnetic susceptibility results; afterwards we present an extensive study of the two (tetragonal and trigonal) symmetry lowering modes, investigated in the cubic paramagnetic phase by means of parastriction and third-order magnetic susceptibility. For the tetragonal symmetry, the quadrupolar pair interactions appear as negative, dominating the magnetoelastic coupling; as a result, the low temperature ordering may be antiferroquadrupolar, a new situation among rare earth intermetallics, the TmZn and TmCd ordering being ferroquadrupolar.     

Incipient orbital order in half-metallic Ba2FeReO6

Largely unquenched Re 5d orbital magnetic moments in half-metallic Ba2FeReO6 drive a symmetry lowering transition from a cubic paramagnet to a compressed tetragonal (c/a < 1) ferrimagnet below Tc ~ 305 K, with a giant linear magnetoelastic constant and the spins lying spontaneously along the unique tetragonal axis. The large orbital magnetization and degree of structural deformation indicate proximity to a metal-insulator transition. These results point to an incipient orbitally ordered state in the metallic ferrimagnetic phase.     

Low to high spin-state transition induced by charge ordering in antiferromagnetic YBaCo2O5

The oxygen-deficient double perovskite YBaCo2O5, containing corner-linked CoO5 square pyramids as principal building units, undergoes a paramagnetic to antiferromagnetic spin ordering at 330 K. This is accompanied by a tetragonal to orthorhombic distortion. Below 220 K orbital ordering and long-range Co(2+)/Co(3+) charge ordering occur as well as a change in the Co2+ spin state from low to high spin. This transition is shown to be very sensitive to the oxygen content of the sample. To our knowledge this is the first observation of a spin-state transition induced by long-range orbital and charge ordering.     

Raman and X‐ray investigations of the incorporation of Ca2+ and Cd2+ in the ZrO2 structure

The formation of solid solution and ZrO₂ phase stabilization were investigated by Raman spectroscopy and X‐ray diffraction (XRD) in calcium‐containing and cadmium‐containing zirconium oxide samples heated at 1073 K in air. The adopted preparation procedure led to the incorporation of calcium and cadmium in solid solution into the zirconia structure. The solid solution favored the tetragonal and cubic zirconia phases at the expense of the thermodynamically stable monoclinic modification. Combined macro‐ and micro‐Raman spectroscopy disclosed that instead of forming a homogeneous phase t″, intermediate between the tetragonal t′ and the cubic phase, the tetragonal and cubic phases coexisted in the range 9.49–13.89 mol% for Ca and 11.88–17.23 mol% for Cd. At higher dopant contents the cubic form stabilized. The impurity content necessary to stabilize the high‐symmetry phases was defined. Copyright © 2007 John Wiley & Sons, Ltd.     

First-Principles Investigations of Pb_(0.5)Ba_(0.5)TiO_3 Alloys Based on Structure Predictions

Crystal structure predictions of Pb_(0.5)Ba_(0.5)TiO_3 alloys under different pressures are performed based on the particle swarming optimization algorithm.The predicted stable ground-state and high-pressure phases are tetragonal ferroelectric(I4mm) and cubic para-electric(Fm3m),respectively,whose structural details have not been reported.The pressure-induced colossal enhancements in piezoelectric response are associated with the mechanical and dynamical instabilities instead of polarization rotation.The band gap of the tetragonal phase is indirect and that of the cubic phase is always direct.As pressure increases,the alloy displays the similar band-gap behaviors to PbTiO_3,while different from BaTiO_3,which is attributed to the different orbital contributions to the valence bands.Our calculated results are in good agreement with the available data.     

Stability of tetragonal crystalline carbon nitrides: the nitrogen content dependence

We report a theoretical study of structural stability of two tetragonal phases of carbon nitride, the tetragonal rocksalt phase and the β-tin phase, using a self-consistent-charge density-functional tight-binding method. The stability of these tetragonal phases relative to their cubic counterparts is evaluated at several nitrogen contents. The tetragonal rocksalt phase, which is recently observed in experiment, is found to be more stable than the cubic rocksalt phase at all nitrogen contents considered. On the contrary, the cubic zincblende phase is always energetically competitive with its tetragonal counterpart β-tin phase. The variation of the equilibrium volume and the bulk modulus with changing nitrogen content is investigated. The bond lengths and bond strengths are systematically examined to understand the underlying mechanism.