No abstract

Disordered fluorite-related solid solutions ͑AF 2 ͒ 1−x−y ͑LnЈF 3 ͒ x ͑LnЉF 3 ͒ y , where A = Ca, Sr, Ba; LnЈ = Er, Tm, Yb; LnЉ = Lu, Y; and x Ӷ y = 0.001-0.4, were studied by both optical detection and conventional electron paramagnetic resonance ͑EPR͒ techniques. The EPR spectra of paramagnetic rare-earth ions Er 3+ , Tm 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 ʈ , approaching the theoretical limits, and small values of factors g Ќ ϳ 0 in the ground states of the paramagnetic 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 fluoriterelated superstructures ͑AF 2 ͒ 1−y ͑LnF 3 ͒ y with compositions y =5/m, where m is an integer in the range of 13-19. The structure of "symmetric" hexameric clusters in CaF 2 , SrF 2 , and BaF 2 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.

Manganites with general formulas RMnO 3 and R 0.5 A 0.5 MnO 3 , where R 3ϩ is a rare-earth ion, and A 2ϩ is an alkaline-earth ion, are known as insulating pure or charge ordered compounds at low temperatures with some R and A. In the present work, it is shown how the charge ordering and crystal structure form the orbital structure, which strongly effects the superexchange interaction. An explicit space group and the magnitudes of the distortions of each crystal were used in the calculations. We assume an ideal insulating state of the crystals. The model presented in this work allows one to explain the CE and A magnetic structures in terms of charge and orbital structures using quantitative estimations of the exchange parameters. The model also allows one to find easy axis and noncollinear components of a multisublattice magnetic structure of manganites.

DFT calculations with six LDA, GGA, and hybrid functionals have been performed using the CRYS-TAL09 code to describe the crystal structure and vibrational spectra of Y 2 CaGe 4 O 12 cyclotetragermanate, a new optical host. Two space groups P4/nbm and Cmme have been considered. The former corresponds to a mixed (0.5 Ca + 0.5 Y) distribution at the octahedral sites found from the results of Rietveld refinement of room temperature powder XRD pattern; the latter refers to the model of crystallographically nonequivalent calcium and yttrium atomic setting in distorted oxygen octahedrons. The most accurate geometry description has been obtained with the WC1LYP and PBE (n = 6) hybrid functionals, while the B3LYP calculation provides the best agreement between the recorded infrared and Raman spectra and their computed counterparts. Assignments of most of the observed bands to vibrational modes are given. The comparison between calculated and experimental frequencies shows a general good agreement for the spectra below 600 cm −1 . The relationship between selected infrared bands and Raman lines, internal vibrations of the [Ge 4 O 12 ] unit, and external modes is briefly discussed.

The paper presents an investigation in the field of orbital physics of strongly correlated oxides. The theoretical study of vibronic mechanism of orbital and magnetic structures forming in BiMnO 3 crystal is carried out. An effect of orbital structure upon superexchange interaction is described. Nonlinear and second-neighbor terms in vibronic interaction on manganese ions play an important role in magnetic ordering of frustrated BiMnO 3 . It is shown that the linear vibronic interaction is insufficient to describe the experimentally detected ferromagnetic structure of bismuth manganite. The new approach to orbital structure formation, presented in the paper, could be used not only in manganite physics but also in other Jahn-Teller compounds.