Structures of Impurity Defects in Lithium Niobate and Tantalate Derived from Electron Paramagnetic and Electron Nuclear Double Resonance Data

Point intrinsic and extrinsic defects, especially paramagnetic ions of transition metals and rare-earth elements, have essential influence on properties of lithium niobate, LN and tantalate, LT, and often determine their suitability for numerous applications. Discussions about structures of the defects in LN/LT have lasted for decades. Many experimental methods facilitate progress in determining the structures of impurity centers. This paper gives current bird’s eye view on contributions of Electron Paramagnetic Resonance (EPR), and Electron Nuclear Double Resonance (ENDOR) studies to the determination of impurity defect structures in LN and LT crystals for a broad audience of researchers and students. Symmetry and charge compensation considerations restrict a number of possible structures. Comparison of measured angular dependences of ENDOR frequencies with calculated ones for Li and Nb substitution using dipole–dipole approximation allows unambiguously to determine the exact location of paramagnetic impurities. Models with two lithium vacancies explain angular dependencies of EPR spectra for Me3+ ions substituting for Li+ like Cr, Er, Fe, Gd, Nd, and Yb. Self-compensation of excessive charges through equalization of concentrations of Me3+(Li+) and Me3+(Nb5+) and appearance of interstitial Li+ in the structural vacancy near Me3+(Nb5+) take place in stoichiometric LN/LT due to lack of intrinsic defects.

Базовые элементы структуры границ зерен наклона. Часть 2. Оси разориентации [110] и [111]-=SUP=-*-=/SUP=-

The computer simulation methods have been applied to calculate structure and energy of symmetric tilt grain boundaries (GB) with the misorientation axes [110] and [111]. The calculations have been carried out with the use of the structural-vacancy model. The study of the atomic structure has been carried out within the entire range of misorientation angles. The reverse density of coincidence sites in special grain boundaries has amounted Σ≤57. The calculations have been carried out with the use of the Morse pair potential and the Cleri-Rosato many-body potential. When calculated with different potentials, the dependence of GB energy on the misorientation angle has a similar form, and the atomic structure completely coincides. It has been shown that the structure of any GB with the misorientation axes [110] and [111] may be represented by a limited number of basic structural units. All found basic structural units defined as units of A, B, C and D types are based on the structures of special grain boundaries. Such special GBs shall be Σ3(111), Σ3(112), Σ11(113) and Σ9(114) for GBs with the misorientation axis [110], and as regarding GBs with the misorientation axis [111], such special GBs shall be Σ3(112), Σ7(123) and Σ13(134). Ranges of angles within which certain basic structural units are found have been defined.

Базовые элементы структуры границ зерен наклона. Часть I. Ось разориентации [100]

The computer simulation methods have been applied to calculate structure and energy of symmetric tilt grain boundaries with the misorientation axis [100]. The calculations have been carried out with the use of the structural-vacancy model previously developed by the authors. The misorientation angles of grain boundaries of common type have varied from 0º up to 90º while the increment has amounted 1º. The reverse density of coincidence sites in special grain boundaries has amounted Σ ≤ 53. The calculations have been carried out with the use of the Morse pair potential and the Cleri-Rosato many-body potential. It is shown that the dependence of the energy of grain boundaries on the misorientation angle when calculated with different potentials has a similar form, and their structure does not depend on the choice of potential and is in good agreement with high-resolution electron microscopic images. Only one special grain boundary is distinguished on the energy curve which is Σ5(013). It has been found that the structure grain boundaries may be represented by a limited number of atomic groups that have been named basic structural units. The structure of low-angle grain boundaries with the misorientation angle less than 8º is described by alternation of basic structural units of D types and ideal crystal, while the structure of low-angle grain boundaries with the misorientation angle ranging from 8º up to 13º is described by alternation of structural units of C and D types, from 14º up to 23º – B and C types. High-angle grain boundaries in the range of misorientation angles from 24º up to 37º contain only of basic structural units of B types, from 38º to 50º – A and B, more than 50º – only A.

Evolution of superconductivity in K2−xFe4+ySe5: Spectroscopic studies of X-ray absorption and emission

This study investigates the evolution of superconductivity in K2−xFe4+ySe5 using temperature-dependent X-ray absorption and resonant inelastic X-ray scattering techniques. Magnetization measurements show that polycrystalline superconducting (SC) K1.9Fe4.2Se5 has a critical temperature (Tc) of ∼31 K with a varying superconducting volume fraction, which strongly depends on its synthesis temperature. An increase in Fe-structural/vacancy disorder in SC samples with more Fe atoms occupying vacant 4d sites is found to be closely related to the decrease in the spin magnetic moment of Fe. Moreover, the nearest-neighbor Fe–Se bond length in SC samples exceeds that in the non-SC (NS) sample, K2Fe4Se5, which indicates a weaker hybridization between the Fe 3d and Se 4p states in SC samples. These results clearly demonstrate the correlations among the local electronic and atomic structures and the magnetic properties of K2−xFe4+ySe5 superconductors, providing deeper insight into the electron pairing mechanisms of superconductivity.