V. I. Simonov

Structure of superconducting single crystals of 2201 thallium cuprate (Tl1.85Cu0.15)Ba2CuO6, Tc = 110 K

Abstract Single crystals of 2201 phase with various superconducting transition temperatures ranging from nonsuperconducting to 110K have been obtained. An X-ray structural study of tetragonal single crystals with Tc = 110 K, space group 14/mmm, a = 3.8686(3), c = 23.259(3) A, has been carreid out. We have found that thallium atoms are partially replaced by copper atoms. We have also found that Tl atoms and O(3) atoms linked with Tl are displaced from their symmetrical sites. A correlation between the structure and properties of Tl-2201 cuprate crystals has been established on the basis of our data and the data reported in the literature.

Threshold concentrations in zinc-doped lithium niobate crystals and their structural conditionality

On the basis of precise X-ray diffraction study of lithium niobate single crystals of congruent composition and four zinc-doped (at 2.8, 5.2, 7.6, and 8.2 mol %) crystals, structural conditionality of the threshold concentrations of the dopant has been established. At these concentrations, the mechanism of zinc incorporation into crystal changes. As the zinc concentration increases, this element first substitutes excess niobium, localized in lithium positions, with a simultaneous decrease in the number of vacancies in these positions. Then zinc substitutes lithium with formation of new lithium vacancies. When a certain limit on the number of vacancies is reached, zinc begins to substitute niobium in its main positions. This process is naturally accompanied by a decrease in the number of vacancies to their complete disappearance and formation of a self-compensating crystal. The character of the dependence of the crystal physical properties on the dopant concentration changes specifically when the impurity concentration passes through the threshold values.

Zn Atoms in Lithium Niobate and Mechanism of Their Insertion into Crystals

Precision X-ray structural studies were carried out for LiNbO3:Znx single crystals with x=0.0, 2.87, 5.20, and 7.60 at. %. It was found that the insertion of the Zn atoms into the Li position was accompanied by a decrease in the concentration of intrinsic NbLi defects. At x=7.6%, the Zn atoms change their locations in the lattice and partially occupy the Nb positions. This clarifies the structural nature of the “threshold” Zn concentration, which manifests itself as singularities in the concentration dependences of various optical properties. The structural origin of the threshold concentration is likely a common feature of all nonphotorefractive impurities (Mg, Zn, In, and Sc) in LiNbO3. A change in the intrinsic defect structure of the LiNbO3 crystals with different Zn concentrations is discussed.

Crystal structure and optical activity of La3Nb0.5Ga5.5O14 and Sr3Ga2Ge4O14 single crystals of the langasite family

Precision X-ray structural studies of the La3Nb0.5Ga5.5O14 and Sr3Ga2Ge4O14 single crystals were carried out. The space group P321 was confirmed. The anomalous X-ray scattering was taken into account to establish the absolute structures (chirality) of the crystals; they proved to be of different sign. The structural features responsible for the optical activity of crystals were revealed, and the dependence of the magnitude and sign of specific rotation on the structural parameters of these compounds were determined.

Comparative study of Tl-2201 single crystals with Tc = 30 and 110 K by means of X-ray structural analysis and NMR

Abstract The crystal structure of a tetragonal single crystal of Tl-2201 with Tc = 30 K has been determined by the X-ray diffraction technique (a = 3.8717(3), c = 23.224(3) A, space group I4/mmm, Z = 2). The chemical composition is found to be Tl1.90(1)Cu.0.10(1)Ba2CuO6.18(4). Displacements of Tl and O3 atoms from their symmetric sites have been observed. Structural parameters of Tl-2201 cuprates with various Tc have been compared. The superconducting transition temperature is shown to correlate most strongly with the parameter c, with the length of the bond between Cu and apical oxygen as well as with the distance between Ba2+ and the cuprate plane.

Structural conditionality of the piezoelectric properties of langasite family crystals

The atomic displacements upon isomorphic substitutions in crystals of the langasite family have been analyzed. The thermal parameters are determined and the probability density function of atoms is analyzed. Local potential energy minima are found which can be occupied by atoms under external effects. The contributions of cations in all four independent crystallographic positions and anions in all three such positions to the piezoelectric properties are established. One specific structural feature is the constant (at isomorphic substitutions) or possible (under external effects) but always opposite displacements of two cations along symmetry axis 2. Large cations in eight-vertex polyhedra make the main contribution to the piezoelectric properties. The cations in the tetrahedra on symmetry axis 2 weaken these properties. The cations in the octahedra in the origin of coordinates and in the tetrahedra on symmetry axes 3 only slightly affect the piezoelectricity.

Structure and properties of niobium-doped potassium titanyl phosphate crystals

A series of potassium titanyl phosphate crystals, KTiOPO4, with various concentrations of niobium dopant has been grown, and some of their physical properties and structural characteristics have been studied. The incorporation of a small amount of niobium results in considerable changes in the electrical conductivity of KTP: Nb crystals and the temperature of the ferroelectric phase transition. Thus, the presence of 3–4 at. % of niobium results in an increase of conductivity by more than an order of magnitude, whereas TC decreases from 930 to 620°C. The X-ray diffraction study of the crystals has been performed at room temperature; the neutron diffraction analysis was made at temperatures of 20, 330, and 730°C. It was revealed that two crystallographically independent positions are statistically (by 90%) occupied by potassium cations, which results in the concentration of potassium atoms in the structure higher than it was expected from the condition of preservation of crystal electroneutrality. At high niobium concentrations, the monoclinic compound of the composition K2TiNb2P2O3 is formed.

Refinement of the atomic structure of CdTe single crystals

A comparative refinement of the anharmonic and disordered models of the atomic structure was performed based on precision X-ray diffraction data sets collected from a CdTe single crystal at 295, 376, 491, and 583 K. Although both models provide the asymmetric contributions of Cd and Te to the structure factors, the anharmonic model seems to be more advantageous.

Accurate crystal structure refinement of La3Ta0.25Ga5.25Si0.5O14

An accurate X-ray diffraction study of an La3Ta0.25Ga5.25Si0.5O14 single crystal has been performed using two data sets obtained independently for the same sample in different orientations on a diffractometer with a 2D CCD detector. This structure was refined with an averaged set of these data (a = 8.1936(15) Å c = 5.1114(6) Å, sp. gr. P321, Z = 1, R/wR = 0.75/0.71%, 4030 independent reflections). This analysis was aimed at determining the character of the occupancies of the cation position in the structure. The octahedra at the origin of coordinates turned out to be statistically occupied by gallium and tantalum ions of similar sizes, whereas the tetrahedra on the threefold symmetry axes are occupied by gallium and silicon whose ionic radii differ significantly. The latter circumstance caused the splitting of oxygen positions and made it possible to reliably establish the structural position of statistically located [SiO4] and [GaO4] tetrahedra of different sizes.

Atomic structure of (Sr0.50Ba0.50)Nb2O6 single crystals in the series of (SrxBa1 − x)Nb2O6 compounds

Czochralski grown (Sr0.50Ba0.50)Nb2O6 single crystals have been studied by the method of precision X-ray diffraction analysis. The structural characteristics of (SrxBa1 − x)Nb2O6 compounds with x = 0.33, 0.50, 0.61, and 0.75 were analyzed. The distributions of the Sr and Ba atoms over the crystallographic positions are considered depending on their concentration. The establishment of the mechanisms of isomorphous replacements in these solid solutions allows the variation, within certain limits, of crystal properties by changing the Ba/Sr ratio.