Space Group P4bm Research Articles

Synthesis and X-ray powder diffraction characterization of a new niobate crystal Ba4Na2VNb9O30

A new niobate crystal Ba4Na2VNb9O30 was synthesized in BaO−Na2O−V2O5−Nb2O5 system for the first time. Its shape, optical properties, melting point and chemical stability wer estudied. X-ray powder diffraction study determined that this compound assumes a tetragonal tungsten bronze structure with space group P4 bm (100) and lattice parameters a=b=12.4275(2), c=3.970(3). The new compound may be one kinds of photorefractive crystals.

Structure Refinement of (Sr,Ba)Nb2O6 Ceramic Powder from Neutron and X-Rays Diffraction Data

The structure of polycrystalline strontium barium niobate at room temperature was refined by the Rietveld method. Sintered ceramic samples were used to collect powder neutron and X-ray diffraction data. The ratio Sr/Ba ~ 64/36 was found from the initial batch composition Sr0.61Ba0.39Nb2O6, corroborating with the quantitative X-ray dispersive spectroscopy (EDS) measurements. The structure is tetragonal with cell parameters a, b = 12.4504(3) Å and c = 3.9325(1) Å and space group P4bm. It was not necessary to introduce any positional disorder for the oxygen atoms. Cation Nb+5 displacements not parallel to the c direction are presented, which can influence the behavior of the ferroelectric properties.

Crystal structure of high- Tc related NdBaCuO 2BO 3: TEM and neutron powder diffraction study

The layered high- T c related oxyborate NdBaCuO 2BO 3 has been investigated by ED, HREM and neutron powder diffraction. The crystal structure was refined using neutron data (space group P4bm, Z=2, a=5.52807(5) Å, c=7.44636(8) Å; S=0.98, R p=0.042, R wp=0.056, R I=0.030, R F=0.018). The main features of this structure are deformed (CuO 5) square pyramids and triangular (BO 3) groups which alternate along the 4-fold axis. Ba and Nd atoms are completely ordered and coordinated by 12 and 8 oxygen atoms, respectively.

Absorption Spectrum and Optical Parameters of Nd-Doped Barium Sodium Niobate Crystals

Nd-doped barium sodium niobate, (Ba2Na)1-xNdxNb5O15 (Nd:BNN), crystallizes with x=0.025 in a tetragonal system with a filled tungsten bronze structure and the point group 4 mm, conforming to the space group P4bm and z=2. The Curie temperature was 537±2°C. The lattice constants were a=b=1.2446(1) and c=0.3991(1) nm at room temperature. The measured absorption of Nd:BNN was compared to the Judd-Ofelt (JO) theory. When applied, the JO theory of parity-forbidden electric-dipole transitions of rare-earth ions on non-centrosymmetric sites demonstrates good agreement. An absorption band around 808 nm to match laser diode pumping is about three times broader than that of Nd-doped yttrium aluminum garnet crystal. The three intensity parameters Ωt were fitted as 5.677×10-20, 2.231×10-20, and 4.106×10-20 cm2 at t=2, 4 and 6, respectively. The fluorescence branch ratio at 1060 nm is 52.38%. The largest integrated emission cross section ∑(4F3/2→4I11/2) is 10.36×10-18 cm. These results indicate the crystal's potential applications as a practical laser material.

The tetragonal phase of Na(0.5)Bi(0.5)TiO3--a new variant of the perovskite structure

The structure of the tetragonal phase of the A-site-substituted perovskite sodium bismuth titanate, Na(0.5)Bi(0.5)TiO3, has been determined by neutron powder diffraction at 698 K. The structure was refined in space group P4bm with a (= b) = 5.5191 (1), c = 3.9085 (1) A, V= 119.055 (5) A3, Z = 2 and Dx = 5.91 Mg m(-3). The structure exhibits an unusual combination of in-phase (a0a0c+) tilts and antiparallel cation displacements along the polar c axis, which results in a new variant of the perovskite structure.

Raman and Infrared Spectra of Nd-Doped Barium Sodium Niobate Crystals

Neodymium-modified barium sodium niobate (Ba2Na)1-xNdxNb5O15(BNN:Nd), with x = 0.025, crystallizes with a filled tungsten bronze (TB) type structure in the tetragonal system, conforming to the space group P4bm and Z = 2. The lattice constants are a = 1.2446(1) and c = 0.3991(1) nm at room temperature. Tc = 537 +- 2 °C. The experiments show that the Nd-ions effectively improve crystal structural quality. The lattice vibrational spectrum, studied by Raman and infrared reflectivity spectroscopy, shows that the crystal microscopic structure and lattice dynamics, thereby the microstrain and microstress, are very stable in a temperature range from 22 to approaching 500 °C.

Structural Investigations of Ammonium Vanadium Diphosphates by X-Ray Powder Diffraction

The crystal structures of three ammonium vanadium diphosphates have been investigated by means of X-ray powder diffraction including ab initio structure determination and Rietveld refinement. The compound NH4VP2O7crystallizes in the monoclinic space groupP21/cwitha=7.5149(2) Å,b=10.0384(3) Å,c=8.2422(2) Å,β=105.988(3)°, andZ=4. The two ammonium vanadyl diphosphates are (NH4)2VOP2O7, which has a tetragonal unit cell, space groupP4bm, a=8.3039(2) Å,c=5.7658(2) Å, andZ=2, andα-(NH4)2(VO)3(P2O7)2, which has an orthorhombic structure, space groupPnam,a=17.4973(4) Å,b=11.3655(3) Å,c=7.2769(2) Å, andZ=4. The first and the third structures consist of a framework of corner-sharing VOxpolyhedra and PO4tetrahedra and intersecting channels, whereas (NH4)2VOP2O7is arranged in layers built-up from VO5square pyramids and P2O7groups and the NH+4cations in between. The three NH4compounds are isostructural to the equivalent alkali vanadium diphosphates.

Ba6CoNb9O30 and Ba6FeNb9O30: Two New Tungsten-Bronze-Type Ferroelectrics. Centrosymmetry of Ba5.2K0.8U2.4Nb7.6O30 at 300 K

Ba6CoNb9O30 and Ba6FeNb9O30 in space group P4bm are shown to satisfy the structural criteria for ferroelectricity. Ba6CoNb9O30 undergoes a diffuse phase transition at 660 (11) K, as observed calorimetrically, in addition to a dielectric permittivity anomaly with an onset temperature of 685 (10) K. The demonstration of dielectric hysteresis at room temperature under the application of a varied DC field reaching a maximum of ± 300 kV m−1, corresponding to a spontaneous polarization of 1.2 (5) × 10−2C m−2, provides unambiguous verification that it is a new ferroelectric. Ba6FeNb9O30 also undergoes a diffuse phase transition at 605 (16) K, with a dielectric anomaly at 583 (5) K, and exhibits dielectric hysteresis at room temperature under a varied DC field ranging to ± 310 kV m−1 corresponding to a spontaneous polarization of 2.2(5) × 10−2Cm−2; it too is demonstrably a new ferroelectric. Although Ba5.2K0.8U2.4Nb7.6O30 has also been reported in space group P4bm, all atomic displacements from the corresponding centrosymmetric positions are less than their refined root-mean-square thermal or static amplitudes. Such an arrangement is likely to be thermodynamically unstable. Either its space group has been incorrectly assigned, and reinvestigation will show the space group is P4/mbm, or the structural refinement is incomplete.

New ferroelectric inorganic materials predicted in point group 4mm

The Inorganic Crystal Structure Database containing a total of 209 entries for 128 different materials reported in point group 4 mm, of which eight are for previously known ferroelectrics. Analysis of the remaining entries, assuming the structure determinations reported are correct, leads to the prediction of an additional 43 new ferroelectric materials. 15 were determined in space group P4mm, 11 in P4bm, one in P42 nm, one in P4cc, two in P42 mc, one in P42 bc, seven in I4mm, two in I4cm, one in I41 md and two in I41 cd. Numerous other structures are shown to have been assigned, most likely, to an incorrect space group. All but one of the materials reported in space group P4bm are predicted to be ferroelectric. New ferroelectrics predicted in point group 4 mm include K2[Pt(CN)4]Br0.3.3H2O, Nd0.33TaO3, YBaCuFeO5+δ, Ba2(TiO)(Si2O7), K2(NbO)2(SiO3)4, Ba6CoNb9O30, Sr2SbMnO6, Hg2AlF5.(H2O)2, Rb5Nb3OF18 and SrNi2(VO4)2.

Laser heated pedestal growth and spectroscopic properties of neodymium doped Ba 2NaNb 5O 15 single crystal fibers

The elaboration of Nd 3+-doped Ba 2NaNb 5O 15 single crystal fibers by the laser heated pedestal growth method is described. Slightly doped fibers are orthorhombic with Pba2 space group and heavier doped fibers are tetragonal with P4bm space group. Low-temperature spectroscopy reveals that Nd 3+ ions probably substitute both Ba 2+ and Na + ions. The stimulated emission cross-section near 1060 nm of the 4 F 3 2 → 4 I 11 2 channel and its branching ratio is determined with the help of a Judd-Ofelt analysis.

On the structure and properties of the layered lanthanide (Ln) barium copper oxyborates, LnBaCuO2BO3

An ordered structure model has been derived for the new series of layered lanthanide (Ln) cuprates, LnBaCuO 2 BO 3 , and some of its members have been synthesized. The cuprates were prepared under ambient pressure by reacting BaO 2 , Ln 2 O 3 , CuO and B 2 O 3 at 1190 K. The derived model, with the tetragonal space-group symmetry P4bm and a=5.5743 (4), c=7.5053 (6) A for the La compound, has been successfully tested against X-ray powder diffraction data. The model has reasonable estimated bond-valence values. Measured magnetic susceptibilities of powder specimens of LaBaCuO 2 BO 3 indicate a transition to a diamagnetic state at ca 29 K

High resolution electron microscopy study of Sr 2NdNbCu 2O 8

The structure of Sr 2NdNbCu 2O 8 (tetragonal, space group P4 bm, a = 0.549, c = 1.166 nm) and crystallographic defects occurring therein have been studied with high resolution electron microscopy and electron diffraction. The structure of Sr 2NdNbCu 2O 8 consists of a layer sequence NdCuO 2SrONbO 2SrOCuO 2 having a perovskite-like structure. This structure is very similar to that of YBa 2Cu 3O 7: Y is replaced by Nd and the CuO layer by a NbO 2 layer. Sr 2NdNbCu 2O 8 has an a p√2, a p√2, c superstructure, due to rotation of the NbO 6 octahedra about the c-axis, leading to a space group P4 bm. This superstructure is often short range. Intergrowths occur with an approximate composition of Sr(Nb, Cu)O 3, which adopts a modified simple perovskite structure. Also, examples are shown of [110] 90° rotation twins and edge dislocations.

The potassium niobyl cyclotetrasilicate K 2(NbO) 2Si 4O 12

The single-phase compound K 2(NbO) 2Si 4O 12 was prepared by solid-state reaction. It crystallizes in the tetragonal system, space group P4 bm with a = 8.7404(8)Å and c = 8.136(1)Å, Z = 2. The structure was determined from 484 reflections collected with a Nonius CAD4 diffractometer with Mo Kα radiation. The final R index and weighted R w index are 0.021 and 0.022, respectively. This framework structure is built up from chains of corner-shared NbO 6 octahedra running parallel to the four-fold axes and linked together by four-membered Si 4O 12 single rings. This structure is very similar to that of K 4(ScOH) 2 Si 4O 12.

K2(NbO)2Si4O12: A new material for non-linear optics

Abstract The potassium niobyl cyclotetrasilicate K2(NbO)2Si4O12 crystallizes in the non-centrosymmetrical polar space group P4bm with a = 8.7404(8) A, c = 8.136(1) A, Z = 2. The acentric character of the structure is mainly due to a sequence of short (niobyl) and long Nb-O bonds along the 4-fold axes. Optical studies on small crystals have shown that the transparency range extends from 0.3 to 4.5 μm and that they are positively uniaxial. Second harmonic generation tests, as a function of temperature, have been performed on a polycrystalline sample with use of the 1.06 μm line of a YAG-Nd laser. They show that phase matching conditions are still obtained at 1080[ddot]C (limit of the equipment). K2(NbO)2Si4O12 melts congruently at 1180[ddot]C and appears as rather inert and chemically stable.

Raman scattering studies of Na DOPED SrxBa1-xNb2O6ceramics

Abstract Strontium barium niobate is a ferroelectric solid solution with tetragonal tungsten bronze structure. SBN described by the formula (SrxBa1-x) Nb2O6 with x = 0.25 to 0.75 is a room temperature ferroelectric crystal of space group P4bm. We have investigated the Raman spectrum and ferroelectric properties in order to study the effect of doping Na+ and the variation of Sr/Ba ratio. The 6471-Å line of a coherent radiation Krypton ion laser having an output power of 200 mW was used as the Raman source.

Synthesis and Unit Cell Parameter Refinement of 25 Tungsten Bronze Ferroelectrics

AbstractSynthesis and unit cell parameter refinement of 25 ferroelectric compounds with the tungsten bronze structure are reported. A general chemical formula for these compounds is (A1, A2, C) B10 O30, where specifically A1 and A2 = K, Na, Ba, Sr, Pb, La, Eu, Sm, Y, Bi; C = Li; and B = Nb, Ta, Ti, W. All compounds were prepared by solid state sintering at temperatures ranging from 1100°C to 1380°C. Refined cell parameters (tetragonal with space group P4bm[100]), I/Icor values, calculated densities and Z values are included for the 25 compounds.

Synthese et structure d'un bronze oxygene quadratique contenant de l'uranium a la valence six

(Nb 7.6U 2.4)(Ba 5.2K 0.8)O 30 is a quadratic bronze with space group P4 bm and lattice parameters a = 12.608(5) and c = 4.012(2) A ̊ , Z = 1. The structure of a single crystal was refined to R= 0.056 ( R w = 0.072) for 967 independent reflections with I ⩾ 3 σI. The U 6+ ion is located inside octahedral sites with Nb 5+ and Nb 4+.

X-ray structure refinement and pyroelectric investigation of fresnoite, Ba2TiSi2O8

Crystal structure parameters have been refined from single crystal x-ray intensity data collected on Ba2TiSi2O8 at 24 and 300°C. Anisotropic refinements in space group P4bm yielded residuals of 0.035 and 0.042 at 24 and 300°C, respectively. Pyroelectric measurements give a room temperature pyroelcctric coefficient of + 10 μC m -2 K -1. A structural mechanism for the pyroelectric effect is discussed in terms of large oxygen displacements. A sign reversal in the pyroeleetric coefficient occurs at 160°C, perhaps caused by the cancellation of primary and secondary effects.

Ferroelectric Tungsten Bronze-Type Crystal Structures. I. Barium Strontium Niobate Ba0.27Sr0.75Nb2O5.78

Ferroelectric Ba0.27Sr0.75Nb2O5.78, with Tc = 348° ± 15°K, is a tungsten bronze-type structure crystallizing in the tetragonal system, with lattice constants a = 12.43024 ± 0.00002 and c = 3.91341 ± 0.00001 Å at 298°K, space group P4bm, and five formulas in the unit cell. The integrated intensities of 6781 structure factors were measured with PEXRAD, 875 symmetry-independent structure factors being significantly above background. The metal-atom positions were determined from the three-dimensional Patterson function and the oxygen atoms from subsequent Fourier series. The final agreement factor between measured and calculated structure factors is 0.0508. The structure consists of close-packed slightly puckered layers of oxygen atoms separated by nearly c / 2. The Nb atoms are slightly displaced from one layer, the Ba and Sr atoms from the other and in the same sense. The oxygen atoms in the Ba and Sr layer are disordered. Neither of the two independent sites occupied by the Ba and Sr atoms is fully filled. The 2a site is preferred by Sr, which is in distorted cubo-octahedral coordination; the Sr–O distances range from 2.547 ± 0.008 Å to 2.919 ± 0.008 Å. The 4c site, occupied by Ba and Sr, has distorted tricapped trigonal prismatic symmetry; the Ba/Sr–O distances range from 2.604 ± 0.009 Å to 3.035 ± 0.010 Å. The two independent Nb atoms fully occupy the 2b and 8d sites and are each in relatively regular octahedral coordination. The 2b site has Nb–O distances ranging from 1.92 ± 0.03 Å to 2.03 ± 0.03 Å; the 8d-site Nb–O distances range from 1.88 ± 0.03 Å to 2.11 ± 0.03 Å. A ferroelectric mechanism is proposed in which the metal atoms in the 2a, 2b, and 4c sites move, at the Curie temperature, into the oxygen layers. These metal atoms then acquire 4̄,222, or 2 point symmetry, respectively, as the space group transforms to P4̄b2. The remaining Nb atoms, in the 8d position, also rearrange so as to have half above and half below the oxygen layer. The absolute displacements of the metal atoms from their nearest mean planes of oxygen atoms, in space group P4̄bm, are directly related to the ferroelectric polarization sense.