Showing papers on "Orthorhombic crystal system published in 2003"

Dimensional Control of Cobalt-hydroxide-carbonate Nanorods and Their Thermal Conversion to One-Dimensional Arrays of Co3O4 Nanoparticles

Abstract: We report a detailed investigation on the formation of cobalt-basic-carbonate compounds [Co(OH)x(CO3)05(2-x)·nH2O] with dimensional and morphological controls Two complementary precipitation methods developed in this work produce cobalt-basic-carbonate in the form of nanorods across a wide diameter span from 2 to 200 nm and a length up to 5 μm Various controlling parameters were examined, such as anions in starting reagents, reaction temperature, aging time, and chemical compositions between the solid products and starting reagents The resultant solids were characterized using XRD, FTIR, CHN, and TGA/DrTGA methods with respect to their formation process and thermal decomposition More importantly, morphological aspects of the Co(OH)x(CO3)05(2-x)·nH2O and their heat-treated product Co3O4 were investigated with TEM/ED methods It has been found that the Co(OH)x(CO3)05(2-x)·nH2O nanorods are grown along the [010] direction (orthorhombic) Upon thermal decomposition at ≥300 °C, the above nanorods self-a

Helix Distortion and Crystal Structure of the α-Form of Poly(l-lactide)

Abstract: The crystal structure of the α-form of poly(l-lactide) was analyzed by the linked-atom least-squares refinements for the X-ray fiber diffraction data. The space group was orthorhombic P212121, and ...

Multifunctionality of Active Centers in (Amm)oxidation Catalysts: From Bi–Mo–Ox to Mo–V–Nb–(Te, Sb)–Ox

Abstract: Catalytic centers in selective (allylic) oxidation and ammoxidation catalysts are multimetallic and multifunctional. In the historically important bismuth molybdates, used for propylene (amm)oxidation, they are composed of (Bi3+)(Mo6+)2 complexes in which the Bi3+ site is associated with the α-H abstraction and the (Mo6+)2 site with the propylene chemisorption and O or NH insertion. An updated reaction mechanism is presented. In the Mo–V–Nb–Te–Ox systems, three crystalline phases (orthorhombic Mo7.5V1.5NbTeO29, pseudohexagonal Mo6Te2VO20, and monoclinic TeMo5O16) were identified, with the orthorhombic phase being the most important one for propane (amm)oxidation. Its active centers contain all necessary key catalytic elements (2V5+/Mo6+, 1V4+/Mo5+, 2Mo6+/Mo5+, 2Te4+) for this reaction wherein a V5+ surface site (V5+ = O ↔ 4+V•–O•) is associated with paraffin activation, a Te4+ site with α-H abstraction once the olefin has formed, and a (Mo6+)2 site with the NH insertion. Four Nb5+ centers, each surrounded by five molybdenum octahedra, stabilize and structurally isolate the catalytically active centers from each other (site isolation), thereby leading to high selectivity of the desired acrylonitrile product. A detailed reaction mechanism of propane ammoxidation to acrylonitrile is proposed. Combinatorial methodology identified the nominal composition Mo0.6V0.187Te0.14Nb0.085Ox for maximum acrylonitrile yield from propane, 61.8% (86% conversion, 72% selectivity at 420 °C). We propose that this system, composed of 60% Mo7.5V1.5NbTeO29, 40% Mo6Te2VO20, and trace TeMo5O16, functions with a combination of compositional pinning of the optimum orthorhombic Mo7.5V1.5±xNb1±yTe1±zO29±δ phase and symbiotic mop-up of olefin intermediates through phase cooperation. Under mild reaction conditions, a single optimum orthorhombic composition might suffice as the catalyst; under demanding conditions this symbiosis is additionally required. Improvements in catalyst performance could be attained by further optimization of the elemental distributions at the active catalytic center of Mo7.5V1.5NbTeO29, by promoter/modifier substitutions, and incorporation of compatible cocatalytic phases (preferably epitaxially matched). High-throughput methods will greatly accelerate the rational catalyst design processes.

The silica-like extended polymorphism of cobalt(II) imidazolate three-dimensional frameworks: X-ray single-crystal structures and magnetic properties.

Abstract: Five polymorphous frameworks of cobalt(II) imidazolates (1-5) have been prepared by solvatothermal syntheses. Of these, compound 3 has already been synthesized in a gas-phase reaction by Seel et al. in 1969 and structurally characterized by Sturm et al. in 1975. The new synthetic strategy affords four polymorphous frameworks of cobalt(II) imidazolates (1, 2, 4, 5) of crystalline substances, of which the compound 4 (a = b = 23.450(3), c = 12.460(3) A, tetragonal, I4(1)cd, Z = 16) is an isomorphous compound of [Zn(im)(2)]( infinity ), which was also synthesized in a gas-phase reaction in 1980. The frameworks of compounds 1 and 2 are porous and isostructural; they have the same framework topology that represents a novel uninodal (6,4)-net: 1: a = 18.513(4), b = 24.368(5), c = 9.2940(19) A, orthorhombic, Fdd2, Z = 16; 2: a = 17.635(4), b = 27.706(6), c = 9.0810(18) A, orthorhombic, Fdd2, Z = 16. The framework of compound 5 exhibits a topology of zeolitic structure with the unit-cell parameters: a = 24.3406(8), b = 9.4526(3), c = 24.8470(8) A, beta = 91.977(1) degrees, monoclinic, P2(1)/n, Z = 4. All polymorphous frameworks of cobalt(II) imidazolates reflect the structural features of silica (SiO(2)) and also exhibit different magnetic behaviors, although the imidazolates transmit the antiferromagnetic coupling between the cobalt(II) ions in all cases. However, the uncompensated antiferromagnetic couplings arise from spin-canting are sensitive to the structures: compound 1 is an antiferromagnet with T(N) = 13.11 K; compounds 2-4 are weak ferromagnets (canted antiferromagnets): 2 shows a very weak ferromagnetism below 15 K, 3 exhibits a relatively strong ferromagnetism below 11.5 K and a coercive field (H(C)) of 1800 Oe at 1.8 K, and 4 displays the strongest ferromagnetism of the three cobalt imidazolates and demonstrates a T(C) of 15.5 K with a coercive field, H(C), of 7300 Oe at 1.8 K. However, compound 5 seems to be a hidden canted antiferromagnet with a magnetic ordering temperature of 10.6 K.

Direct water splitting into H2 and O2 under visible light irradiation with a new series of mixed oxide semiconductor photocatalysts

Abstract: The research on a new series of solid photocatalysts with different crystal structures was reviewed. The first system is A2B2O7 pyrochlore-crystal type: Bi2MNbO7 (M=Al, Ga, In and Y, rare earth, and Fe), which is cubic system and space group Fd3m. The second system is ABO4 stibotantalite-crystal type: BiMO4 (M=Nb5+, Ta5+), in which both the triclinic system with space group P1 in the case of M=Ta and the orthorhombic system with space group Pnna in the case of M=Nb. The third system is ABO4 wolframite-crystal type: InMO4 (M=Nb5+, Ta5+), which is monoclinic system and space group P2/a. Although these photocatalysts crystallize in the different crystal structure, they contain the same octahedral TaO6 and/or NbO6 in the different photocatalysts. The band structure of the photocatalysts is defined by Ta/Nb d-level for a conduction band and O 2p-level for a valence band. The band gaps of the photocatalysts were estimated to be between 2.7 and 2.4 eV. Metal doped InTaO4 photocatalysts were also investigated. Under visible light (λ>420 nm) or ultra-violet irradiation, the H2 and/or O2 evolutions were observed from pure water as well as aqueous CH3OH/H2O and AgNO3 solutions. The photocatalytic activity increases significantly by loading co-catalysts such as Pt, RuO2 and NiOx on the surface of the photocatalysts. Finally, direct water splitting into H2 and O2 under visible light irradiation was firstly established using newly synthesized NiOx (partly oxidized nickel) promoted In0.9Ni0.1TaO4 photocatalyst.

Phase transitions in ternary caesium lead bromide

Abstract: Phase transitions in ternary caesium lead bromide (CsPbBr3) were studied by means of DSC, TMA and high temperature X-ray diffraction. The samples were prepared from the solution by water evaporation and from the melt. on the DSC curves as well as on the temperature dependence of the lattice constants of CsPbBr3 only two effects were found belonging to the earlier published phase transitions at 88 and 130°C and no further effects. Linear thermal expansion coefficient α of individual CsPbBr3 modifications were calculated from both TMA and high temperature X-ray diffraction. The structural parameters of the room temperature orthorhombic phase were refined and the results are presented. CsPbBr3 prepared from the solution contained about 10% of CsPb2Br5 and so the DSC curve of pure CsPb2Br5 was also measured and an effect at a temperature of 68.5°C was found.

Structural and magnetic phase diagrams of La 1-x Sr x MnO 3 and Pr 1-y Sr y MnO 3

Abstract: The nuclear and magnetic structures and properties of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ with $0.45l~xl~1$ and ${\mathrm{Pr}}_{1\ensuremath{-}y}{\mathrm{Sr}}_{y}{\mathrm{MnO}}_{3}$ $(0.58l~yl~1)$ were investigated using neutron powder diffraction, resistivity and magnetic measurements. In this paper, we present the full magnetic and structural phase diagram for ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ and a partial phase diagram for ${\mathrm{Pr}}_{1\ensuremath{-}y}{\mathrm{Sr}}_{y}{\mathrm{MnO}}_{3}.$ At low temperatures, we observe a series of structural phase transitions evolving as a function of increasing x as follows: orthorhombic $\mathrm{Pbn}\stackrel{\ensuremath{\rightarrow}}{m}\mathrm{rhombohedral}$ $R3\ifmmode\bar\else\textasciimacron\fi{}\stackrel{\ensuremath{\rightarrow}}{c}\mathrm{orthorhombic}$ $\mathrm{Fmm}\stackrel{\ensuremath{\rightarrow}}{m}\mathrm{tetragonal}$ $I4/mc\stackrel{\ensuremath{\rightarrow}}{m}\mathrm{cubic}$ $\mathrm{Pm}3\ifmmode\bar\else\textasciimacron\fi{}m.$ At higher temperatures, the orthorhombic Fmmm structural region disappears. We also identify four magnetic states (FM, A, C, and G type) evolving as a function of increasing x. The magnetic and nuclear structural transitions coincide for samples with $0.8l~xl~0.95.$ In the $0.5l~xl~0.9$ region, resistivity and magnetic measurements show a substantial hysteresis near ${T}_{N}.$ This hysteresis of a few K is indicative of a first order antiferromagnetic transition in good agreement with the neutron diffraction results. Our data also suggest the existence of a strong competition between the ferromagnetic and A-type states for $x\ensuremath{\sim}0.5,$ and between the C- and G-type states for $x\ensuremath{\sim}0.95.$ A similar behavior is observed for ${\mathrm{Pr}}_{1\ensuremath{-}y}{\mathrm{Sr}}_{y}{\mathrm{MnO}}_{3}.$

α- and β-A2Hg3M2S8 (A = K, Rb; M = Ge, Sn): Polar quaternary chalcogenides with strong nonlinear optical response

Abstract: The closely related phases α- and β-A2Hg3M2S8 (A = K, Rb; M = Ge, Sn) have been discovered using the alkali polychalcogenide flux method and are described in detail. They present new structure types with a polar noncentrosymmetric crystallographic motif and strong nonlinear second-harmonic generation (SHG) properties. The α-allotropic form crystallizes in the orthorhombic space group Aba2 with a = 19.082(2) A, b = 9.551(1) A, c = 8.2871(8) A for the K2Hg3Ge2S8 analogue, and a = 19.563(2) A, b = 9.853(1) A, c = 8.467(1) A for the K2Hg3Sn2S8 analogue. The β-form crystallizes in the monoclinic space group C2 with a = 9.5948(7) A, b = 8.3608(6) A, c = 9.6638(7) A, β = 94.637° for the K2Hg3Ge2S8 analogue. The thermal stability and optical and spectroscopic properties of these compounds are reported along with detailed solubility and crystal growth studies of the α-Κ2Hg3Ge2S8 in K2S8 flux. These materials are wide gap semiconductors with band gaps at ∼2.40 and ∼ 2.64 eV for the Sn and Ge analogues, respectively...

Structural Characterization of the Orthorhombic Phase M1 in MoVNbTeO Propane Ammoxidation Catalyst

Abstract: The structure of the orthorhombic phase in the MoVNbTeO propane ammoxidation catalyst system has been characterized and refined using a combination of TEM, synchrotron X-ray powder diffraction (S-XPD), and neutron powder diffraction (NPD). This phase, designated as M1 by Ushikubo et al. [1], crystallizes in the orthorhombic space group Pba2 (No. 32) with a = 21.134(2) A, b = 26.658(2) A, and c = 4.0146(3) A. The formula unit is Mo7.5V1.5NbTeO29. Bond valence sum calculations indicate the presence of d 1 metal sites neighbored by d 0 metal sites. The d 1 sites are occupied by a distribution of Mo5+ and V4+, whereas the d 0 sites are occupied by a distribution of Mo6+ and V5+. Out-of-center distortions in d 0 octahedra are consistent with the second-order Jahn–Teller effect and lattice effects. We argue that the V5+–O–V4+/Mo5+ moieties adjacent to Te4+ and Mo6+ sites in the [001] terminal plane provide a spatially isolated active site at which the selective ammoxidation of propane occurs.

Hafnium nitride with thorium phosphide structure: physical properties and an assessment of the Hf-N, Zr-N, and Ti-N phase diagrams at high pressures and temperatures.

Abstract: The physical properties of the new cubic phase of Hf3N4 as well as of isomorphic Zr3N4 and Ti3N4 are studied using first-principles calculations. Hf3N4, Zr3N4, and Ti3N4 are semiconductors with band gaps of 1.8, 1.1, and 0.6 eV, respectively. The band structure is characterized by the simultaneous presence of steep and extremely flat bands. The calculated shear modulus G indicates that the cubic Hf3N4 will be harder than the mononitride HfN. At ambient conditions, the cubic modifications of M3N4 (M=Hf, Zr, Ti) are metastable with respect to orthorhombic M3N4 phases, but the orthorhombic phases of Hf3N4 and Zr3N4 are stable with respect to the mononitrides and nitrogen.

Orientation dependence of ferroelectric properties of pulsed-laser-ablated Bi4−xNdxTi3O12 films

Abstract: Epitaxial (001)-, (118)-, and (104)-oriented Nd-doped Bi4Ti3O12 films have been grown by pulsed-laser deposition from a Bi4−xNdxTi3O12 (x=0.85) target on SrRuO3 coated single-crystal (100)-, (110)-, and (111)-oriented SrTiO3 substrates, respectively. X-ray diffraction illustrated a unique epitaxial relationship between film and substrate for all orientations. We observed a strong dependence of ferroelectric properties on the film orientation, with no ferroelectric activity in an (001)-oriented film; a remanent polarization 2Pr of 12 μC/cm2 and coercive field Ec of 120 kV/cm in a (118)-oriented film; and 2Pr=40 μC/cm2, Ec=50 kV/cm in a (104)-oriented film. The lack of ferroelectric activity along the c-axis is consistent with the orthorhombic nature of the crystal structure of the bulk material, as determined by powder neutron diffraction.

Solvent-induced polymorphism of three-dimensional hydrogen-bonded networks of hexakis(4-carbamoylphenyl)benzene.

Abstract: The crystal structures for three types of three-dimensional (3-D) hydrogen-bonded networks of hexakis(4-carbamoylphenyl)benzene (1), the network morphologies of which depend greatly on crystallization conditions, have been determined. When this compound is crystallized from hot DMSO, the resulting crystals, 1.12DMSO (orthorhombic, Pca2(1)), showed a 3-D hydrogen-bonded porous network (type A) via 1-D catemer chains as a hydrogen-bonding motif of six primary amide groups. The type A network creates chambers surrounded by six molecules of 1 and channels along the c axis to give the highest porosity among the network polymorphs of 1 investigated here. Crystallization from a boiling mixture of n-PrOH and water gave 1.6n-PrOH (monoclinic, P2(1)/c), which exhibits another type of 3-D hydrogen-bonded porous network (type B) via cyclic dimers as another hydrogen-bonding motif of six primary amide groups. The type B network leads to triangle-like channels along the a axis having a cross section of ca. 9.2 x 9.7 x 9.7 A (including van der Waals radii). The crystal structure of 1.H(2)O (monoclinic, P2(1)/c), which was produced under hydrothermal conditions, showed a nonporous 3-D hydrogen-bonded network chain of amide groups (type C) composed of a mixed hydrogen bonding motif of helical catemer chains/cyclic dimer/catemer. Solvent-induced topological isomerism of these 3-D hydrogen-bonded networks of 1 arises from (i) the guest inclusion ability based on a radially functionalized hexagonal structure of 1, (ii) the correlation between the hydrogen bond donor ability of the syn and anti protons of the primary amide group in host 1 and the hydrogen bond acceptor ability of the oxygen atoms of 1 and guest solvents, and (iii) the polarity of the bulk crystallization solvents.

Orientation Dependence of Ferroelectric Properties of Pulsed-Laser-Ablated Bi4-xNdxTi3O12 Films

Abstract: Epitaxial (001)-, (118)-, and (104)-oriented Nd-doped Bi4Ti3O12 films have been grown by pulsed-laser deposition from a Bi4-xNdxTi3O12 (x=0.85) target on SrRuO3 coated single-crystal (100)-, (110)-, and (111)-oriented SrTiO3 substrates, respectively. X-ray diffraction illustrated a unique epitaxial relationship between film and substrate for all orientations. We observed a strong dependence of ferroelectric properties on the film orientation, with no ferroelectric activity in an (001)-oriented film; a remanent polarization, 2Pr, of 12 microC/cm2 and coercive field, Ec, of 120 kV/cm in a (118)-oriented film; and 2Pr = 40 microC/cm2, Ec = 50 kV/cm in a (104)-oriented film. The lack of ferroelectric activity along the c-axis is consistent with the orthorhombic nature of the crystal structure of the bulk material, as determined by powder neutron diffraction.

Polymorphism in 2-4-6 trinitrotoluene

Abstract: Two crystal structures of 2-4-6 trinitrotoluene (TNT) are given, the monoclinic form (a0 ) 1.49113 (1) nm, b0 ) 0.60340 (1) nm, c0 ) 2.08815(3) nm, â ) 110.365 (1)°, V ) 1.76137 (4) nm3, space group ) P21/a, T ) 100 K) and the orthorhombic form (a0 ) 1.4910 (2) nm, b0 ) 0.6031 (2) nm, c0 ) 1.9680 (4) nm, V ) 1.7706 (7) nm3, space group ) Pca21, T ) 123 K). Of these two forms, the most stable is the monoclinic and the less stable is the orthorhombic form. These two polymorphs are shown to be orientational, rather than configurational in character. Due to their restricted molecular motifs, no strong hydrogen bonding exists and the crystalline form is dominated by van der Waals type forces. The two structures are shown to be closely related and an analysis of the two structures shows that they are effectively large scale polytypes. Calorimetric studies show that the two polymorphs are monotropic and that the enthalpy of transformation is very low, concurring with the similarity shown by the diffraction data and calculated lattice energies. The thermal expansion coefficients are defined, and it is shown that both polymorphs have similar thermal expansions.

Trapping of oxygen vacancies on twin walls of CaTiO3: a computer simulation study

Abstract: We have studied the atomic structure of [001]90 ◦ rotation twin walls in orthorhombic CaTiO3 (symmetry Pbnm )a t low temperature (10 K) and their effects on oxygen vacancies. The wall thickness was found to be 2.3 nm at

Photostimulated luminescence in Eu-doped fluorochlorozirconate glass ceramics

Abstract: We report the synthesis of Eu2+- and chlorine-doped fluorozirconate glass-ceramics that show an intense photostimulated luminescence (PSL) after x-ray irradiation at room temperature. The PSL efficiency is up to 80% of that found in the well-known crystalline x-ray storage phosphor BaFBr:Eu2+, and it is the largest thus far reported for a glass-ceramic. We attribute the PSL to crystallites of orthorhombic BaCl2 that are formed after annealing above the glass temperature. Hexagonal BaCl2 crystallites are also observed after short annealing times, but they do not provide a measurable PSL signal. The photoluminescence peak from glass-ceramics containing orthorhombic BaCl2 crystallites occurs at 402 nm, and the stimulation band is centered at about 560 nm.

Ferroelectric and Piezoelectric Properties of KNbO3 Ceramics Containing Small Amounts of LaFeO3

Abstract: Dense KNbO3 ceramics have been successfully synthesized by pressure-less sintering under optimized heat-treatment conditions using a small amount of La2O3 and Fe2O3 additives KNbO3 forms (K1-xLax)(Nb1-xFex)O3 solid solutions and changes in the crystal system, depending on the additive content, from orthorhombic to tetragonal at x of 0020, and from tetragonal to cubic at x of 0200 or higher When only 0002 mol of La2O3 and Fe2O3 (x=0002) was added into KNbO3, the highest value (988%) of the theoretical density was obtained This specimen showed orthorhombic symmetry with a high Curie temperature of 420°C, and demonstrated a well-saturated ferroelectric hysteresis loop with large remanent polarization (Pr) of 18 µC/cm2, which is comparable to the value reported for pure KNbO3 ceramics fabricated by hot pressing Furthermore, the x=0002 specimen showed a planar electromechanical coupling ratio (kp) of 017 and piezoelectric d33 constant of 98 pC/N, regardless of the unsaturated poling state

The crystal structure of synthetic autunite, Ca[(UO2)(PO4)]2(H2O)11

Abstract: Autunite, Ca[(UO 2 )(PO 4 )] 2 (H 2 O) 11 , is amongst the most abundant and widely distributed of the uranyl phosphate minerals, yet because of its pseudo-tetragonal symmetry and rapid dehydration in air, the details of its symmetry, stoichiometry, and structure were previously uncertain. The crystal structure of synthetic autunite was solved by direct methods and refined by full-matrix least-squares techniques to agreement indices R 1 = 0.041, calculated for the 1497 unique observed reflections (| F o | ≥ 4σ F ), and wR 2 = 0.119 for all data. Autunite is orthorhombic, space group Pnma , Z = 4, a = 14.0135(6), b = 20.7121(8), c = 6.9959(3) A, V = 2030.55(15) A 3 . The structure contains the well-known autunite type sheet with composition [(UO 2 )(PO 4 )] − , resulting from the sharing of equatorial vertices of the uranyl square bipyramids with the phosphate tetrahedra. The calcium atom in the interlayer is coordinated by seven H 2 O groups and two longer distances to uranyl apical O atoms. Two symmetrically independent H 2 O groups are held in the structure only by hydrogen bonding. Bond-length-constrained refinement provided a crystal-chemically reasonable description of the hydrogen bonding.

Cation-size control of structural phase transitions in tin perovskites

Abstract: The structural evolution at 300 K of the series SrxBa1?xSnO3 and SrxCa1?xSnO3 at x = 0.2 intervals has been determined by powder neutron diffraction. All SrxCa1?xSnO3 samples (x = 0?1) have the Pbnm superstructure. In the series SrxBa1?xSnO3, the x = 1.0 and 0.8 samples have the Pbnm superstructure. The x = 0.6 sample has a second orthorhombic structure with space group Imma. There is a tetragonal I4/mcm phase at x = 0.4 and the undistorted cubic structure for x = 0.2 and BaSnO3. The octahedral tilt angles show a smooth variation with average A cation radius over the range of superstructures described by mean field theory close to the transition. The transitions are also analysed via the variation of spontaneous strains.

First principles study of oxygen vacancy defects in tantalum pentoxide

Abstract: First principles total energy calculations were performed to characterize oxygen vacancy defects in tantalum pentoxide (Ta2O5). A simplified version of the crystalline orthorhombic phase of Ta2O5 was used in this study. Results indicate that O vacancies in Ta2O5 can be broadly classified based on their location in the lattice. One type of vacancy that occupies the “in-plane” sites displays deep or midgap occupied states and shallow unoccupied states, while a second type occupying “cap” sites results in shallow occupied states. For a wide range of Fermi levels or chemical potentials, the neutral and +2 charged states of the in-plane type vacancy and the +2 charge state of the cap type vacancy are found to be most stable.

Ferroelectric phase transitions in SrBi2Nb2O9 and Bi5Ti3FeO15: A powder neutron diffraction study

Abstract: Structural phase transitions in the Aurivillius phase ferroelectrics SrBi 2 Nb 2 O 9 and Bi 5 Ti 3 FeO 1 5 , containing two and four perovskite layers, respectively, have been studied using powder neutron diffraction. At temperatures below the ferroelectric Curie temperature both materials crystallize in the polar orthorhombic space group A2 1 am. On passing through T C , both phases appear to transform directly to a tetragonal paraelectric phase, space group I4/mmm. This behavior contrasts with that of the analogs Sr 0 . 8 5 Bi 2 . 1 Ta 2 O 9 and SrBi 4 Ti 4 O 1 5 , both of which have been shown unambiguously to transform via an intermediate paraelectric orthorhombic phase, space group Amam.