Transparent oxyfluoride glass ceramics

The ferromagnetic structure of ${\mathrm{BiMnO}}_{3},$ ${T}_{c}=105 \mathrm{K},$ has been determined from powder neutron-diffraction data collected at 20 K on a sample synthesized at high pressures using a cubic anvil press. ${\mathrm{BiMnO}}_{3}$ is a distorted perovskite that crystallizes in the monoclinic space group $C2$ with unit-cell parameters $a=9.5317(7) \AA{},$ $b=5.6047(4) \AA{},$ $c=9.8492(7) \AA{},$ and $\ss{}=110.60(1)\ifmmode^\circ\else\textdegree\fi{}$ $(Rp=6.78%,$ $wRp=8.53%,$ reduced ${\ensuremath{\chi}}^{2}=1.107).$ Data analysis reveals a collinear ferromagnetic structure with the spin direction along [010] and a magnetic moment of $3.2\ensuremath{\mu}B.$ There is no crystallographic phase transition on cooling the polar room-temperature structure to 20 K, lending support to the belief that ferromagnetism and ferroelectricity coexist in ${\mathrm{BiMnO}}_{3}.$ Careful examination of the six unique Mn-O-Mn superexchange pathways between the three crystallographically independent ${\mathrm{Mn}}^{3+}$ sites shows that four are ferromagnetic and two are antiferromagnetic, thereby confirming that the ferromagnetism of ${\mathrm{BiMnO}}_{3}$ stems directly from orbital ordering.

Orbital ordering as the determinant for ferromagnetism in biferroic BiMnO 3

a b s t r a c t Cu-doped LaTiO3 perovskite was prepared with a sol–gel method and characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The introduction of Cu induced the formation of LaTiO3 perovskite in which titanium existed as Ti 3+ , resulting in the coexistence of Ti 3+/4+ and Cu +/2+ in the perovskite structure. LaTi0.4Cu0.6O3 showed highly Fenton activity and stability for the degradation of RhB with H2O2 in the initial pH range of 4–9. Moreover, the catalyst buffered the solution with the tested initial pH to around 7 during the adsorption process. The studies of electron spin resonance, the effect of radical scav

Enhanced Fenton degradation of Rhodamine B over nanoscaled Cu-doped LaTiO3 perovskite

A detailed spectroscopic study of ${\text{Yb}}^{3+}:{\text{CaF}}_{2}$ is made by using laser selective excitation, low-temperature time-resolved fluorescence, and Raman techniques. Three dominant types of ${\text{Yb}}^{3+}$ isolated ion centers are identified at very low $(0.03\text{ }\text{at}\text{.}\text{ }%)$ dopant concentration: two of them with cubic and tetragonal symmetries and the third one not clearly identified but likely of trigonal symmetry. Fluorescence decays strongly differ from one site to another. The overall measured positions of the Stark sublevels of the ${^{2}F}_{7/2}$ and ${^{2}F}_{5/2}$ multiplets are satisfactorily explained by the crystal-field calculations. On the other hand, at higher dopant concentrations, only one dominant class of ${\text{Yb}}^{3+}$ centers occurs which corresponds to some hexameric cluster in which the ${\text{Yb}}^{3+}$ ions site in nearly tetragonal crystal-field environments and whose energy-level positions are discussed and compared with those previously reported in the literature.

Spectroscopy of Yb 3+ :CaF 2 : From isolated centers to clusters

Er3+-doped PbF2: Comparison between nanocrystals in glass-ceramics and bulk single crystals

Disordered fluorite-related solid solutions AF21xy LnF3x LnF3y, where A=Ca, Sr, Ba; Ln=Er, Tm, Yb; Ln=Lu, Y; and x y=0.001-0.4, were studied by both optical detection and conventional electron paramagnetic resonance EPR techniques. The EPR spectra of paramagnetic rare-earth ions Er 3+ ,T m 3+ , and Yb 3+ in clusters of diamagnetic Y 3+ and Lu 3+ ions were recorded. It appears that the crystalline electric field at the sites of Ln ions in the clusters is of "nearly" tetragonal symmetry and provides for high values of factors g, approaching the theoretical limits, and small values of factors g0 in the ground states of the paramag- netic Ln ions. It was assumed that all the clusters of Ln ions in the solid solutions appear to be similar in structure to the hexameric clusters, which are the basic structural units of the homologous series of fluorite- related superstructures AF21yLnF3y with compositions y=5/m, where m is an integer in the range of 13-19. The structure of "symmetric" hexameric clusters in CaF2, SrF2, and BaF2 hosts was established by computer simulation. The crystalline electric field and the spectroscopic ground-state parameters for Er 3+ , Tm 3+ , and Yb 3+ ions in the hexameric clusters were calculated and found to be in agreement with the experimental data, being totally different from those known for the "isolated" simple cubic and tetragonal centers in the fluorite crystals.

EPR spectra and crystal field of hexamer rare-earth clusters in fluorites

In solid solutions of alkaline-and rare-earth fluorides with a fluorite structure, ions of most elements of the rare-earth (RE) row form hexameric clusters that assimilate the minor component of the solid solutions (fluorine) and build it into the cubic fluorite lattice without changing its shape. An analysis of the EPR spectra of paramagnetic RE ions (Er3+, Tm3+, Yb3+) in clusters of diamagnetic ions (Lu3+, Y3+) confirms their hexagonal structure, which was established when studying the superstructures of the compounds under study. In such a cluster, a RE ion is in a nearly tetragonal crystal field, with the parameters of this field differing radically from those of single cubic and tetragonal RE centers in crystals with a fluorite structure. In particular, this field causes high (close to limiting) values of the g∥ factors of the ground states of the paramagnetic RE ions. Computer simulation is used to determine the atomic structure of a hexameric cluster in MF2 crystals (M = Ca, Sr, Ba). The crystal field and energy spectrum of Er3+, Tm3+, and Yb3+ ions in such clusters are calculated, and the spectroscopic parameters of the ground states of these ions are determined. The calculations confirm the earlier assumption that the unusual EPR spectra of nonstoichiometric fluorite phases are related to RE ions in hexameric clusters.

Crystal fields of hexameric rare-earth clusters in fluorites

A model is proposed for the LaTiO3 compound, which allows for the strong coupling of the lattice, orbital, and magnetic subsystems of this crystal. The interaction of the lattice with the electronic structure of the LaTiO3 compound is described by the vibronic Hamiltonian with the constants calculated using the ab initio Hartree-Fock-Roothaan method with the inclusion of the electron correlations. It is demonstrated that a considerable contribution to the formation of the orbital structure of the LaTiO3 compound is made by the next-nearest neighbors of the Ti3+ ion. The orbital-dependent superexchange interactions in LaTiO3 are described within the Hubbard model and agree well with experiment.

Structure and the electronic and magnetic properties of LaTiO 3

A model of the low-temperature orthorombic phase (O′) of LaMnO 3 was constructed using pair potentials with explicitly allowing for the many-body Jahn–Teller contribution to the crystal energy. The analysis of the structure, elastic constants and phonon frequencies is based on the comparison of the structural phases that obtained with including and “switching-off” Jahn–Teller contribution to the energy and dynamical matrix of the crystal.

The influence of the Jahn–Teller effect on phonons in LaMnO3

The relation between the orbital ordering and magnetic structure of the crystal LaMnO3 is investigated. The dependence of the exchange parameters on the angle Φ of the orbital structure is determined. When the isotropic exchange interaction and the single-ion anisotropy, which depends on the angle Φ and the rotational distortions, are introduced into the spin Hamiltonian, a four-sublattice structure (AX, FY, GZ) is obtained with orientation of the magnetic moments of the sublattices near the long axis of the orthorhombic cell of the crystal in the basal plane of the crystal (AX ≫ GZ, FY). The effect of a magnetic field on the magnetic structure and the antiferromagnetic resonance spectrum are investigated taking account of the nonequivalent, anisotropic, orbitally-dependent g tensors. The spin-flop and spin-flip transition fields are calculated.

A. E. Nikiforov

Papers

EPR spectra and crystal field of hexamer rare-earth clusters in fluorites

Crystal fields of hexameric rare-earth clusters in fluorites

Structure and the electronic and magnetic properties of LaTiO 3

The influence of the Jahn–Teller effect on phonons in LaMnO3

Antiferromagnetic resonance spectrum in LaMnO 3 : Interrelation of the orbital structure and the magnetic properties