O. A. Alekseeva

Crystal structure of the oxygen conducting compound Nd5Mo3O16

Abstract Structure of the Nd5Mo3O16 single crystal grown in the Nd2O3–MoO3 system was studied using the X-rays diffraction technique at 293 K and 110 K temperatures. The unit-cell values were always cubic relating to that of CaF2 fluorite as a ≈ 2af (af = 5.5 Å). The structure was solved within the Pn-3n symmetry group. It was found that the Nd5Mo3O16 compound has a fluorite-like structure with all atoms disordered. An indirect confirmation for the violation of translational periodicity in the distribution of Mo and Nd atoms was obtained. The possible oxygen diffusion paths were analyzed using the one-particle potentials of the oxygen atoms. The ionic conductivity of Nd5Mo3O16 compound is associated with the disordering of the oxygen atoms in several positions, and their deficiency in comparison with the initial fluorite.

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.

Electrical and nonlinear optical properties of KTiOPO4 single crystals doped with niobium, antimony, and tantalum

Single crystals of K1 − xTi1 − xNbxOPO4 (KTP: Nb), K1 − xTi1 − xSbxOPO4 (KTP: Sb), and K1 − xTi1 − xTaxOPO4 (KTP: Ta) solid solutions are grown and their dielectric, conducting, and nonlinear optical properties are investigated. The maximum contents x of niobium, antimony, and tantalum impurities in the crystals are equal to 0.11, 0.23, and 0.25, respectively. The doping of the KTiOPO4 crystals with niobium, antimony, and tantalum brings about the formation of additional potassium vacancies and additional potassium positions and, as a consequence, an increase in the ionic conductivity σ33. An increase in the doping level leads to a smearing of the ferroelectric phase transitions and a decrease in the phase transition temperatures. The permittivity of the doped crystals exhibits a broad relaxation peak in the temperature range 200–600°C.

Structure of KTiOAsO4 single crystals at 293 and 30 K

The unit cell parameters of KTiOPO4 and KTiOAsO4 single crystals are measured in the temperature range from room temperature to 20 K. It is found that the unit cell volume of the single crystals changes smoothly. With a decrease in temperature, the c parameter remains almost unchanged. In a certain temperature range, the linear temperature dependence of the a and b parameters is violated. An X-ray diffraction study of KTiOAsO4 single crystals is performed at T = 293 and 30 K. With a decrease in temperature, the electron density in the channels of the structure undergoes a redistribution, suggesting that at room temperature the state of the potassium ions is characterized by the dynamic and static disordering. The nonuniformity of the distribution of the electron density at the junctions of TiO6 octahedra and AsO4 tetrahedra is significantly enhanced in relation to that at the corresponding junctions in the KTiOPO4 structure. It has been experimentally established that the geometry of the tetrahedral anions makes a decisive contribution to the nonlinearity of KTiOAsO4 single crystals.

Crystal structure of La2Mo2O9 single crystals doped with bismuth

Precision X-ray diffraction studies of La2 − xBixMo2O9 (x = 0.04, 0.06, and 0.18) single crystals are performed. It is found that in the compounds doped with bismuth, analogously with the structure of the metastable βms phase of pure La2Mo2O9 (LM), the La, Mo1, and O1 atoms deviate from the threefold axis on which they are located in the high-temperature β phase. It is shown that bismuth atoms substitute for part of lanthanum atoms and occupy a position at the threefold axis in the neighborhood of the split lanthanum position. The implantation of bismuth atoms in the LM structure results in the return of a part of the molybdenum atoms to the position at the threefold axis. The occupancy of this position is equal to the occupancy of the bismuth atomic position.

Nanorelief of the natural cleavage surface of triglycine sulphate crystals with substitutional and interstitial impurities

Ferroelectric triglycine sulphate crystals (TGS) with substitutional (LADTGS+ADP, DTGS) and interstitial (Cr) impurities have been studied by atomic-force microscopy, X-ray diffraction, and X-ray fluorescence. The nanorelief parameters of the mirror cleavage TGS(010) surface have been measured with a high accuracy. A correlation between the crystal defect density in the bulk and the cleavage surface nanorelief is revealed at the submicrometer level.

Crystal structure of the metastable cubic βms phase of La2Mo2O9 single crystal at T = 33 K

The structure of the cubic metastable βms phase of La2Mo2O9 single crystal has been precisely investigated by X-ray diffraction at 33 K for the first time. The measurement of the unit-cell parameter of this crystal in the range from room temperature to 33 K showed that the unit-cell parameter and volume change continuously in this range. The crystal has a similar structure at T = 33 K and at room temperature. A local lowering of the symmetry for La and Mo atoms, caused by their displacement, is confirmed, and a similar displacement (which was not observed at room temperature) is revealed for O(1) atoms. The thermal parameters for O(2) and O(3) atoms do not change with a decrease in temperature, in contrast to the thermal parameters of Mo, La, and O(1) atoms. This fact indicates that the O(2) and O3 atoms in this crystal are statically disordered.

Synthesis, properties, and structure of potassium titanyl phosphate single crystals doped with hafnium

Single crystals of potassium titanyl phosphate doped with hafnium are grown by spontaneous flux crystallization. Their physical properties are studied, and the structure of three KTi1 − xHfxOPO4 crystals (x = 0.01, 0.03, and 0.12) is determined. In the crystals studied, hafnium mostly occupies the second titanium position. The doping of KTP crystals with hafnium results in an elongation of K-O bonds in the potassium polyhedra and, as a consequence, in a considerable (by approximately 180°C) decrease in the temperature of ferroelectric phase transition. The magnitude of anomalous permittivity substantially decreases. The electrical conduction in the specimens studied decreases by approximately half an order of magnitude in the low-temperature region but remains almost unchanged in the high-temperature region. Even at minor concentrations, the presence of a hafnium additive in the specimens considerably (by 35%) enhances the intensity of the second harmonic generation of laser radiation.

Structural reasons for the nonlinear optical properties of KTi0.96Zr0.04OPO4 single crystals

This paper reports on the results of the precision X-ray structural investigations of KTi0.96Zr0.04OPO4 single crystals at room temperature. It is established that the incorporation of zirconium atoms into the structure of KTiOPO4 (KTP) crystals does not lead to substantial changes in the framework structure and results only in an insignificant decrease in the scatter of the distances in the PO4 tetrahedra and the formation of more symmetric (TiZr)O6 octahedra as compared to the TiO6 octahedra. However, the incorporation of zirconium atoms into the KTP structure is accompanied by the redistribution of the electron density in the crystal as a whole, so that the electron density increases in the region of the positions occupied by the potassium atoms. This changes the nonlinear optical properties of the given series of crystals, which are estimated from the intensity of the second harmonic generation signals.

Structure of the metastable cubic B1 phase of the La2Mo2O9 single crystal

The structure of the metastable B1 phase of the La2Mo2O9 single crystal is investigated using X-ray diffraction. It is established that the crystal structure of the compound under investigation is described by the cubic unit cell with the parameter a = 7.158(5) Å, which makes it possible to index approximately 84% of the reflections measured for this single crystal. The structure of the metastable cubic B1 phase is characterized by a local lowering of the symmetry for the La and Mo atoms, which are displaced from their positions on the threefold axis, thus forming three sites around it with an occupancy of 0.333(2). The O(1) atom in the structure of the metastable cubic B1 phase remains in the 4a position on the threefold axis but occupies it by only 86%. The O(2) and O(3) atoms located in a general position occupy their own sites with occupancies of 0.77(2) and 0.35(2), respectively. The final R factor of the refinement of this structural model is 2.52%.