Propagation of longitudinal ultrasonic waves in RbH2PO4 near its ferroelectric phase transition temperature

Ferroelectric phase transition in RbH₂PO₄ has been investigated using propagation of longitudinal acoustic waves along the polar axis near the transition temperature. The velocity of this mode is continuous across the transition temperature. Velocity data in the ferroelectric phase are analyzed in terms of coupled soft modeacoustic mode model of Pytte to obtain the temperature dependence of the soft mode frequency. The attenuation data in the ferroelectric phase show power law dependence. It follows scaling behaviour of the type predicted by Kawasaki from the mode-mode coupling theory and the dynamical scaling.     

Theory studies of the local lattice distortions and the EPR parameters for Cu2+, Mn2+ and Fe3+ centres in ZnWO4

The local lattice distortions and the electron paramagnetic resonance (EPR) parameters (g factors, hyperfine structure constants and zero-field splittings) for Cu²⁺, Mn²⁺ and Fe³⁺ in ZnWO₄ are theoretically studied based on the perturbation calculations for rhombically elongated octahedral 3d⁹ and 3d⁵ complexes. The impurity centres on Zn²⁺ sites undergo the local elongations of 0.01, 0.002 and 0.013 Å along the C₂ axis and the planar bond angle variations of 8.1°, 8.0° and 8.6° for Cu²⁺, Mn²⁺ and Fe³⁺, respectively, due to the Jahn–Teller effect and size and charge mismatch. In contrast to the host Zn²⁺ site with obvious axial elongation (～0.31 Å) and perpendicular (angular) rhombic distortion, all the impurity centres demonstrate more regular octahedral due to the above local lattice distortions. The copper centre exhibits significant Jahn–Teller reductions for the spin-orbit coupling and orbital angular momentum interactions, characterised by the Jahn–Teller reduction factor J (≈0.29 ? 1). The calculated EPR parameters agree well with the experimental results. The local structures of the impurity centres are analysed in view of the corresponding lattice distortions.     

Investigations on the EPR parameters and tetragonal distortions for Cu2+ in alkali lead tetraborate glasses

The electron paramagnetic resonance parameters (g factors and hyperfine structure constants) and local structures are theoretically investigated for Cu²⁺ in alkali lead tetraborate 90R₂B₄O₇·9PbO·CuO (R = Li, Na and K) glasses based on the high-order perturbation calculations for a tetragonally elongated octahedral 3d⁹ complex. The [CuO₆]^10? complexes are found to experience the relative tetragonal elongation ratios 18%, 23% and 30% for R = Li, Na and K, respectively, due to the Jahn–Teller effect, much larger than those for similar ARbB₄O₇ (A = Li, Na and K) glasses. This point is attributed to the lattice expansion (longer A–O bond lengths) with doped PbO, yielding lower force constants and more intense Jahn–Teller elongations in the 90R₂B₄O₇·9PbO·CuO glasses. The increasing tendency (Li > Na > K) of the relative elongation ratio λ, covalency and the ratio Δg_///Δg_⊥ for g-shifts are systematically analysed in a uniform way.     

Defect Structure and EPR Parameters of the Cu2+Center in MNB Ternary Glasses

The electron paramagnetic resonance (EPR) parameters (g factors g _//, g _⊥ and hyperfine structure constants A _//, A _⊥) for 15MgO-15Na₂O-69B₂O₃ (MNB):Cu²⁺ ternary glasses were calculated based on the high-order perturbation formulae of 3d⁹ ion in a tetragonal symmetry. From the calculations, the defect structures of MNB:Cu²⁺ ternary glasses were obtained and a negative sign for A _// and A_⊥ for the Cu²⁺ center is suggested in the discussion.     

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.     

Theoretical investigations of the spin Hamiltonian parameters and local tetragonal distortions for Cu2+ in crystalline and amorphous TeO2 and GeO2

The spin Hamiltonian parameters (i.e., anisotropic g factors and hyperfine structure constants) and local tetragonal distortions for Cu²⁺ in crystalline and amorphous TeO₂ and GeO₂ are theoretically investigated using the high-order perturbation formulas of these parameters for a tetragonally elongated octahedral 3d⁹ cluster. The impurity Cu²⁺ occupying the octahedral sites are found to experience the relative tetragonal elongation ratios of about 11.4% and 9.5% for crystalline TeO₂ and GeO₂ and 10.8% and 6.6% for amorphous TeO₂ and GeO₂, respectively, along the C₄ axis due to the Jahn–Teller effect. This reveals the larger tetragonal elongation distortions for the Cu²⁺ centres in crystalline than amorphous systems (especially TeO₂). The theoretical spin Hamiltonian parameters show good agreement with the experimental data. The results are discussed.     

An EPR Study on the Chemical form of Mn2+ in the Chinese Loess Samples

Electron paramagnetic resonance (EPR) spectroscopy in combination with thermal methods were used to identify and characterize Mn²⁺ in the Chinese loess that is a multimineral system. EPR spectra of the loess samples from the classic loess-paleosol section in central China show the presence of trace amounts of Mn²⁺; whereas paleosol samples present no Mn²⁺ EPR signal. The spectral changes upon step heating from room temperature to 1000 °C suggest that this EPR signal in the loess arises from Mn substituted into CaCO₃. This study provides a direct evidence that the loess-paleosol profiles were formed under the changing redox conditions caused by a past climatic change.     

X-ray study of the low-temperature phase transition in K2CoCl4

Precise lattice parameter measurements and intensity measurements of selected main and satellite reflections of K₂CoCl₄ have been performed in the temperature range 100 to 300 K in the vicinity of the low-temperature phase transition (commensurate-commensurate phase transition, T _c = 142 K). A broadening of the FWHM for the h01 reflections was observed below 142 K which suggests a transition from an orthorhombic phase to a monoclinic phase.     

Theoretical analysis of spin-Hamiltonian parameters for the rhombic Cu2+ centres in CuGaSe2 crystals

The spin-Hamiltonian parameters (g factors g_i and hyperfine structure constants A_i, where i = x, y, z) of the rhombic Cu²⁺ centres in the CuGaSe₂ crystal are determined from the high-order perturbation formulae based on the cluster approach (sometimes also called two-spin-orbit parameter model). In the studies, some parameters in the analysis of g factors for the same centre within the tetragonal symmetry approximation in the previous paper are used, and the parameter due to the perturbation of rhombic crystal field caused by a charge compensator at, e.g., [110] direction are considered. As the result of a fitting process, the determined spin-Hamiltonian parameters are in reasonable agreement with the experimental values. The results are discussed.     

EPR g factors and tetragonal distortion for the isoelectronic Ni and Cu centers in the CuGaSe2 crystal

The EPR g factors, g_|| and g_⊥, for the isoelectronic 3d⁹ ions Ni⁺ and Cu²⁺ at the tetragonal Cu⁺ site of the CuGaSe₂ crystal are calculated from the high-order perturbation formulas based on a two-spin-orbit-parameter model. In the model, both the contributions to g factors from the spin-orbit parameter of central 3d⁹ ion and that of ligand ion are contained. The calculated results appear to be consistent with the experimental values. The tetragonal distortions (characterized by θ−θ₀, where θ is the angle between the metal-ligand bond and C₄ axis, and θ₀≈54.74° is the same angle in cubic symmetry) of Ni⁺ and Cu²⁺ centers, which are different from the corresponding angle in the host CuGaSe₂ crystal and from impurity to impurity, are obtained from the calculations. The difference of the sign of g_||−g_⊥ between the isoelectronic Ni⁺ and Cu²⁺ centers is found to be due to the different tetragonal distortions of both centers in the CuGaSe₂ crystal.     

The Interaction Between Levofloxacine Hydrochloride and DNA Mediated by Cu2+

Levofloxacin (LEV) is a fluoroquinolone antimicrobial agent. LEV also inhibits DNA synthesis and is bactericidal. The mechanisms of the interaction among LEV, DNA and Cu2+ are studied by fluorescence method. In the paper, the results show that LEV and Cu2+ can form binary complex, and also LEV and DNA can form complex mediated by Cu2+. The composition of the complex is determined. The affects to the reaction are also found.     

Theoretical studies of the spin Hamiltonian parameters and local structures for WO3 doped Zn3(PO4)2ZnO nanopowders

The spin Hamiltonian parameters (SHPs) and the local structures for impurity W⁵⁺ in the Zn₃(PO₄)₂ZnO nanopowders doped with WO₃ under different concentrations are theoretically investigated using the perturbation calculations of these parameters. The exponential functions of the related quantities (cubic field parameter Dq, covalency factor N, relative tetragonal compression ratio τ and core polarisation constant κ) of concentration x with totally four adjustable coefficients a, b, c and d are adopted to fit the concentration dependences of the experimental d-d transition bands and SHPs. The impurity W⁵⁺ centres demonstrate moderate tetragonal compression ratios τ (～3.1%) due to the Jahn–Teller effect. With the increase of WO₃ concentration, Dq and N show moderately decreasing rules, while τ and κ exhibit slightly and moderately increasing tendencies with x, respectively. The mechanisms of the above concentration dependences of these quantities are analysed from the modifications of the local crystal-field strength and electron cloud density around the impurity W⁵⁺ with the variation of x. Present theoretical studies would be useful to the exploration of the structural properties and optical applications for WO₃ doped Zn₃(PO₄)₂ZnO nanopowders.     

Theoretical investigation of the spin-Hamiltonian parameters and optical spectra for the doped Cu2+ in ZnCdO nanopowder

The spin-Hamiltonian (SH) parameters (g factors g_||, g_⊥ and hyperfine structure constants A_||, A_⊥) and d–d transitions for ZnCdO:Cu²⁺ are calculated based on the perturbation formulas for a 3d⁹ ion in tetragonally elongated octahedra. Good agreement between the calculated results (four SH parameters and three optical absorption bands) and the experimental results can be obtained. Since the SH parameters are sensitive to the local structure of a paramagnetic impurity center, the tetragonal distortion (characterized by the relative elongation ratio ρ ≈ 3.5% along the C₄ axis) of the impurity center due to the Jahn–Teller effect is also acquired from the calculations. The negative and positive signs of hyperfine structure constants A_|| and A_⊥ for ZnCdO:Cu²⁺, respectively, are also suggested in the discussions.     

Theoretical studies of the local structures and electron paramagnetic resonance parameters for Cu2+ center in Zn(C3H3O4)2(H2O)2 single crystal

The electron paramagnetic resonance (EPR) parameters (g factors g_xx, g_yy, g_zz and hyperfine structure constants A_xx, A_yy, A_zz) are interpreted by taking account of the admixture of d-orbitals in the ground state wave function of the Cu²⁺ ion in a Zn(C₃H₃O₄)₂(H₂O)₂ (DABMZ) single crystal. Based on the calculation, local structural parameters of the impurity Cu²⁺ center were obtained (i.e. R_a≈1.92 Å, R_b≈1.96 Å, R_c≈1.99 Å). The theoretical EPR parameters based on the above Cu²⁺?O^2? bond lengths in the DABMZ crystal show good agreement with the observed values and some improvements have been made as compared with those in the previous studies.     

Pressure-induced change of the interlayer exchange interaction from ferromagnetic to antiferromagnetic in CS2CuF4 observed by neutron scattering experiments up to 2 GPa

Abstract

Neutron scattering experiments on the two-dimensional Heisenberg ferromagnets Cs₂ CuF₄ and K₂CuF₄ have been performed around 2 ～ 3 GPa over 1·4–15 K. At ambient pressure both the intralayer and the interlayer exchange interactions in these two compounds are ferromagnetic. At about 2 GPa, the interlayer exchange coupling in Cs₂ CuF₄ is found to change from ferromagnetic to antiferromagnetic, while the ferromagnetic intralayer exchange interaction is maintained. Contrary to Cs₂CuF₄, the ferromagnetism in K₂Cuf₄ is not destroyed by pressure up to 9 GPa, that was confirmed in the early study of the magnetic susceptibility measurements.