Pairs Of Transition Metal Ions Research Articles

Ba3MxTi3−xO9(M=Ir, Rh): A family of5d/4d-based diluted quantum spin liquids

We report the structural and magnetic properties of the 4}\textit{\textcolor{black}{d }}\textcolor{black}{(M = Rh) based and 5}\textit{\textcolor{black}{d }}\textcolor{black}{(M = Ir) based systems Ba$_{3}$M$_{x}$Ti$_{3-x}$O$_{9}$ (nominally $x$ = 0.5, 1). The studied compositions were found to crystallize in a hexagonal structure with the centrosymmetric space group }$P6_{3}/mmc$.\textcolor{black}{{} The structures comprise of A$_{2}$O$_{9}$ polyhedra (with the A site (possibly) statistically occupied by M and Ti) in which pairs of transition metal ions are stacked along the crystallographic }\textit{\textcolor{black}{c}}\textcolor{black}{-axis. These pairs form triangular bilayers in the $ab$-plane. The magnetic Rh and Ir ions occupy these bilayers, diluted by Ti ions even for $x$ = 1. These bilayers are separated by a triangular layer which is dominantly occupied by Ti ions. From magnetization measurements we infer strong antiferromagnetic couplings for all of the materials but the absence of any spin-freezing or spin-ordering down to 2~K. Further, specific heat measurements down to 0.35~K show no sign of a phase transition for any of the compounds. Based on these thermodynamic measurements we propose the emergence of a quantum spin liquid ground state for Ba$_{3}$Rh$_{0.5}$Ti$_{2.5}$O$_{9}$, and Ba$_{3}$Ir$_{0.5}$Ti$_{2.5}$O$_{9}$, in addition to the already reported Ba$_{3}$IrTi$_{2}$O$_{9}$. }

Half-Metallic Diluted Antiferromagnetic Semiconductors

The possibility of half-metallic antiferromagnetism, a special case of ferrimagnetism with a compensated magnetization, in the diluted magnetic semiconductors is highlighted on the basis of the first-principles electronic structure calculation. As typical examples, the electrical and magnetic properties of II-VI compound semiconductors doped with 3d transition metal ion pairs--(V, Co) and (Fe, Cr)--are discussed.

Luminescence of Exchange Coupled Pairs of Transition Metal Ions

The influence of exchange interaction between paramagnetic ions on the luminescence properties is investigated for and pairs. Two effects are reported: shortening of the lifetime of the spin-forbidden emission and a shift of emission lines or bands to a longer wavelength. The shortening of the lifetime of the spin-forbidden emission is important in doped phosphors. For application of -doped phosphors in displays the long lifetime of the emission is a problem. Shortening of the lifetime is possible by exchange coupling with neighbors, or possibly other paramagnetic neighbors. The results reported here show that the lifetime shortening depends on the exchange-coupling parameter J. The strongest coupling is observed in pairs and the lifetime shortening is strong (e.g., in and the lifetime is about 30 times shorter for the pair emission). The coupling in and pairs is weak. Typically, values for J are around 1 cm−1 and the lifetime is reduced by a factor of 2-10 (e.g., J is −1.0 cm−1 for exchange coupling between and the lifetime of the spin-forbidden emission is 14 ms in compared to 56 ms in The lifetime shortening for the emission is not related to the observed red shift for the pair emission. For the pair the exchange interaction is very weak and no significant lifetime shortening could be measured for pairs (24 ms for the emission in vs. 22 ms in © 2001 The Electrochemical Society. All rights reserved.

Force field calculations for nearest neighbor transition metal ion pairs in cubic ZnS

Force field calculations for the totally symmetric a 1 vibrations of nearest neighbor transitionmetal (TM) ion pairs in cubic ZnS ( M 2 S 7 10−, M = Mn, Fe, Co, Ni) have been performed by the INDO/2 method. The calculated force constants are considerably lower compared to those obtained for isolated MS 4 6− clusters. This finding is explained by the presence of M-M bonding and charge transfer from the bridging S atom to the metal ions. The trends in the force constants and equilibrium M-S distances are explained in terms of the bonding parameters for TM ion doped II–VI compounds previously reported.

ChemInform Abstract: Factors Affecting the EPR Spectra of Exchange Coupled Pairs of Transition Metal Ions

Abstract A brief review of the factors affecting the EPR spectra of pairs and of larger clusters of exchange coupled metal ions is reported. The role of exchange contributions in determining the zero-field splitting of pairs is discussed and a clear distinction is made between the ground state exchange coupling constant, J, and the corresponding ones which involve excited states and are relevant to the zero-field splitting tensor.

Factors affecting the epr spectra of exchange coupled pairs of transition metal ions

A brief review of the factors affecting the EPR spectra of pairs and of larger clusters of exchange coupled metal ions is reported. The role of exchange contributions in determining the zero-field splitting of pairs is discussed and a clear distinction is made between the ground state exchange coupling constant, J, and the corresponding ones which involve excited states and are relevant to the zero-field splitting tensor.

The effect of antisymmetric exchange on the E.P.R. spectra of coupled pairs of transition metal ions

The antisymmetric exchange term (AET) in the exchange spin hamiltonian, d. S1×S2 , can have sizeable effects on the EPR spectra of hetero-dinuclear transition metal complexes. In this paper we review briefly the symmetry aspects of the AET and give a general formulation for its matrix elements for any pair of S 1, S 2 values. The effects of the AET on the E.P.R. spectra are discussed and one case where its presence can be recognized is shown.

On Kinetic Exchange Theory

AbstractThe kinetic exchange in transition metal ion pairs is analyzed for the case of crystal fields of simply reducible symmetry groups. The effects of non‐orthogonality of molecular orbitals belonging to different centres are treated explicitly for the first time in the multi‐electron theory. The kinetic exchange parameters are related to the covalency parameters of the metal—metal “bond” and charge transfer energies. The particular cases of low and high charge transfer energies are considered. It is shown that with low charge transfer energies, strong ferromagnetic kinetic exchange may predominate. Certain differences in the operator forms of the direct and kinetic exchange Hamiltonians are discussed. The results obtained are applied to interpret experimental data.

Two‐Centre Exchange Interactions between Orbitally Degenerate Ions. I. One Ion in Orbitally Degenerate State

AbstractA multielectron theory of exchange interactions in transition metal ion pairs in cubic matrices and ions in orbitally degenerate states is developed. The orbitally dependent part of the exchange Hamiltonian is expressed by Pauli operators determined in the orbital space for the case of an orbital doublet and by fictitious orbital operators L̃ = 1 in the case of a triplet state. It is shown that the anisotropic exchange parameters may be estimated from isotropic ones. The theory described is compared with experiment.

High resolution phosphorescence excitation spectroscopy of exchange coupled Mn2+pairs in KMgF3and KZnF3

A study of the absorption of light by pairs of exchange coupled manganese ions in the perovskite fluorides KMgF3 and KZnF3 has been made using the technique of high resolution phosphorescence excitation spectroscopy. Transitions to the following pair states have been observed: 6 A 1g 4 T 1g a ; 6 A 1g 4 E g a ; 6 A 1g 4 A 1g ; 6 A 1g 4 E g b ; 4 T 1g a 4 T 1g a ; 4 E g a 4 T 1g a and 4 A 1g 4 T 1g a . Quantitative estimates of intensities have been made for some of these transitions as well as estimates of the effective exchange parameters. A detailed analysis of the transition 6 A 1g 6 A 1g → 6 A 1g 4 A 1g provides confirmation of the Tanabe mechanism of exchange induced absorption of light by pairs of transition metal ions. It is shown that absence of MCD in this pair absorption is predicted by the Tanabe mechanism.