Shiou-Jyh Hwu

A new barium titanium (III) pyrophosphate: BaTi2(P2O7)2

The new barium titanium (III) pyrophosphate BaTi2(P2O7)2 has been prepared by conventional high temperature solid state reaction at 900°C in a fused silica tube. BaTi2(P2O7)2 crystallizes with four formula units in a cell with dimensions a = 10.680(3) A, b = 10.564(4) A, c = 9.834(4) A/, β = 102.88(3)° and V = 1081.6(6) A3 in space group C62h  C2c (No. 15) of the monoclinic system. The single crystal structure refinement gives a final structure solution with R index on F2o of 0.018 for 99 variables and GOF = 1.05. BaTi2(P2O7)2 displays a new layered type structure which consists of layers of slightly distorted TiO6 octahedra with (P2O7) and (BaO10) polyhedra between the layers. The structural framework is built up from corner-sharing TiO6 octahedra and P2O7 pyrophosphate groups, to give rise to [Ti2(P2O7)2]2− units and to form tunnels where the barium cations reside. Magnetic susceptibility measurements on selected single crystals confirm the presence of Ti3+ (d1) ions with spin S = 12. The high oxidative thermal stability of BaTi2(P2O7)2 versus the stabilities of other trivalent titanium cation-containing compounds, including Ti2O3, TiPO4, and BaTi2(PO4)3, owing to possible anion matrix effects are discussed.

Li3−xTi2(PO4)3 (0 ≤ x ≤ 1): A new mixed valent titanium(III/IV) phosphate with a NASICON-type structure

Abstract A new NASICON-related structure of lithium titanium phosphate Li 2.72 Ti 2 (PO 4 ) 3 has been determined. This compound crystallizes in an orthorhombic system, Pbcn , with a = 12.064 (3) A, b = 8.663 (3) A, c = 8.711 (4) A, V = 910.4 (8) A 3 , and Z = 4. The single crystal structure of this novel mixed valent titanium(III/IV) phosphate reveals one titanium atom per asymmetric unit. Two lithium sites are characterized by a pair of distorted polyhedra, Li(1)O 4 and Li(2)O 5 , which share a common edge resulting in a short Li(1) … Li(2) distance, i.e., 2.29 (5) A. Magnetic susceptibility and microprobe analysis confirmed the structural composition. The room temperature ionic conductivity is comparable with that of the known Li 1+ x Ti IV 2− x In III x (PO 4 ) 3 , which suggests possible fast ionic conductivity.

A layered framework characterized by two independent tunnels: The single crystal structure of strontium vanadium (III) pyrophosphate, SrV2(P2O7)2

Abstract The crystal structure of strontium vanadium (III) pyrophosphate, SrV2(P2O7)2, has been analyzed by the single crystal X-ray diffraction method. The title compound crystallizes in a triclinic (P1) unit cell, with lattice dimensions: a = 4.8006 (7)A, b = 7.117 (1)A, c = 7.8569 (8)A, α = 89.92 (1)°, β = 92.38 (1)°, γ = 106.57 (1)°, V = 257.05 (6)A3, and Z = 1. The single crystal structure refinement gives a final structure solution with R/Rw indexes on F20 of 0.021/0.030 for 100 variables and GOF = 1.38. The layered type structural framework, built up from corner-sharing VO6 octahedra and P2O7 pyrophosphate groups, possesses two types of tunnel structures of which the larger tunnels are occupied by strontium atoms. The strontium atoms are centered at the origin of the lattice and exhibit an irregular SrO10 polyhedron which can be best described as a tetra-face-capped octahedron. The structure comparison with previously reported BaTi2(P2O7)2 and NaxMoP2O7 (0.25 ≤ x ≤ 0.50) is also discussed.

Synthesis and structural characterization of Ba6Nb14Si4O47. The first member of the new barium niobium(IV/V) oxosilicate series(Ba3Nb6Si4O26)n(Ba3Nb8O21), n = 1

Abstract The crystal structure of the mixed-valence barium niobium(IV/V) oxosilicate, Ba 6 Nb 14 Si 4 O 47 , has been determined by single crystal X-ray diffraction. The title compound crystallizes in the hexagonal crystal system with the space group P 6 3 cm (No. 185) and Z = 2. The lattice parameters are a = 9.022(2)A˚, c = 27.890(6)A˚, and V = 1966(1)A˚ 3 . The structure can be described as the intergrowth of two structure types, barium niobium oxosilicate and barium niobium oxide, which form the series (Ba 3 Nb 6 Si 4 O 26 ) n (Ba 3 Nb 8 O 21 ). Ba 6 Nb 14 Si 4 O 47 corresponds to the first member of the series, n = 1. The unit cell consists of columns of ReO 3 -type quasi-one-dimensional, fused NbO 6 octahedra. The parallel octahedral columns are interconnected through shared corner oxygen atoms with isolated NbO 6 octahedra and Si 2 O 7 pyrosilicate units. The framework is closely related to that of theBa 6+ x Nb 14 Si 4 O 47 ( x = 0.23) phase, reported by Evans and Katz in 1973 ( l ), yet the title compound is stoichiometric and has an acentric unit cell. A detailed structure analysis and structure comparison with the nonstoichiometric phase, as well as a brief comparison with the fully oxidized compound Ba 6 Nb 10 Ti 4 Si 4 O 47 , are presented.

Framework composed of interconnected quadruple-octahedra infinite chains: Synthesis and structure of calcium ytterbium sulfide, CaYb2S4, with the Yb3S4-type structure

Abstract Single crystals of the ternary calcium ytterbium compound, CaYb2S4, have been synthesized using an eutectic, halide flux and have been structurally characterized by the X-ray single-crystal diffraction method. The compound crystallizes in an orthorhombic unit cell, Pnma (No. 62), with cell dimensions of a = 12.807 (3) A, b = 3.836 (2) A, c = 12.964 (3) A, V = 636.9 (7) A3, and Z = 4. The least-squares refinement of 658 observed reflections (with F20 > 3σ (F20)) gives a final structure solution of R = 0.026, Rw = 0.030, and GOF = 1.06 for 45 variables. This compound has adopted a Yb3S4-type structure in which the 7-coordinated divalent Yb2+ sites are occupied by Ca2+ cations. The refined occupancy factor on the calcium site suggests some nonstoichiometry, which is attributed to the solid solution of a ytterbium-rich phase. The crystal formula found is Ca1−xYb2+xS4 (x = 0.04). The CaYb2S4 structure is a three-dimensional lattice which consists of interconnected quadruple-octahedra infinite chains. Each quadruple chain is built up from four edge-sharing octahedra that are composed of two pairs of asymmetric Yb(1)S6 and Yb(2)S6 groups. Structure comparison with another ternary rare-earth sulfide bearing a larger alkaline earth metal cation, BaSm2S4 (CaFe2O4-type), gives rise to some important insight for structure formation of rare-earth chalcogenide compounds.

CaNb2P2O11: A new calcium niobium (V) oxophosphate with a quasi-one-dimensional structure

Abstract The crystal structure of a new calcium niobium (V) oxophosphate, CaNb2P2O11, has been determined by single crystal X-ray diffraction. The title compound crystallizes in the monoclinic crystal system with the space group C2 c (No. 15) and Z = 4. The lattice parameters are a = 14.930 (3)A, b = 10.824 (3)A, c = 5.214 (2)A, β = 93.59 (2) A, and V = 840.9 (4)A3. The structure contains a double layer of zig-zag corner-sharing NbO6 octahedral chains. These octahedral chains are structurally isolated by PO4 tetrahedral groups, each of which shares three oxygen atoms with NbO6 octahedra to form a quasi-low-dimensional network structure. The interconnected octahedral chains are arranged parallel to each other so that channel structures are formed. The fourth oxygen atoms of the PO4 tetrahedra are pointing into the channel where the calcium atoms are located. A structure comparison with the isotypic compound BaNb2P2O11 and a discussion of the NbO framework formation related to that in the purple molybdenum bronze are presented.

Completion of the alkali metal titanium (III) pyrophosphate series: Synthesis and structure of AITiP2O7 (AI = K, Rb, Cs)

The structure of a new titanium (III) pyrophosphate RbTiP{sub 2}O{sub 7} has been determined. It crystallizes in a monoclinic unit cell (P2{sub 1}/c, Z=4) with dimensions a=7.542(7){angstrom}, b=10.256(2){angstrom}, c=8.270 (3){angstrom}, {beta}=105.59(5){degree}, V=616.2 (6){angstrom}{sup 3}. The single crystal structure refinement gives a final structure solution with R index on F{sub 0}{sup 2} of 0.046 for 101 variable and GOF=1.76. RbTiP{sub 2}O{sub 7} is isostructural with KAlP{sub 2}O{sub 7}, whose structure is also adopted by trivalent transition metal containing pyrophosphates; namely M{sup I}MoP{sub 2}O{sub 7} (M{sup I} = K, Rb, and Cs) and KFeP{sub 2}O{sub 7}. The potassium and cesium analogues are also synthesized. The structure differences between LiTiP{sub 2}O{sub 7}, NaTiP{sub 2}O{sub 7} ({beta}-phase), and (K, Rb, Cs)TiP{sub 2}O{sub 7} are contrasted by the connectivity of the TiO{sub 6} octahedra and P{sub 2}O{sub 7} pyrophosphate groups. The structural relationship between RbTiP{sub 2}O{sub 7} and previously reported BaTi{sub 2}(P{sub 2}O{sub 7}){sub 2} versus the role of electropositive cations in the formation of Ti(P{sub 2}O{sub 7}){sub n}O{sub 6-2n} units (n is 1 for Rb{sup +} and 2 for Ba{sup 2+}) are discussed.