Impurity Cu2 Research Articles

Theoretical studies of the spin Hamiltonian parameters and local structures for the tetragonal Cu2+ and Ni3+ centers in Mg2TiO4

The local structures of the two impurity Cu2+ (and Ni3+) centers with low spin (S=1/2) in Mg2TiO4 are theoretically studied by using the perturbation formulas of the spin Hamiltonian parameters for 3d9 (and 3d7) ions in tetragonally elongated octahedra. In these formulas, the tetragonal field parameters are quantitatively determined using the superposition model and the local structures of the impurity Cu2+ (and Ni3+) centers, and the ligand orbital and spin–orbit coupling contributions are included on the basis of the cluster approach in view of moderate covalency for the studied systems. The [CuO6]10− and [NiO6]9−clusters on the substitutional Mg2+ site are found to suffer relative elongations by about ΔZCu (≈3.2%) and ΔZNi (≈0.7%) for the impurity Cu2+ and Ni3+ centers, respectively, along the C4 axis due to the Jahn–Teller effect. The calculated spin Hamiltonian parameters based on the above Jahn–Teller elongations show good agreement with the observed values. The results are discussed.

Theoretical Investigations on the Anomaly of the g Factors and Local Structure for LaCuO3−δ

The anomaly of the g factors and local structure for LaCuO3-delta are theoretically investigated from the perturbation formulas of the g factors for a 3d (9) ion in tetragonally distorted tetrahedra based on the cluster approach including the ligand contributions. The anomaly (g (ayen)> 2 > g (aSyen)) of the g factors is ascribed to the paramagnetic Cu2+ (originating from the host diamagnetic Cu3+ capturing one electron under oxygen deficiency) in a tetragonally elongated tetrahedron. The tetragonal distortion angle is found to be about 3.56A degrees (i.e., the Cu2+-O2- bond angle related to the C-4 axis is 3.56A degrees smaller than that (a parts per thousand 54.74A degrees) of a regular tetrahedron) due to the Jahn-Teller effect. The coordination number decreases from the original six for the host octahedral Cu3+ in LaCuO3-delta to four for the tetrahedral impurity Cu2+ in LaCuO3-delta arising from incomplete Cu2+-O2- bonding due to oxygen deficiency and some Cu2+ locating in the surface of the sample.

Investigations on the spin Hamiltonian parameters and the local structures for the orthorhombic [CuX4(H2O)2]2− centers in NH4X (X = Cl, Br)

The spin Hamiltonian parameters (the g factors and the hyperfine structure constants) and the local structures for the orthorhombic [CuX4(H2O)2]2− clusters in NH4X (X=Cl, Br) are theoretically investigated from the perturbation formulas of these parameters for a 3d9 ion in orthorhombically compressed octahedra. The related molecular orbital coefficients are quantitatively determined from the cluster approach in a uniform way. The studied defect centers are attributed to the impurity Cu2+ occupying the interstitial site associated with two adjacent water molecules along the [100] (or C2) axis. The ligand octahedra are found to suffer the relative planar bond length variations of about 0.19 and 0.20Å along X and Y axes for X=Cl and Br, respectively, due to the Jahn–Teller effect. The calculated spin Hamiltonian parameters based on the above local structures show reasonable agreement with the observed values for both the centers. The validity of the results is also discussed.

Magnetic susceptibility and heat capacity of a novel antiferromagnet:LiNi2P3O10 and the effect of doping

We report the synthesis, x-ray diffraction, magnetic susceptibility andspecific heat measurements on polycrystalline samples of undopedLiNi2P3O10 and samples withnon-magnetic impurity (Zn2 + , S = 0) and magnetic impurity (Cu2 + , S = 1/2) at the Ni site. The magnetic susceptibility data show a broad maximum at around10 K and a small anomaly at about 7 K in the undoped sample. There is aλ-like anomaly in the specific heat at 7 K, possibly due to the onset ofantiferromagnetic ordering in the system. The magnetic entropy changeat the ordering temperature is close to the value corresponding toRln(2S + 1) expectedfor an S = 1 system. The temperature corresponding to the broad maximum and the orderingtemperature both decrease on Zn and Cu substitutions and also in applied magnetic fields.

Theoretical investigations of the local structures and the g factors for 3d9 ions in CdS

The local structures and the g factors g // and g ⊥ for the isoelectronic 3d9 ions Cu2+ and Ni+ in CdS are theoretically investigated from the perturbation formulas of these parameters for a 3d9 ion under trigonally distorted tetrahedral environments. In consideration of significant covalency of the [MS4] combinations (M = Cu and Ni), the ligand orbital and spin–orbit coupling contributions are taken into account using the cluster approach. Based on the studies, the substitutional impurity Cu2+ (or Ni+) on Cd2+ site is found to undergo a small inward displacement 0.026 A (or a slight outward shift 0.017 A) towards (or away from) the ligand triangle along C 3 axis. The theoretical g factors for both ions based on the above impurity displacements are in good agreement with the experimental data.

Theoretical investigations of the EPR parameters and local structure for Cu2+ in BeO

The electron paramagnetic resonance (EPR) parameters (the anisotropic g factors, the hyperfine structure parameters and the quadrupole coupling constant Q) and local structure for Cu2+ in BeO are theoretically investigated from the perturbation formulas of these parameters for a 3d9 ion under trigonally distorted tetrahedra. The ligand orbital and spin-orbit coupling contributions are included in the basis of the cluster approach, in view of the strong covalency of the [CuO4]6− cluster. From the calculations, the impurity Cu2+ is suggested not to occupy exactly the ideal Be2+ site but to suffer a slight inward displacement (≈0.024 Å) toward the ligand triangle along the C3 axis. The theoretical EPR parameters show good agreement with the experimental data.

Investigations of the EPR Parameters and Local Structure for one Cu<sup>2+</sup> Center in CdS

The EPR g factors g// and g and the hyperfine structure constants for one substitutional Cu2+ center in CdS are theoretically investigated from the perturbation formulas of these parameters for a 3d9 ion under trigonally distorted tetrahedra. In view of the significant covalency of the [CuS4]6 cluster, the ligand contributions are taken into account on the basis of the cluster approach. According to the calculations, the impurity Cu2+ is found not to occupy exactly the ideal Cd2+ site but to suffer a small inward displacement 0.12 Å toward the ligand triangle along C3 axis. The theoretical results by considering the above impurity displacement show reasonable agreement with the experimental data. The defect structure of this Cu2+ center is also discussed.

INVESTIGATIONS ON THE LOCAL STRUCTURE AND THE EPR PARAMETERS FORCu2+-DOPEDGaN

The local structure and the EPR parameters (g factors and the hyperfine structure constants) for Cu2+in GaN are theoretically studied from the perturbation formulas of these parameters for a 3d9ion in trigonally distorted tetrahedra. In these formulas, the ligand orbital and spin-orbit coupling contributions are taken into account from the cluster approach, in view of the strong covalency effect of the system. Based on the studies, the impurity Cu2+is found not to occupy exactly the host Ga3+site but to suffer a slight displacement (≈ 0.004 Å ) towards the ligand triangle along C3axis due to charge and size mismatching substitution. The theoretical EPR parameters show good agreement with the experimental data. The validity of the impurity displacement is also discussed.

THEORETICAL EXPLANATIONS OF THE SPIN HAMILTONIAN PARAMETERS FORCu2+ION INLaSrAl1-xCuxO4SOLID SOLUTION

It is of interest for layered oxides containing copper because they are related to high temperature superconductivity. Among them, LaSrAl1-xCuxO4solid solution has received much attention due to its elastic and elasto-optic properties and application as promising substrate for high- Tcsuperconducting thin films. So many theoretical and experimental works have been done to understand the several characteristics of the LaSrAl1-xCuxO4materials. In this paper, the electron paramagnetic resonance g factors g//, g⊥and hyperfine structure constants A//, A⊥for the tetragonal Cu2+center in LaSrAl1-xCuxO4solid solution are theoretically explained by the method of diagonalizing the full Hamiltonian matrix. The related crystal field parameters are calculated from the superposition model and the local structural parameters of the impurity Cu2+occupying the host Al3+site. The superposition model parameters used in this work are comparable with those for similar tetragonal ( CuO6)10-clusters in the previous works. The calculated results are in reasonable agreement with the observed values. The results are discussed.

Theoretical Studies on the Gyromagnetic Factors and the Hyperfine Structure Constants for the Tetragonal Copper Center in KTaO3

The gyromagnetic factors g∥, g⊥ and the hyperfine structure constants A∥ and A⊥ of the tetragonal Cu2+ center in KTaO3 are theoretically studied in this work. Based on the analyses of the electron paramagnetic resonance results of this center, it is found that the impurity Cu2+ occupies the octahedral Ta5+ site, associated with a nearest-neighbouring oxygen vacancy VO along the C4 axis. Due to the electrostatic repulsion of VO, Cu2+ is displaced away from VO by ΔZ(≈ −0.29 Å ) along the C4 axis. The theoretical values of the g and A factors based on the above defect structure and the impurity displacement agreee reasonably with the experimental data.

Jahn-Teller impurity dependence of the transition temperature Tc1, critical exponent and pseudo-Jahn-Teller well splitting in ZnTiF6.6H2O

The 182 K (-91.2 degrees C) trigonal-to-monoclinic phase transition in ZrTiF6.6H2O single crystal has been studied by EPR, for various concentrations of doped Jahn-Teller (JT) impurity Cu2+ ions and also for non-JT impurity Ni2+ ions. The transition temperature Tc1 which decreases with increase in impurity concentration, is more strongly affected by the JT impurity. The critical exponent beta as well as the JT potential well splitting E0 have been determined from the temperature variation in EPR signal intensity in the immediate neighborhood of Tc1 for the Cu2+ impurity. The critical exponent beta and E0 are dependent strongly on the JT impurity concentration.