N.I. Sorokina

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 of fluorite-like compound based on Nd5Mo3O16 with lead partly substituting for neodymium

A single crystal of Nd5Mo3O16 with lead partly substituting for neodymium, which has a fluorite-like structure, was studied by precision X-ray diffraction, high-resolution transmission microscopy and EDX microanalysis. The crystal structure is determined in the space group Pn3¯n. It was found that the Pb atoms substitute in part for Nd atoms in the structure and are located in the vicinity of Nd2 positions. Partial substitutions of Mo cations for Nd positions and of Nd for Mo positions in crystals of the Ln5Mo3O16 oxide family are corroborated by X-ray diffraction for the first time. The first experimental verification of the location of an additional oxygen ion in the voids abutting MoO4 tetrahedra was obtained.

Single-crystal structure of Nd{sub 5}Mo{sub 3}O{sub 16} at T = 30 K

A precision X-ray diffraction study of Nd5Mo3O16 single crystals is performed for the first time at 30 K. Measurements in the range from room temperature to 30K showed that the unit-cell parameters and volume change smoothly. The crystal structure at T = 30 K is similar to that at room temperature. The model of splitting of the atomic positions is confirmed.

Growth and structure of K{sub 2}Ni{sub x}Co{sub (1–x)}(SO{sub 4}){sub 2} · 6H{sub 2}O single crystals

Single crystals of the K2NixCo(1–x)(SO4)2 · 6H2O composition are grown by spontaneous flux crystallization. More exact chemical formulas of the single crystals are determined based on the diffraction data as K2Co(SO4)2 · 6H2O (I), K2(Co0.657Ni0.343)(SO4)2 · 6H2O (II), K2(Co0.226Ni0.774)(SO4)2 · 6H2O (III), K2(Co0.216Ni0.784)(SO4)2 · 6H2O (IV), and K2Ni(SO4)2 · 6H2O (V). The substitution of nickel atoms for cobalt atoms in structure I results in a shortening of all (Co,Ni)–O interatomic distances. With increasing Ni concentration, the (Co,Ni)–O2 distance shortens to a lesser degree than the (Co,Ni)–O1 and (Co,Ni)–O3 distances and, as a consequence, the distortion of (Co,Ni)O6 octahedra decreases. NiO6 polyhedra are less distorted than CoO6 octahedra. The analysis of difference syntheses of electron density shows that the number of uninterpretable peaks on the maps of mixed crystals II, III, and IV, as well as on the map of K2Co(SO4)2 · 6H2O, is larger with respect to those of structure K2Ni(SO4)2 · 6H2O.Single crystals of the K{sub 2}Ni{sub x}Co{sub (1–x)}(SO{sub 4}){sub 2} · 6H{sub 2}O composition are grown by spontaneous flux crystallization. More exact chemical formulas of the single crystals are determined based on the diffraction data as K{sub 2}Co(SO{sub 4}){sub 2} · 6H{sub 2}O (I), K{sub 2}(Co{sub 0.657}Ni{sub 0.343})(SO{sub 4}){sub 2} · 6H{sub 2}O (II), K{sub 2}(Co{sub 0.226}Ni{sub 0.774})(SO{sub 4}){sub 2} · 6H{sub 2}O (III), K{sub 2}(Co{sub 0.216}Ni{sub 0.784})(SO{sub 4}){sub 2} · 6H{sub 2}O (IV), and K{sub 2}Ni(SO{sub 4}){sub 2} · 6H{sub 2}O (V). The substitution of nickel atoms for cobalt atoms in structure I results in a shortening of all (Co,Ni)–O interatomic distances. With increasing Ni concentration, the (Co,Ni)–O2 distance shortens to a lesser degree than the (Co,Ni)–O1 and (Co,Ni)–O3 distances and, as a consequence, the distortion of (Co,Ni)O{sub 6} octahedra decreases. NiO{sub 6} polyhedra are less distorted than CoO{sub 6} octahedra. The analysis of difference syntheses of electron density shows that the number of uninterpretable peaks on the maps of mixed crystals II, III, and IV, as well as on the map of K{sub 2}Co(SO{sub 4}){sub 2} · 6H{sub 2}O, is larger with respect to those of structure K{sub 2}Ni(SO{sub 4}){sub 2} · 6H{sub 2}O.

X-ray diffraction study of oxygen-conducting compounds Ln₂Mo₂O₉ (Ln = La, Pr).

The La2Mo2O9 (LM) and Pr2Mo2O9 (PM) single crystals are studied using precision X-ray diffraction and high-resolution transmission microscopy at room temperature. The crystal structures are determined in the space group P2(1)3. La and Pr atoms, as well as Mo1 and O1 atoms, are located in the vicinity of the threefold axes rather than on the axes as in the high-temperature cubic phase. In both structures studied, the O2 and O3 positions are partially occupied. The coexistence of different configurations of the Mo coordination environment facilitates the oxygen-ion migration in the structure. Based on the X-ray data, the activation energies of O atoms are calculated and the migration paths of oxygen ions in the structures are analysed. The conductivity of PM crystals is close to that of LM crystals. The O2 and O3 atoms are the main contributors to the ion conductivity of LM and PM.

Growth, structure, and properties of ferroelectric-ferroelastic-superionic K3Nb3B2O12 and K3-xNaxNb3B2O12 crystals

The potassium niobate-borate K3Nb3B2O12 (KNB) crystals and their solid solutions with partial substitution of potassium by sodium (KNB: Na) are grown from flux and their physical properties are studied. The specific feature of the crystals grown is a complicated polymorphism and the unique combination of ferroelectric and ferroelastic properties with superionic conductivity with respect to potassium ions.

GROWTH, MORPHOLOGY AND SUPERCONDUCTING PROPERTIES OF TMBA2CU3O7-X SINGLE-CRYSTALS

Nearly twin-free TmBa2Cu3O7−x single crystals of high crystalline perfection were grown from nonstoichiometric melts in the system Tm2O3BaO(BaO2CuO(Cu2O) in air atmosphere. The superconducting transition temperatures and widths depend on crystallization conditions and are Tc = 89–91 K and ΔTc = 0.2 K for the best samples. Morphology of the crystals and growth mechanisms of different faces were studied and are discussed.

Polymorphism and structure of ion-conducting rare earth molybdates

Alexander Antipin1, Olga Alekseeva2, Natalia Sorokina2, Alexander Dudka2, Valentina Voronkova3 1National Research Center Kurchatov Institute, Moscow, Russian Federation, 2Shubnikov Institute of Crystallography of Federal Scientific Research Centre “Crystallography and Photonics” of Russian Academy of Sciences, Moscow, Russian Federation, 3Lomonosov Moscow State University, Moscow, Russian Federation E-mail: antipin@physics.msu.ru

Growth and structure of K2CoxNi(1-x)(SO4)2·6H2O mixed single crystals

To date, crystals of Tutton salts with the general formula (М+)2М2+(SО4)2*6Н2О (where М+alkali metal or ammonium, М2+ bivalent metal Co2+, Ni2+) are used as a materials for ultraviolet (UV) filters. Only in recent year effort of ternary crystal growth has been taken. The main problem of mixed crystal growth from liquid solution is high level of the crystal inhomogeneity, which leads to generation of the elastic stress, inclusion trapping and microand macrocrack formation in the bulk crystal. For the first time the optically homogeneous mixed K2CoxNi(1 – x)(SO4)2 ⋅ 6H2O (KCNSH) large single crystals have been grown from solutions of different compositions by the temperature-reduction technique. Precise X-ray experiment of three mixed crystals was carried out by four-circle diffractometer CAD-4F and XcaliburS diffractometer with two-dimensional CCD detector at the room temperature. KCNSH crystals belong to the monoclinic space group P2(1)/c. Each Co2+ or Ni2+ ion is coordinated with six H2O molecules, forming a distorted octahedral (Co(H2O)6)2+ and (Ni(H2O)6)2+ unit. With increasing content of nickel ions in the crystal, the octahedral unit is narrowed and the unit cell volume is decreased. Chemical formulas refined using diffraction data are K2Co0.657Ni0.343(SO4)2•6H2O, K2Co0.226Ni0.774(SO4)2•6H2O and K2Co0.216Ni0.784(SO4)2•6H2O. Ratios of isomorphic cobalt and nickel components in the mixed crystals are conformed to data obtained by atomic emission spectroscopy. Effect of the Co2+ and Ni2+ ion ratio in KCNSH single crystal to crystal quality is considered.

Structure of KTP crystals grown by top-seeded solution and spontaneous flux crystallization methods

1 all play. Although we are all using the same ball, confusion reigns because we each play the game according to our own set of rules. A good chemical model requires well defined concepts that lead to quantitative predictions. The ionic model meets these criteria. The electroneutrality rule (the sum of all atomic valences in a compound is zero) is the only assumption the model makes and it applies to all compounds that obey this rule (mostly inorganic compounds, but also aqueous chemistry, hence much of biological chemistry). From the Coulomb field of the ionic model one can derive rigorous concepts of atom, bond, atomic valence, bond valence and electronegativity as well as Lewis acid and base strength. The result is a predictive model expressed in terms of the familiar chemical concept of localized bonds linking nearest-neighbour atoms.