Ab initio SCF and limited CI calculations of the d-d transitions in cobalt oxide

Ab initio calculations using an extended set of gaussian-type orbitals have been performed for the CoO₆ ^10- cluster in the Hartree-Fock approximation and with a limited configuration interaction. The results have been compared with those of crystal-field theory. CoO is found to be a very ionic system with localized d-electrons. The excited d-states may be described using a simple configuration interaction scheme, which is found to be almost equivalent to a crystal-field parameterized configuration interaction built on Hartree-Fock results. Localization and delocalization phenomena in openshell systems have been discussed.     

Crystal-field excitations in Pr2CuO4

Neutron spectroscopy has been employed to study the crystal-field excitations in Pr₂CuO₄. The energetic ordering of the crystal-field levels and the crystal-field parameters are determined from the observed transitions. Our results are consistent with the large magnetic anisotropy observed in single-crystal susceptibility data.     

Spectra energy levels and symmetry assignments of Sm3+ doped in YAl3(BO3)4 single crystal

Single crystals of YAl₃(BO₃)₄ doped with 5% Sm³⁺ were grown by spontaneous nucleation from K₂Mo₃O₁₀ and B₂O₃ flux. Photoluminescence spectra at room and low temperatures were measured. Four emission transitions have been observed from the ⁴G_5/2 excited state to the lower lying states ⁶H_5/2, ⁶H_7/2, ⁶H_9/2 and ⁶H_11/2 upon exciting the sample with 400 nm wavelength. Polarized absorption spectra in UV, visible and NIR regions were recorded for two light polarizations (π and σ) relative to the C₃ crystal axis.Energy levels scheme of the doping ion has been compiled after a careful analysis of the spectra by reproducing the observed transitions by means of theoretical calculations based on a Hamiltonian including the free-ion and the crystal-field (CF) terms. Free-ion and crystal-field analyses are performed based on a D₃ point-group symmetry Hamiltonian. A best fit between theoretical and experimental energy levels is obtained with a root-mean-square (rms) deviation of 16 cm^?1. The general trend of the crystal-field parameters of rare earth ions in YAB host is confirmed. The calculated N_V crystal-field strength parameter presents a linear variation with the rare earth ionic radii and with the 4fⁿ electron number.     

Epitaxially stabilized AgGaSe2 for high-efficiency spin-polarized electron source

High-quality spin-polarized electron source (SPES) is of fundamental importance in the investigation of spin-dependent phenomena. Generally speaking, an ideal material for SPES application should have both large spin–orbit and positive crystal-field splitting. Currently, almost all sources in use with accelerators are based on photoemission from GaAs and related materials such as strained GaAs grown on GaAsP or InGaAs grown on GaAs. Nevertheless, the reduced critical layer thickness of these strained films leads to poor material quality and, consequently, low quantum efficiency. Besides other ordered ternary semiconductor compounds, tetragonal chalcopyrite ternary compounds have also been considered. However, since all these compounds have zero or negative crystal-field splitting, the achieved polarization and quantum efficiency are rather low. Here we propose a new material, AgGaSe₂ in the CuAu phase, as a high-quality SPES. We show that it is possible to grow epitaxially strain-free AgGaSe₂ in the CuAu phase on ZnSe substrate. Since this material has a direct-band gap, a large spin–orbit splitting, as well as a large positive crystal-field splitting, it is predicted to be a promising material for SPES with 100% spin polarization.     

An ESCA study of Sn organometallic compounds: Solid state broadening effects

X-ray photoelectron spectra have been recorded for a number of solid and gaseous organotin compounds. Solid-state charging effects (such as differential charge broadening) can be large, though we have almost eliminated them by subliming very thin films onto platinum metal. In this way, we have obtained solid-state Sn 3d line widths that are within 10% of the analogous gas-phase values. The quality and reproducibility of the solid-state spectra likewise approach those of gases.The general correlation between ESCA Sn 3d line widths and Mössbauer quadrupole splittings indicates that the broadening of the Sn 3d lines is due to crystal-field splitting from the C₂⁰ term (the quadrupole term) in the crystal-field expansion. There is a general correlation between ESCA binding energies and Mössbauer isomer shifts for the solids.     

Tetragonal distortion from cubic symmetry at lower temperatures in the Cs2NaLnCl6 compounds (elpasolites) with light lanthanides

Single crystal magnetization- and NMR-measurements show that the Cs₂NaLnCl₆ compounds with the light rare earth Ce and Nd, contrary to that with the heavy rare earth Ho, are tetragonally distorted from cubic symmetry at lower temperatures. Because of this distortion, detailed single crystal magnetization data are required in order to evaluate the Ln crystal-field level scheme. The size of the distortion is such that it splits the ground quadruplet of Nd⁺³ into doublets nearly two Kelvin apart.     

Crystal-field splitting of the fundamental multiplet of Tb3+ in terbium aluminum garnet

The low-temperature magnetic behaviour of ‘non-Kramers’ ions depends strongly on the low-lying energy levels, particularly for the case of Tb³⁺ in Terbium Aluminum garnet, where two lowest-lying singlets are well separated from upper levels. Calculations of the crystal-field splitting of the fundamental multiplet ⁷F₆ of Tb³⁺ in a field of tetragonal symmetry have been performed and compared to spectroscopy and magnetization experimental results available. A set of five crystal-field parameters is derived and is found sufficient to give a satisfactory agreement between calculation and experimental results, including susceptibility measurements between 1 and 24K reported here.     

Magnetic susceptibility of the singlet ground-state system (LaPr)Sn3

Recent experimental work on La_1-xPr_xSn₃ alloys shows anomalous low-temperature properties which have been tentatively ascribed to Kondo and crystalline-electric-field effects. In order to examine in this paper the soundness of this hypothesis we calculate the magnetic susceptibility of La_1-xPr_xSn₃ compounds, taking consistently into account crystal-field and Kondo effects. Our theoretical results are in good agreement with the experimental data, giving strong support to the interpretation of La_1-xPr_xSn₃ as the first-observed singlet ground-state system in which the Kondo effect arises from excited crystal-field levels.     

Crystal-field excitations in the heavy-fermion alloys CeCu6−x Au x studied by specific heat and inelastic neutron scattering

The specific heat of CeCu_6–x Au _x withx=0,0.3, and 0.9, and of the corresponding La homologues has been measured between 1.5 K and 150 K. With increasingx we find progressively better-defined Schottky anomalies arising from the crystal-field splitting, which is attributed to the decrease of the Kondo temperature. Both observations are in line with the results of inelastic neutron-scattering data performed on samples withx=0.5. Overall, the crystal-field splitting does not change very much upon alloying with Au. A detailed analysis shows that forx=0.5 the quasielastic line width is smaller and the inelastic crystal-field transitions are better resolved compared to CeCu₆.     

Crystal-field induced mixing of electron states in C60 crystals at high pressure

Optical absorption spectra of thin fullerene (C₆₀) crystals in the range 1.7 to 3.8 eV have been measured at T=300 K and at pressures up to 2.5 GPa. The spectrum shifts toward the red with pressure, and the electron absorption intensity is redistributed among its bands. The intensity of the band associated with the lowest direct electron interband transition monotonically increases with pressure, whereas the intensity of the upper interband feature decreases. Bands related to weak edge absorption in the range between 1.7 and 2.2 eV gradually merge with the band associated with the lowest interband transition, whose intensity rises with pressure. A similar redistribution of intensity among electron transition bands has been observed when comparing the spectrum of an isolated C₆₀ molecule and that of a C₆₀ crystal. The results indicate that the crystal-field induced mixing of electron states is present in solid C₆₀, and they can be discussed in terms of the Craig-McClure model, which was suggested to describe crystal-field induced mixing of electron states in anthracene and naphthalene molecular crystals. Zh. éksp. Teor. Fiz. 113, 313–322 (January 1998)     

DV-X2计算红宝石的晶场谱

本文利用文献中的一种DV-X₂计算过渡金属络合物的晶场谱的方法,非经验计算红宝石的全部晶场跃迁光谱项能量,得到了与实验值较一致的结果。 关键词：     

Estimation of single-ion anisotropies, crystal-field and exchange interactions in Gd-based frustrated pyrochlore anti-ferromagnets Gd2M2O7 (M=Ti, Sn, Hf, Zr)

The temperature dependence of the experimental results of dc (macroscopic) magnetic susceptibility and nuclear hyperfine properties of frustrated magnetic Gd-based pyrochlore compounds, Gd₂Ti₂O₇, Gd₂Sn₂O₇, Gd₂Hf₂O₇ and Gd₂Zr₂O₇, are analyzed within the frame work of appropriate crystal-field theory and a mean field approximation by introducing effective anisotropic molecular field tensors, and formulating an exact relation between single-ion susceptibility tensors and site susceptibility tensors. Components of the calculated susceptibility along and perpendicular to the local 〈1 1 1〉 axis of the tetrahedral sublattice of pyrochlore structure show that these pyrochlores are easy-planar anisotropic magnetic systems. The crystal-field parameters and anisotropic exchange coupling have been determined and their systematic variations over the Gd-based pyrochlores studied here are discussed.     

Theoretical and comparative investigations of Yb ion in MWO4 and M′MoO4 scheelites crystals (M=Sr, Pb, Ca, Ba) and (M′=Sr, Pb, Ca, Cd)

The crystal-field model is applied to a series of scheelites crystals (CaWO₄, SrWO₄, PbWO₄, BaWO₄, CdMoO₄, CaMoO₄, SrMoO₄ and PbMoO₄) doped with the Yb³⁺ ion. The calculated crystal-field parameters present a general trend of variation with M²⁺ ionic radius of the host cation. The maximum splitting ΔE of the ²F_7/2 manifold of the Yb³⁺ ion is then obtained as a function of N_V crystal-field strength parameters. The agreement between experimental results and theoretical predictions for all investigated systems is very satisfactory. The crystal-field effects are very important for the prediction of emission energies of the Yb³⁺ ion in different scheelites.     

Crystal-field hamiltonian and spin-orbit interaction for d and d ions at low C3 symmetry sites: V:Al2O3 and Ni:LiNbO3

The experimental studies have provided evidence of the occurrence of transitions from the ³T_1g(³F) ground state to the crystal-field levels ³T_2g(³F), ³T_1g(³P) and ³A_2g(³F) for the V³⁺ centres in Al₂O₃ crystal; and from the ³A_2g(³F) ground state to the crystal-field levels ³T_2g(³F), ³T_1g(³F) and ³T_1g(³P) for the Ni²⁺ centres in LiNbO₃ crystal (levels are assigned to irreps of the O_h point symmetry group). Using the experimental spectroscopic data, theoretical calculations of the crystal-field levels of V³⁺:Al₂O₃ and Ni²⁺:LiNbO₃ are carried out based on the Racah theory. The observed crystalline-field splittings of the V³⁺ and Ni²⁺ terms were accounted for using a C₃ symmetry Hamiltonian. The spin-orbit interaction was taken into account in this work. The Racah, crystal-field and spin-orbit parameters, which fit experimental and theoretical energy levels, have been reliably obtained. A good agreement between the theoretical and experimental results for the energy levels of V³⁺:Al₂O₃ and Ni²⁺:LiNbO₃ has been obtained.     

Electron Spin Resonance and Ground State Wave Functions of Vanadyl Ion in LiNaSO<Subscript>4</Subscript>

Electron spin resonance (ESR) of VO²⁺ doped into LiNaSO₄ has been studied at 300 K on an X-band spectrometer. The ESR powder spectra have been analyzed and spin Hamiltonian parameters have been evaluated. Using the crystal-field approach, a theoretical estimate of the ground state wave functions of VO²⁺ in LiMSO₄ (M = Na, K) has been made from the ESR data. The dipolar hyperfine coupling constant P and Fermi contact parameter K have also been estimated.     

Optical properties of trivalent chromium ions in the LiNbO<Subscript>3</Subscript> crystal

The formulas of the crystal-field theory have been adapted to a system with the symmetry group C _3v. A simple method has been proposed for including the polarization of the local environment of the Cr³⁺ impurity ion in LiNbO₃. A model dependent on one parameter has been proposed for a distortion of the niobium octahedron due to the incorporation of the trivalent chromium ion. This parameter has been determined from experimental data. The parameters of the intraionic and interionic interactions have been obtained for the Cr³⁺ ion in the lithium and niobium positions of the crystal lattice of lithium niobate.     

Ionization energies and optical spectra of 5d-metal hexafluorides as calculated in the Dirac-Slater model

A relativistic self-consistent Dirac-Slater model has been used in a study of the electronic structure of 5d-metal hexafluorides. Experimental absorption spectra have been compared with calculated energies obtained as one-electron energy differences. The calculated “crystal field” splitting between the relativistic analog oft _2g ande _g levels, as well as spinorbit splitting of thet _2g level, has been found to be in good agreement with experimental data. Ionization energies which agree well with available spectra have been calculated using a transition state procedure. From a Mulliken population analysis of the molecular levels and ground state charge densities the validity of the classical crystal-field model is discussed.     

Magnetic properties of CeB6 single crystal

Magnetic measurements of CeB₆ have been made in fields up to 80 kOe at temperatures up to 1000 K. Below 2.3 K, each magnetization vs field curve shows several critical points depending on the direction of field, but between 2.3 and 3 K, the curve is smooth. Above about 3 K, the magnetization curve has a break point at a certain field which increases monotonically with increasing temperature. The paramagnetic susceptibility has been analysed by assuming appropriate values of crystal-field splitting and exchange interactions. From these results, a magnetic phase diagram for CeB₆ has been proposed.     

Crystal-field analysis of Tb3+ doped yttrium aluminium garnet using site-selective polarized spectroscopy

The optical isotropy of the cubic garnet was overcome by crystal-site selective excitation and fluorescence spectroscopy. Thus the symmetries of the crystal-field states of the free ion levels⁷F _J and⁵D₄ could be determined. On this basis a crystal field analysis was performed to derive the crystal-field parameters for the Tb³⁺ ions replacing the Y³⁺ ions on the D₂ sites.     

Structural analysis and crystal-field calculations of Nd^3＋ in Gdx Lu1-x TaO4（x = 0.85） polycrytalline

The crystal structural parameters of Nd 3+-doped rare earth orthotantalate Gd x Lu 1 x TaO 4(x = 0.85) are determined by applying the Rietveld refinement to its X-ray diffraction,and its emission and excitation spectra at 77 K are analysed.The relativistic model of ab initio self-consistent DV-Xα method,which is applied to the cluster NdO 8 in Gd x Lu 1 x TaO 4,and the effective Hamiltonian model are used to investigate its spin-orbit and crystal-field parameters.The free-ions and crystal-field parameters are fitted to the experimental energy levels at 77 K with a root-mean-square deviation of 14.92 cm 1.According to the crystal-field calculations,96 levels of Nd 3+ are assigned.Finally,the fitting results of free-ions and crystal-field parameters are compared with those already reported for Nd 3+:YAlO 3.The results indicate that the free-ion parameters are similar to those of the Nd3+ in Gdx Lu1-x TaO4 and YAlO 3 hosts,and the crystal-field interaction of Nd3+ in Gdx Lu1-x TaO4 is stronger than that in YAlO 3.