ABSTRACT
The system Bi(2(n + 2))Mo(n)O(6(n + 1)) is described within the superspace formalism. Two superspace models are proposed for the different members of this family, depending on the parity of the parameter n. The superspace model for the odd members is constructed through the embedding of the cationic distribution of the member with n = 3, and the modification of a superspace model previously proposed for the compound Bi(2)MoO(6). However, this model cannot be applied to the even members of the family. Performing the appropriate transformations, a suitable superspace model for the even members is obtained. The atomic structure of the different compounds of the family have been refined through the Rietveld method combining synchrotron X-ray and neutron powder diffraction data.
ABSTRACT
The crystal structure of the title aluminium barium lanthanum ruthenium strontium oxide has been solved and refined using neutron powder diffraction to establish the parameters of the oxygen sublattice and then single-crystal X-ray diffraction data for the final refinement. The structure is a cubic modification of the perovskite ABO(3) structure type. The refined composition is Ba(0.167)La(0.548)Sr(1.118)Ru(0.377)Al(0.290)O(3.480), and with respect to the basic perovskite structure type it might be written as (Ba(8)La(13.68)Sr(34.32))(Al(13.92)La(12.64)Ru(18.08)Sr(19.36))O(192-x), with x = 24.96. The metal atoms lie on special positions. The A-type sites are occupied by Ba, La and Sr. The Ba atoms are located in a regular cuboctahedral environment, whereas the La and Sr atoms share the same positions with an irregular coordination of O atoms. The B-type sites are divided between two different Wyckoff positions occupied by Ru/Al and La/Sr. Only Al and Ru occupy sites close to the ideal perovskite positions, while La and Sr move away from these positions toward the (111) planes with high Al content. The structure contains isolated RuO(6) octahedra, which form tetrahedral substructural units.
ABSTRACT
The compounds poly[di-mu(4)-succinato-mu(2)-1,2-di-4-pyridylethane-dicopper(II)], [Cu(2)(C(4)H(4)O(4))(2)(C(12)H(12)N(2))](n), (I), and poly[di-mu(4)-succinato-mu(2)-1,3-di-4-pyridylpropane-dicopper(II)], [Cu(2)(C(4)H(4)O(4))(2)(C(13)H(14)N(2))](n), (II), exhibit polymeric structures with the dicopper units doubly bridged by bis-bidentate succinate groups and crosslinked by the separator bis(pyridyl) molecules. In (I), the molecule exhibits a centre of inversion located midway between the core Cu-dimer atoms and another that relates half of the bis(pyridyl)ethane ligand to the other half. Compound (II) has a similar molecular packing but with a doubled lattice constant and noncentrosymmetric core units. An antiferromagnetic interaction due to the dinuclear copper units was deduced from magnetic subsceptibility measurements, and spin triplet signals were detected in the electron paramagnetic resonance spectra for both compounds.
ABSTRACT
The structure of the monoclinic phase of the compound InAl(1 - x)Ti(x)O(3 + x/2) with x = 0.701 (1) has been analyzed within the (3 + 1)-dimensional superspace formalism. Two different models were refined describing the structure as an incommensurate modulated layer and modulated composite, respectively. Both models include the same composition-structure relation. In the composite approach it is derived from the mismatching between the two subsystems. In the incommensurate modulated system, it is derived from a closeness condition between O atomic domains. The distribution and coordination of the cations is discussed and compared with previously proposed models for similar compounds.
ABSTRACT
Sodium tetracalcium pentaniobium heptadecaoxide, NaCa(4)Nb(5)O(17), corresponds to the n = 5 term of the homologous A(n)B(n)O(3n+2) family of structures composed of ABX(3) perovskite layers. The structure consists of perovskite-type blocks of n = 5 layers of NbO(6) octahedra, separated by an interblock region. Successive blocks along the b axis are displaced by 1/2c with respect to each other. The deformation of the NbO(6) octahedra is a minimum at the middle of each block, and increases along the direction of the b axis to a maximum at each end of the block. Ca and Na share the same intrablock sites, coordinated by 12 O atoms, whereas only Ca atoms are found in the interblock cavities, at sites with different coordination geometries.
