Space Group P4bm Research Articles

Room temperature multiferroic Ba4Bi2Fe2Nb8O30: Structural, dielectric, and magnetic properties

A room temperature multiferroic compound Ba4Bi2Fe2Nb8O30 was synthesized using a solid state reaction technique. Rietveld analysis of x-ray diffraction data shows that Ba4Bi2Fe2Nb8O30 has a tetragonal (space group P4bm) tungsten bronze structure. In this structure, the Fe3+ and Nb5+ statistically occupy the octahedral center while the Ba2+ ions and the Bi3+ ions occupy the pentagonal channels and the square channels, respectively. Diffuse dielectric peaks with strong frequency dispersion in the temperature range from 150 to 300 K can be attributed to the random distribution of Fe3+ and Nb5+ at B sites. Magnetic hysteresis loop at room temperature is also obtained, which suggests that Ba4Bi2Fe2Nb8O30 is a room temperature multiferroic compound. The coupling between the relaxor behavior and magnetic ordering is verified by observing an increase of magnetization near the maximum dielectric constant temperature.

X-ray, scanning electron microscopy and electrical properties of synthetic fresnoite (Ba2TiSi2O8) ceramics

Polycrystalline ceramic samples of fresnoite (Ba2TiSi2O8 or BTS) have been prepared by a standard solid-state reaction method using high-purity oxides and carbonates. For one set of compounds, in stoichiometric ratio BaCO3, TiO2 and SiO2 were melted at 1300°C and then sintered into pellet form, whereas the other set of compounds have been prepared without melting and sintered into pellet form at 1250°C. The formation of the single-phase compound and its structural parameters were investigated by X-ray diffraction followed by Rietveld refinement and scanning electron microscopic (SEM) techniques. A better agreement between the observed and calculated X-ray diffraction patterns was obtained by performing the Rietveld refinement with a structural model using the non-centrosymmetric space group P4bm. A better agreement between observed and calculated d-values shows that the lattice parameters calculated using the Rietveld refinement analysis are better than that of the earlier report so far. The activation energies of both compounds were calculated by measuring its dc electrical conductivities. The results are discussed in detail.

SINGLE GRAINS HOSTING TWO SPACE GROUPS — A TRANSMISSION ELECTRON MICROSCOPY STUDY OF A LEAD-FREE FERROELECTRIC

In this letter, conclusive experimental evidence is presented for the coexistence of two phases within individual grains in the system ( Bi 0.5 Na 0.5 TiO 3)0.91-( BaTiO 3)0.06-( K 0.5 Na 0.5 NbO 3)0.03. Electron diffraction data unequivocally reveal the presence of a rhombohedral and a tetragonal phase with space group R3c and P4bm, respectively, both on the nanoscale level. Superlattice reflections, observed in the corresponding selected area electron diffraction, were used to identify the rhombohedral and tetragonal phase present at different regions within one grain. It is thought that the rhombohedral phase acts as a nucleation site for the phase transformation monitored under applied electrical field. The coexistence of these two phases in conjunction with the triggered tetragonal to rhombohedral phase transformation rationalizes the high bipolar and unipolar strain recorded for this lead-free ferroelectric material.

Crystal Structure and Ferroelectric Behaviors of Ba5SmTi3Ta7O30 and Ba4Sm2Ti4Ta6O30 Tungsten Bronze Ceramics

The crystal structure and dielectric characteristics were evaluated for Ba4Sm2Ti4Ta6O30 and Ba5SmTi3Ta7O30 ceramics. The X‐ray diffraction patterns confirmed the tetragonal tungsten bronze structure with space group P4bm. A normal ferroelectric phase transition was observed at 332 K for the former, while the typical relaxor ferroelectric behavior with a spin glass nature was determined for the latter, where a broad permittivity peak with a strong frequency dispersion was observed around 150 K, which followed the Vogel–Fulcher relationship well. These different dielectric behaviors originated from the different crystal structures governed by the occupancy configuration of cations at A1 and A2 sites.

Structural and Raman properties of the tetragonal tungsten bronze ferroelectric [formula omitted

Using X-ray diffraction and Raman spectroscopy we studied the Pb 2 ( 1 − x ) K 1 + x Gd x Nb 5 O 15 (PKGN) ceramics over the composition range 0 ≤ x ≤ 1 . The results reveal two orthorhombic phases, with space group Cm2m and Pba2 for 0 ≤ x ≤ 0.2 and 0.3 ≤ x ≤ 0.6 , respectively, and a tetragonal phase, with space group P4bm, for 0.7 ≤ x ≤ 1 . These suggest a rotation of polarization of 90 ∘, when x increases, from the ab-plane ( 0 ≤ x ≤ 0.2 ) to the c -axis ( 0.3 ≤ x ≤ 1 ). The evolution of Raman spectra as a function of x confirms the structural changes in PKGN compounds and shows a strong dependence of the concentration of Gd mainly in the low-frequency part ( < 170 cm − 1 ), which reflects the vibrations of cations in the tetragonal tungsten bronze (TTB) structure. The behavior of the O–Nb–O bending vibrations seems to be a signature of the phase modifications as a function of x in PKGN. The Raman results provide strong evidence of the reduction of the disorder in the TTB structure when going from Pb to Gd. This disorder is connected to the orthorhombic distortion, which is reduced when going from orthorhombic to the tetragonal phase.

Crystal growth and dielectric characterization of crystals derived from the solid-solution Ba (1− x) Na xTi (1− x) Nb xO 3 (BTNN)

Single crystals of (1 − x)BaTiO 3 + xNaNbO 3 (BTNN) for x = 0.84 were obtained by high temperature solution growth using Na 2B 4O 7 as solvent. The room temperature crystal structure of BTNN 16/84-phase was determined from X-ray single crystal diffraction data, in the tetragonal system with space group P4 bm. The refinement from 246 independent reflections led to the following parameters: a = b = 5.5845(3) Å, c = 3.9453(2) Å, V = 123.041(11) Å 3, Z = 2, with final cR wp = 0.150 and R B = 0.041. The structure of BTNN 16/84-phase can be described as a three-dimensional framework built up from (Nb–Ti)O 6 octahedra with Na and Ba in the dodecahedral site of perovskite-like type. Some mm 3-sized crystals have been selected and various dielectric measurements (ferroelectric, pyroelectric, and piezoelectric) have been performed. Transition from paraelectric to ferroelectric state at around 460 K has been observed to be in good agreement with ceramics of closer composition. Dielectric, piezoelectric and pyroelectric measurements on crystal confirm the ferroelectric behaviour of BTNN 16/84.

Dielectric relaxations, ultrasonic attenuation, and their structure dependence in Sr4(LaxNd1-x)2Ti4Nb6O30 tungsten bronze ceramics

The dielectric anomalies and their structure dependence were evaluated and discussed in Sr4(LaxNd1-x)2Ti4Nb6O30 ceramics, together with the analysis of ultrasonic velocity shift and attenuation spectra in the low-temperature range. The room-temperature structure was confirmed as the tetragonal in space group P4bm for all compositions. One diffuse ferroelectric peak and three relaxor ferroelectric peaks corresponding to the commensurate/incommensurate modulation of oxygen octahedra, polar clusters of A-site ion ordering, and B-site ion ordering, respectively, were observed in the composition with x = 0.25. With decreasing the radius difference between A1- and A2-ions (increasing x), the dielectric relaxations, especially the one originating from the polar clusters of A-site ion ordering, tended to increase significantly and overlap the diffuse ferroelectric peak, which was completely overlapped for x ⩾ 0.75. This process just reflected the increased disordering degree of both A- and B-site ions, and the analysis of ultrasonic attenuation strongly supported the above conclusions on dielectric relaxations and their structural origins. The ultrasonic attenuation peak at approximately 100 K corresponded to the freezing process of the dielectric relaxations, and the fluctuation with composition of the ultrasonic attenuation peaks between 150 and 260 K suggested the possible structure variation.

Precise Lattice Parameters of the Sr&lt;sub&gt;0.61&lt;/sub&gt;Ba&lt;sub&gt;0.39&lt;/sub&gt;Nb&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;6&lt;/sub&gt; Single Crystal

Single crystals of composition Sr0.61Ba0.39Nb2O6 (space group P4bm) were prepared using the Czochralski method. The Bond's method of the precise lattice parameter measurements was used to study structure of SBN 61 at room temperature. In this way lattice parameters a = b = 12.45676 ± 6*10-5Å and c = 3.93541 ± 5*10-5Å were determined.

Structural modulation in K 2V 3O 8

To elucidate the phase transition at 115 K in the fresnoite-type compound K 2V 3O 8, we undertook temperature-dependent neutron powder diffraction and single-crystal X-ray diffraction (XRD). For structure refinements in the nominal space group P4 bm, the most dramatic change is evidenced by the a cell edge, which initially expands on cooling, then abruptly begins to contract at 115 K. The c cell edge contracts monotonically. The atomic displacement parameters (ADPs) also deviate from their expected temperature dependence at 115 K, where the oxygen atoms in the vanadium oxide plane exhibit an increase in apparent positional disorder. Similar changes in lattice parameters and ADPs are observed from the single-crystal XRD refinements. Below 115 K, weak superlattice reflections are clearly evident in XRD patterns recorded by a CCD detector, and these extra reflections can be indexed with the wave vector ±1/3〈110〉*+1/2 c*. Possible space groups for the modulated structure are P4 2 bc and P4 nc.

Crystal structures of KBa 2Nb 5O 15 and LaK 2Nb 5O 15 with the TTB-type structure

Single crystals of two niobates, KBa 2Nb 5O 15 and LaK 2Nb 5O 15, were synthesized by high-temperature reaction and the crystal structures were determined by single crystal X-ray diffraction data. Although the space groups for these compounds were different (the non-centrosymmetrical space group P4 bm (#100) for KBa 2Nb 5O 15 and the centrosymmetrical one P4/ mbm (#127) for LaK 2Nb 5O 15), both compounds had the same tetragonal tungsten bronze-type (hereafter TTB-type) structure. The lattice parameters and R-factors of KBa 2Nb 5O 15 (LaK 2Nb 5O 15) were a = 12.533(2) (12.563(2)) and c = 4.0074(9) (3.9179(9)) Å, and R 1 = 0.040 (0.047) and w R 2 = 0.131 (0.120), respectively. From the crystal structural analysis, it was clarified that distribution of two large cations was different from each other in the way that K and Ba atoms in KBa 2Nb 5O 15 were distributed statistically at two crystallographic sites and K and La atoms in LaK 2Nb 5O 15 were ordered.

Crystal Chemistry of a New Solid Solution in the BaO—Bi2O3—Nb2O5 System: Ba5x/2Bi(1‐x)5/3Nb5O15.

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Crystal Chemistry of a New Solid Solution in the BaO−Bi2O3−Nb2O5 System: Ba5x/2Bi(1-x)5/3Nb5O15

A solid solution of the type Ba5x/2Bi(1-x)5/3Nb5O15 has been identified in the BaO-Bi2O3-Nb2O5 system for the first time. The limits of the solid solution are within the range 0.52 <= x <= 0.80. The compositions x = 0.52, 0.60, 0.72, 0.77, 0.78, and 0.80 were synthesized by the solid-state technique from the starting materials in stoichiometric quantities. The powder X-ray patterns of all the phases in the domain indicate a structural similarity to tetragonal tungsten bronzes (TTBs). The compositions below x = 0.52 and those above x = 0.80 exhibit barium niobate and bismuth niobate impurities, respectively. Single crystals of the composition x = 0.77 were obtained by the melt cooling technique. The crystal structure of Ba3.85/2Bi1.15/3Nb5O15 (x = 0.77) was solved in the tetragonal space group P4bm (No. 100) with a = 12.4938 (14) angstrom, c = 3.9519 (2) A, V = 616.87 (10) angstrom(3), and Z = 2 and was refined to an R index of 0.034. Dielectric measurements on all the phases indicate a typical relaxor behavior with a broad phase transition at T-m approximate to 300 K.

Preparation, characterization and dielectric properties of Ba6Mg0.67Nb9.33O30 ceramic

A new niobate Ba6Mg0.67Nb9.33O30 was synthesized in the BaO−MgO−Nb2O5 system by solid state reaction at 1350°C for 48 h. The structure and dielectric properties of Ba6Mg0.67Nb9.33O30 were determined by X-ray powder diffraction, scanning electron microscopy and dielectric measurements. The results show that Ba6Mg0.67Nb9.33O30 belongs to ferroelectric phase of filled tetragonal tungsten bronze structure at room temperature with unit cell parameters: a=1.26052(4) nm, c=0.40045(2) nm, space group P4bm, calculated density 5.707 g·cm−3. Ba6Mg0.67Nb9.33O30 belongs to relaxor ferroelectrics undergoing diffusive phase transition, and the transition temperature (Tc) is 30°C observed at 10 kHz. At room temperature, Ba6Mg0.67Nb9.33O30 ceramic has a high dielectric constant 850 at 1 MHz and a low dielectric loss of 0.0052.

Síntese e caracterização estrutural do niobato de potássio e estrôncio com estrutura tipo Tetragonal Tungstênio Bronze TTB

A fase nanoestruturada KSr2Nb5O15, foi sintetizada pelo método de mistura mecânica de óxidos por moagem de alta eficiência e por via química, utilizando o método dos precursores poliméricos, em forno tipo tubo sob atmosfera de O2 durante 10 h. A análise por difração de raios X dos pós calcinados a 1150 ºC foi realizada pelo método de Rietveld utilizando-se o software FullProf. A análise dos parâmetros estruturais obtidos via método de Rietveld mostrou um sistema tetragonal com estrutura tipo tetragonal tungstênio bronze. A partir das reflexões observadas, há apenas evidência da regra [(0kl) k = 2n], a qual é compatível com o grupo espacial centrossimétrico P4bm, o qual foi utilizado para o refinamento. O volume da célula unitária obtido na análise dos pós de KSr2Nb5O15 obtidos por síntese química e por moagem de alta eficiência foi 611,90 ų e 611,95 ų, respectivamente.

Synthesis and characteristic of a new niobate: Ba5NaCr0.5Nb9.5O30

A new niobate crystal was synthesized for the first time in BaO−NaO−Cr2O3−Nb2O3, ystem by the solid state reaction and the flux method. The new compound was studied by X-ray diffraction, electron probe, X-ray microanalysis, chemical analysis and SEM. The result of X-ray powder diffraction show that Ba5NaCr0.5 Nb9.5O30 belongs to the tetragonal tungsten bronze structure, with space group P4 bm(100) and lattice parameters a=b=12.4450(2)A, c=3.9931(2)A and Z=1. The X-ray powder diffraction lines of the compound were well indexed.

Synthesis, crystal structures and characterization of Ba5LiTiNb9O30

AbstractA new niobate compound with the chemical composition of Ba5LiTiNb9O30 was synthesized by doping Li+ into the system BaO‐TiO2‐Nb2O5 in conventional solid state reaction method. The crystalline structure was determined by X‐ray dilation analysis (XRD). The results showed that crystal structure of Ba5LiTiNb9O30 belongs to tetragonal tungsten bronze structure with space group P4bm and its unit cell parameters: a=b= 1.2512(2) nm, c= 0.4008(5) nm. The microstructure of reaction products was observed by scanning electron microscopy (SEM).

The Synthesis and Structure of New Perovskite-type Niobate Processed in Millimeter Wave Field

ABSTRACTIn present paper a new niobate materials Ba5LixTixNb10-xO30 were synthesized by doping Li+ in the system BaO- TiO2- Nb2O5 in millimeter wave field. X-ray diffraction (XRD) quantitative and scanning transition spectroscopy (SEM) analysis were employed to study crystal structure and microstructure of reaction products. It was found that pure products could be obtained at temperature 900°, 8 min which is lower comparing with that by conventional method. The XRD data shown the crystal belongs to tetragonal tungsten bronze structure with space group P4bm. The grain size synthesized in millimeter wave field had smaller size, narrower distribution, better sinter-ability, and without hard agglomeration comparing whit that obtaining from conventional synthesis. At lower temperature Ba5LiTiNb9O30 is a tetragonal ferroelectric phase.

Synthesis, X-ray crystal structure and dielectric measurements of a tetragonal tungsten bronze: Pb0.75K1.80Li1.70Nb5O15

Abstract Typical crystals Pb0.75K1.80Li1.70Nb5O15, belonging to tetragonal tungsten bronze materials, were grown by the slow cooling technique. The crystals were characterized at room temperature by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HREM). The structure, solved combining information obtained by HRTEM images and Direct Methods application to single crystal XRD data, was refined by full-matrix least-squares technique in the P4bm space group to a final R = 0.031. The lattice constants are a = 12.540(2), c = 4.037(1) Å, V = 634.8(2) Å3, at 293 K. There are two formulas per unit cell, for a calculated density of 4.928 Mg/m3. Dielectric measurements, carried out along the [001] direction in the frequency 103 Hz, showed the existence of the paraelectric-ferroelectric transition at 628 K and of a further maximum of ε′r (with a very weak intensity) at T 1 = 150 K.

A structural study of the (Na1−xKx)0.5Bi0.5TiO3 perovskite series as a function of substitution (x) and temperature

Rietveld neutron powder profile analysis of the (Na1−xKx)0.5Bi0.5TiO3 (NKBT) series (x=0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) is reported over the temperature range 293–993 K. A detailed characterization of the structures and phase transitions occurring across this series as a function of temperature has been made. Room-temperature refinements have revealed a rhombohedral phase, space group R3c for x=0, 0.2, and 0.4, which exhibits an antiphase, a−a−a− oxygen tilt system with parallel cation displacements along [111]p. An intermediate zero-tilt rhombohedral phase, space group R3m possessing cation displacements along [111]p, has been established for x=0.5 and 0.6. At the potassium-rich end of the series at x=0.8 and 1.0, a tetragonal phase, space group P4mm is observed possessing cation displacements along [001]. At the sodium-rich end of the series for x=0.2, the unusual tetragonal structure with space group P4bm is seen for Na0.5Bi0.5TiO3 which possesses a combination of in-phase a0a0c+ tilts and antiparallel cation displacements along the polar axis. Temperature-induced phase transitions are reported and structural modifications are discussed.

Investigation of the structure and phase transitions in the novel A-site substituted distorted perovskite compound Na(0.5)Bi(0.5)TiO(3).

Rietveld neutron powder profile analysis of the compound Na(0.5)Bi(0.5)TiO(3) (NBT) is reported over the temperature range 5-873 K. The sequence of phase transitions from the high-temperature prototypic cubic structure (above 813 K), to one of tetragonal (673-773 K) and then rhombohedral structures (5-528 K) has been established. Coexisting tetragonal/cubic (773-813 K) and rhombohedral/tetragonal (with an upper temperature limit of 145 K between 528 and 673 K) phases have also been observed. Refinements have revealed that the rhombohedral phase, space group R3c, with a(H) = 5.4887 (2), c(H) = 13.5048 (8) A, V = 352.33 (3) A(3), Z = 6 and D(x) = 5.99 Mg m(-3), exhibits an antiphase, a(-)a(-)a(-) oxygen tilt system, omega = 8.24 (4) degrees, with parallel cation displacements at room temperature. The tetragonal phase, space group P4bm, with a(T) = 5.5179 (2), c(T) = 3.9073 (2) A, V = 118.96 (1) A(3), Z = 2 and D(x) = 5.91 Mg m(-3), possesses an unusual combination of in-phase, a(0)a(0)c(+) oxygen octahedra tilts, omega = 3.06 (2) degrees, and antiparallel cation displacements along the polar axis. General trends of cation displacements and the various deviations of the octahedral network from the prototypic cubic perovskite structure have been established and their systematic behaviour with temperature is reported. An investigation of phase transition behaviour using second harmonic generation (SHG) to establish the centrosymmetric or non-centrosymmetric nature of the various phases is also reported.