Sergey Yu. Stefanovich

Spin and Dipole Ordering in Ni2InSbO6 and Ni2ScSbO6 with Corundum-Related Structure

The complex metal oxides Ni2InSbO6 (NISO) and Ni2ScSbO6 (NSSO) have been prepared in form of polycrystalline powders by a solid state reaction route. The crystal structure and magnetic properties of the compounds were investigated using a combination of X-ray and neutron powder diffraction, electron microscopy, calorimetric and magnetic measurements. The compounds adopt a trigonal structure, space group R3, of the corundum related Ni3TeO6 (NTO) type. Only one of the octahedral Ni positions (Ni(2)) of the NTO structure was found to be occupied by In (Sc). NTO has non-centrosymmetric structure and is ferroelectric below 1000 K, dielectric and second harmonic measurements suggest that also NISO and NSSO are correspondingly ferroelectric. Magnetization measurements signified antiferromagnetic ordering below TN=60 K (NSSO) and 76 K (NISO). The magnetic structure is formed by two antiferromagnetically coupled incommensurate helices with the spiral axis along the b-axis and propagation vector k = [0, ky,0] with ky= 0.036(1) (NSSO) and ky= 0.029(1) (NISO). The observed structural and magnetic properties of NISO and NSSO are discussed and compared with those of NTO.

Processing and characterization of ferro/piezoelectrics in the still wellite family

Abstract New family of stillwellite-like (CeBS1O5, sp.gr. P31) ferroelectric borosilicates and borogermanates is found, their properties and possible applications are discussed. Borophosphates ABPO5 (sp.gr. P3121, A=Ca, Sr, Ba, Pb) which are very similar in structure are classified as piezoelectrics. Structural, dielectric and optical non-linear characteristics of new ferroelectrics are investigated on single crystals obtained either by flux or Czochralski methods. Optical SHG data on powders and ceramics in an interval 300-1300 K, combined with dielectric and X-ray data, allow to define ferroelectric phase boundaries on the x-T phase diagrams. Crystallization from glasses in ternary systems Ln2O3-B2O3-S1O2 (GeO2) is investigated and good opportunity is demonstrated for formation of polar glassceramic textures with ferroelectric and pyroelectric properties.

Syntheses, Crystal Structures, and Nonlinear Optical Activity of Cs2Ba[AnO2(C2H5COO)3]4 (An = U, Np, Pu) and Unprecedented Octanuclear Complex Units in KR2(H2O)8[UO2(C2H5COO)3]5 (R = Sr, Ba)

X-ray diffraction was applied to the elucidation of crystal structures of single crystals of Cs2Ba[AnO2(C2H5COO)3]4, where An = U(I), Np(II), Pu(III), and KR2(H2O)8[UO2(C2H5COO)3]5, where R = Sr(IV), Ba (polymorphs V-a and V-b). FTIR spectra were analyzed for the uranium-containing crystals I, IV, and V-b. Isostructural cubic crystals I-III are constructed of typical mononuclear anionic complex units [AnO2(C2H5COO)3]- and charge-balancing Cs and Ba cations. Features of actinide contraction in the six U-Np-Pu isostructural series known to date are analyzed. In crystal structures of IV and V two typical complexes [UO2(C2H5COO)3]- bind with a hydrated Sr or Ba cation to form the rare trinuclear neutral complex unit {R(H2O)4[UO2(C2H5COO)3]2}, where R = Sr, Ba. Two such trinuclear units and one typical mononuclear unit further bind with a K cation to form the unprecedented octanuclear neutral complex unit K[UO2(C2H5COO)3]{R(H2O)4[UO2(C2H5COO)3]2}2. As the derived polynuclear complexes of uranyl ion with carboxylate ligands in the crystal structures of IV and V are not the first but are rare examples, the equilibrium between mono and polynuclear complex units in aqueous solutions is discussed. The two polymorphic modifications V-a and V-b were studied at 100 K and at room temperature, respectively. Peculiarities of noncovalent interactions in crystal structures of the two polymorphs are revealed using Voronoi-Dirichlet tessellation. The nonlinear optical activity of noncentrosymmetric crystals I was estimated by its ability for second harmonic generation.

Ferroelectric crystal Ca9Yb(VO4)7 in the series of Ca9R(VO4)7 non-linear optical materials (R = REE, Bi, Y)

The crystal structure, thermal, dielectric and second harmonic generation (SHG), and nonlinear optical activity data for whitlockite-type Ca9Yb(VO4)7 single crystals were obtained on one and the same sample produced by means of the Czochralski method. The crystal structure refinement has revealed that Yb3+ cations substitute for Ca2+ ions only in the M1, M2 and M5 positions of the whitlockite-type structure. Dielectric, differential thermal analysis and SHG data have shown that Ca9Yb(VO4)7 belongs to the family of high-temperature Ca3(VO4)2 ferroelectrics with Curie temperature Tc = 1221 K, where the symmetry changes from R3c to Rc. At higher temperatures a previously unknown complementary phase transition is discovered at T2 = 1276 K and is associated with the symmetry change during heating from Rc to Rm. Unlike other whitlockites, two phase transitions in Ca9Yb(VO4)7 are separated by a broad interval (ΔT = 55 K) which allows one to register two phase transitions by DSC and dielectric measurements. According to the thermal type both transitions are classified as first-order transformations and their structural mechanisms are considered. Inhomogeneity in the cation distribution is argued to have a crucial influence on the optical quality and ferroelectric domain structures of Ca9Yb(VO4)7 and other whitlockite-type laser crystals.

Polar-to-centrosymmetric phase transition in Ca1.5Sr1.5(VO4)2 and the polar phase structure

Abstract Ca 1.5 Sr 1.5 (VO 4 ) 2 was synthesized by the solid-state method at 1150°C followed by quenching in air. Crystal structure of Ca 1.5 Sr 1.5 (VO 4 ) 2 was refined by Rietveld method: space group R 3c, a = 11.0152(1) A, c = 38.8937(2) A, Z = 21. The structure of Ca 1.5 Sr 1.5 (VO 4 ) 2 is isotypic with Ca 3 (VO 4 ) 2 . Here the M 5 site is fully occupied by calcium cations and the M 4 site is half-occupied by strontium cations. The M 1– M 3 sites are occupied by strontium and calcium cations. The amount of strontium cations in the M 1– M 3 sites correlates with the size of these positions in the host structure of Ca 3 (VO 4 ) 2 . The second harmonic generation in Ca 1.5 Sr 1.5 (VO 4 ) 2 (I 2ω /I 2ω (SiO 2 ) ≈ 8) is close to Ca 3 (VO 4 ) 2 (I 2ω /I 2ω (SiO 2 ) ≈ 10) but substantially less than in Ca 9 R(VO 4 ) 7 (for R = Dy I 2ω /I 2ω (SiO 2 ) ≈ 30).

New alluaudite-related triple molybdates Na25Cs8R5(MoO4)24 (R = Sc, In): synthesis, crystal structures and properties

New triple molybdates Na25Cs8R5(MoO4)24 (R = Sc, In) were prepared as powders and ceramics by solid state reactions, and their single crystals were also obtained from melts by spontaneous crystallization. The structures were determined by single crystal XRD analysis. The electrical conductivity of ceramics was measured by impedance spectroscopy. The crystal structures were determined in monoclinic sp. gr. P21/c, a = 14.0069(3) A, b = 12.6498(3) A, c = 28.6491(6) A, β = 90.007(1)° (Sc) and a = 14.0062(2) A, b = 12.6032(2) A, c = 28.7138(4) A, β = 90.001(1)° (In). Together with triclinic Na25Cs8Fe5(MoO4)24, the titled compounds form a distinctive family of pseudo-orthorhombic alluaudite-related structures with the parent sp. gr. Pbca. Its structural features are alluaudite-like polyhedral layers composed of pairs of edge-shared (R, Na)O6 and NaO6 octahedra connected by bridging MoO4 tetrahedra. The layers are joined together by means of interlayer MoO4 tetrahedra, thus forming open 3D frameworks with cavities filled with Cs+ and Na+ ions. The manner of stacking layers is somewhat different from the alluaudite type. The compounds undergo phase transitions at 668 (Sc) and 725 (In) K accompanied by an abrupt increase of electrical conductivity presumably Na+-ionic in nature. Above these transitions, the conductivity is as high as 10−3 S cm−1, which makes Na25Cs8R5(MoO4)24 (R = Sc, In) promising solid state electrolytes.

Complex Structural Behavior of BiMn7O12 Quadruple Perovskite

Structural properties of a quadruple perovskite BiMn7O12 were investigated by laboratory and synchrotron X-ray powder diffraction between 10 and 650 K, single-crystal X-ray diffraction at room temperature, differential scanning calorimetry (DSC), second-harmonic generation, and first-principles calculations. Three structural transitions were found. Above T1 = 608 K, BiMn7O12 crystallizes in a parent cubic structure with space group Im3̅. Between 460 and 608 K, BiMn7O12 adopts a monoclinic symmetry with pseudo-orthorhombic metrics (denoted as I2/m(o)), and orbital order appears below T1. Below T2 = 460 K, BiMn7O12 is likely to exhibit a transition to space group Im. Finally, below about T3 = 290 K, a triclinic distortion takes place to space group P1. Structural analyses of BiMn7O12 are very challenging because of severe twinning in single crystals and anisotropic broadening and diffuse scattering in powder. First-principles calculations confirm that noncentrosymmetric structures are more stable than centrosymmetric ones. The energy difference between the Im and P1 models is very small, and this fact can explain why the Im to P1 transition is very gradual, and there are no DSC anomalies associated with this transition. The structural behavior of BiMn7O12 is in striking contrast with that of LaMn7O12 and could be caused by effects of the Bi3+ lone electron pair.

Iodide-Iodates M3[IO3]12·Ag4I, M = Bi, Tb, with a Framework Structure and High Second-Harmonic Generation Optical Response

Single crystals of two new iodide-iodates, Bi3[IO3]12·Ag4I and Tb3[IO3]12·Ag4I, are synthesized in hydrothermal systems. The anionic parts in both iodide-iodates are characterized as a complex charged framework of isolated IO3 umbrella-like groups and large Bi(Tb)-O polyhedra similar to those previously found in La3[IO3]12[IO3](Pb3O). Broad channels along the c-axis contain compensators: (Ag3I)2+ umbrella-like groups and additional Ag+ ions which form Ag44+ tetrahedral clusters augmented with I- halogen. New iodates possess significantly higher second-order nonlinear optical characteristics compared to the previously known lead-containing compounds REE3[IO3]12[IO3](Pb3O), REE = La, Pr, Nd. The difference is related to the polar ordering of umbrella-like (Ag3I)2+ groups in the channels in the new iodide-iodate. Additionally, planar-coordinated Ag atoms add three Ag atoms in umbrellas forming [Ag4I]3+ polar clusters in the channels.

Pure, lithium- or magnesium-doped ferroelectric single crystals of Ca9Y(VO4)7: cation arrangements and phase transitions

Abstract Single crystals of Ca9Y(VO4)7 (1), Ca9Y(VO4)7:Li+ (2) and Ca9Y(VO4)7:Mg2+ (3) were grown by the Czochralski method. Their chemical composition was analyzed by ICP spectroscopy and their crystal structure was examined by single crystal X-ray analysis. The crystals are characterized by trigonal symmetry, space group R3c. Hexagonal unit-cell parameters are as follows: a=10.8552(1) Å, c=38.0373(2) Å, V=3881.65(1) Å3 for 1; a=10.8570(1) Å, c=38.0161(3) Å, V=3880.77(4) Å3 for 2; a=10.8465(1) Å, c=38.0366(2) Å, V=3875.36(3) Å3 for 3. All crystals are characterized by β-Ca3(PO4)2-type structure with statistical distribution of Ca2+ and Y3+ over M1, M2 and M5 sites in different ratios and with completely empty M4-cationsite. The impurity of Mg2+cations in structure 2 has been detected in octahedral M5 site. Ferroelectric phase transitions are evidenced by DSC and SHG. At about 1220 and 1300 K, they demonstrate phase transitions. Upon heating the symmetry of the crystal structure changes according to the scheme R3c→R3̅c→R3̅m and is restored during consequent cooling. The first of them is of ferroelectric and the second of non-ferroelectric nature. Even a small amount of impurities in Ca9Y(VO4)7 structure is accompanied by a noticeable decrease in the temperature of the ferroelectric-paraelectric phase transition.

A new hydrogen-containing whitlockite-type phosphate Ca9(Fe0.63Mg0.37)H0.37(PO4)7: hydrothermal synthesis and structure

Abstract A new hydrogen-containing Ca9(Fe0.63Mg0.37)H0.37(PO4)7 phosphate with the whitlockite-type structure has been synthesized by a hydrothermal method and its structure has been studied by the single-crystal X-ray diffraction. The compound crystallizes in the trigonal space group R3c (traditional for compounds with the whitlockite-type structure) with unit-cell parameters: a=10.3533(1) Å, c=37.1097(4) Å. The structure has been determined using the “charge flipping” method. Ca9(Fe0.63Mg0.37)H0.37(PO4)7 structure is similar to that of other members of the whitlockite-type family. The presence of hydrogen in the structure leads to the formation of OH-group with one of the oxygen of PO4-tetrahedra. Based on an analysis of the bond valence sums (BVS) a conclusion has been made about localization of H atoms in the structure. Smaller values of BVS for O1 and O10 atoms than ones for other oxygen atoms indicate localization of H atoms between them in a position with site symmetry 18b.