Pentagonal Channels Research Articles

Ba6-3XNd8+2XTi18O54 Microwave dielectric resonators

The Ba6-3xNd8+2xTi18O54 solid solution (BNT) is well established as a useful microwave resonator material due to its high dielectric constant (εr), high quality factor (Q = 1/tanδ), and tunable temperature coefficient of resonant frequency (τf). Its crystal structure is similar to that of the tungsten bronzes, containing large pentagonal channels parallel to [001], which are only partly occupied by the divalent species. These pentagonal sites are separated by Nd+3 and TiO6 octahedra arranged in an essentially perovskite-like manner. Ceramics of this phase have been processed in this study by a mixed-oxide route, and the variation in properties caused by doping with either Ca, Sr, or Pb was investigated. Most samples showed a multiphase character, with Nd4Ti9O24 (ss) being the main secondary phase. For × < 0.75, single-phase BNT was always obtained, while phase purity was only attained at 1400°C when × = 0.75. Doping with PbO was found to usefully reduce τf whilst leaving both τr and Q at reasonable levels. Pb-doping lead to a moderately lower Q and, contrary to earlier reports, slightly lowered τr

Structure and Properties of Nd5Ni6In11

The title compound was prepared from the elements by a reaction in an arc melting furnace and subsequent annealing at 970 K. Nd5Ni6In11crystallizes with the orthorhombic Pr5Ni6In11-type structure:Cmmm,a=1455.9(3) pm,b=1453.3(3) pm,c= 438.3(1) pm,V=0.9274(2) nm3,wR2=0.0551, 1086F2values, and 42 variables. The structure of Nd5Ni6In11is built up from a complex three-dimensionally infinite [Ni6In11] polyanionic network in which the neodymium atoms occupy pentagonal and hexagonal channels. Magnetic susceptibility measurements indicate Curie–Weiss behavior above 150 K with an experimental magnetic moment of 3.60(5)μB/Nd and a paramagnetic Curie temperature of −18(1) K. Antiferromagnetic ordering is detected atTN=11.4(1) K. Field- and temperature-dependent magnetization data indicate a complex magnetic phase diagram with spin reorientations into three antiferro- and one ferrimagnetic phase. The magnetic data are supported by specific heat measurements. Nd5Ni6In11is a metallic conductor with a specific resistivity of 120 μΩ cm at room temperature.

Structural investigation of Ba6−3xLn8+2xTi18O54 (x = 0.27, Ln = Sm) by single crystal x-ray diffraction in space group Pnma(No. 62)

Single crystals of barium samarium titanium oxide Ba6−3xSm8+2xTi18O54 (x = 0.27) have been synthesized and studied using x-ray diffraction. Superstructure reflections, which cause a doubling of the cell along the short axis, were taken into account and the refinement was conducted in the orthorhombic space group Pnma. Unit cell parameters from single crystal x-ray diffraction were a = 22.289(1), b = 7.642(1), and c = 12.133(1) Å. Refinement on F resulted in R1 = 5.37% for 1410 Fo &gt; 4σ with the thermal parameters of the Sm and Ba atoms refined anisotropically and the thermal parameters of the Ti and O atoms refined isotropically. The structure is made up of a network of corner sharing TiO6−2 octahedra creating rhombic (perovskite-like) and pentagonal channels. The two pentagonal channels are fully occupied by Ba atoms. The refinement suggests that one rhombic channel is fully occupied by Sm atoms (Sm3/Sm4), one rhombic channel is partially occupied by Sm atoms (100% Sm1/86.25% Sm5), and one rhombic channel is shared by BaySm atoms (59.25% Ba3/40.75% Sm2), resulting in a formula of Ba10.38Sm17.08Ti36O108 with Z = 1. The above site occupancies differ from the site occupancies previously reported in the literature for refinements conducted with the short axis approximately equal to 3.8 Å.

New crystal structure in the BaO-Ce 2O 3-TiO 2 system

A new structure located in the TiO 2-rich corner of the BaO-Ce 2O 3-TiO 2 system crystallizes with an orthorhombic unit cell, space group Pnma, and with lattice parameters a = 27.88(1), b = 7.745(2), and c = 17.350(4)Å. Refinement on F resulted in R1 = 6.46% for 3868 F 0 > 4σ. The structure is made up of a network of corner-sharing TiO 6 octahedra creating rhombic (perovskite-like) and pentagonal channels. In this new structure, there are eight unique rhombic positions (occupied chiefly by Ce atoms), four unique pentagonal positions (occupied chiefly by Ba atoms), and eight unique Ti positions. Full occupation of the sites would correspond to the formula Ba 16Ce 32Ti 64O 192; however, refinement of the site occupancies of the Ce and Ba atoms suggests some substitution of the cations and vacancies. Electron probe wavelength dispersive spectroscopic microanalysis averaged from twenty measurements on three crystals' results in 4.68 ± 0.11 at% Ba, 9.63 ± 0.30 at% Ce, and 18.57 ± 0.35 at% Ti.

Symmetry relationships in the homeotypical aluminium-rich structures with pentagonal channels

Symmetry relationships in the homeotypical aluminium-rich structures with pentagonal channels

Powder diffraction data for the intermetallic compounds Co2Al5, monoclinic m-Co4Al13 and orthorhombic o-Co4Al13

A significant feature for the atomic structure of the intermetallic compounds Co2Al5 (hP28), monoclinic m-Co4Al13(mC(102-7.2) [notation according to Parthé etal. (1993)] and orthorhombic o-Co4Al13 (oP102) is the occurring of the filled pentagonal channels. Least-square refined lattice parameters as well as powder diffraction data are reported for these homeotypic phases.

Refinement of the Fe4Al13 structure and its relationship to the quasihomological homeotypical structures

Abstract The crystal structure of Fe4Al13 was refined using single crystal diffractometer data: Pearson symbol mC102, space group C2/m; a = 15.492(2) Å, b = 8.078(2) Å, c = 12.471(1) Å, β = 107.69(1)°; RF = 0.053, RF (w) = 0.044 for 1127 reflections and 137 refined parameters. The coordination numbers of atoms are 9, 10. 11 for iron and 10, 12, 13, 14 for aluminium. The shortest interatomic distances are: Fe–Fe – 2.902 Å, Fe–Al – 2.374 Å, Al–Al – 2.533 Å. A preferred occupation of pentagonal prismatic coordinated positions by aluminium was found. The structural relationship between the Fe4Al13 structure and chemically homologous and homeotypical structures of aluminium and gallium containing systems with the 3d transition metals is discussed. The greatest similarity was found concerning the coordination polyhedra, especially that of transition metal atoms. The main common feature of these homeotypical structures is the presence of pentagonal “channels”, which is strongly dependent on the chemical composition. With increasing atomic number of the 3d transition metal, the stability range of these structures shifts to the transition metal-rich concentration. It is concluded that there is a connection between the occurrence of aluminium and gallium-containing decagonal and icosahedral phases and the existence of the infinite one-dimensional pentagonal channels in the intermetallic compounds showing a similar chemical composition.

Guest-dependent [Cd(CN)2]nhost structures of cadmium cyanide–alcohol clathrates: two new [Cd(CN)2]nframeworks formed with PrnOH and PriOH guests

Crystallization of Cd(CN)2 from the geometric isomers of propanol, PrnOH and PriOH, yielded new cadmium cyanide clathrates whose [Cd(CN)2]n frameworks are topologically and geometrically different not only from those of all the previously known cadmium cyanide clathrates but also from each other: the former has both hexagonal and pentagonal channels whereas the latter has only pentagonal channels.

Five-dimensional structure analysis of decagonal Al65Cu20Co15

A single-crystal X-ray structure analysis of decagonal Al65Cu20Co15 has been performed using the n-dimensional approach. Five-dimensional space group P10(5)/mmc, a(i)* = 0.2656 (2), a5* = 0.24107 (3)-angstrom-1, d(i) = 3.368 (1), d5 = 4.1481 (3)-angstrom, alpha-ij = 60, alpha-i5 = 90-degrees (i, j = 1,..,4), volume of the five-dimensional unit cell-OMEGA = 298.2-angstrom-5, M(r) = 39.08, F(00000) = 15.0, D(x) = 4.4 Mg m-3, mu = 12.0 mm-1, Mo K-alpha, lambda = 0.71069-angstrom, wR = 0.098 for 259 independent reflections with I > 2-sigma-(I) and 11 variables. The structure was solved by Patterson analysis and subsequent least-squares refinement, both in the five-dimensional description. There are three atoms in the asymmetric unit. Five-dimensional Fourier and difference Fourier syntheses show the pentagonal shape and chemical structure of the atoms parallel to the internal space. The three-dimensional decagonal structure is built up of periodically stacked sandwiches which consist of two planar quasiperiodic layers related by the tenfold screw axis. Characteristic large motifs of the crystalline Al13Fe4 structure type are found in a new arrangement in the quasiperiodic layers. The appearance of pentagonal channels which are partly empty or occupied statistically is noteworthy.

Five-dimensional structure refinement of decagonal Al65Cu20Co15

Abstract A single-crystal X-ray structure analysis of decagonal Al65 Cu20Co15 has been performed using the n-dimensional approach. The structure has been solved by Patterson analysis and subsequent least-squares refinement, both in the five-dimensional description. A final value of wR=0.097 for 259 independent reflections was achieved with I≥2σ(1) and 12 variables. There are three atoms in the asymmetric unit. The calculated density of the refined, structure model is ρc = 4.8 M gm −3. The three-dimensional decagonal structure consists of two planar quasiperiodic layers, related by the tenfold screw axis, stacked periodically one upon the other. Characteristic large elements of the crystalline Al13 Fe4 structure type can be found in new combinations in the quasiperiodic layers. It is remarkable that pentagonal channels appear which are empty or occupied statistically.

Silicotantalates et siliconiobates non-stoechiometriques : Les composes K 6−2xBa xTa 6Si 4O 26 (0 ≤ x ≤ 3) et K 8M 14Si 4O 47 (M = Ta, Nb)

New oxides: K 6−2xBa xTa 6Si 4O 26 (0 ≤ x ≤ 3) and K 8M 14Si 4O 47 (M = Ta, Nb) are non stoichiometric-compounds. Distorted pentagonal channels have a variable occupancy for barium and potassium of K 6−2xBa xTa 6Si 4O 26 phases. The structure of K 8M 14Si 4O 47 (M = Ta, Nb) shows an intergrowth of (M 6Si 4O 26) and M 8O 21 layers. It is related to that of K 6M 6Si 4O 26 (M = Ta, Nb) as evidenced by the “transformation” K 6Ta 6Si 4O 26 → K 8Ta 14Si 4O 47 observed at 1350°C.

De nouveaux oxydes a reseau forme d'octaedres NbO 6 (TaO 6) et de groupes Si 2O 7: Les phases A 3Ta 6Si 4O 26 (A = Ba, Sr) et K 6M 6Si 4O 26 (M = Nb, Ta)

New oxides, A 3 Ta 6 Si 4 O 26 (A = Ba, Sr) and K 6 M 6 Si 4 O 26 (M = Nb, Ta) have been synthesized. Their structure shows an interesting feature: Si 2 O 7 groups are linked with Nb O 6 (Ta O 6) octahedra so as to create distorted pentagonal channels. These channels have an half occupancy for A 3 Ta 6 Si 4 O 26; they are full for K 6 M 6 Si 4 O 26. The stuctures are basically the same for the two types of compounds. Supplementary potassium of K 6 M 6 Si 4 O 26 have two additionnal neighbor oxygen, of the Si O Si bridge.