Structural, electronic and vibrational properties of InN under high pressure

The structural, electronic and vibrational properties of InN under pressures up to 20 GPa have been investigated using the pseudo-potential plane wave method (PP-PW). The generalized-gradient approximation (GGA) in the frame of density functional theory (DFT) approach has been adopted. It is found that the transition from wurtzite (B4) to rocksalt (B1) phase occurs at a pressure of approximately 12.7 GPa. In addition, a change from a direct to an indirect band gap is observed. The mechanism of these changes is discussed. The phonon frequencies and densities of states (DOS) are derived using the linear response approach and density functional perturbation theory (DFPT). The properties of phonons are described by the harmonic approximation method. Our results show that phonons play an important role in the mechanism of phase transition and in the instability of B4 (wurtzite) just before the pressure of transition. At zero pressure our data agree well with recently reported experimental results.     

Spin--polarized electronic properties of NiHe0.25 under pressures

This paper studies the effects of He atom on the spin-polarized electronic properties of nickel under pressures using ab initio pseudopotential plan-wave method.Under high pressures,the compound of NiHe0.25 can exist and helium-bubble can not create in Ni.A pressure-induced ferromagnetic to paramagnetic phase transition has been predicted in NiHe0.25 at about 218 GPa.It is found that under pressures,the magnetic property of Ni atoms is more strongly affected by He atom than by H atom and that the behaviour of He atom in Ni are completely different from that of H atom,like the bonding characteristics and the electron transfer.     

Off-stoichiometry effects on BiFeO3 thin films

The effects of Bi and Fe-excess on the structure, ferroelectric, leakage current and magnetic properties of BiFeO₃ (BFO) thin films are reported. BFO with 5% excess exhibits no change in the structure with an improvement in leakage current properties in comparison to stoichiometric BFO. Raman spectroscopy of 10% Bi excess suggests a structural change from monoclinic to rhombohedral accompanied with an improvement of resistivity and ferroelectric polarization switching. A higher Fe-excess leads to the formation of pyrochlore Bi₂Fe₄O₉ and gamma-Fe₂O₃ that cause an increase in conductivity at the macroscopic scale. The results are discussed in terms of Fe and Bi-excess effects on the defect structure of BFO.     

Structural and electronic properties of type-I and type-VIII Ba8Ga16Sn30 clathrates under compression

The total energy and electronic structures for type-I (β phase) and type-VIII (α phase) Ba₈Ga₁₆Sn₃₀ clathrates under hydrostatic pressure have been investigated using density functional theory (DFT) calculations. It was found that the type-VIII phase is more stable than the type-I one at ambient conditions and that β→α phase transition can not occur under hydrostatic pressure. The band structures show that the type-I and type-VIII Ba₈Ga₁₆Sn₃₀ are indirect semiconductors with band gaps of 0.24 eV and 0.19 eV, respectively. The results suggested that type-I clathrate Ba₈Ga₁₆Sn₃₀ has a larger value of the thermoelectric (TE) power than that of type-VIII clathrate. We found that pressure tuning changes the k-point of conduction band minimum (CBM) in the Brillouin zone for β-phase, but it is not the case for α-phase. Furthermore, the results show that the pressure can change the interaction between guest atoms and the host lattice, and consequently results in the decrease of the band gap of β-phase and the increase of the band gap of α-phase, indicating that the pressure effect can play an important role in the magnitude of the TE power.     

Soft-mode driven tetragonal-to-monoclinic phase transition in multiferroic BiFeO3

The phase transition of BiFeO₃ (BFO) from tetragonal to monoclinic induced by pressure was investigated by first-principles method. The sequential monoclinic phase, _Ma$M_{\mathrm{a}}$, which is favorable during low compression with respect to the tetragonal phase, was characterized. The order parameters were calculated in the vicinity of the phase transition, showing that phase transition has a second-order character. The results demonstrated that the pressure-induced tetragonal-to-monoclinic phase transition in BFO is related to the softening behavior of the E mode, which are very helpful in further investigations of the morphotropic phase boundary (MPB) in lead-free materials.     

Temperature and field dependent electronic structure and magnetic properties of LaCoO3 and GdCoO3

The transformation of the band structure of LaCoO₃ in the applied magnetic field has been theoretically studied. If the field is below its critical value B_C≈65 T, the dielectric band gap decreases with the field, thus giving rise to negative magnetoresistance that is highest at T≈300÷500 K. The critical field is related to the crossover between the low- and high-spin terms of Co³⁺ ions. The spin crossover results in an insulator–metal transition induced by an increase in the magnetic field. Similar calculations have been done for GdCoO₃ which is characterized by large spin gap∼2000 K.     

Ab initio study on phase transition and magnetism of BiFeO3 under pressure

In this paper the first-principles calculations within local spin density approximation (LSDA)+U show that BiFeO3 experiences a mixed phase state with P4mm structure being the intermediate phase before the pressure of phase transition is reached. The critical pressure for the insulator-metal transition (IMT) is found to be about 50 GPa. A pressure induced crossover of high-spin states and low-spin states is observed close to the IMT pressure in R3c structure. The LSDA+U calculations account well for the mechanism of the IMT and crossover of spin states predicted in recent experiment (Ref.[1]).     

The structural, elastic and electronic properties of BiI3: First-principles calculations

The structural, elastic and electronic properties of BiI₃ are investigated using the first-principles pseudopotential calculations within the framework of density functional theory. The calculated equilibrium structural parameters agree well with the experimental values. The results show that rhombohedral R-3 structure is low enthalpy structure at zero pressure. R-3 structure will transform into SbI₃-type structure (space group P2₁/c) at about 7.0 GPa. At zero pressure, BiI₃ with R-3 symmetry meets the mechanical stability criteria, but BiI₃ with P-31 m symmetry is an unstable one mechanically. For R-3 structure, the obtained bulk, shear, and Young’s moduli are 25.6, 15.3 and 38.3 GPa, respectively. BiI₃ presents large elastic anisotropy. Debye temperature of R-3 structure calculated is 181 K. The metallization pressure of R-3 structure is about 133 GPa and that of predicted high pressure phase P2₁/c structure is about 61 GPa, indicating BiI₃’s potential application as a nuclear radiation detector under high pressure environment.     

Structural transitions of NaAlH4 under high pressure by first-principles calculations

First-principles calculations have been performed on NaAlH₄ using the generalized gradient approximation pseudopotential method. The predicted β-NaAlH₄ (α-LiAlH₄-type) structure is energetically more favorable than α-NaAlH₄ for pressures over 15.9 GPa, which is apparently correlated with the experimental transition pressure 14 GPa. This transition is identified as first-order in nature with volume contractions of 1.8%. There is no pressure-induced softening behavior from our calculated phonon dispersion curves near the phase transition pressure. Based on the Mulliken population analysis, the β-NaAlH₄ structure is expected to be the most promising candidate for hydrogen storage.     

Phase transition, structural and electrical properties of Pb(Zn1/3Nb2/3)O3-doped Pb(Ni1/3Nb2/3)O3-PbTiO3 ceramics prepared by solid-state reaction method

Phase pure perovskite (1−x−y)Pb(Ni_1/3Nb_2/3)O₃-xPb(Zn_1/3Nb_2/3)O₃-yPbTiO₃ (PNN-PZN-PT) ferroelectric ceramics were prepared by conventional solid-state reaction method via a B-site oxide mixing route. The PNN-PZN-PT ceramics sintered at the optimized condition of 1185 °C for 2 h exhibit high relative density and rather homogenous microstructure. With the increase of PbTiO₃ (PT) content, crystal structure and electrical properties of the synthesized PNN-PZN-PT ceramics exhibit successive phase transformation. A morphotropic phase boundary (MPB) is supposed to form in (0.9−x)PNN-0.1PZN-xPT at a region of x=32-36 mol% confirmed by X-ray diffraction (XRD) measurement and dielectric measurement. The MPB composition can be pictured as providing a “bridge” connecting rhombohedral ferroelectric (FE) phase and tetragonal one since crystal structure of the MPB composition is similar to both the rhombohedral and tetragonal lattices. Dielectric response of the sintered PNN-PZN-PT ceramics also exhibits successive phase-transition character. 0.64PNN-0.1PZN-0.26PT exhibits broad, diffused and frequency dependent dielectric peaks indicating a character of diffused FE-paraelectric (PE) phase transition of relaxor ferroelectrics and 0.40PNN-0.1PZN-0.50PT exhibits narrow, sharp and frequency independent dielectric peaks indicating a character of first-order FE-PE phase transition of normal ferroelectrics. The FE-PE phase transition of 0.56PNN-0.1PZN-0.34PT is nearly first-order with some diffused character, which also exhibits the largest value of piezoelectric constant d₃₃ of 462pC/N.     

The reversal of magnetization driven by the Dzyaloshinskii-Moriya interaction in Bi2FeMnO6

Ab initio calculations show that the coupling between antiferrodistortive (AFD) distortions and magnetization in perovskite Bi₂FeMnO₆ is prohibited to make magnetization rotate as effective Hubbard parameter is larger than 2.7 eV, where anomalies in antiferromagnetic (AFM) vectors and band gap varying with on-site Coulomb interaction can be observed. This coupling is attributed to the antisymmetric Dzyaloshinskii-Moriya interaction driven by the e_g-e_g states AFM interaction and charge redistribution with respect to different AFD distortions.     

Optical properties of CH3NH3PbI3 crystal grown using inverse temperature crystallization

Perovskite CH₃NH₃PbI₃ (MAPbI₃) single crystal was grown using inverse temperature crystallization method. Crystallinity of the perovskite was confirmed by X-ray diffraction. Photoluminescence (PL) spectra revealed abnormal behavior due to a temperature-induced orthorhombic to the tetragonal phase transition. Four PL emission peaks, A, B, C, and D, were observed in the low temperature regime. Peaks A and B were observed at 756 and 776?nm?at 12?K, and were blue-shifted and disappeared at 130 and 70?K, respectively. Peaks C and D were observed at 789 and 807?nm?at 40?K and were also blue-shifted to 780 and 794?nm?at 100?K. On the other hand, the peak C red-shifted to 799?nm from 100 to 140?K because of an orthorhombic to the tetragonal phase change and was also blue-shifted above 140?K. From the excitation intensity- and temperature-dependent PL results, peaks A and B were assigned to the free-exciton and bound-exciton of the orthorhombic phase crystal, respectively. In addition, peaks C and D were associated with the free-exciton and bound-exciton of the tetragonal phase crystal, respectively. The activation energy of peak C was calculated to be 98?meV from temperature dependence of the PL intensity.     

Polymorphism in Cu-substituted GaV4S8: structural and electronic properties

The hexagonal and cubic phases of Ga_1−xCu_xV₄S₈ are obtained by different methods of preparation. The reaction of elements above 900 °C gives hexagonal phases for large range of x=0.02-0.5. These are metallic and show enhanced paramagnetism. The reduction of oxides by H₂S at a lower temperature of 700 °C gives non-stoichiomertic compositions of cubic-V₄ cluster compound GaV₄S₈. The solubility of Cu-atoms in cubic phase is less than 10% and above x=0.2 the samples contain a mixture of phases, Cu_xVS₂, Ga_xVS₂ and CuGaS₂. The cubic phases are insulating and show Mott's Variable Range Hopping conduction. The non-stoichiomerty and the Cu-substitution reduce the resistivity and thermopower. For x=0.15 and 0.20, the additional peaks are observed in X-ray patterns. These compositions showed a sharp metal to insulator transition on cooling below 180 K. The transitional behaviour is very similar to that previously reported intercalated VS₂ compound Al_xVS₂. The transport and magnetic properties of these phases are discussed in terms of the clustering interactions among V-atoms and the localisation of carriers on the metallic clusters frequently found in V-chalcogenides.     

Phase transition and thermodynamic properties of BiFeO3 from first-principles calculations

The first-principles projector-augmented wave method employing the quasi-harmonic Debye model,is applied to investigate the thermodynamic properties and the phase transition between the trigonal R3c structure and the orthorhombic Pnma structure.It is found that at ambient temperature,the phase transition from the trigonal R3c phase to the orthorhombic Pnma phase is a first-order antiferromagnetic-nonmagnetic and insulator-metal transition,and occurs at 10.56 GPa,which is in good agreement with experimental data.With increasing temperature,the transition pressure decreases almost linearly.Moreover,the thermodynamic properties including Grneisen parameter,heat capacity,entropy,and the dependences of thermal expansion coefficient on temperature and pressure are also obtained.     

Magnetic properties of Ni-substituted BiFeO3

BiFe_1−xNi_xO₃ ceramic powders with x up to 0.10 have been prepared by the sol-gel technique. The band gap of BiFeO₃ is 2.23 eV, and decreases to 2.09 eV for BiFe_0.95Ni_0.05O₃ and BiFe_0.90Ni_0.10O₃. The Mössbauer spectra show sextet at room temperature, indicating the magnetic ordering and the presence of only Fe³⁺ ions. Superparamagnetism with blocking temperature of 31 K for BiFe_0.95Ni_0.05O₃ and 100 K for BiFe_0.90Ni_0.10O₃ was observed. Enhanced magnetization at room temperature have been observed (1.0 emu/g for BiFe_0.95Ni_0.05O₃ and 2.9 emu/g for BiFe_0.90Ni_0.10O₃ under magnetic field of 10,000 Oe), which is one order larger than that of BiFeO₃ (0.1 emu/g under magnetic field of 10,000 Oe). The enhanced magnetization was attributed to the suppression of the cycloidal spin structure by Ni³⁺ substitution and the ferrimagnetic interaction between Fe³⁺ and Ni³⁺ ions.     

Effect of Co substitution on the magnetic properties of BiFeO3

Bismuth ferrite is doped with a dilute concentration of cobalt, _{BiFe1−XCoXO3}${\mathrm{BiFe}}_{1-X}{\mathrm{Co}}_X{\mathrm{O}}_3$; X=0, 0.01, 0.02 is prepared by sol-gel auto combustion technique. X-ray diffraction data refined via Reitveld method shows single phase and shrinkage in cell volume for Co doped BiFeO₃. Various magnetic ground states viz. superparamagnetic, glassy antiferromagnetic and glassy ferrimagnetic behavior is observed for X=0, 0.01, 0.02, respectively. A first-order magnetic transition is observed in the Arrott plot data of cobalt doped BiFeO₃. Possibility of thermally induced magnetic transition is also seen in the magnetization data of cobalt doped BiFeO₃. A model based on the existing spinoidal cyclic arrangement of spins is proposed to explain the observed data. Finally, a very dilute concentration of Co⁺³${\mathrm{Co}}^{+3}$ in BiFeO₃ is found sufficient to tailor the magnetic properties.     

Sulfur-catalyzed phase transition in MoS2 under high pressure and temperature

We report phase transition and stability of MoS₂ with and without the presence of sulfur melt under high-pressure and high-temperature conditions. Rhombohedral (3R) phase is found to be a high-temperature phase of MoS₂ at high pressures. Excess sulfur melt catalyzes the hexagonal (2H) to rhombohedral (3R) phase transformation and lowers the conversion temperature by more than 280 K. Boundary between 2H and 3R phases has been delineated with a negative slope. Based on experimental observations, sulfur-catalyzed 2H→3R transformation mechanisms are proposed involving atomic exchange between MoS₂ and sulfur, which is different from the case of without excess sulfur that proceeds through rotation and translation of the S–Mo–S sandwich layers.     

Ba(Mg1/3Nb2/3)O3电子结构第一性原理计算及光学性能研究

Ba(Mg_1/3Nb_2/3)O₃ (BMN)复合钙钛矿陶瓷具有高介电常数和高品质因子等介电性能, 预示了其在光学领域的应用前景. 本文采用第一性原理方法计算了BMN的电子结构, 对其本征光学性能进行分析和预测. 对固相合成六方相BMN的XRD 测试结果进行Rietveld精修(加权方差因子R_wp=6.73%, 方差因子R_p=5.05%), 在此基础上建立晶体结构模型并对其进行几何优化. 运用基于密度泛函理论(DFT)的平面波赝势方法, 对六方相BMN晶体模型的能带、态密度和光学性质进行理论计算. 结果表明BMN的能带结构为间接带隙, 禁带宽度E_g=2.728 eV. Mg-O和Ba-O以离子键结合为主, Nb-O以共价键结合为主, 费米面附近的能带主要由O-2p和Nb-4d 态电子占据, 形成了d-p轨道杂化. 修正带隙后, 计算了BMN沿[100]和[001]方向上的复介电函数、吸收系数和反射率等光学性质. 结果表明, BMN近乎光学各向同性, 在可见光区, 其本征透过率为77%< T <83%, 折射率为1.91< n <2.14, 并伴随一定的色散现象. 实验测试结果与理论计算结果相吻合.     

Phase transition in multiferroic YMnO3 and its solid solution YMn(0.93)Fe(0.07)O3

Ceramics of YMnO₃ and its Fe substituted YMn_(0.93)Fe_(0.07)O₃ solid solution were synthesized by solid state reaction of the oxides at 1200 °C. Hexagonal phase was identified in both cases by X-ray powder diffraction. Rietveld refinement of cell parameters showed an increase of the parameter values for the solid solution. Dielectric permittivity measurements versus temperature showed a phase transition at 655 °C for yttrium manganite, however, for the solid solution no phase transition was detected on heating up to 700 °C. Dielectric loss measurements showed higher slope changes and better defined local maxima for the solid solution than for the pure phase.     

First-principles calculations for elastic and electronic properties of ZnSnO3 under pressure

The LiNbO₃ (LN)-type structure and the ilmenite (IL)-type structure of ZnSnO₃ are investigated with the ultrasoft pseudopotential scheme in the frame of the local density approximation (LDA). The calculated lattice parameters of ZnSnO₃ under zero pressure and zero temperature are in very good agreement with the existing experimental data. The pressure dependences of the elastic constants, Debye temperatures, Poisson's ratio, sound velocity, mechanical stability and mechanical anisotropy of the LN-type structure of ZnSnO₃ have also been investigated. We find that the LN-type structure of ZnSnO₃ is a mechanically stable phase under pressures up to 21 GPa; however, the mechanical anisotropy weakens with the increasing pressures. In addition, the calculated band structure indicates that LN-ZnSnO₃ has a direct band gap of 1.669 eV, and the total and partial densities of states, under diverse pressures of the LN-type structure ZnSnO₃ have also been obtained.