Crystal structures of the novel hydrated borates Ba2B5O9(OH), Sr2B5O9(OH) and Li2Sr8B22O41(OH)2

Three novel hydrated borates Ba₂B₅O₉(OH) (1), Sr₂B₅O₉(OH) (2) and Li₂Sr₈B₂₂O₄₁(OH)₂ (3) have been synthesized hydrothermally and their structures determined. Compounds (1) and (2) are isostructural, crystallizing in space group P2₁/c and having lattice parameters of a=6.6330(13) Å, b=8.6250(17) Å, c=14.680(3) Å, β=93.46(3)° and a=6.4970(13) Å, b=8.4180(17) Å, c=14.177(3) Å, β=94.35(3)°, respectively. Compound (3) crystallizes in P-1 with lattice parameters of a=6.4684(13) Å, b=8.4513(17) Å, c=14.881(3) Å, α=101.21(3)°, β=93.96(3)°, γ=90.67(3)°. While similar in their axis lengths, (3) differs greatly in structure and formulation from (1) and (2). The structure of (1) and (2) is contrasted to that of the well-known mineral hilgardite (Ca₂B₅O₉Cl·H₂O).     

Non-centrosymmetric Ba3Ti3O6(BO3)2

The compound previously reported as Ba₂Ti₂B₂O₉ has been reformulated as Ba₃Ti₃B₂O₁₂, or Ba₃Ti₃O₆(BO₃)₂, a new barium titanium oxoborate. Small single crystals have been recovered from a melt with a composition of BaTiO₃:BaTiB₂O₆ (molar ratio) cooled between 1100°C and 850°C. The crystal structure has been determined by X-ray diffraction: hexagonal system, non-centrosymmetric space group, a=8.7377(11) Å, c=3.9147(8) Å, Z=1, wR(F²)=0.039 for 504 unique reflections. Ba₃Ti₃O₆(BO₃)₂ is isostructural with K₃Ta₃O₆(BO₃)₂. Preliminary measurements of nonlinear optical properties on microcrystalline samples show that the second harmonic generation efficiency of Ba₃Ti₃O₆(BO₃)₂ is equal to 95% of that of LiNbO₃.     

Crystal and Magnetic Structures of Ba4Mn3O10

A polycrystalline sample of Ba₄Mn₃O₁₀ has been prepared and characterized by X-ray diffraction (290 K), neutron diffraction (290, 80, 5 K) and magnetometry (5≤T(K)≤1000). At 290 K the compound is paramagnetic and isostructural with Ba₄Ti₂PtO₁₀. Mn₃O₁₂ trimers, built up from MnO₆ octahedra, are linked through common vertices to form corrugated sheets perpendicular to the y-axis of the orthorhombic unit cell (Space group Cmca, a=5.6850(1), b=13.1284(1), c=12.7327(1) Å); Ba atoms occupy the space between the layers. On cooling, the magnetic susceptibility shows a broad maximum at ∼130 K, and a sharp transition at 40 K. Neutron diffraction has shown that long-range antiferromagnetic order is present at 5 K but not at 80 K, although magnetometry at 5 K has revealed a remanent magnetization (0.002 μ_B per Mn) which is below the detection limit of the neutron experiment.     

Synthesis, crystal structure and magnetic properties of the open framework compound Co3Te2O2(PO4)2(OH)4

The new compound Co₃Te₂O₂(PO₄)₂(OH)₄ was synthesized using hydrothermal techniques. It crystallizes in the monoclinic space group C2/m with the unit cell a=19.4317(10) Å, b=6.0249(3) Å, c=4.7788(2) Å, β=103.139(5)°. The crystal structure is an open framework having chains of edge sharing [Co(1)O₆] octahedra. Other building blocks are [TeO₃(OH)₂], [PO₄] and [Co(₂)O₂(OH)₄] connected mainly via corner sharing. The –OH groups protrude into channels in the structure. The magnetic susceptibility measured from 2 to 300 K shows two broad anomalies at around 21 K and 4 K, respectively. The peak at ∼20 K is ascribed to a two-dimensional antiferromagnetic ordering of linear [Co(1)O₆] chains coupled by interchain interaction via [PO₄] groups in the Co(1) sheets. The second transition at 4 K is ascribed to a second antiferromagnetic ordering of the moments of the Co(2) entities via super–super exchange involving [PO₄] and [TeO₃(OH)₂] groups. This assignment is strongly supported by low-temperature heat capacity measurements indicating an entropy removal within the high-temperature transition of about twice the magnitude of the low-temperature transition.     

Crystal structure and electrical properties of the mixed valent titanium oxysulfide Sm2Ti2S2O4.5

A new phase Sm₂Ti₂S₂O_4.5 was synthesized and its crystal structure was solved by single-crystal X-ray diffraction. This compound crystallizes in the monoclinic system (C2/m) with lattice constants a=17.9987(11) Å, b=3.71607(14) Å, c=12.6172(8) Å and β=133.645(4)°. The structure is built up from double chains of Ti-centered octahedra between which Sm-polyhedra develop. In spite of very close formulations, the structure of Sm₂Ti₂S₂O_4.5 differs completely from that of the defect Ruddelsden-Popper phase Sm₂Ti₂S₂O₅ previously reported. The title compound presents a mixed valence for titanium with Ti(III) (d¹) and Ti(IV) (d⁰) located in different crystallographic sites. However, conductivity measurements show that this compound is non-metallic.     

Crystal structures of lazulite-type oxidephosphates TiTi3O3(PO4)3 and M4Ti27O24(PO4)24 (M=Ti, Cr, Fe)

Single crystals of the oxidephosphates Ti^IIITi^IV₃O₃(PO₄)₃ (black), Cr^III₄Ti^IV₂₇O₂₄(PO₄)₂₄ (red-brown, transparent), and Fe^III₄Ti^IV₂₇O₂₄(PO₄)₂₄ (brown) with edge-lengths up to 0.3 mm were grown by chemical vapour transport. The crystal structures of these orthorhombic members (space group F2dd ) of the lazulite/lipscombite structure family were refined from single-crystal data [Ti^IIITi^IV₃O₃(PO₄)₃: Z=24, a=7.3261(9) Å, b=22.166(5) Å, c=39.239(8) Å, R₁=0.029, wR₂=0.084, 6055 independent reflections, 301 variables; Cr^III₄Ti^IV₂₇O₂₄(PO₄)₂₄: Z=1, a=7.419(3) Å, b=21.640(5) Å, c=13.057(4) Å, R₁=0.037, wR₂=0.097, 1524 independent reflections, 111 variables; Fe^III₄Ti^IV₂₇O₂₄(PO₄)₂₄: Z=1, a=7.4001(9) Å, b=21.7503(2) Å, c=12.775(3) Å, R₁=0.049, wR₂=0.140, 1240 independent reflections, 112 variables). For Ti^IIITi^IVO₃(PO₄)₃ a well-ordered structure built from dimers [Ti^III,IV₂O₉] and [Ti^IV,IV₂O₉] and phosphate tetrahedra is found. The metal sites in the crystal structures of Cr₄Ti₂₇O₂₄(PO₄)₂₄ and Fe₄Ti₂₇O₂₄(PO₄)₂₄, consisting of dimers [M^IIITi^IVO₉] and [Ti^IV,IV₂O₉], monomeric [Ti^IVO₆] octahedra, and phosphate tetrahedra, are heavily disordered. Site disorder, leading to partial occupancy of all octahedral voids of the parent lipscombite/lazulite structure, as well as splitting of the metal positions is observed. According to Guinier photographs Ti^III₄Ti^IV₂₇O₂₄(PO₄)₂₄ (a=7.418(2) Å, b=21.933(6) Å, c=12.948(7) Å) is isotypic to the oxidephosphates M^III₄Ti^IV₂₇O₂₄(PO₄)₂₄ (M^III: Cr, Fe). The UV/vis spectrum of Cr₄Ti₂₇O₂₄(PO₄)₂₄ reveals a rather small ligand-field splitting Δ_o=14,370 cm⁻¹ and a very low nephelauxetic ratio β=0.72 for the chromophores [Cr^IIIO₆] within the dimers [Cr^IIITi^IVO₉].     

Structural and physical properties of 1:2 B-site-ordered perovskite Ba3CaIr2O9

The high-pressure phase of iridium-based compound Ba₃CaIr₂O₉ was synthesized using high-pressure sintering. Being different from the distorted hexagonal BaTiO₃ structure of the ambient Ba₃CaIr₂O₉, the high-pressure phase crystals into the 1:2 B-site-ordered perovskite structure with the space group P-3m1 (Z=1). Through fitting the X-ray powder diffraction (XRD) data with Rietveld analysis, in which the obtained R_p, R_wp, and R_exp factors are 7.49%, 11.4%, and 4.82%, respectively, the lattice parameters are a=5.8296(1) Å and c=7.1659(2) Å. The atomic coordinates and the main interatomic distances and bond angles were also obtained. The relationship of electrical resistivity versus temperature shows that the high-pressure phase of Ba₃CaIr₂O₉ is a semiconductor in the temperature range of 5-300 K. The measurement of temperature dependence of magnetic susceptibility indicates that it is paramagnetic.     

Crystal structure of BaMg2Si2O7 and Eu luminescence

Crystal structure of BaMg₂Si₂O₇ was determined and refined by a combined powder X-ray and neutron Rietveld method (monoclinic, C2/c, no. 15, Z=8, a=7.24553(8) Å, b=12.71376(14) Å, c=13.74813(15) Å, β=90.2107(8)°, V=1266.44(2) Å³; R_p/R_wp=3.38%/4.77%). The structure contains a single crystallographic type of Ba atom coordinated to eight O atoms with C₁ (1) site symmetry. Under 325-nm excitation Ba_0.98Eu_0.02Mg₂Si₂O₇ exhibits an asymmetric emission band around 402 nm. The asymmetric shape of the emission band is likely associated with a small electron-phonon coupling in BaMg₂Si₂O₇. The integrated intensity of the emission band was observed to remain constant over the temperature range 4.2-300 K.     

Crystal structures of two new oxysulfides La5Ti2MS5O7 (M=Cu, Ag): evidence of anionic segregation

Two new compounds, La₅Ti₂MS₅O₇ (M=Cu, Ag) were synthesized and their structures solved from single crystal X-ray data. Both compounds are isotypic. They crystallize in the orthorhombic system (space group Pnma, Z=4) with lattice constants a=19.423(1) Å, b=3.9793(2) Å, c=18.1191(9) Å for La₅Ti₂CuS₅O₇, and a=19.593(2) Å, b=3.9963(1) Å, and c=18.2973(15) Å for La₅Ti₂AgS₅O₇. The structure of these compounds is built from fragments of the rock-salt, perovskite and fluorite types and a clear anionic segregation of the anions appears in the structure. La₅Ti₂CuS₅O₇ and La₅Ti₂AgS₅O₇ exhibit an orange-yellow color and measurement of their optical band gap gave 2.02 and 2.17 eV, respectively.     

Low-temperature structural phase transition in synthetic libethenite Cu2PO4OH

Low-temperature structural properties of the synthetic mineral libethenite Cu₂PO₄OH were investigated by single-crystal X-ray diffraction, synchrotron X-ray powder diffraction, specific heat measurements, and Raman spectroscopy. A second-order structural phase transition from the Pnnm symmetry (a=8.0553(8) Å, b=8.3750(9) Å, c=5.8818(6) Å at 180 K) to the P2₁/n symmetry (a=8.0545(8) Å, b=8.3622(9) Å, c=5.8755(6) Å, β=90.0012(15) at 120 K) was found at 160 K during cooling. At 120 K, the monoclinic angle is 90.0012(15) from single crystal X-ray data vs 90.083(1) from powder X-ray diffraction data. The P2₁/n–to–Pnnm transition may be a general feature of the adamite-type compounds, M₂XO₄OH.     

Two new titanium molybdenum arsenides: Ti2MoAs2 and Ti3MoAs3, ternary substitution variants of V3As2 and β-V4As3

The title compounds were prepared by arc-melting pre-annealed mixtures of Ti, Mo, and As. Both Ti₂MoAs₂ and Ti₃MoAs₃ adopt structures formed by the corresponding binary vanadium arsenides, V₃As₂ and β-V₄As₃. Ti₂MoAs₂ crystallizes in the tetragonal space group P4/m, with a=9.706(4) Å, c=3.451(2) Å, V=325.1(3) Å³ (Z=4), and Ti₃MoAs₃ in the monoclinic space group C2/m, with a=14.107(3) Å, b=3.5148(7) Å, c=9.522(2) Å, β=100.66(3)°, V=464.0(2) Å³ (Z=4). In both cases, the metal atoms form infinite chains of trans edge-condensed octahedra, and the As atoms are located in (capped) trigonal prismatic voids. While most metal atom sites exhibit mixed Ti/Mo occupancies, the Mo atoms prefer the sites with more metal atom and fewer As atom neighbors. Ti₂MoAs₂ and Ti₃MoAs₃ are metallic entropy-stabilized materials that decompose upon annealing at intermediate temperatures.     

Synthesis and characterization of sodium titanates Na2Ti3O7 and Na2Ti6O13

Na₂Ti₃O₇ and Na₂Ti₆O₁₃ were synthesized by sol-gel method in order to obtain pure phases. Different heat-treatments were applied on powders and pellets of these materials. The effects were studied by XRD, dilatometry, TGA-DTA, SEM and electrochemical impedance spectroscopy. Pure Na₂Ti₃O₇ was obtained at 973 K. Sintering at 1373 K caused a partial decomposition into Na₂Ti₆O₁₃. The Na₂Ti₃O₇ powder sintered at 1273 K showed polygonal microstructure. Na₂Ti₃O₇ pellets sintered at 1323 K for 10 h exhibited large structures. This latter microstructure decreased the electrical conductivity of Na₂Ti₃O₇. Pure Na₂Ti₆O₁₃ was obtained at 873 K. Sintering at 1073 K caused a partial decomposition into TiO₂ (rutile). Na₂Ti₆O₁₃ pellets sintered at 1323 K for 10 h exhibited common shrinkage behavior. This shrinkage process increased the electrical conductivity of this material. The presence of TiO₂ resulted in a oxygen partial pressure dependence of the electrical conductivity.     

New process of preparation, structure, and physicochemical investigations of the new titanyl phosphate Ti2O(H2O)(PO4)2

New titanyl phosphate Ti₂O(H₂O)(PO₄)₂ has been prepared and characterized by X-ray and neutron diffraction, nuclear magnetic resonance, infrared and Raman spectroscopies and thermogravimetric analysis. The crystal structure has been solved from neutron powder diffraction data at 300 K by Rietveld method in P2₁ space group. The refinement led to satisfactory profile factors (R_p=2.7%, R_wp=3.2%) and crystal structure model indicators (R_B=5.8%, R_F=3.2%). The cell is monoclinic with a=7.3735 Å, b=7.0405 Å, c=7.6609 Å and β=121.48°, Z=4. The structure can be described as a three-dimensional framework built up by chains of [TiO₅(OH₂)] octahedra with alternative short bonds [Ti₍₁₎-O₍₁₂₎; Ti₍₂₎-O₍₁₂₎, 1.88-1.84 Å] and long ones [Ti₍₁₎-O_W; Ti₍₂₎-O_W, 2.25-2.23 Å] along c-axis and connected via [PO₄] tetrahedra. Oxygen atom denoted O₍₁₂₎ is only linked to two titanium atoms and Oxygen atom denoted O_W is linked to two titanium atoms and two hydrogen atoms. O₍₁₂₎ and O_W are not linked to P atoms and justify the titanyl phosphate formulation Ti₂O(H₂O)(PO₄)₂. The infrared and Raman spectra presents peaks due to vibrations of Ti-O, P-O and O-H bonds. The ³¹P MAS NMR spectrum reveals two ³¹P resonance lines, in agreement with the structure which showed two crystallographic sites for phosphorus. The thermogravimetric analysis show that Ti₂O(H₂O)(PO₄)₂ is thermally stable until 400 °C. Above this temperature, it losses water and decomposes to Ti₅O₄(PO₄)₄ and TiP₂O₇.     

High-Temperature Synchrotron X-Ray Powder Diffraction Study of the Orthorhombic-Tetragonal Phase Transition in La0.63(Ti0.92,Nb0.08)O3

The crystal structure of La_0.63(Ti_0.92,Nb_0.08)O₃ has been refined by the Rietveld analysis of CuKα X-ray powder diffraction data collected at 23°C. This material was confirmed to have an A-site deficient orthorhombic perovskite-type structure with double ideal perovskite ABO₃ units along the c-axis (space group Pmmm, Z=2, a=3.86036(5) Å, b=3.87222(5) Å, c=7.82609(9) Å). Lattice parameters of the same sample have been investigated in situ in the temperature range from 25°C to 496°C by 1.37873(3) Å synchrotron X-ray powder diffraction. The synchrotron X-ray powder diffraction technique was found to be very powerful to determine precise lattice parameters around a phase transition temperature. This compound exhibited a reversible phase transition between the orthorhombic and tetragonal phases at around 370°C. (1) The lattice parameters increased continuously with temperature, while the b/a ratio decreased continuously with temperature and became unity at the orthorhombic-tetragonal transition point. (2) No hysteresis was observed in the lattice parameter values between heating and cooling. These results of (1) and (2) suggest that the orthorhombic-tetragonal phase transition is continuous.     

Layered perovskite-related ruthenium oxychlorides: crystal structure of two new compounds Ba5Ru2Cl2O9 and Ba6Ru3Cl2O12

Single crystals of the title compounds were prepared using a BaCl₂ flux and investigated by X-ray diffraction methods using MoKα radiation and a charge coupled device (CCD) detector. The crystal structures of these two new compounds were solved and refined in the hexagonal symmetry with space group P6₃/mmc, a=5.851(1) Å, c=25.009(5) Å, ρ_cal=4.94 g cm⁻³, Z=2 to a final R₁=0.069 for 20 parameters with 312 reflections for Ba₅Ru₂Cl₂O₉ and space group , a=5.815(1) Å, c=14.915(3) Å, ρ_cal=5.28 g cm⁻³, Z=1 to a final R₁=0.039 for 24 parameters with 300 reflections for Ba₆Ru₃Cl₂O₁₂. The structure of Ba₅Ru₂Cl₂O₉ is formed by the periodic stacking along [001] of three hexagonal close-packed BaO₃ layers separated by a double layer of composition Ba₂Cl₂. The BaO₃ stacking creates binuclear face-sharing octahedra units Ru₂O₉ containing Ru(V). The structure of Ba₆Ru₃Cl₂O₁₂ is built up by the periodic stacking along [001] of four hexagonal close-packed BaO₃ layers separated by a double layer of composition Ba₂Cl₂. The ruthenium ions with a mean oxidation degree +4.67 occupy the octahedral interstices formed by the four layers hexagonal perovskite slab and then constitute isolated trinuclear Ru₃O₁₂ units. These two new oxychlorides belong to the family of compounds formulated as [Ba₂Cl₂][Ba_n+1Ru_nO_3n+3], where n represents the thickness of the octahedral string in hexagonal perovskite slabs.     

Crystal growth of a novel oxygen-deficient layered perovskite: Ba7Li3Ru4O20

Single crystals of both Ba₇Li₃Ru₄O₂₀ and Ba₄NaRu₃O₁₂ were grown from reactive molten hydroxide fluxes. Ba₇Li₃Ru₄O₂₀ is a 7L-layer perovskite-related phase resulting from the stacking of six [AO₃] layers and one oxygen deficient [AO₂] layer, thereby creating LiO₄ tetrahedra in addition to the LiO₆ octahedra and face-sharing Ru₂O₉ bi-octahedra formed from the [AO₃] layers. The compound crystallizes in the space group with a=5.7927(1) Å and c=50.336(2) Å, Z=3. Ba₄NaRu₃O₁₂ crystallizes in the space group P6₃mc with lattice parameters of a=5.8014(2) Å and c=19.2050(9) Å, Z=2. Ba₄NaRu₃O₁₂ is identical to a previously reported neutron refinement structure. The magnetic properties of Ba₇Li₃Ru₄O₂₀ are also reported.     

Synthesis, crystal structure, and magnetic properties of new layered hexagonal perovskite Ba8Ta4Ru8/3Co2/3O24

A new hexagonal perovskite-type oxide Ba₈Ta₄Ru_8/3Co_2/3O₂₄ was synthesized by the solid-state method at 1573 K and characterized by electron diffraction (ED), time-of-flight (TOF) neutron powder diffraction, and magnetic susceptibility. Structure parameters of Ba₈Ta₄Ru_8/3Co_2/3O₂₄ were refined by the Rietveld method from the TOF neutron powder diffraction data on the basis of space group P6₃/mcm and lattice parameters a=10.0075(1) Å and c=18.9248(2) Å as obtained from the ED data (Z=3). The crystal structure of Ba₈Ta₄Ru_8/3Co_2/3O₂₄ consists of 8-layered (cchc)₂ close-packed stacking of BaO₃ layers along the c-axis. Corner-shared octahedra are filled by Ta only and face-shared octahedra are statistically occupied by Ru, Co, and vacancies. Similar compounds Ba₈Ta₄Ru_8/3M_2/3O₂₄ with M=Ni and Zn were also prepared. Magnetic susceptibility measurements showed no magnetic ordering down to 5 K.     

Pb2.85Ba2.15Fe4SnO13: A new member of the AnBnO3n−2 anion-deficient perovskite-based homologous series

Pb_2.85Ba_2.15Fe₄SnO₁₃, a new n=5 member of the anion-deficient perovskite based A_nB_nO_3n−2 (A=Pb, Ba, B=Fe, Sn) homologous series, was synthesized by the solid state method. The crystal structure of Pb_2.85Ba_2.15Fe₄SnO₁₃ was investigated using a combination of neutron powder diffraction, electron diffraction, high angle annular dark field scanning transmission electron microscopy and Mössbauer spectroscopy. It crystallizes in the Ammm space group with unit cell parameters a=5.7990(1) Å, b=4.04293(7) Å and c=26.9561(5) Å. The Pb_2.85Ba_2.15Fe₄SnO₁₃ structure consists of quasi two-dimensional perovskite blocks separated by 1/2[110](1?01)_p crystallographic shear (CS) planes. The corner-sharing FeO₆ octahedra at the CS planes are transformed into edge-sharing FeO₅ distorted tetragonal pyramids. The octahedral positions in the perovskite blocks between the CS planes are jointly taken up by Fe and Sn, with a preference of Sn towards the position at the center of the perovskite block. The chains of FeO₅ pyramids and (Fe,Sn)O₆ octahedra of the perovskite blocks delimit six-sided tunnels at the CS planes occupied by double chains of Pb atoms. The compound is antiferromagnetically ordered below T_N=368±15 K.     

A new ordered triple Hollandite: Ba1.33Sb2.66Al5.33O16

Ba_1.33Sb_2.66Al_5.33O₁₆ is a triple Hollandite, which crystallizes in a tetragonal cell (I4/m no. 87) with a=b=9.86090(5) Å and c=8.77612(6) Å. Its crystal structure was characterized using electron diffraction and X-ray powder diffraction; it is isotypic to K_1.33Mg_3.11Sb_4.89O₁₆, K_1.76Mg_3.25Sb_4.75O₁₆ and to K_1.8Li_2.45Sb_5.55O₁₆. In the rutile chains of Ba_1.33Sb_2.66Al_5.33O₁₆, the ordering of Al and Sb atoms into unmixed sites induces the tripling of the c parameter compared to a ‘single’ Hollandite structure. The Ba²⁺ cations are dispersed along c, in the largest tunnels on non-split and fully occupied sites. They lie into Ba-Ba pairs separated by vacancies. Their regular arrangement has been confirmed by high resolution electron microscopy. Electrochemical experiments have also been performed in Li-ion cell but no Li insertion was detected.