ChemInform Abstract: Synthesis and Crystal Structure of the A6B5O18 Perovskite-Like Compounds.

Single phase Ba₅KNb₅O₁₈ (I) is prepared by solid state reaction of Ba₅Nb₄O₁₅ and KNbO₃ (1370 K, 2 h), while Sr₆Nb₄SnO₁₈ (II) results from calcination of the coprecipitated metal hydroxycarbonates (1570 K, 2 h).     

HRTEM study of cation-deficient perovskite-related AnBn−δO3n (n?4δ) microphases in the Ba5Nb4O15-BaTiO3 system

The occurrence of coherent intergrowths of cation-deficient perovskites in the Ba₅Nb₄O₁₅-BaTiO₃ system has been examined by high-resolution transmission electron microscopy and selected area electron diffraction. Because of their structural similarity, the simple members Ba₅Nb₄O₁₅ (n=5) and Ba₆TiNb₄O₁₈ (n=6) form coherent intergrowths—noted 5^P6¹—by the juxtaposition along the c-axis of P perovskite-like blocks n=5 and one perovskite-like block n=6, with P=1, 2 and 3. More generally, the ability to form intergrowths in the hexagonal perovskite systems is discussed considering the structural characteristics of the simple members. Examples taken from various systems show that the formation of such intergrowths is highly dependent on the size of the A cation present in simple members.     

Preparation and Properties of Highly Oriented Sr0.3Ba0.7Nb2O6 Thin Films by a Sol-Gel Process

We succeeded in the preparation of epitaxial or highly oriented strontium-barium niobate (Sr_0.3Ba_0.7Nb₂O₆) thin film by a sol-gel process. A homogeneous coating solution was prepared with Sr and Ba acetates and Nb(OEt)₅ as raw materials, and acetic acid and diethylene glycol monomethyl ether as solvents. Sr_0.3Ba_0.7Nb₂O₆ film sintered at 900°C on MgO(1 0 0) was oriented with c-axis perpendicular to the substrate surface. Sr_0.3Ba_0.7Nb₂O₆ film sintered at 700°C on SrTiO₃(1 0 0) was an epitaxial and oriented with c-axis in parallel to the substrate surface. Transmittance of Sr_0.3Ba_0.7Nb₂O₆ film (film thickness: 144 nm) was more than 60% at the range from 400 to 800 nm. Refractive index was 2.33 at 633 nm. Dielectric constant and dielectric loss of the Sr_0.3Ba_0.7Nb₂O₆ thin films prepared on polycrystal Pt substrates were 600 and 0.06 at room temperature and 1 kHz, respectively. The curie temperature (T_c) of polycrystalline Sr_0.3Ba_0.7Nb₂O₆ thin films was about 200°C. At room temperature and 50 kHz, remanent polarization (P_r) and coercive field (E_c) of the polycrystalline thin films were 1.79 C/cm² and 2.69 kV/cm, respectively.     

Preparation and comparison of the photoelectronic properties of Sr2Nb2O7 and Ba0.5Sr0.5Nb2O6

The two alkaline earth niobates Sr₂Nb₂O₇ and Ba_0.5Sr_0.5Nb₂O₆ have been prepared, their electronic properties measured, and their photoresponses compared. The indirect band gap in Sr₂Nb₂O₇ is 3.86 eV compared with 3.38 eV for Ba_0.5Sr_0.5Nb₂O₆. Hence, photoanodes composed of Sr₂Nb₂O₇ respond to much less of the “white” light spectrum than those made from Ba_0.5Sr_0.5Nb₂O₆. Nevertheless, their electrical outputs at an anode potential of 0.8 eV with respect to SCE in 0.2 M sodium acetate under “white” xenon arc irradiation of 1.25 W/cm² are comparable.     

Tuning luminescent properties through solid-solution in (Ba1−xSrx)9Sc2Si6O24:Ce3+,Li+

A solid-solution of cerium-substituted alkaline earth scandium silicate phosphors, (Ba_1−xSr_x)₉Sc₂Si₆O₂₄:Ce³⁺,Li⁺ (x = 0, 0.25, 0.50, 0.75, 1), have been prepared by solid-state reaction. The structures, characterized using synchrotron X-ray powder diffraction, show the solid-solution closely follows Vegard's law. The substitution of Sr for Ba results in a decrease of the alkaline earth–oxygen bond distances by more than 0.1 Å at all three crystallographic sites, leading to changes in optical properties. The room temperature photoluminescent measurements show the structure has three excitation peaks corresponding to Ce³⁺ occupying the three independent alkaline earth sites. The emission of (Ba_1−xSr_x)₉Sc₂Si₆O₂₄:Ce³⁺,Li⁺ is red-shifted from the near-UV (λ_max = 384 nm) for x = 0 to blue (λ_max = 402 nm) for x = 1. The red-shifted photoluminescent quantum yield also increases when Sr is substituted for Ba in these compounds.     

Structural and magnetic study of the cation-ordered perovskites Ba2−xSrxErMoO6

A series of perovskite phases have been prepared from the appropriate carbonates and oxides by heating under reducing conditions at temperatures up to 1300 °C. Complete ordering between ErO₆ and MoO₆ octahedra and a disordered distribution of Sr²⁺ and Ba²⁺ occur in all compounds. Neutron powder diffraction experiments show that the substitution of Sr²⁺ into Ba₂ErMoO₆ introduces a progressive reduction in symmetry from Fm3¯m (x=0) to I4/m (x=0.5, 0.8) to P2₁/n (x=1.25, 1.75, 2.0). Magnetic susceptibility measurements indicate that all of these compounds show Curie-Weiss paramagnetism and that for x<1.25 this behaviour persists down to 2 K. The monoclinically distorted compounds show magnetic transitions at low temperature and neutron diffraction has confirmed the presence of long-range antiferromagnetic order below 2.5 and 4 K in Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆, respectively. Ba_0.75Sr_1.25ErMoO₆, Ba_0.25Sr_1.75ErMoO₆ and Sr₂ErMoO₆ do not undergo structural distortion on cooling from room temperature.     

Ein weiteres Gemischtvalentes Oxoniobat: Sr5Nb34+Nb25+O16

About a Mixed Valence Oxoniobate: Sr₅Nb₃⁴⁺Nb₂⁵⁺O₁₆ The hitherto unknown compound Sr₅Nb₅O₁₆ was prepared and examined by X-ray single crystal work. It crystallizes with orthorhombic symmetry (space group D–Pmn2₁, a = 3.992(1), b = 32.476(10), c = 5.677(2) Å; Z = 2). Sr₅Nb₅O₁₆ consists of stacked perovskite-like blocks cut by a plane perpendicular to the cube face diagonal of the perovskite structure. The coordination relations of the intersections between those blocks and the distribution of Nb⁵⁺ and Nb⁴⁺ are discussed.     

Blue photoluminescence in Ti-doped alkaline-earth stannates

Blue photoluminescence properties of Ti-doped alkaline-earth stannates, A₂(Sn_1−xTi_x)O₄ (A=Ca, Sr, Ba) (x=0.005-0.15), were examined at room temperature. These stannates showed intense broad emission bands peaking at 445 nm for Ca₂SnO₄, at 410 nm for Sr₂SnO₄, and at 425 nm for Ba₂SnO₄ under UV excitation. Emission intensities were relatively insensitive to Ti concentration and no sharp concentration quenching was observed. Mixing alkaline-earth ions in the crystal structures did not increase the emission intensities in the A₂(Sn_1−xTi_x)O₄ system. The excitation spectra of these stannates exhibited broad bands just below the fundamental absorption edges, implying that luminescence centers do not consist of the component elements in the host materials. It was suggested that the isolated TiO₆ complexes are possible luminescence centers in these materials, as previously proposed in other Ti-doped stannates such as Mg₂SnO₄ and Y₂Sn₂O₇.     

Über hexagonale Perowskite mit Kationenfehlstellen. XX. Ba6Nb4Zr□O18 – eine neue Stapelvariante mit rhomboedrischer 18 L-Struktur

On Hexagonal Perovskites with Cationic Vacancies. XX. Ba₆Nb₄Zr□o₁₈ - a New Stacking Polytype with a Rhombohedra1 18 L Structure The white Ba₆Nb₄Zr□O₁₈ crystallizes in a rhombohedral 18 L structure (a = 5.82₁ Å; c = 42.6₃ Å; space group R3 m) with three formula units for the trigonal setting (?_exp = 6.0₅ g/cm³; ?_calc = 6.27₁ g/cm³). The corresponding Ti^IV and Hf^IV compounds, Ba₆Nb₄Zr□O₁₈ and Ba₆Nb₄Hf□O₁₈, are isotypic.     

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.     

Photo-luminescence studies of strontium barium niobate crystals doped with Cr3+ ions

This paper reports the photo-luminescence spectroscopic results of Strontium–Barium–Niobate, Sr_x,Ba_1−xNb₂O₅ (SBN, x = 0.61 for near congruent composition) crystals doped with Cr₂O, at cryogenic temperature (20 K). The experimental results reveal the need of re-assignment of the Cr³⁺ ions defect centres in this material. For first time, a broad emission band in the near infrared region centred at ca. 950 nm is reported. This emission band has micro-seconds decaytime constant and a FWHM band-width > 1700 cm⁻¹ and has been ascribed to the vibronically assisted ⁴T₂ → ⁴A₂ transition. A much narrower emission band centred at ca. 764 nm with milli-seconds decaytime constant and a FWHM band-width of ca. 170 cm⁻¹ is correlated to the ²E → ⁴A₂ radiative transition (R-line).     

Cluster intergrowth of perovskite and defect sodium chloride type structures in the K---Nb---O system: The structure of KNb4O6

A new reduced potassium niobate (KNb₄O₆) of intergrowth type structure containing condensed Nb₆O₁₂ clusters has been found. The structure has been determined from HREM images. The atomic positions have been refined with the Rietveld technique using X-ray powder diffraction data. The space group of KNb₄O₆ is P4/mmm; Z = 1, and its unit cell parameters are a = 4.1393(1) and c = 8.2537(2). KNb₄O₆ consists of alternating slabs of KNbO₃ (perovskite) and NbO (ordered deficient NaCl-type) both being a single unit thick. The structure is closely related to that of A₂Nb₅O₉ (A = Ba, Sr). Both phases can be considered as members (n = 1 and 2 respectively) of a homologous series A_nNb_3+nO_3+3n. Electron microscopy studies show the presence of defects, both as extra perovskite layers and missing NbO slabs, together with areas of more disordered intergrowth. The profile refinement and microanalysis of individual crystal fragments both indicate the structure to be niobium deficient according to the formula K_1+x/2Nb_4−xO₆.     

Redox behavior and transport properties of La0.5−2xCexSr0.5+xFeO3−δ and La0.5−2ySr0.5+2yFe1−yNbyO3−δ perovskites

The effects of doping the mixed-conducting (La,Sr)FeO_3−δ system with Ce and Nb have been examined for the solid-solution series, La_0.5−2xCe_xSr_0.5+xFeO_3−δ (x = 0–0.20) and La_0.5−2ySr_0.5+2yFe_1−yNb_yO_3−δ (y = 0.05–0.10). Mössbauer spectroscopy at 4.1 and 297 K showed that Ce⁴⁺ and Nb⁵⁺ incorporation suppresses delocalization of p-type electronic charge carriers, whilst oxygen nonstoichiometry of the Ce-containing materials increases. Similar behavior was observed for La_0.3Sr_0.7Fe_0.90Nb_0.10O_3−δ at 923–1223 K by coulometric titration and thermogravimetry. High-temperature transport properties were studied with Faradaic efficiency (FE), oxygen-permeation, thermopower and total-conductivity measurements in the oxygen partial pressure range 10⁻⁵–0.5 atm. The hole conductivity is lower for the Ce- and Nb-containing perovskites, primarily as a result of the lower Fe⁴⁺ concentration. Both dopants decrease oxide-ion conductivity but the effect of Nb-doping on ionic transport is moderate and ion-transference numbers are higher with respect to the Nb-free parent phase, 2.2 × 10⁻³ for La_0.3Sr_0.7Fe_0.9Nb_0.1O_3−δ cf. 1.3 × 10⁻³ for La_0.5Sr_0.5FeO_3−δ at 1223 K and atmospheric oxygen pressure. The average thermal expansion coefficients calculated from dilatometric data decrease on doping, varying in the range (19.0–21.2) × 10⁻⁶ K⁻¹ at 780–1080 K.     

Crystal chemistry and crystallography of the Aurivillius phase Bi5AgNb4O18

Bi₅AgNb₄O₁₈ is a new phase, which was discovered during the phase equilibrium study of the Bi₂O₃-Ag₂O-Nb₂O₅ system. Bi₅AgNb₄O₁₈ was prepared at 750°C and is stable in air up to its melting temperature of 1160.1±5.0°C (standard error of estimate). Results of a Rietveld refinement using neutron powder diffraction confirmed that Bi₅AgNb₄O₁₈ is isostructural with Bi₃TiNbO₉, Bi₅NaNb₄O₁₈, and Bi₅KNb₄O₁₈. The structure was refined in the orthorhombic space group A2₁am, Z=2, and the lattice parameters are a=5.4915(2) Å, b=5.4752(2) Å, c=24.9282(8) Å, and V=749.52(4) Å³. The structure can be described as the m=2 member of the Aurivillius family, (Bi₂O₂)²⁺ (A_m−1B_mO_3m+1)²⁻ (where A=Bi and B=Ag, Nb), which is characterized by perovskite-like (A_m−1B_mO_3m+1)²⁻ slabs regularly interleaved with (Bi₂O₂)²⁺ layers. The octahedral [NbO₆] units are distorted with Nb-O distances ranging from 1.856(4) to 2.161(2) Å and the O-Nb-O angles ranging from 82.6(3)° to 98.5(3)°. These octahedra are tilted about the a- and c-axis by about 10.3° and 12.4°, respectively. Ag was found to substitute exclusively into the Bi-site that is located in the layer between the two distorted [NbO₆] units. Although the Ag substitutes into the Bi-site with the Bi:Ag ratio of 1:1, the existence of a superlattice was not detected using electron diffraction. A comparison of (Bi₂O₂)²⁺(A_m−1Nb_mO_3m+1)²⁻ structures (where A=Ag, Na, and K) revealed a relation between the pervoskite tolerance factor, t, and structural distortion. The reference pattern for Bi₅AgNb₄O₁₈ has been submitted to the International Centre for Diffraction Data (ICDD) for inclusion in the Powder Diffraction File.     

Zur Kristallchemie der tetragonalen Wolframbronze: Ba6FeNb9O30

Solid state reaction of BaCO₃, FeC₂O₄·2H₂O and Nb₂O₅ gave single crystals of Ba₆FeNb₉O₃₀. The crystal structure was solved by X-ray investigations (a=12.597,c=3.990Å, space group P 4 bm-C _4v ² ,Z=1). Ba₆FeNb₉O₃₀ crystallyzes in the tetragonal bronze type with a statistical distribution of Fe³⁺ and Nb⁵⁺ in the octahedral framework. The anisotropic temperature factors of barium are discussed with respect to the oxygen coordination.

     

A new type of orthoborates: ASr4La3(BO3)6 (A = Li,Na)

Two new rare earth containing orthoborate crystals ASr₄La₃(BO₃)₆ (A = Li, Na) have been obtained by spontaneous nucleation from high-temperature melts of A₂O–SrO–La₂O₃–B₂O₃–AF. X-ray diffraction analyses show that they both crystallize in the rhombohedral space group R-3 with cell parameters of a = 12.309(7) Å, c = 9.316(7) Å and a = 12.4049(13) Å, c = 9.348(2) Å for the Li and Na compounds respectively. Similar to the large A′₆MM′(BO₃)₆ family, these compounds are all related to the structure of Sr₃Y(BO₃)₃ with La and Sr statistically occupy the Sr site, and the alkaline elements and remaining Sr enter the ordered Y1 and Y2 sites, which can be approximately represented as (La_2.91Sr_3.09)(La_0.09Sr_0.91)Li[B₆O₁₈] and (La_2.85Sr_3.15)(La_0.15Sr_0.85)Na[B₆O₁₈]. The characteristic of the structure is that the La/Sr and isolated BO₃ groups form a network with tunnels along the c-axis where the alkaline A and Sr ions alternatively reside. The optical transmission spectrum shows that the ultraviolet absorption edge of NaSr₄La₃(BO₃)₆ crystal is about 193 nm and Raman spectra reveal that both crystals possess sharp peaks at 930 cm⁻¹.     

Structural characterization and Curie temperature determination of a sodium strontium niobate ferroelectric nanostructured powder

The Curie temperature and its correlation with the magnitude of the displacement of the niobium atom from the center of [NbO₆] octahedra in NaSr₂Nb₅O₁₅ nanostructured powder were investigated. A single powder was prepared by high-energy ball milling. A powder with an average crystallite size of 37 nm was prepared by calcining the precursor at 1423 K. The refinement of the structural parameters was carried out by the Rietveld method. NaSr₂Nb₅O₁₅ exhibits tetragonal symmetry with the tungsten bronze structure (a=b=12.3495 (6) Å, c=3.8911 (2) Å, V=593.432 (5) Å³, and Z=2). The site occupancy of the Na⁺ and Sr²⁺ cations and the interatomic distances between the niobium and oxygen atoms were derived. The [NbO₆] octahedron undergoes both rotation and tilting depending on the crystallographic site. The Curie temperature of the powder was derived using both the impedance and infrared spectroscopy methods.     

Structural and magnetic characterisation of Aurivillius material Bi2Sr2Nb2.5Fe0.5O12

The n=3 Aurivillius material Bi₂Sr₂Nb_2.5Fe_0.5O₁₂ is investigated and combined structural refinements using neutron powder diffraction (NPD) and X-ray powder diffraction data (XRPD) data reveal that the material adopts a disordered, tetragonal (I4/mmm) structure at temperatures down to 2 K. Significant ordering of Fe³⁺ and Nb⁵⁺ over the two B sites is observed and possible driving forces for this ordering are discussed. Some disorder of Sr²⁺ and Bi³⁺ over the M and A sites is found and is consistent with relieving strain due to size mismatch. Highly anisotropic thermal parameters for some oxygen sites suggest that the local structure may be slightly distorted with some rotation of the octahedra. Magnetic measurements show that the material behaves as a Curie-Weiss paramagnet in the temperature range studied with no evidence of any long-range magnetic interactions. Solid solutions including Bi_3−xSr_xNb₂FeO₁₂, Bi₂Sr_2−xLa_xNb₂FeO₁₂ and Bi₂Sr₂Nb_3−xFe_xO₁₂ were investigated but single-phase materials were only successfully synthesised for a narrow composition range in the Bi₂Sr₂Nb_3−xFe_xO₁₂ system.     

A new promising scintillator Ba3InB9O18

We report on the synthesis, crystal structure and scintillation property of a new compound Ba₃InB₉O₁₈. This compound crystallizes in space group P6₃/m with unit cell of dimensions a=7.1359(3) Å, c=16.6151(8) Å and V=732.697 Å³ with two Ba₃InB₉O₁₈ molecular formula. Its crystal structure is made up of planar B₃O₆ groups parallel to each other along the 〈0001〉 direction, regular InO₆ octahedra, irregular BaO₆ hexagons and BaO₉ polyhedra to form an analog structure of Ba₃YB₉O₁₈. DTA and TGA curves for Ba₃InB₉O₁₈ show that it is a chemically stable and congruent melting compound. Its X-ray excited luminescence spectra show an intense emission band in the range of 360-500 nm with a maximum at 400 nm. Light yield for Ba₃InB₉O₁₈ is about 75% as large as that for BGO under the same measurement conditions. There may exist a correlation between the scintillation properties and the crystal structural features of Ba₃InB₉O₁₈.     

Structural and optical absorption studies of barium substituted strontium ferrite powder

Attempt has been made to synthesize Ba_xSr_1−xFeO_3−ξ (x = 0–1.0) ferrite powder by decomposition of sol–gel derived oxalate at 800–1000 °C for 5–10 h to study the effect of barium insertion with regard to phase(s), stability, optical behavior, oxidation states of iron, and oxygen deficiency. It is shown that these ferrites possess a perovskite-type cubic phase (a = 3.877–4.020Å, Z = 1, space group Pm3m) for 0.1 ≤ x ≤ 0.94, a mixture of 82% rhombohedral (a_R = 5.666Å and α_R = 59.761°, Z = 2, space group R3c) and 18% hexagonal phases for x = 0.96 and a pure hexagonal (a = 5.689Å, c = 13.944Å, Z = 6, space group P6₃/mmc) phase for x = 1. Barium substitution in SrFeO_3−ξ system leads to lattice expansion, weakening of the metal-oxygen bond, reduction of tetravalent iron ions (as evident from Mossbauer analysis), and decrease of oxygen content. The optical absorption peaks observed in the range 3.17–4.11 eV are attributed to charge transfer transitions from O²⁻ (2p) to Fe (3d) band. The values of optical energy band gap of Ba_xSr_1−xFeO_3−ξ are found to be ∼5.48 and ∼4.04 for x = 0.1 and 1.0, respectively. A stable perovskite-type cubic phase in Ba_xSr_1−xFeO_3−ξ system with significant anion deficiency (ξ = 0.26–0.32) may possibly act as an oxygen permeable membrane.