S. M. Orel

Spin state transformations of a 3d ion in the pyramidal environment and under lattice distortions

Spin state (SS) transitions in metal-containing pyramidal complexes that originate from crystallographic distortions are the research subject of this work. The transition elements are considered to be in the 3d 6 configuration (Fe 2+ ,C o 3+ ), and they display three different SSs: low-spin (LS, S = 0), intermediate-spin (IS, S = 1) and high-spin (HS, S = 2) states. They have been studied in the frame of the crystal field approximation via the effective nuclear charge Z eff and under oxygen cage distortions. The features of the SS stability have been calculated with/without accounting for spin‐orbit coupling. Some critical points at which an accidental degeneracy of the SSs exist have been revealed. Near the critical points, negligible distortions can crucially change the SS. It is demonstrated that the ground SS of the pyramidal MeO5 complex is very sensitive to the symmetry and magnitude of its distortions. SS diagrams in the parameter space ‘effective charge’‐‘distortion magnitude’ have been established. It is revealed that the IS state exists as a ground state for all considered distortions with a corresponding choice of Z eff. Jahn‐Teller distortions stabilize the IS state in a wide range of Z eff. The orbital-like reorientation of the electron density distribution was observed for the IS state at some threshold magnitude of Jahn‐Teller distortions. (Some figures in this article are in colour only in the electronic version)

Magnetic properties of Ce3+ ion in iron-containing oxypnictide CeFeAsO

Magnetic properties of a Ce3+ ion in two crystallographic phases of a nonsuperconducting oxipnictide CeFeAsO were studied. Ce3+-ion energy levels, g-factor values, temperature dependences of magnetic susceptibility components and the forth-order anisotropy constant were calculated within a modified crystal field approach. It was shown that in the tetragonal and orthorhombic phases, the easy direction for the magnetic moment of Ce3+ ions corresponds to [110] axis. Comparison of the calculated temperature dependences of susceptibility components to the respective experimental data showed that at temperatures below 100 K, it is necessary to take into account the internal field produced by the magnetic subsystem of iron ions.

Evolution of the spin state of a 3d ion in a pyramidal complex

The evolution of the spin state of a metallic 3d ion in a pyramidal MeO5 environment is studied. Ions of metals with 3d6 (Fe2+,Co3+) and 3d4 configurations (Mn3+,Cr2+) are studied. The change in the spin state of the metal ion from S=0 to S=1 and S=2 with a variation of the effective charge Zeff of the metal ion and the displacements of the oxygen ions is investigated. The stability of the spin state as a function of the magnitude and symmetry of the displacements of the ligands is checked. Breathing-mode distortions, displacements of the 3d ion along the Z axis, Jahn-Teller distortions of the basal plane of the pyramid, and fluting distortions of the plane of the pyramid are studied. It is shown that different types of distortions produce different behaviors of the terms corresponding to a change in the spin states of all three types. It is found that the spin state of the coordination complex MeO5 with the 3d4 configuration of the metal ion is most sensitive to breathing-mode distortions and Jahn-Teller...

Multi-minimum adiabatic potential in the single crystal normal spinel ZnAl2O4, doped by Cu2 + ions

Spectroscopic investigations of a ZnAl(2)O(4) spinel doped with bivalent copper ions of 0.05% concentration have been carried out in the temperature range 4.2-290 K using a 3 cm(-1) range electron paramagnetic resonance (EPR) spectrometer having an operational frequency f = (9.241 ± 0.001) GHz. The spectrum can be represented as a superposition of two components: a low-temperature (LT) and a high-temperature (HT) one. Redistribution of integrated intensity between HT and LT components of the spectra occurs with temperature change that is typical of systems with multi-minimum adiabatic potential. Spectra observed are explained within the modified theory of crystalline field (MTCF). The electron levels of a Cu(2+) ion placed in an octahedral coordination center with trigonal distortion [CuO(6)](10-) have been calculated. The influence of possible types of oxygen octahedron distortions and possible displacement of copper ions from the symmetry center on the electron spectrum, as well as the shape of the adiabatic potential, has been analyzed. It is shown that in the low-temperature phase the multiple minima of the adiabatic potential occur due to tetragonal distortions while the depth of a minimum is determined by the degree of trigonal octahedron distortions. Tetragonal distortion values and multi-minimum potential barrier heights have been determined.

Determination of the effective nuclear charge from EPR data using a modified crystal-field theory

We propose a scheme for determining the effective nuclear charge ZefCF of a 3d ion placed in a crystal field of arbitrary symmetry. As an example, we consider the Co2+ ion in a matrix MCO3 (M = Ca, Cd). We show that the effective nuclear charge ZefCF correlates with a change in the degree of the bond covalence.

Temperature dependence of the spin state of a Co3+ Ion in RCoO3 (R = La, Gd) cobaltites

Changes in the spin state of Co3+ ions in LaCoO3 and GdCoO3 compounds are studied through the use of the temperature dependence of the magnetic susceptibility and the modified crystal field theory. It is shown that the spin subsystem of Co3+ ions in LaCoO3 and GdCoO3 undergoes the spin-crossover type transition between the high-spin (S = 2) and low-spin (S = 0) states without any contribution of the intermediate-spin state (S = 1).

The fluctuations of the spin state of 3d-ions near the “triple point”

The spin states of 3d ions placed in an arbitrarily distorted coordination complex are investigated within the semiempirical modified crystal-field theory. A scheme for constructing spin-state diagrams is presented. Spin-state diagrams are obtained for transition metal ions with electron configurations 3d4, 3d5, and 3d6 placed in a pyramidal environment. The spin-state diagrams for all configurations studied display so-called “triple points”, i.e., regions where the spin states are mixed in equal proportions. The spin-state evolution of a five-coordinated Co3+ ion in the layered rare-earth cobaltite GdBaCo2O5.5 is studied. It is found that the intermediate spin state of the Co3+ ion is stabilized by the apex oxygen displacement along the pyramidal axis.

Local magnetic anisotropy of rare-earth elements in the iron-containing oxypnictides RFeAsO (R = Ce, Nd, Sm)

This is a detailed study of the local magnetic anisotropy of rare-earth ions in RFeAsO (R = Ce, Nd, Sm) compounds. A modified crystal field theory method is used to calculate the energy levels of the rare-earth ions and the g-factors. The temperature dependences of the components of the magnetic susceptibility are constructed, along with the fourth order anisotropy constants. A method is proposed for determining the “easy” direction of the magnetic moment. The calculations show that the “easy” directions of the magnetic moments of Ce3+ and Nd3+ ions in the orthophase are the [100] and [010] axes, respectively, and in the tetraphase the [110] axis is the “easy” axis for both ions. In the case of Sm3+ the “easy” direction is the [001] axis regardless of the symmetry of the phase.

Effective nuclear charge approximation for free rare-earth ions

ABSTRACT A semi-empirical method to describe the electron structure of free many-electron rare-earth ions has been proposed. The method takes into account the spin–orbit interaction as well as relativistic interactions known as Breit’s terms. It is shown that a set of parameters to calculate the electronic spectrum of a free rare-earth ion with accuracy less than 5% can be limited to an effective nuclear charge of the rare-earth ion and a constant related to the Breit’s terms. Using the known experimental spectra of free rare-earth metals these parameters have been calculated for basic terms of all rare-earth ions.

An investigation of the adiabatic potential surface in single crystals with copper ions

We performed a comprehensive study of the EPR spectra of Cu 2+ ions embedded in single crystals of inverse spinel LiGa5O8 and zinc tungstate ZnWO4. The EPR spectra were interpreted using a modified crystal field theory. It was found that the adiabatic potential of a Cu 2+ ion in LiGa5O8 exhibits multiple minima, whereas the adiabatic potential of a Cu 2+ ion in ZnWO4 has a single minimum. It was shown that the presence of low-symmetry distortions leads to the suppression of frustration and, ultimately, as in ZnWO4, to the formation of a global minimum of the adiabatic potential.