Space Group Cmmm Research Articles

Synthesis, structure and physical properties of U3Co2Si7 and Np3Co2Si7 compounds

A new ternary compound Np3Co2Si7 was found to crystallize in the orthorhombic La3Co2Sn7-type structure, space group Cmmm (N°65) with room temperature lattice parameters determined from powder diffraction as follows: a = 4.000 (1) Å, b = 24.491 (8) Å, c = 4.020 (2) Å, (V = 393.8 Å3). Np3Co2Si7 orders ferromagnetically with a Curie temperature of about 32 K. U3Co2Si7 has also been prepared for comparison. It is isostructural to Np3Co2Si7with lattice parameters a = 4.010 (6) Å, b = 24.389 (2) Å, c = 4.022 (5) Å. An annealing of U3Co2Si7 at 1223 K for 10 days is required to obtain it as a single phase. U3Co2Si7 was reported in the literature as a ferromagnet. However, our measurements show a paramagnetic behavior and the presence of a weak ferromagnetic impurity.

Phase Transition in Weberite-Type Gd3NbO7

Gd3NbO7 has an orthorhombic weberite-type crystal structure and undergoes a phase transition at about 340 K. The phase transition was confirmed by second harmonic generation, heat capacity measurement, and high-resolution X-ray diffraction. Above 340 K, Gd3NbO7 has a centrosymmetric weberite-type structure with space group Cmcm (No. 63) and transforms at about 340 K into a non-centrosymmetric structure with space group Cm2m (No. 38). The phase transition is mainly due to off-center shifts of Nb5+ ions within NbO6 octahedra, as well as off-center shifts of one-third of the Gd3+ ions within GdO8 polyhedra.

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.

High pressure synthesis and properties of ternary titanium (III) fluorides in the system KF–TiF 3 containing regular pentagonal bipyramids [TiF 7

Titanium trifluoride TiF 3 has the distorted ReO 3 structure composed of corner sharing TiF 6 octahedra linked with Ti–F–Ti bridges. Potassium fluoride KF was inserted into the bridges using high-pressure and high-temperature conditions (5 GPa, 1000–1200 °C). When the molar ratio KF/TiF 3≥1, a few low dimensional compounds were obtained forming non-bridged F ions. At the composition KF/TiF 3=1/2, a new compound KTi 2F 7 was formed, which crystallizes with the space group Cmmm and the lattice parameters of a=6.371(3), b=10.448(6), c=3.958(2) Å, consisting of edge-sharing pentagonal bipyramids [TiF 7] forming ribbons running along the a axis. The ribbons are linked by corners to construct a three-dimensional framework without forming non-bridged F ions. The compound is antiferromagnetic with the Néel temperature T N =75 K, and the optical band gap was 6.4 eV. A new fluoride K 2TiF 5 (KF/TiF 3=2) with the space group Pbcn and the lattice parameters of a=7.4626(2), b=12.9544(4) and c=20.6906(7) Å was also obtained by the high pressure and high temperature treatment (5 GPa at 1000 °C) of a molar mixture of 2 KF+TiF 3. The compound contains one-dimensional chains of corner-sharing TiF 6 octahedra.

The Crystal Structure of LaAlO3‐Stabilized La2/3TiO3 Ceramics: An HRTEM Investigation

LaAlO3‐stabilized La2/3TiO3 (LT) ceramics were prepared by the conventional mixed oxide route. Small amounts of manganese oxide were added to eliminate Ti4+ reduction. The powders were calcined at 1150°C and sintered at 1400°–1500°C for 4 h and cooled at rates of 900°–15°C/h. The products were high density and single phase, with an average grain size of 6 μm. The LaAlO3‐stabilized LT ceramics exhibited a relative permittivity (ɛr) of 64, a positive temperature coefficient of resonant frequency (τf) of 84, and dielectric Q value × resonant frequency (Q×f) values of 16 400 GHz. The crystal structure and microstructures have been investigated using high‐resolution transmission electron microscopy (HRTEM) in conjunction with X‐ray diffraction (XRD). One candidate crystal structure, a≈2ap (where ap is the lattice parameter of the high‐temperature form of the cubic perovskite), b≈2ap, and c≈2ap with a space group Cmmm (65), has been confirmed by XRD, electron diffraction, and lattice imaging techniques. Microtwins, with twin boundaries parallel to the {100} planes, were observed in the microstructures.

Order–disorder character and twinning in the structure of a new synthetic titanosilicate: (Ba,Sr)4Ti6Si4O24·H2O

Prismatic crystals of the title compound, up to 100 microm long and {001} twinned by metric merohedry, were obtained as a side-product of a hydrothermal run devoted to synthesizing the heterophyllosilicate lamprophyllite. Single-crystal X-ray diffraction data were collected on a Bruker-AXS Smart Apex diffractometer from a crystal with approximate composition (Ba(0.80)Sr(0.20))(4)Ti6Si4O24 x H2O. The structure was solved and refined as a disordered structure in the space group Cmmm [a(o) = 5.906 (2), b(o) = 20.618 (8), c(o) = 16.719 (6) A, R = 0.089 for 682 reflections with I(o) > 2sigma(I(o))], and then deciphered by the order-disorder (OD) theory as an ordered structure and refined in the space group P2/c [a = 5.906, b = 16.719, c = 10.724 A, beta = 105.99 degrees , R = 0.083 for 1090 reflections with I(o) > 2sigma(I(o))]. The discussion based on the OD theory shows that the refined ordered structure corresponds to one (2M) of two maximum degree of order (MDO) polytypes. The structure of the second MDO polytype (4O) was modelled but not refined because it does not substantially occur in our sample. In the structure, infinite (001) ribbons of Ti octahedra elongated along the [100] direction are connected by (SiO3)4 four-membered rings, thus realising a new type of heteropolyhedral framework. The ribbons are three-octahedra wide and one-octahedron thick; they are formed by linking, via edge-sharing, rutile-type edge-sharing rows of octahedra. This ribbon represents a slice of the octahedral sheet that occurs in perrierite-(Ce), Ce4MgFe2Ti2O8(Si2O7)2. The (Ba,Sr) ions are hosted within two independent [100] narrow channels both delimited by four Ti octahedra and four Si tetrahedra. The disorder of H2O is discussed on the basis of a Raman spectrum.

Polar Hexagonal Tungsten Bronze-Type Oxides: KNbW2O9, RbNbW2O9, and KTaW2O9

The synthesis, crystal structures, second-harmonic generation (SHG), piezoelectric, pyroelectric, and ferroelectric properties of three polar noncentrosymmetric (NCS) hexagonal tungsten bronze-type oxides are reported. The materials KNbW 2O 9, RbNbW 2O 9, and KTaW 2O 9 were synthesized by standard solid-state techniques and structurally characterized by laboratory powder X-ray diffraction. The compounds are isostructural, crystallizing in the polar NCS space group Cmm2. The materials exhibit a corner-shared MO 6 (M = Nb (5+)/W (6+) or Ta (5+)/W (6+)) octahedral framework, with K (+) or Rb (+) occupying the "hexagonal" tunnels. The d (0) transition metals, Nb (5+), Ta (5+), and W (6+), are displaced from the center of their oxide octahedra attributable to second-order Jahn-Teller effects. SHG measurements using 1064 nm radiation revealed frequency-doubling efficiencies ranging from 180 to 220 x alpha-SiO 2. Converse piezoelectric measurements resulted in d 33 values ranging from 10 to 41 pm V (-1). The total pyroelectric coefficient, p, at 50 degrees C ranged from -6.5 to -34.5 muC K (-1) m (-2). The reported materials are also ferroelectric, as demonstrated by hysteresis loops (polarization vs electric field). Spontaneous polarization values, P s, ranging from 2.1 to 8.4 muC cm (-2) were measured. The magnitudes of the SHG efficiency, piezoelectric response, pyroelectric coefficient, and ferroelectric polarization are strongly dependent on the out-of-center distortion of the d (0) transition metals. Structure-property relationships are discussed and explored. Crystal data: KNbW 2O 9, orthorhombic, space group Cmm2 (No. 35), a = 21.9554(2) A, b = 12.60725(15) A, c = 3.87748(3) A, V = 1073.273(13) A (3), and Z = 6; RbNbW 2O 9, orthorhombic, space group Cmm2 (No. 35), a = 22.00985(12) A, b = 12.66916(7) A, c = 3.8989(2) A, V = 1086.182(10) A (3), and Z = 6; KTaW 2O 9, orthorhombic, space group Cmm2 (No. 35), a = 22.0025(2) A, b = 12.68532(14) A, c = 3.84456(4) A, V = 1073.05(2) A (3), and Z = 6.

The structure of a new ternary aluminide Th 4Fe 3Al 32

The atomic structure of a new ternary Th 4Fe 3Al 32 phase appearing in the alloy with composition Al–5 at.%Th–2.5 at.% Fe was solved using electron crystallography and X-ray diffraction techniques. Unit cell dimensions, type of Bravais lattice and the symmetry of the new compound were determined in transmission electron microscope by applying selected area and micro-beam electron diffraction techniques. The single-crystal X-ray diffraction data were used for developing the structural model. The Th 4Fe 3Al 32 compound crystallizes in a new type of crystal structure which is orthorhombic and belongs to Cmmm space group with the unit cell dimensions a = 10.03 Å, b = 16.44 Å and c = 8.21 Å. The unit cell contains 77 atoms which occupy 78 positions: 4i and 4j (for Th atoms), 4h and 2d (for Fe atoms) and 16r, 8m, 8n, 8q, 8p, 8o, 4g, 2b and 2c (for Al atoms). Refinement of the structure with isotropic thermal parameters for all atoms led to agreement factor R1 = 0.11. Refinement with anisotropic thermal parameters for Th atoms converged at R1 = 0.071.

Crystal Structure of the Terbium Borocarbide Tb2B2C3

The title compound was prepared from the elements by arc-melting followed by annealing in silica tubes at 1270 K for one week. The crystal structure was investigated by means of single-crystal X-ray diffraction: space group Cmmm, a = 3.412(1), b = 13.699(8), c = 3.669(1) Å, V = 171.5(1) Å3, Z = 2, R1 = 0.032; wR2 = 0.087 for 123 unique reflections with Io ≥ 2σ (Io) and 11 refined parameters.

The crystal structure of seeligerite, Pb3IO4Cl3, a rare Pb-I-oxychloride from the San Rafael mine, Sierra Gorda, Chile

AbstractThe crystal structure of seeligerite, Pb3IO4Cl3, from the San Rafael mine, Sierra Gorda, Chile, was solved in the space group Cmm2, and refined to R = 3.07%. The unit-cell parameters are: a = 7.971(2), b = 7.976(2), c = 27.341(5) Å, V = 1738.3(6) Å3 and Z = 8. The crystal structure consists of a stacking sequence along [001] of square-net layers of O atoms and square-net layers of Cl atoms with Pb+ and I+ cations located in the voids of the packing. As is typical of cations with a stereoactive lone-pair of electrons, Pb2+ and I5+ adopt strongly-asymmetrical configurations. Pb2+ cations occur in a variety of coordination polyhedra, ranging from anticubes and monocapped anticubes to pyramidal ‘one-sided’ coordinations. I5+ is coordinated by a square of four oxygen atoms: I1 and I3 exhibit a ‘one-sided’ coordination, whereas I2 has square-planar coordination.The TEM investigation has revealed additional superlattice reflections (which were not registered by X-ray diffraction (XRD)) in the hk0 diffraction pattern of seeligerite based upon a 0.158 Å-1 square net, which can be interpreted as arising from a 20-cation super-sheet motif (12.6 Å x 12.6 Å), likely related to a further level of Pb-I order superimposed upon the 8-site motif identified by XRD.

ChemInform Abstract: Synthesis, Structure and Physical Properties of the New Uranium Ternary Phase U3Co2Ge7.

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Nd 11Pd 4In 9 compound – A new member of the homological series based on AlB 2 and CsCl types

The Nd 11Pd 4In 9 compound was prepared by arc melting of pure metals under an argon atmosphere. Crystal structure was refined from X-ray single crystal diffractometer data (space group Cmmm, a = 14.843(3), b = 22.284(3), c = 3.7857(6) Å, Z = 2, R I = 0.0584, 653 F 2 values). It has own structure type and together with Mn 2AlB 2, Cr 3AlB 4, Mo 2FeB 2 and Lu 5Ni 2In 4 structure types belongs to homological series based on AlB 2 and CsCl structure types with common formula R m + n M 2 n X m .

Electron microscopy and structural studies of Nd1/3NbO3

Abstract The crystal structure of the deficient perovskite Nd1/3NbO3 has been determined at room temperature, using electron microscopy observations and X-ray powder diffraction data. Electron diffraction study and HRTEM images evidence the doubling of a and b parameters and confirm one of the c-parameter with respect to perovskite unit cell. The structure refinement has been carried out using the orthorhombic Cmmm space group. The Nd3+ cations occupy randomly and alternatively the (00l) planes and Nb5+ cations distorted octahedral sites. This structure is characterized by the presence of empty Nd3+ sites in every second layer and the tilting of the octahedral sites around the b-axis. In addition, electron diffraction patterns exhibit weak additional diffuse reflections, which supposes some ordering of the Nd vacancies.

Centrosymmetric silicide ScNiSi 3 – Structure, chemical bonding, and 45Sc solid state NMR

The SmNiGe 3 type silicide ScNiSi 3, previously reported to crystallize in the non-centrosymmetric space group Amm2, was synthesized from the elements by arc-melting. Two different single crystals were investigated on the basis of X-ray diffractometer data: a = 382.0(1), b = 2058.2(1), c = 381.0(1) pm, wR2 = 0.0531, 947 F 2 values for ScNi 0.913(2)Si 3 and a = 382.5(1), b = 2058.7(1), c = 380.8(1) pm, wR2 = 0.0494, 554 F 2 values for ScNi 0.929(4)Si 3 with 23 variable parameters per refinement. The single crystal data unambiguously confirm the centrosymmetric space group Cmmm. The structure consists of a three-dimensional [NiSi 3] polyanion in which the scandium atoms fill distorted hexagonal channels. The polyanion contains three crystallographically independent silicon sites which are all involved in Si–Si bonding with Si–Si distances ranging from 232 to 235 pm. Each nickel atom has a square-pyramidal silicon coordination with rather short Ni–Si distances (217–222 pm). The nickel sites of both crystals show significant underfilling. The 45Sc magic-angle spinning NMR spectrum reveals the presence of a single scandium site, thereby confirming the centrosymmetric structure. In addition, a second, weaker resonance is attributed to scandium nuclei in the vicinity of a nickel vacancy. The fractional area of this resonance is quantitatively consistent with the nickel site occupancy of 92% determined from single crystal X-ray diffraction. Electronic structure calculations reveal metallic behaviour with strong Ni–Si and Si–Si bonds as expected, but additionally significant Sc–Ni bonding in contrast to YNiSi 3, where Y–Ni bonding is reported to be absent.

Crystallochemistry of the novel two-layer RECuMg 4 ( RE=La, Tb) ternary compounds

The crystal structures of the new ternary compounds LaCuMg 4 and TbCuMg 4 were studied by X-ray powder diffraction and single-crystal methods, respectively. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDXS) was used for examining microstructure and phase composition. LaCuMg 4 crystallizes in the UCoAl 4 structure type (space group P6¯2 m, Pearson code hP18, a=1.03911(1), c=0.45126(1) nm, Z=3, R F=0.0654), while TbCuMg 4 exhibits a new structure (space group Cmmm, Pearson code oS48, a=1.35797(6), b=2.03333(9), c=0.39149(2) nm, Z=8, w R 2=0.0426). Both structures represent a family of two-layer compounds. All interatomic distances indicate metallic type bonding. The structural peculiarities of these compounds and their relations are discussed.

Synthesis, structure and physical properties of the new uranium ternary phase U 3Co 2Ge 7

A new ternary compound, U 3Co 2Ge 7, has been synthesized from the corresponding elements by a high temperature reaction using molten tin flux. It crystallizes in the orthorhombic La 3Co 2Sn 7-type (Pearson's symbol oC24, space group Cmmm, No. 65) with lattice parameters determined from single-crystal X-ray diffraction as follows: a=4.145(2) Å; b=24.920(7); c=4.136(2) Å, V=427.2(3) Å 3. Structure refinements confirm an ordered structure having two crystallographically inequivalent uranium atoms, occupying sites with dissimilar coordination. U 3Co 2Ge 7 orders ferromagnetically below 40 K and undergoes a consecutive magnetic transition at 20 K. These results have been obtained from temperature- and field-dependent magnetization, resistivity and heat-capacity measurements. The estimated Sommerfeld coefficient γ=87 mJ/mol-U K 2 suggests U 3Co 2Ge 7 to be a moderately heavy-fermion material.

Na2.4CeSi6O15·2.6H2O: Hydrothermal synthesis, characterization and properties of a new luminescent microporous cerium silicate

A new microporous sodium cerium silicate Na2.4CeSi6O15 · 2.6H2O containing mixed valence of CeIII/CeIV (denoted CeSiO-CJ2) was successfully synthesized under mild hydrothermal conditions. Its structure was determined by single-crystal X-ray diffraction (XRD) analysis, and characterized by SEM, powder XRD, ICP, TGA, and XPS analyses. CeSiO-CJ2 crystallizes in orthorhombic space group Cmm2 (No. 35) with a = 7.4165(15) Å, b = 30.966(6) Å, and c = 7.1539(14) Å, and Z = 4. It consists of flexuous [Si2O5]n2n- silicate layers that are connected by seven-coordinated Ce polyhedra to form a three dimensional (3D) open framework. There exist two different types of irregular 8-ring channels along [1 0 0] and 6-ring channels along [0 0 1] in which Na+ ions and water molecules reside. Magnetic measurement shows that CeSiO-CJ2 exhibits a Curie–Weiss behavior over the temperature range of 2–300 K with a Weiss constant θ of −5.34(1) K, and a Curie constant C of 0.12(7) K. The data of susceptibility show that there exists at least 31.2% trivalent Ce in CeSiO-CJ2 which is consistent with the XPS analysis. CeSiO-CJ2 exhibits a strong broad emission band containing two overlapped peaks centered at about 384 and 420 nm, respectively. Ion-exchange capacity of CeSiO-CJ2 was investigated by measuring the exchange amount of Na+ ions with various cations. Studies indicate that the Na+ ions can be nearly completely (97.2%) exchanged with NH4+ ions. The Na+ ionic conductivity measurement shows that its conductivity activation energy is 0.25 eV and that the conductivity at 573 K is 2.7 × 10−5 (Ω cm)−1.

Synthesis, Structural Characterization, Electronic Structure, and Magnetic Properties of the Zintl Phase Eu10Cd6Bi12

A new transition-metal-containing Zintl phase, Eu(10)Cd(6)Bi(12), was synthesized by combining the elements in excess molten Cd. Single-crystal X-ray diffraction studies indicated that this compound crystallizes in the orthorhombic space group Cmmm (No. 65) with a=7.840(2), b=24.060(7), and c=4.7809(14) A. The crystal structure of Eu(10)Cd(6)Bi(12) can be viewed as a stacking of a series of [Cd(6)Bi(12)] double layers, which are arranged alternately along the b axial direction. The layers are composed of corner- and edge-shared CdBi(4) tetrahedra, a common feature in the crystal chemistry of many transition-metal Zintl phases. Electronic-band-structure calculations confirm the closed-shell configuration of all constituent elements and corroborate the electron count inferred by the Zintl formalism, that is, [Eu(2+)](10)[Cd(2+)](6)[Bi(3-)](8)[Bi(2-)](4). Magnetic-susceptibility measurements confirm the divalency of europium and show the existence of a long-range antiferromagnetic order of the Eu spins below 12.3 K.

Microporous titanosilicates Synthesis and structural characterization of a new orthorhombic-type labuntsovite

The crystal structure of a new orthorhombic-type labuntsovite with composition Na 5.36 Ti 4 (Si 4 O 12 ) 2 (O 0.34 OH 0.66 ) 4 · 10.31H 2 O, obtained as side product of hydrothermal runs devoted to the synthesis of rhodesite-type microporous silicates, has been solved and refined in the space group Cmmm ( a = 7.278, b = 14.134, c = 7.118 A; Z = 1) with X-ray single-crystal diffraction data collected by a Bruker-AXS Smart Apex diffractometer equipped with a CCD area detector. The crystal structure shows the overall features known for the heteropolyhedral framework (tetrahedral/octahedral) structures of the labuntsovite-group minerals, like systems of channels and disorder. The simpler chemical composition of the synthetic compound, which does not show isomorphous replacements, allows a detailed discussion of the zeolite-like disorder in the channels, which is related to the charge of the framework [Ti 4 (Si 4 O 12 ) 2 (OH 4−x O x )] (4+x)− and to the system of hydrogen bonding. The different space group Pbam shown by the two known natural labuntsovites (nenadkevichite and korobitsynite) is attributed to the presence in the natural samples of the Nb → Ti substitution.

Crystal structure of Ln1/3NbO 3 ( Ln=Nd, Pr) and phase transition in Nd 1/3NbO 3

The crystal structure of the A-site deficient perovskite Ln 1/3NbO 3 ( Ln=Nd, Pr) at room temperature has been determined, for the first time, as orthorhombic in space group Cmmm using high-resolution neutron powder diffraction. Pertinent features are the alternation of unoccupied layers of A-sites and layers partly occupied by Ln cations, as well as out-of-phase tilting of the NbO 6 octahedra around an axis perpendicular to the direction of the cation/vacancy ordering. The phase transition behaviour of Nd 1/3NbO 3 has also been studied in situ. This compound undergoes a continuous phase transition at around 650 °C to a tetragonal structure in space group P4/ mmm due to the disappearance of the octahedral tilting. The analysis of spontaneous strains shows that this phase transition is tricritical in nature.