Research on the defect structure and g factors for the tetragonal (CrO4)3− impurity center in ZrSiO4: Cr5+ crystal

ABSTRACT

The EPR g factors g// and g⊥ for the tetragonal (CrO₄)^3? impurity center in ZrSiO4: Cr⁵⁺ crystal are studied from the high-order perturbation formulas based on the two-mechanism (the crystal field and charge-transfer mechanisms) model. The studies indicate that differing from the tetragonally-elongated host (SiO4)^4- tetrahedron, the dominant defect structure of the substitutional (CrO₄)^3? tetrahedron is tetragonally- compressed with the ground state |d_z² due to the Jahn-Teller distortion. Furthermore, the agreement of g factors between calculation and experiment requires a small admixture of the first excited state |d_x²_?y² to the ground state |d_z² due to the vibrational motion of ligands, which leads a compressed (CrO₄)^3? tetrahedron to become a twinkling elongated one, These results are discussed.     

Investigations of the Spin-Hamiltonian Parameters for the Rhombic Mo5+ Centers in Ca1−x Y x MoO4 Crystal

The spin-Hamiltonian parameters (g factors g _i and hyperfine structure constants A _i , where i = x, y, z) of the rhombic Mo⁵⁺ center in Ca_1?x Y _x MoO₄ crystal are calculated from the high-order perturbation formulas based on the two-mechanism model for the rhombic d¹ tetrahedral clusters with the ground state |d _z 2〉. In these formulas, besides the contributions due to the widely applied crystal-field (CF) mechanism concerning CF excited states, those due to the charge-transfer (CT) mechanism (which is omitted in CF theory) concerning CT excited states are considered. The calculated results are in reasonable agreement with the experimental values. The calculations show that because of the great relative importance of CT mechanism for the components of spin-Hamiltonian parameter along x and y axes, the accurate and complete calculations of spin-Hamiltonian parameters for Mo⁵⁺ and other high valence state dⁿ ions in crystals should take account of both the CF and CT mechanisms. The defect model of the rhombic Mo⁵⁺ center is also confirmed from the calculations.     

Magnetic phase diagram of the Bi<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript>x</Subscript>MnO<Subscript>3</Subscript> manganites

The magnetic and elastic properties of the Bi_1-xCa_xMnO₃ manganites are studied. The phase transformations revealed are ferromagnet-spin glass (x≥0.15) and spin glass-charge-ordered antiferromagnet (x≥0.25). The ferromagnetic state is characterized by ordering of the Mn³⁺d _x ²-y orbitals. It is suggested that thespin glass state originates from local static Jahn-Teller distortions. The antiferromagnetic charge-ordered and the spin-glass disordered phases coexist in samples with 0.25<x<0.32, which may be due to the charge order-disorder phase transformation being martensitic in character. The magnetic phase diagram is constructed.     

Analysis of the lattice,strong coupling series for g0φ4 field theory in d dimensions

We analyze the renormalized strong coupling series for lattice g₀φ⁴ field theory which is a double series in x = M⁴a⁴/g₀a_4?dandy = 1/M²a², where M is the renormalized mass, a the lattice spacing, g₀ the bare coupling constant and d the dimension of space-time. We extrapolate to large y for fixed x by using a Padé-like extrapolation technique. We study the dimensionless renormalized coupling constant G/M^4?d and find that as we approach the continuum(x → 0, y → ∞) the entire spectrum of g₀ from zero to infinity can be studied. Our results for d = 1,2,3,4 based on a series in y up to y⁵ and in x up to x³ show that for fixed lattice spacing a, G/M^4?d is a monotonic function of g₀ ranging from zero at g₀ = 0 to a maximum at g₀ = ∞. Using the high temperature expansion results, we have also derived 9 terms in y on 8 lattices of dimension 1,2,3 and 4 for the linear term in x, and studied this series to see if one can see a breakdown in this monotonic behavior of G for large y. The analysis of this latter series is inconclusive.     

Multielectron approach to the electronic structure and mechanisms of superconductivity in high-Tc cuprates

A review of recent results for the doping dependence of the Fermi surface and mechanism of pairing in LaSrCuO obtained within the LDA+GTB multielectron approach is given. Both magnetic and phonon contributions to the d_x²-y²-gap are considered.     

Etude rpe du glycylglycinate de cuivre II trihydrate: Calcul des valeurs de g moleculaires de monocristaux a quatre molecules non equivalentes par maille

Molecular g values for glycylglycinatocopper(II) trihydrate are determined from crystalline g values. The calculation method used is a generalization of Hathaway's formulae (see Refs. [1, 2]). A d_x²?_y² ground state is suggested on the basis of the molecular g values (g_x = 2.086, g_y = 2.001, g_z = 2.276) and of the orientations of the principal axes. This is an usual situation for the Cu II ion in approximately square-pyramidal stereochemistry.     

AlxOy(x=1—2,y=1—3)分子几何结构与稳定性的DFT研究

用密度泛函理论(DFT)的B31yp方法在6-311++g^水平上对Al_xO_y(x=1—2,y=2—3)分子的几何构型、电子结构、振动频率等性质进行了理论研究. 通过对基态结构的几何参数分析发现，它们的基态结构趋于直线或平面结构. 对基态结构的绝热电离能讨论表明，金属铝原子数一定时，氧原子数从1增加到3，其气态分子越来越稳定，铝原子数少的分子体系更稳定. 系统给出了该系列分子基态的几何参数、电子结构、光谱性质. Al₂O₃的C_2V三重态是该分子的能量最低结构. 关键词： xO_y分子')" href="#">Al_xO_y分子 密度泛函理论 结构与稳定性     

Transport and magnetic properties of disordered LixVyO2 (x=0.8 and y=0.8)

The magnetic and electron transport properties of rhombohedral Li_xV_yO₂ (x=0.8 and y=0.8) are studied. The dc susceptibility of Li_xV_yO₂ can be well fitted to the modified Curie-Weiss law, which verified the paramagnetic ground state. The magnetic hysteresis and ac susceptibility also confirm this paramagnetism. The Li_xV_yO₂ exhibits semiconducting behavior, which is explained by thermal activated process at high temperature and variable-range hopping mechanism at low temperature. Anderson localization plays an important role in both the electron transport behavior and the magnetic behavior due to the site disorder between the Li⁺ ion and V⁴⁺ ion.     

ESR study of some Cu(II)-oxazepam complexes

The CuL₂X₂ (L = 7-chlor-l,3-dihydro-3-hydroxi-5-phenyl-2H-l,4-benzodiazepin-2-one, also known as oxazepam, X = Cl, Br) complexes were prepared and investigated by ESR (electron spin resonance) method. Powder ESR spectra of CuL₂X₂ compounds at room temperature show the presence of monomeric species having an axial symmetry with a small rhombic distortion. In case of compounds with Cl there is a superposition of two nonequivalent mononuclear species, one with ad_{x² - y²} d_{x^2 - y^2 } ground state and another with ad_z² d_{z^2 } ground state. In pyridine (Py) and dimethylformamide (DMF) solutions the monomeric species prevail. Two different monomeric species, one with pseudotetrahedral (T_d) and the other with elongated tetrahedral-octahedral symmetry, were evidenced in DMF solutions adsorbed on NaY zeolite. In Py-DMF solutions two monomeric species were also identified. Dimeric species appear in DMF and Py solutions adsorbed on NaY zeolite through the coordination of two Cu(II) ions at the same keto-oxygen from one oxazepam molecule.     

Effect of next-nearest-neighbour interaction on dx2?y2-wave superconducting phase in 2D t-J model

An exact diagonalization calculation of the t-J model on 2D square cluster has been studied for the ground state properties of HTSC. Effect of next-nearest-neighbour hopping and magnetic (both antiferromagnetic and ferromagnetic) interaction on d _x ²−y ²-wave pairing has been shown. Relative strength of the next-nearest-neighbour interaction with respect to that of near-neighbour interaction for the strongest d _x ²−y ²-wave pairing has been estimated. A schematic phase diagram is shown. It is shown that a two-sublattice model with antiferromagnetic interaction between them and a small intra-ferromagnetictype interaction in one sublattice favours d _x ²−y ²-wave superconductivity and moderate negative type NNN hopping adds flavours to this phase.     

An interpretation of the g factors for the tetragonally-compressed Cr centers in YVO4 and YPO4 crystals

The g factors g_∥ and g_⊥ for the tetragonally-compressed (CrO₄)³⁻ clusters in YMO₄ (M=V, P) crystals are calculated from the high-order perturbation formulas based on the two-mechanism model for the compressed d¹ tetrahedra with the ground state |d_z²〉. From the calculated values and by considering a small admixture of the first excited state |d_x²−y²〉 to the ground state |d_z²〉 due to the vibrational motion of ligands (which leads a twinkling compressed tetrahedron to become an elongated one), the observed g_∥ and g_⊥ for Cr⁵⁺ centers in YMO₄ crystals are explained reasonably. The difficulty of the large deviations of g_∥ from g_e (≈2.0023) in the two systems is therefore removed and the above dynamic effect may be the cause which results in the large deviation of g_∥ from g_e for some (CrO₄)³⁻ clusters in crystals.     

Formation and lithuim ionic conduction of the antifluorite type solid electrolytes,Li2+x-y-z[(NH)1-x-y-zNxClyHz]

Antifluorite type solid electrolytes, Li_2+x-y-z[(NH)_1-x-y-zN_xCl_yH_z] have been synthesized from Li₃N, LiCl and LiCl·H₂O or $\text{Li}{}_2\text{[(}\text{NH}\text{)}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{N}{}_{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{H}{}_{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$]. Either of the last two compounds introduces NH^2- ions into the product and is indispensable to form the antifluorite type structure. The lattice parameter decreases with increasing amount of NH^2- ions in the compound, reflecting the difference of anion sizes between Cl^- and NH^2- ions. Activation energies of Li⁺ diffusive motion, derived from both conductivity and NMR measurements, increase with concentration of NH^2- ions, although the values differ by a factor of ≈2 netween the both methods.     

Temperature dependent g-shift and exchange interaction in β: BIS(N-methyl salicylaldimine) copper (II)

EPR Studies have been performed on β: bis(N-methyl salicylaldimine) copper (II) in the temperature range 300–1.5 K and are compared with EPR of other two isomers α and λ. Only in β-form g_∥ increases and g_⊥ decreases continuously with lowering temperature. This is explained in terms of temperature dependent vibronic mixing of d_z2 state with ground d_x²?y² state.Only at 1.5 K some structures appear. These may be identified as hyperfine structures. Also linewidths increases gradually at low temperatures. These observations show the existence of a positive temperature coefficient of exchange interaction in this salt.     

Resonant Raman scattering of polymeric sulfur nitride modified by iodine

Resonance Raman spectra of (SNI_y)_x crystals have been measured at 150 K. The energies of the principal Raman lines of the chromophore (109 and 154 cm^?1) and their intensity behaviour with exciting laser frequency are consistent with the formation of a charge-transfer complex in which iodine enters the interfiber regions of the (SN)_x lattice as I₅^- or I₃^- linked to distorted I₂ units. This structural model is supported by comparison of these Raman data with the excitation profiles of the chromophoric group in starch-iodine and α-cyclodextrine-iodine complexes. Possible mechanisms for the conductivity increase on the basis of the proposed charge transfer model are also discussed.     

Spin-0 and spin-1/2 particles in a constant scalar-curvature background

We study the Klein-Gordon and Dirac equations in the presence of a background metric ds²=−dt²+dx²+e^−2gx(dy²+dz²) in a semi-infinite lab (x>0). This metric has a constant scalar-curvature R=6g² and is produced by a perfect fluid with equation of state p=−ρ/3. The eigenfunctions of spin-0 and spin-1/2 particles are obtained exactly, and the quantized energy eigenvalues are compared. It is shown that both of these particles must have nonzero transverse momentum in this background. We show that there is a minimum energy E²_min=m²c⁴+g²c²?² for bosons (E_KG>E_min), while the fermions have no specific ground state (E_Dirac>mc²).     

X-ray photoelectron diffraction (XPED) patterns from: III-V compound mixed crystals (Ga1-xAlxAs AND GaAs1-yPy). Comparison of experiment and single scattering calculations

X-ray photoelectron diffraction (XPED) measurements, together with single scattering calculations, were made for two III—V group compound semiconductor mixed crystals (Ga_1-xAl_xAs(110) and GaAs_1-yP_y(001)). Each pair of photoelectrons excited at equivalent atomic sites in the crystal (Ga 3d and Al 2p in Ga₁-xAl_xAs, and As 3d and P 2p in GaAs₁-yP_y) showed essentially the same XPED patterns. Single scattering calculations reproduced the observed XPED patterns fairly well. An obvious site dependence of XPED patterns was observed in both the experimental and calculated results. These results clearly show that, for a given kinetic energy, XPED patterns mainly depend on the site of emitter atoms in the crystal, not on the species of emitter atoms. Thus, XPED measurements can be used for crystal site determinations in a fingerprint manner.     

Magnetic properties of Mn-doped GaN with defects: ab-initio calculations

According to first-principles density functional calculations,we have investigated the magnetic properties of Mn-doped GaN with defects,Ga 1-x-y V Gx Mn y N 1-z-t V Nz O t with Mn substituted at Ga sites,nitrogen vacancies V N,gallium vacancies V G and oxygen substituted at nitrogen sites.The magnetic interaction in Mn-doped GaN favours the ferromagnetic coupling via the double exchange mechanism.The ground state is found to be well described by a model based on a Mn 3+-d 5 in a high spin state coupled via a double exchange to a partially delocalized hole accommodated in the 2p states of neighbouring nitrogen ions.The effect of defects on ferromagnetic coupling is investigated.It is found that in the presence of donor defects,such as oxygen substituted at nitrogen sites,nitrogen vacancy antiferromagnetic interactions appear,while in the case of Ga vacancies,the interactions remain ferromagnetic;in the case of acceptor defects like Mg and Zn codoping,ferromagnetism is stabilized.The formation energies of these defects are computed.Furthermore,the half-metallic behaviours appear in some studied compounds.     

Effect of crystal field on the spin density at the copper nucleus in copper complexes

The role of lower symmetry component of the crystal field in causing a mixing of excited 3d ^x−1 4s with the ground 3d ^x configuration and leading to spin density at the nucleus for iron group ions was suggested by Griffith and Orgel. This mechanism has been examined in detail for the two low-symmetry copper complexes, one square planar (D_4h symmetry) and the other distorted tetrahedron (D_2d symmetry) and the calculation has been performed using the powerful Racah method and tensor operator technique. It is found that for the two types of copper complexes, copper pthalocyanin (square planar, D_4h symmetry) and cesium copper chloride (distorted tetrahedron, D_2d symmetry) the contribution from this mechanism to the spin density at the nucleus vanishes identically.     

A comment on the structure of K2CuF4

The structure of K₂CuF₄ has been investigated by X-ray diffraction. Some samples displayed a multi-domain structure which arises from the possibility of the co-existence of the x and y axes in the [110]_p direction of a pseudo K₂NiF₄-type structure. The structure was determined using a single-domain crystal to be of orthorhombic symmetry, D¹⁸_2h-Bbcm, with four molecules per unit cell. The displacement of F^- ions can be explained by the orbital ordering model assumed by Khomskii and Kugel.     

Fine structure in the low temperature luminescence of Zn2SiO4:Mn and Mg4Ta2O9:Mn

The luminescence of powder samples of the well-known green-emitting Zn₂SiO₄:Mn and the red-emitting Mg₄Ta₂O₉:Mn phosphor shows a considerable fine structure at 4° K in appropriately prepared samples containing a sufficiently low Mn concentration. For (Zn_1-xMn_x)₂SiO₄ (0.0005?x?0.05) two sharp lines were found which are interpreted as due to zero phonon transitions between the ⁴T₁ and ⁶A₁ levels of Mn²⁺ ions on the two crystallographically different zinc sites. The remaining structure is ascribed to vibronic sidebands. The decay times of the luminescence bands associated with the two sites differ; they are 12 and 15 ms for the high and low energy bands respectively. The experimental results of Vlam are confirmed by our data. In addition some (Zn_1-yBe_y)₂SiO₄:Mn (0.025? y ?1) samples were investigated. In Mg₄Ta₂O₉:Mn two zero phonon lines could be identified, indicating that in this material Mn²⁺ is distributed over two inequivalent Mg sites. Most of the phonon replicas were found at intervals of 15 meV. Raman scattering experiments showed that this energy corresponds to one of the lattice vibrations. The decay time of this luminescence band is 1.0 ms.