Two-Dimensional Antiferroelectricity in Nanostripe-Ordered In_{2}Se_{3}.

Two-dimensional (2D) layered materials have been an exciting frontier for exploring emerging physics at reduced dimensionality, with a variety of exotic properties demonstrated at 2D limit. Here, we report the first experimental discovery of in-plane antiferroelectricity in a 2D material ß^{'}-In_{2}Se_{3}, using optical and electron microscopy consolidated by first-principles calculations. Different from conventional 3D antiferroelectricity, antiferroelectricity in ß^{'}-In_{2}Se_{3} is confined within the 2D layer and generates the unusual nanostripe ordering: the individual nanostripes exhibit local ferroelectric polarization, whereas the neighboring nanostripes are antipolar with zero net polarization. Such a unique superstructure is underpinned by the intriguing competition between 2D ferroelectric and antiferroelectric ordering in ß^{'}-In_{2}Se_{3}, which can be preserved down to single-layer thickness as predicted by calculation. Besides demonstrating 2D antiferroelectricity, our finding further resolves the true nature of the ß^{'}-In_{2}Se_{3} superstructure that has been under debate for over four decades.

Growth, structure, and properties of BiFeO3/-BiCrO3 films obtained by dual cross beam PLD.

The properties of epitaxial Bi(2)FeCrO(6) thin films, recently synthesized by pulsed laser deposition, have partially confirmed the theoretical predictions (i.e., a magnetic moment of 2 micro(B) per formula unit and a polarization of approximately 80 microC/cm(2) at 0 K). The existence of magnetic ordering at room temperature for this material is an unexpected, but very promising, result that needs to be further investigated. Because magnetism is assumed to arise from the exchange interaction between the Fe and Cr cations, the magnetic behavior is strongly dependent on both their ordering and the distance between them. We present here the successful synthesis of epitaxial Bi(2)Fe(x)CryO(6) (BFCO x/y) films grown on SrTiO3 substrates using dual crossed-beam, pulsed-laser deposition. The crystal structure of the films has different types of (111)-oriented superstructures, depending on the deposition conditions. The multiferroic character of BFCO (x/y) films is proven by the presence of both ferroelectric and magnetic hysteresis at room temperature. The oxidation state of Fe and Cr ions in the films is shown to be 3+ only, and the difference in macroscopic magnetization with Fe/Cr ratio composition could only be due to ordering of the Cr(3+) and Fe(3+) cations to the modification of the exchange interaction between them.

Electron diffraction and microscopy study of the structure and microstructure of the hexagonal perovskite Ba3Ti2MnO9.

This paper reports a structural and microstructural investigation of the hexagonal perovskite Ba(3)Ti(2)MnO(9) using electron microscopy and diffraction. Convergent-beam electron diffraction (CBED) revealed the structure has the non-centrosymmetric space group P6(3)mc (186) at room temperature and at approximately 110 K. Compared with the centrosymmetric parent structure BaTiO(3), with space group P6(3)/mmc, this represents a break in mirror symmetry normal to the c axis. This implies the Ti and Mn atoms are ordered on alternate octahedral sites along the 0001 direction in Ba(3)Ti(2)MnO(9). Using high-resolution electron microscopy (HREM), we observed occasional 6H/12R interfaces on (0001) planes, however, no antiphase boundaries were observed, as were seen in Ba(3)Ti(2)RuO(9). Using powder X-ray Rietveld refinement we have measured the lattice parameters from polycrystalline samples to be a = 5.6880 +/- 0.0005, c = 13.9223 +/- 0.0015 A at room temperature.

Structure and microstructure of hexagonal Ba3Ti2RuO9 by electron diffraction and microscopy.

We have used electron microscopy and diffraction to refine the structure and investigate the microstructure of Ba(3)Ti(2)RuO(9). The parent compound is hexagonal BaTiO(3) with the space group P6(3)/mmc. Using convergent-beam electron diffraction (CBED) combined with electron-sensitive image plates we have found that the space group of Ba(3)Ti(2)RuO(9) is the non-centrosymmetric group P6(3)mc at room temperature and at approximately 110 K. This is consistent with the Ru and Ti atoms occupying alternate face-sharing octahedral sites in the 0001 direction. This maintains the c-glide, but breaks the mirror normal to the c axis and consequently removes the centre of symmetry. Using powder X-ray diffraction, we have measured the lattice parameters from polycrystalline samples to be a = 5.7056 +/- 0.0005, c = 14.0093 +/- 0.0015 A at room temperature. Using high-resolution electron microscopy (HREM) we observed highly coherent, low-strain {10\bar 10} grain boundaries intersecting at 60 and 120 degrees . From CBED we deduce that adjacent grains are identical but for the relative phase of the Ti and Ru atom ordering along the c axis. HREM also revealed occasional stacking faults, normal to the c-axis.

Voltage refinement by deficit HOLZ line geometry.

A method of refining electron beam voltage from deficit line intersections, using a quasi-kinematic scattering approximation, is described and applied to a silicon standard. It is shown that dynamical corrections are necessary for higher-index zone axes, such as 310 and 411, where a single deficit line associated with one dominant Bloch state is visible. This leads to a substantial difference in the refined voltage compared with that obtained from a purely kinematic approximation. Neglect of this dynamical correction term effectively invokes a systematic error that may lead to high precision but poor accuracy in higher-order Laue zone measurements of beam voltage or lattice parameters. Although relatively small, differences in the dynamical correction necessary for these two zones are confirmed experimentally for 100-300 keV electrons.