G.E. Barberis

Synthesis, crystal structure and spectroscopic properties of the NH4NiPO4·nH2O (n= 1,6) compounds; magnetic behaviour of the monohydrated phase

NH4NiPO4·6H2O and NH4NiPO4·H2O have been obtained by adding different concentrations of H3PO4 to dilute solutions of NiCl2·6H2O with special attention to the control of the pH in the solvent medium, which was regulated by addition of NH4OH. The NH4NiPO4·6H2O compound crystallizes in the orthorhombic Pmn21space group with cell parameters a= 6.9032(8), b= 6.0907(5) and c= 11.1402(8)A, V= 468.39(7)A3, Z= 2, R= 23 and Rw= 2.3%. The structure is three-dimensional and consists of Ni[O(w)]6 octahedra [O(w)= oxygen from a water molecule] linked to PO4 and NH4 tetrahedra by hydrogen bonds. All polyhedra are quite regular in this compound. The NH4NiPO4·H2O phase crystallizes in the Pmn2l space group with cell parameters a= 5.5698(2), b= 8.7668(2) and c= 4.7460(2)A. The structure of this compound has been refined with the Rietveld method using the coordinates of the KMnPO4·H2O phase as a starting model. The final residual factors were Rwp= 7.37, RB= 2.68%. The structure is formed from sheets of distorted NiO6 corner-sharing octahedra bridged through the oxygen atoms of the phosphate tetrahedra. These layers are pillared along the b direction and are interconnected by hydrogen bonds with the NH4+ cations, which are inserted between the sheets. The spectroscopic properties of both compounds are in good agreement with the symmetry observed in each phase. The values of the nephelauxetic ratio, β, are 0.89 and 0.94 for the hexahydrated and monohydrated compounds respectively. Magnetic susceptibility and specific heat results obtained for NH4NiPO4·H2O show an essentially two-dimensional antiferromagnetic exchange coupling, which becomes of a more three-dimensional behaviour with decreasing temperature.

Evidence for dynamic effects in the ESR spectra of the high Tc superconductor YBa2Cu3O7

Abstract The ESR spectra of the high Tc superconductor YBa2Cu3O7 show the structure corresponding to the Cu2+ (3d9, S= 1 2 , I= 3 2 ) ions in an anisotropic environment and in the presence of rapid oscillating distortions.

Intermediate valence effects in the magnetic behaviour of Ce (RhxPt1−x)2 laves phase compounds

Abstract The complete series of solid solutions Ce (Rh x Pt 1−x ) 2 (0 ≤ × ≤ 1) present a single cubic phase C15. The magnetic susceptibility and lattice parameters were measured, showing a big change in the temperature dependence of the susceptibility, which is explained as a change in the valence of the Ce ions, from 3+ (CePt 2 ) to 3.5+ (CeRh 2 ). This is supported by the lattice parameter study, which shows a non-linear relationship of the lattice parameter with the concentration x. We compare our results with previous works, and we also found a T 2 dependence for the susceptibility in the low temperature region for the compounds presenting intermediate valence. This can be explained in the light of the Fermi liquid theory.

Unexpected substitution in the Li1 − 3xFexNiPO4 (0 < x < 0.15) solid solution. Weak ferromagnetic behaviour

Li1 − 3xFexNiPO4 (0 < x < 0.15) solid solution phases have been synthesized by solid state reaction. X-Ray studies on polycrystalline samples show that all phases are isostructural with the parent LiNiPO4 compound, crystallizing in the orthorhombic system, space group Pnma. The evolution of the lattice parameters with the iron content follows Vegards law. The IR spectra show split bands for the phosphate groups, in good agreement with the distortion observed in the crystal structure. The EPR spectra for all phases show a broad band centred at zero field. The absence of a signal corresponding to Fe3+ ions suggests the presence of FeIII–NiII interactions strongly affected by the zero field splitting of the nickel(II) ions. The magnetic behaviour of the Li1 − 3xFexNiPO4 (0 < x < 0.15) phases can be described as antiferromagnetic with the presence of weak ferromagnetism below the ordering temperature. The magnitude of the remanent ferromagnetic moment is indicative of the existence of iron clustering in the samples. The FeO6 octahedra form finite chains in which the magnetic moments of the Fe3+ ions are antiferromagnetically aligned with a small canting angle.

Fully relativistic electronic structure of the semiconductors PbTe, PbSe and PbS

Abstract We present a fully relativistic band structure and the corresponding electronic density of states for the lead salts PbTe, PbSe and PbS, calculated using the parametrized linear combination of atomic orbitals of Slater and Koster, and the Vogl basis {sp 3 s ∗ } . We substitute for the pure atomic orbitals, symmetrically orthogonalized orbitals, through the Lowdin process; we consider only nearest-neighbours, and replace the actual hamiltonian with a pseudo-hamiltonian which includes the orthogonalization terms of the basis. The density of states was calculated by the tetrahedral division of the Brillouin zone and linear interpolation. Our results show that with this simple model, diagonalizing a 20 × 20 matrix with only 13 adjustable parameters, it is possible to reproduce the valence and lower conduction bands of a fully relativistic calculation done with the local empirical pseudo potential method.

Electronic Structure of La0.5Ca0.5MnO3

Electronic structure calculations on La0.5Ca0.5MnO3 are reported. Calculations were performed with the Full Potential LAPW method within the Local Density Approximation (LDA) as well as the Generalised Gradient Approximation (GGA). The ferromagnetic phase of La0.5Ca0.5MnO3 is half-metallic, which is important in the relation to the colossal magnetoresistance properties of this compound.

Stress-induced g-anisotropy of Er in Ag thin films

Abstract ESR of diluted Er in epitaxially growth Ag [111] single crystal thin films has been measured. The Er resonance was found to have anisotropic g-value. This anisotropy was interpreted in terms of induced strains on the film by the substrate. The second order orbit-lattice coupling trigonal parameter was estimated. Its sign was found to be in disagreement to that predicted by the point dipole model.

ESR of different Gd3+ sites in CaB6

The environment of Gd 3+ (4f 7 , S = 7/2) ions in single crystals of Ca 1-x Gd x B 6 (0.0001≤x≤0.01) is studied by electron spin resonance (ESR). The spectra for x≤0.001 show a split spectrum due to cubic crystal field effects (CFE). The line shape of each fine structure line is Lorentzian, indicating an insulating environment for the Gd 3+ ions. For 0.003≤x≤0.01, the spectra show a single resonance (g 1.992(4), ΔH 1/2 60 Oe) with no CFE and Dysonian line shape indicating metallic environment for the Gd. 3+ ions. For intermediate concentrations, a coexistence of spectra corresponding to insulating and metallic regions is observed. Most of the measured samples show the weak ferromagnetism (WF) as reported for Ca 0.995 La 0.005 B 6 , but, surprisingly, this WF has no effect in our ESR spectra either for metallic or insulating environments. This result suggests that the WF in these systems might be isolated in clusters (defect-rich regions) and its relationship with metallicity is nontrivial.

Theoretical and experimental investigation of the electron spin resonance of Co2+ in Zn2(OH)PO4 and Mg2(OH)AsO4

Electron spin-resonance experiments were performed on Co2+ substituting for Zn2+ or Mg2+ in powder samples of Zn2(OH)PO4 and Mg2(OH)AsO4. These two compounds are iso-structural and contain the Co2+ in two environments with approximately octahedral and trigonal bipyramidal structures. The observed resonances are described using a theoretical model that considers the departures from the two perfect structures. It is shown that resonance in the penta-coordinated complex is allowed, and the crystal fields that would reproduce the resonance of the Co2+ in the two environments are calculated. The low intensity of the resonance in the penta-coordinated complex is explained by assuming that this site is much less populated than the octahedral one; this assumption was verified by a molecular calculation of the energies of the two environments, with both Co and Zn as central ions in Zn2(OH)PO4.

Electron spin resonance study of Fe3+ in LiNbO3 single crystals: Bulk and fibres

Abstract We present the ESR spectra and angular variations of both the line positions and linewidths, for Fe 3+ in LiNbO 3 single crystals grown by the Czochralski method and by laser heated pedestal growth. The spectra prove the high quality of the fibres and show that ESR is a good method to characterize these optical materials.