Centrosymmetric or Noncentrosymmetric? Case Study, Generalization and Structural Redetermination of Sr5Nb5O17

SHG in a centrosymmetric crystal - can it be possible?

The present report lists selected publications on centrosymmetric compounds that manifest second harmonic generation responses in a laser, along with a few publications that dispute the laser outcomes. Two studies provide a plausible explanation for this apparent contradiction between second-order nonlinear susceptibility and inversion symmetry: the crystals are noncentrosymmetric and are twinned by inversion. If crystal structures of SHG-active compounds are presented in centrosymmetric settings, the authors of the publications may consider stipulating that the true space group is likely to be one of the noncentrosymmetric sub-space groups.

Patterning Oxide Nanopillars at the Atomic Scale by Phase Transformation

Phase transformations in crystalline materials are common in nature and often modify dramatically properties of materials. The ability to precisely control them with a high spatial precision represents a significant step forward in realizing new functionalities in confined dimensions. However, such control is extremely challenging particularly at the atomic scale due to the intricacies in governing thermodynamic conditions with a high spatial accuracy. Here, we apply focused electron beam of a scanning transmission electron microscope to irradiate SrNbO3.4 crystals and demonstrate a precise control of a phase transformation from layered SrNbO3.4 to perovskite SrNbO3 at the atomic scale. By purposely squeezing O atoms out of the vertex-sharing NbO6 octahedral slabs, their neighboring slabs zip together, resulting in a patterning of SrNbO3 nanopillars in SrNbO3.4 matrix. Such phase transformations can be spatially manipulated with an atomic precision, opening up a novel avenue for materials design and processing and also for advanced nanodevice fabrication.

Two-dimensional magnetic clusters in La(n)(Ti(1-x)Fe(x))(n)O(3n+2) (n = 5 with x = 0.2 and n = 6 with x = 0.33)

Crystal structure analyses of the layered compounds La(n)(Ti(1-x)Fe(x))(n)O(3n+2), with nominal compositions x = 0.2 for n = 5 and x = 0.33 for n = 6, show that the iron is concentrated at the centers of the slabs. The spatial arrangements of the iron ions can be approximated by two-dimensional square lattices with strong magnetic interactions between neighboring sites, albeit with fractional occupancies of the sites of an average of 0.67 magnetic ions in n = 6 and 0.42 magnetic ions in n = 5 compounds. A previously described (Lichtenberg et al 2008 Prog. Solid State Chem. 36 253) crossover of magnetic behavior of n = 6 at room temperature is explained by the formation of two-dimensional, ferromagnetically organized magnetic clusters with an average size of 52 Fe(3+) ions. The absence of long-range magnetic order follows from the fractional occupancy of the sites by the magnetic ions. The lower concentration of magnetic ions in n = 5 explains why crossover behavior is not found and clusters do not form in that compound.

Perovskite-related Ca(Nb,Ti)O3.33

Crystals of nominal composition Ca(Nb0.8Ti0.2)O3.4, an n = 5 member of the homologous series A n B n O 3n+2, have been synthesized by floating-zone melting. The material was found to be multiphase consisting also of the n = 6 type. A good single crystal of this n = 6 type was found and the crystal structure at room temperature was determined by X-ray diffraction using synchrotron radiation. Structure refinements indicate a composition of Ca(Nb0.76Ti0.24)O3.33. The crystal structure is monoclinic P21 and consists of slabs of corner-sharing (Nb,Ti)O6 octahedra interrupted by layers of oxygen atoms. Ca atoms within the slabs are 12-fold coordinated whereas those at the borders show irregular coordination geometries. The octahedral distortion is greatest near the borders of the slabs and least near the middle of the slabs. Ti4+ preferentially occupy octahedral sites in the middle of slabs. The non-stoichiometry in the refined composition and the insulating character of the material may be explained in terms of extra oxygen, vacancies on the cation sites, or by localization of the electrons in the 4d orbitals of Nb at the borders of slabs.

Extremely small energy gap in the quasi-one-dimensional conducting chain compound SrNbO3.41

Resistivity, optical, and angle-resolved photoemission experiments reveal unusual one-dimensional electronic properties of highly anisotropic SrNbO3.41. Along the conducting chain direction, we find an extremely small energy gap of only a few meV at the Fermi level. A discussion in terms of typical 1D instabilities (Peierls, Mott-Hubbard) shows that neither seems to provide a satisfactory explanation for the unique properties of SrNbO3.41.

The bootstrap method as a test for non-centrosymmetricity – Application to tazheranite (Zr,Ca,Ti)O2

The bootstrap statistical method is applied to the analysis of single-crystal X-ray diffraction data of tazheranite [(Zr,Ca,Ti)O2; cubic, a = 5.117 Å], in order to establish the significance of statistical estimators of noncentrosymmetricity. The lack of the inversion centre is successfully proved showing that the average structure of tazheranite has space group F[unk]3m, due to a different repartition of oxygen vacancies between two sites of the fluorite-type structure. Instead, classical estimators of the intensity distribution and of the refinements were not able to discriminate between Fm[unk]m and F[unk]3m space groups.