Search

Detailed theoretical calculations for the spin-Hamiltonian parameters (g factors gi and hyperfine structure constants Ai, where i=x, y, z) of the rhombic W5+ center in CaWO4:Y3+ crystal are performed by using the high-order perturbation formulas for d1 ions in rhombic tetrahedral clusters with the ground state |dz2〉. These formulas consist of the contributions from two mechanisms, the crystal-field (CF) mechanism connected with CF excited states in the vastly-used CF theory and the frequently-neglected charge-transfer (CT) mechanism related to CT excited states. The calculated results agree well with the experimental values. The calculations indicate that for W5+ ion (or other high valence state dn ions) in crystals, the model calculations of spin-Hamiltonian parameters should take both the CF and CT mechanisms into account. The signs of hyperfine structure constants Ai are suggested and the forming (or defect model) of rhombic W5+ center in CaWO4:Y3+ crystal is confirmed from the calculations.

The high-order perturbation formulas based on the two-mechanism model are adopted to calculate the spin-Hamiltonian parameters (g factors g//, g⊥ and hyperfine structure constants A//, A⊥) of the tetragonal Cr5+ centers in CaWO4 and CaMoO4 crystals. In the model, besides the vastly used crystal-field (CF) mechanism concerning the CF excited states, the generally omitted charge-transfer (CT) mechanism related to CT excited states are taken into account. The calculated results are rationally coincident with the experimental values. The signs of constants A//, A⊥ and the impurity-induced angular distortion of Cr5+ centers in both crystals are suggested from the calculations. The results (including the relative importance of CT mechanism) are discussed.

The optical band positions and EPR (or spin-Hamiltonian) parameters (g factors g//, g⊥ and zero-field splitting D) of Co2+ ion in the trigonally-distorted tetrahedral Cd2+ site of CdSe crystal are calculated simultaneously from the complete diagonalization (of energy matrix) method (CDM) based on the two-spin–orbit-parameter model. This model contains the contributions to the spectral data from both the spin–orbit parameters of central dn ion (i.e., one-spin–orbit-parameter model) in the conventional crystal-field theory and that of ligand ions via covalence effect. The calculated 12 optical band positions and three spin-Hamiltonian parameters using four adjustable parameters show reasonable agreement with the experimental values. The local atom-position parameter uloc (where the corresponding parameter u in the host CdSe crystal is unlikely) in the Co2+ center is also estimated from the calculations. The calculations of these spectral data from the CDM based on the conventional one-spin–orbit parameter model are also made for comparison. It is found that the calculated optical band positions are also close to the experimental values, but the calculated spin-Hamiltonian parameters are in poor agreement with the observed values. So, for the unified and reasonable calculations of optical and EPR data of dn ions in crystals (in particular, in the cases of ligand with large spin–orbit parameter), the CDM based on the two-spin–orbit-parameter model should be applied.

The optical band positions of W 5+ octahedral clusters (WO 6 ) 7− in tungsten oxide (WO 3 )-based glasses are reassigned and suggested. Based on this, the spin-Hamiltonian parameters ( g factors g // , g ⊥ and hyperfine structure constants A // , A ⊥ ) of these W 5+ clusters are calculated from the high-order perturbation formulas based on the two-mechanism model. In the model, besides the contributions due to crystal-field (CF) mechanism in the extensively-used CF theory, the contributions owing to charge-transfer (CT) mechanism (which are often omitted) are also included. The calculated results are in reasonable agreement with the experimental values and the signs of hyperfine structure constants A i are suggested. The results are discussed.

The complete diagonalisation (of energy matrix) method is applied in this paper to calculate together the optical and electron paramagnetic resonance (EPR) spectral data for Cr3+ ion at the trigonal Ga3+ site of Y3Ga5O12 crystal. The method is founded on the two-spin-orbit-parameter model where in addition to the contributions from the spin-orbit parameter of central dn ion (i.e., one-spin-orbit-parameter model) in the traditional crystal field theory, those from the spin-orbit parameter of ligand ion via covalence effect is also considered. The calculated results propose that by using only four adjustable parameters, the 12 observed spectral data (nine optical band positions and three EPR parameters g//, g⊥ and D) in Y3Ga5O12: Cr3+ are reasonably explained. The impurity-induced local lattice distortion of Cr3+ in Y3Ga5O12 crystal is also estimated through the calculations. The results are discussed.

The spin-Hamiltonian parameters (g factors g‖, g⊥ and hyper ne structure constants A‖(A), A⊥(A), A‖(B) and A⊥(B), A(A) and A(B) belonging to isotopes Mo and Mo) of Mo ion at the tetragonally-compressed tetrahedral W site in CaWO4 crystal are calculated from the high-order perturbation formulae based on the two-mechanism model, where besides the contributions to spin-Hamiltonian parameters due to the crystaleld mechanism concerning the crystaleld excited states in the extensively-applied crystaleld theory, those due to charge-transfer mechanism concerning charge-transfer excited states (which are omitted in crystaleld theory) are included. The calculated results are in reasonable agreement with the experimental values. The calculations show that for the high-valence state d ions (e. g., Mo considered) in crystals, the contributions due to charge-transfer mechanism should be taken into account in the studies of spin-Hamiltonian parameters. The local structure of Mo center in CaWO4 crystal due to the impurity-induced local lattice relaxation is estimated from the calculations. The results are discussed.

In this paper, the weaknesses and mistakes in the assignments of some optical absorption bands and the theoretical calculations of g factors (g// g⊥) for the tetragonally-compressed (CrO4)3− cluster in barium aluminophosphate glasses are also explained in a unified way. The results are discussed.

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 Ca1−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.

In order to establish a succession law of every phase in sintered Nd–Fe–B magnets marked as 38/38H/38SH/38UH, four alloys of Nd33−x Dy x Al0.7Nb0.6Cu0.1B1.05Febal. (at%) were investigated after smelting processing, sintering processing, high-temperature tempering processing, and high- and low-temperature tempering processing. It is found that the four phases: the Nd2Fe14B matrix phase, Nd-rich phase, B-rich phase, and defect phase can be inherited by means of the subsequent processing. These phases might have the special constitution and appearance in the different states. Magnetic properties also have succession law. In every processing except smelting one, the values of remanence and maximum energy product hardly alter, but the value of coercive force increases gradually.

The EPR g factors and local angular distortion of tetragonal (CrO 4 ) 3− centers in Ca 3 Ga 2 Ge 3 O 12 (CGGG):Cr 5+ garnet crystal are calculated from the high-order perturbation formulas based on the two-mechanism model. In the model, besides the contributions to g factors due to the extensively used crystal-field (CF) mechanism concerning CF excited states, those due to the charge-transfer (CT) mechanism (which is often neglected) are contained. The calculated results are in reasonable agreement with the observed values. The calculations indicate that the g -shifts Δ g i CT (= g i − g e , where g e ≈ 2.0023, i = // or ⊥) due to CT mechanism in sign are opposite to and in magnitude are about 30% of the corresponding Δ g i CF due to CF mechanism in CGGG:Cr 5+ crystal and so the CT mechanism cannot be omitted for a high valence state d n ion in crystals. The angular distortion of the tetragonal (CrO 4 ) 3− centers in CGGG:Cr 5+ crystal is also acquired from the calculations. The results are discussed.

This paper reports an unified calculation of optical and EPR spectral data for Cr3+ ion at the trigonal Al3+ site of KAl(MoO4)2 crystal from the complete diagonalization (of energy matrix) method based on the two-spin–orbit-parameter model. In this model, differing from that in the conventional crystal-field theory, not only the contributions from the spin–orbit parameter of central dn ion, but also those from the spin–orbit parameter of ligand ion via covalence effect are taken into account. The calculation shows that by using only three adjustable parameters, the observed twelve optical and EPR spectral data (nine optical band positions and three EPR parameters g||, g⊥ and D) are reasonably explained in a unified way with this method. The results are discussed.

The density functional method PW91P86 with relativistic effective core potentials (RECPs) and LANL2DZ basis set has been used to investigate the possible stable geometrical configurations and relative stabilities of the lowest-lying isomers of rhodium-doped gold clusters, AunRh (n=1–8). Several low-lying isomers were determined. The results indicate that the ground-state AunRh, n=1–8, clusters have planar structures except for n=5 and 8. The stability trend of the AunRh clusters shows that the clusters with even numbers of atoms are more stable than the clusters with odd numbers of atoms, indicating that the Au5Rh cluster is a magic cluster with high chemical stability.

The electron paramagnetic resonance (EPR) g factors g // and g ┴ for the tetragonally-compressed (MoO 4 ) 3− clusters in the γ -irradiated (MoO 4 ) 2− -doped KH 2 PO 4 (KDP) and NH 4 H 2 PO 4 (ADP) crystals are computed from the high-order perturbation formulas based on the two (crystal-field and charge transfer)-mechanism model for the compressed d 1 tetragonal tetrahedra with the ground state |d z 2 ⟩. From the calculated values and by taking account of the mixing of the first-excited state |d x 2 −y 2 ⟩ to the ground state |d z 2 ⟩ caused by the dynamic effect due to the vibration of ligands (which can result in a twinkling change of tetrahedron from compression to elongation), the computed g // and g ┴ for the two crystals are in good accordance with the experimental values. The large deviations of the observed g // from g e (≈2.0023, the g factor of free electron) in both crystals are therefore explained. The results are discussed.

∗corresponding author; e-mail: zhengwc1@163.com [35] S. Ravi, P. Subramanian, Physica B 399, 143 (2007). [36] M. Ciresan, M. Vaida, N.M. Avram, Acta Phys. Pol. A 112, 835 (2007). [37] M. Brik, N.M. Avram, J. Optoelectron. Adv. Mater. 8, 102 (2006). [38] C. Rudowicz, Y.Y. Zhou, J. Magn. Magn. Mater. 111, 153 (1992). [39] M.G. Brik, C.N. Avram, N.M. Avram, Physics B 384, 78 (2006). [40] W.C. Zheng, Radiat. Eff. Def. Solids 140, 329 (1997).

The optical peaks of Mn 4+ ions in Y 3 Al 5 O 12 crystal are analyzed by calculating the crystal field energy levels through the complete diagonalization (of energy matrix) method based on the two-spin-orbit-parameter model. In the model, the contributions from both the spin-orbit parameter of central d n ion and that of ligand ion are considered. From the calculations, the eleven observed optical peaks are assigned and explained reasonably with three adjustable parameters, and the defect structure of Mn 4+ center in Y 3 Al 5 O 12 crystal is estimated.

Arrays of Ni1-xMnxFe2O4 (x = 0.0, 0.25, 0.5, 0.75) nanowires with an average diameter of about 80 nm are prepared by porous anodic aluminum oxide membrane poured sol technique. X-ray diffraction analysis shows that the nickel manganese ferrites nanowires with cubic spinel structure are synthetized. Scanning electron microscopy and transmission electron microscope images indicate that the nanowire arrays are composed of prolate spheroids with different crystal orientations. Magnetic measurements show that saturation magnetization increases and then decreases with Mn increasing. The change is related to the location and the substitution of ion in spinel structure. Compared with of block material NiFe2O4, the saturation magnetization of nickel ferrite nanowire arrays is low. This is due to the fact that the noncollinear magnetic structure in nanowire arrays become predominant.