Theoretical Studies of g Factors and Defect Structures for Cubic, Tetragonal, and Orthorhombic Fe^＋ Centers in Alkali Halides MX （M=Li, Na; X = F, C1）

The EPR 9 factors for cubic, tetragonal and orthorhombic Fe^＋ centers in alkali halides MX （M= Li, Na; X = F, CI） are calculated from second-order perturbation formulas of g factors based on cluster approach for 3d^7 ions in three symmetries. From calculations, the g factors of these Fe^＋ centers in MX crystals are reasonably explained and the defect structural data for the tetragonal and orthorhombic Fe^＋ centers are estimated. The results are discussed.     

GdBa_2Cu_(3-x)Fe_xO_(7-δ)超导体系结构相变及拉曼谱特征

采用固态反应法制备了GdBa2Cu3-xFexO7-δ(x=0.00-0.30)系列样品,利用X射线衍射、拉曼光谱以及电测量技术对体系的晶体结构、拉曼散射谱特征以及电输运特性进行了系统研究。结果表明,当Fe掺杂量在x=0.05-0.10区间时,体系的晶体结构发生了从正交相到四方相的转变。通过对拉曼光谱中典型振动模的指认及振动模随Fe掺杂量的变化规律,得到了拉曼谱随体系正交-四方结构相变的变化特征:对于具有正交相的x=0.00-0.05样品,拉曼谱具有五个与正交相结构相对应的特征峰;而对于具有四方相的x=0.10-0.30样品,随Fe掺杂量增加,振动模强度变弱,且典型振动模发生了不同程度的展宽或频移。电输运测量表明,随Fe掺杂量的增加,超导临界温度Tc降低,正常态电阻率增加且发生了金属-半导体相变。     

Structural phase stability,electronic structure and mechanical properties of alkali metal hydrides AMH4 (A=Li,Na; M=B,AL)

The structural stability of Alkali metal hydrides AMH₄ (A=Li, Na; M=B, Al) is analyzed among the various crystal structures, namely hexagonal (P6₃mc), tetragonal (P4₂/nmc), tetragonal (P-42₁c), tetragonal (I4₁/a), orthorhombic (Pnma) and monoclinic (P2₁/c). It is observed that, orthorhombic (Pnma) phase is the most stable structure for LiBH₄, monoclinic (P2₁/c) for LiAlH₄, tetragonal (P4₂/nmc) for NaBH₄ and tetragonal (I4₁/a) for NaAlH₄ at normal pressure. Pressure induced structural phase transitions are observed in LiBH₄, LiAlH₄, NaBH₄ and NaAlH₄ at the pressures of 4 GPa, 36.1 GPa, 26.5 GPa and 46 GPa respectively. The electronic structure reveals that these metal hydrides are wide band gap insulators. The calculated elastic constants indicate that these metal hydrides are mechanically stable at normal pressure.     

Crystal structure determination of Pb2Sr2YCu3O8+δ (δ=0, 1.67) by powder neutron diffraction

Crystal structures of Pb₂Sr₂YCu₃O_8+δ(δ=0.0 and 1.67) were determined by powder neutron diffraction. For δ=0, it had been indicated by powder X-ray diffraction that the structure was a monoclinic one. The R factors of the present analysis for the monoclinic structure were about 3/4 of those for the previous orthorhombic ones. The structure was almost the same as the previous X-ray result except z(O(2)). For the annealed phase, the tetragonal and the orthorhombic structures had been presented by X-ray diffraction and by neutron diffraction, respectively. The R factors of the present analysis for the orthorhombic structure were about 5/6 of those for the tetragonal structure. The distance between the Pb plane and the oxygen-deficient Cu plane was found to become larger in proportion to δ, though there occurs a phase separation in the low-δ region.     

Structural, Elastic and Electronic Properties of ReO2

Structural, elastic and electronic properties of ReO₂ are investigated by first-principles calculations based on density functional theory. The ground stateof ReO₂ has an orthorhombic symmetry which belongs to space group Pbcn with a=4.7868Å b=5.5736Å, and c=4.5322Å. The calculated bulk moduli are 322GPa, 353GPa, and 345GPa for orthorhombic, tetragonal, and monoclinic ReO₂, respectively, indicating that ReO₂ has a strong incompressibility. ReO₂ is a metal ductile solid and presents large elastic anisotropy. The obtained Debye temperatures are 850K for orthorhombic, 785K for tetragonal, and 791K for monoclinic ReO₂.     

Antiferromagnetic stripe ordering at the tetragonal-orthorhombic transition in LnOFeAs probed by X-ray absorption spectroscopy

Measurements of the Fe K-edge X-ray absorption near edge (XANES) spectra of LnOFeAs high T_c superconductors exhibit small but measurable changes across the tetragonal to orthorhombic phase structural phase transition. Calculations, based on the local structure centered at the Fe site obtained by crystallographic investigations, reproduce the observed changes in the spectra under the change of the crystal structure, indicating variations in the Fe local unoccupied electronic states. Calculated Fe density of states including stripe spin ordering in the orthorhombic phase show significant modifications with respect to Fe 3d density of states in the tetragonal phase or the density of states in the orthorhombic phase calculated ignoring spin ordering. This implies that the striped spin ordering in the orthorhombic phase is necessary to describe its electronic properties not only in the ground state but also for excited states above the Fermi energy.     

PREPARATION,SUPERCONDUCTIVITY AND STRUCTURE OF THE LaBaCaCu3O7-x SUPERCONDUCTORS WITH Tc HIGHER THAN 80 K

In this paper, the preparation process of the tetragonal LaBaCaCu₃O_7-x (LBCCO) superconductor with T_c=84 K is repored. The superconductivity and structure of the LBCCO superconducting phase are investigated by means of X-ray diffraction (XRD), transmission electron microscope(TEM) and superconductivity measurements, The results indicate that the superconducting phase with T_c=82K in the LBCCO system has a tetragonal structure with a=0.3966nm, c=l.1838nm and space group P4/mmm. About 50% of the Ca and La atoms are located in 1d sites (i.e. Y site in the YBCO structure). It is confirmed that there is in fact no orthorhombic to tetragonal transition in this material. The relationship between the oxygen content and T_c is also presented.     

Microwave absorption spectra and the problem of the crystal field in tetragonal compounds HoBa2Cu3O x (x = 6.0, 6.3)

The resonance microwave absorption at wavelengths 871, 406, 305, and 118 μm in tetragonal layered perovskites HoBa₂Cu₃O _x (x = 6.0, 6.3) in pulsed magnetic fields up to 40 T has been detected. This absorption is caused by the electronic transitions between low lying levels of the Ho³⁺ ion in the crystal field. The positions and intensities of the basic resonance absorption lines for the crystal with x = 6.0 in the magnetic field oriented along the tetragonal axes are adequately described in terms of the tetragonal crystal field with known interaction parameters. To explain the weaker absorption lines, it is necessary to take into account the effect of the inhomogeneous orthorhombic and monoclinic components of the crystal field, which are due to disorder in the oxygen subsystem. This effect is more pronounced in the absorption spectra of the crystal with x = 6.3 for which the oxygen disorder is more pronounced.     

High-frequency tunable EPR spectroscopy of non-Kramers ions in AgGaSe<Subscript>2</Subscript>: Cr,AgGaS<Subscript>2</Subscript>: Cr,and CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript>: Cr crystals

Crystals of the chalcopyrite family, AgGaSe₂, AgGaS₂, and CdGa₂S₄, doped with chromium ions have been investigated using high-frequency broad-band EPR spectroscopy in the range 65–530 GHz at T = 4.2 K. It has been revealed that, in the AgGaSe₂ and AgGaS₂ crystals, the Cr²⁺ ions occupy positions with orthorhombic and tetragonal symmetry, whereas the previously investigated CdGaS₄ crystals contain only tetragonal centers. The observed spectra have been described in the framework of the spin-Hamiltonian formalism. Apart from the divalent chromium centers, the EPR lines attributed to non-Kramers ions are observed in the frequency range 300–450 GHz for all the crystals under investigation. The nature of these lines has been discussed.     

Mössbauer studies of YBa2Cu3O7−δ-type high-T c superconductors

The local magnetic, electronic, and structural properties of (RE)Ba₂Cu₃O_7?δ supercondcutors (RE=Gd, Dy, and Eu) were studied by Mössbauer spectroscopy using the resonances of¹⁵⁵Gd,¹⁶¹Dy,¹⁵¹Eu, and⁵⁷Eu. In GdBa₂Cu₃O_7?δ, different magnetic ordering behaviors of the Gd sublattice are found for the orthorhombic (superconducting) and tetragonal (non-superconducting) phases. In DyBa₂Cu₃O_7?δ, the magnetic moments of the respective CEF ground states in the orthorhombic and tetragonal phases are derived from paramagnetic hyperfine splittings at 1.4 K. In both DyBa₂Cu₃O_7?δ and EuBa₂Cu₃O_7?δ, anomalies connected with the superconducting transitions in isomer shift, recoil-free fraction, and relaxation behavior were looked for, but not found. The electric-quadrupole splittings observed for both systems are discussed in connection with the lattice EFGs derived for the Gd system. In GdBa₂ (Cu_0.995Fe_0.005)₃O_7?δ, the local properties of the various Fe sites are investigated over a wide temperature range in both the orthorhombic and tetragonal phase. The magnetic ordering of the Gd sublattice in the orthorhombic phase and of the Cu(2) sublattice in the tetragonal phase, respectively, is monitored via the magnetic splittings at the various Fe sites. Possible assignments of Cu(1) and Cu(2) sites as well as different oxygen configurations around the substituted Fe ions are discussed.     

Non-kramers ENDOR and ESEEM of the S = 2 ferrous ion of [Fe(II)EDTA](2-)

We report here the first non-Kramers (NK) ESEEM and ENDOR study of a mononuclear NK center, presenting extensive parallel-mode ESEEM and ENDOR measurements on the S(t) = 2 ferrous center of [Fe(II)ethylenediamine-N,N,N',N'-tetraacetato](2-); [Fe(II)EDTA)](2-). The results disclose an anomalous equivalence of the experimental patterns produced by the two techniques. A simple theoretical treatment of the frequency-domain patterns expected for NK-ESEEM and NK-ENDOR rationalizes this correspondence and further suggests that the very observation of NK-ENDOR is the result of an unprecedentedly large hyperfine enhancement effect. The mixed nitrogen-carboxylato oxygen coordination of [Fe(II)EDTA](2-) models that of the protein-bound diiron centers, although with a higher coordination number. Analysis of the NK-ESEEM measurements yields the quadrupole parameters for the (14)N ligands of [Fe(II)EDTA](2-), K = 1.16(1) MHz, 0 相似文献   

Anisotropic elastic properties of MB (M = Cr,Mo, W) monoborides: a first-principles investigation

First principles calculations were performed to systematically investigate structure properties, phase stability and mechanical properties of MB (M = Cr, Mo, W) monoborides in orthorhombic and tetragonal structures. The results of equilibrium structures are in good agreement with other available theoretical and experimental data. The elastic properties, including bulk modulus B, shear modulus G, Young’s modulus E and Poisson’s ratio ν were calculated by the Voigt-Reuss-Hill approximation. All considered monoborides are mechanically stable. The results of elastic anisotropies show that elastic anisotropy of orthorhombic structure is larger than that of tetragonal structure. Moreover, the minimum thermal conductivities were also estimated using the Cahill’s model, and the results indicate that the minimum thermal conductivities show a dependence on directions.     

铁基氟化物超导体SrFe_(1-x)Co_xAsF(x=0,0.125)声子特性的第一性原理计算研究

采用基于第一性原理的平面波赝势方法,计算了铁基氟化物及其钴掺杂超导体SrFe1-xCoxAsF(x=0,0.125)在四方非磁态与正交条纹反铁磁态下的声子谱(声子色散曲线、声子态密度)及电-声子耦合常数.计算发现:条纹反铁磁相互作用下的自旋-声子耦合效应强于电-声子耦合作用使声子谱的宽度减小;自旋效应使声子的有效质量增加导致条纹反铁磁态下Fe原子与As原子的耦合振动频率减小.另外,掺杂和自旋效应是提高电-声子耦合常数的两个有效方法,但计算所得超导转变温度远小于实验测量值,表明铁基超导电性非简单的电-声子耦合配对机理.     

Spin Hamiltonian parameters and local structures for tetragonal and orthorhombic Ir2+ centers in AgCl

The perturbation formulae of the spin Hamiltonian parameters (the anisotropic g factors, hyperfine structure constants and superhyperfine parameters) are established for a 5d⁷ ion in an orthorhombically elongated octahedron based on the cluster approach. These formulae are applied to the theoretical studies of the EPR spectra and the local structures for the tetragonal and orthorhombic Ir²⁺ centers in AgCl. For the tetragonal Ir²⁺ center, the uncompensated substitutional [IrCl₆]⁴ cluster is found to experience a relative elongation of about 0.08 Å along the C ₄ axis due to the Jahn–Teller effect. For the orthorhombic center, the ligand octahedron also suffers Jahn–Teller elongation (by about 0.08 Å) along the [001] (or Z) axis. Meanwhile, the ligand Cl intervening in the impurity Ir²⁺ and the next nearest neighbor silver vacancy V_Ag along the [100] (or X) axis may undergo an inward displacement of 0.004 Å towards the center of the octahedron due to electrostatic repulsion of the V_Ag. The calculated spin Hamiltonian parameters based on the above local structures show good agreement with experimental data for both centers.     

Predicting Physical Properties of Tetragonal,Monoclinic and Orthorhombic M_3N_4(M=C,Si,Sn) Polymorphs via First-Principles Calculations

The recently discovered tetragonal,monoclinic and orthorhombic polymorphs of M_3N_4(M=C,Si,Sn) are investigated by using first-principles calculations.A set of anisotropic elastic quantities,i.e.,the bulk and shear moduli,Young's modulus,Poisson ratio,B/G ratio and Vickers hardness of M_3N_4(M=C,Si,Sn) are predicted.The quasi-harmonic Debye model,assuming that the solids are isotopic,may lead to large errors for the non-cubic crystals.The thermal effects are obtained by the traditional quasi-harmonic approach.The dependences of heat capacity,thermal expansion coefficient and Debye temperature on temperature and pressure are systematically discussed in the pressure range of 0-10 GPa and in the temperature range of 0-1100 K.More importantly,o-C_3N_4 is a negative thermal expansion material.Our results may have important consequences in shaping the understanding of the fundamental properties of these binary nitrides.     

Magnetic characteristics of R2(Fe, Co)14B systems (R = Y, Nd and Gd)

R₂(Fe, Co)₁₄B compounds (R = Y, Nd and Gd) were prepared in high purity. The magnetic behavior of R₂(Fe, Co)₁₄B compounds is reported over the temperature range 4 to 300 K. The effects of Fe substitution by Co on the saturation magnetization, Curie temperature and anisotropy are presented. The spin-reorientation temperature is lowered as Co replaces Fe. This also results in a reduced cone angle.

The R₂Fe_14−xCo_xB alloys crystallize in the tetragonal structure over the entire concentration range of 0 x 14. When Fe is substituted by Co, the Curie temperature increases significantly, the saturation magnetization increases to a maximum value around x = 2, and the anisotropy becomes planar for R = Y and Gd. The Nd₂(Fe, Co)₁₄B systems all exhibit uniaxial anisotropy at room temperature and Nd₂Co₁₄B is strongly uniaxial at 77 K. The Nd₂(Fe, Co)₁₄B systems are conical at 77 K.     

Crystal structure determination of Pb2Sr2YCu3O8+δ (δ=0, 1.67) by powder neutron diffraction

Crystal structures of Pb₂Sr₂YCu₃O_8+δ(δ=0.0 and 1.67) were determined by powder neutron diffraction. For δ=0, it had been indicated by powder X-ray diffraction that the structure was a monoclinic one. The R factors of the present analysis for the monoclinic structure were about of those for the previous orthorhombic ones. The structure was almost the same as the previous X-ray result except z(O(2)). For the annealed phase, the tetragonal and the orthorhombic structures had been presented by X-ray diffraction and by neutron diffraction, respectively. The R factors of the present analysis for the orthorhombic structure were about of those for the tetragonal structure. The distance between the Pb plane and the oxygen-deficient Cu plane was found to become larger in proportion to δ, though there occurs a phase separation in the low-δ region.     

<Superscript>57</Superscript>Fe NMR study of amorphous and rapidly quenched crystalline Fe-B alloys

Amorphous and crystalline Fe-B alloys (5–25 at % B) were studied using pulsed ⁵⁷Fe nuclear magneticr esonance at 4.2 K. The alloy samples were prepared from a mixture of the ⁵⁷Fe and ¹⁰B isotopes by rapid quenching from the melt. In the microcrystalline Fe-(5–12 at %) B alloys, the resonance frequencies were measured for local states of ⁵⁷Fe nuclei in the tetragonal and orthorhombic Fe₃B phases and also in α-Fe. The resonance frequencies characteristic of ⁵⁷Fe nuclei in α-Fe crystallites with substitutional impurity boron atoms in the nearest neighborhood were also revealed. In the resonance frequency distribution P(f) in the amorphous Fe-(18–25) at % B alloys, there are frequencies corresponding to local Fe atom states with short-range order of the tetragonal and orthorhombic Fe₃B phases. As the boron content decreases below 18 at %, the P(f) distributions are shifted to higher frequencies corresponding to ⁵⁷Fe NMR for atoms exhibiting a short-range order of the α-Fe type. The local magnetic structure of the amorphous Fe-B alloys is also considered.     

The electronic,optical,and thermodynamical properties of tetragonal,monoclinic,and orthorhombic M_3N_4(M=Si,Ge,Sn):A first-principles study

A detailed study of the M_3N_4(M = Si,Ge,Sn) nitrides in their tetragonal,monoclinic and orthorhombic phases has been performed with the plane-wave pseudo-potential method combined with the quasi-harmonic approximation,including the phononic effects.We rationalize the main puzzle,i.e.,the fundamental properties of these phases are unclear,by calculating the crystal structures,density of states,and optical properties.The direct band gaps of t-Ge_3N_4,m-Si_3N_4,and o-Ge_3N_4 benefit the opto-electrical properties,t-,m-,and o-Si_3N_4 can be used as refractive materials while m-M_3N_4(M = Si,Ge,Sn) are optically transparent in the visible light region.Our results improve the understanding of the detailed electronic structures of all compounds,as well as the influences of electronic structure on their stabilities.Furthermore,we find that thermodynamic quantities are sensitive to structures and,therefore,depend on various temperature and pressure conditions.     

Magnetism of Fe in SrFe2As2: Local investigation by time differential perturbed angular distribution (TDPAD) spectroscopy

Employing the γ-ray perturbed angular distribution technique, we have measured the magnetic hyperfine field of ⁵⁴Fe in tetragonal and orthorhombic structural phases of SrFe₂As₂. In the tetragonal phase, the magnetic response of ⁵⁴Fe shows Curie-Weiss type local susceptibility, indicating the presence of localized moment on Fe. The temperature dependence of the hyperfine field of ⁵⁴Fe reflects quasi-two dimensional first order magnetic transition at 200 K. Our data indicate that Fe moments in the magnetically ordered phase of SrFe₂As₂ may be canted out of the ab-plane.