Stabilization of the high-temperature and high-pressure cubic phase of ZnO by temperature-controlled milling

The reversible transition of wurtzite to rock salt phase under pressure is well reported in literature. The cubic phase is unstable under ambient conditions both in the bulk and in nanoparticles. This paper reports defect-induced stabilization of cubic ZnO phase in sub 20 nm ZnO particles and explores their optical properties. The size reduction was achieved by ball milling in a specially designed mill which allows a control of the milling temperature. The process of synthesis involved both variation of milling temperature (including low temperature similar to 150 K) and impact pressure. We show that these have profound influence in the introduction of defects and stabilization of the cubic phase. A molecular dynamics simulation is presented to explain the observed results. The measured optical properties have further supported the observations of defect-induced stabilization of cubic ZnO and reduction in particle size.

Monoclinic to cubic phase transformation in combustion synthesized gadolinium oxide

Gadolinium oxide, cerium oxide, and 10 mol% gadolinia doped ceria ceramic powders have been synthesized using combustion technique. Though the cubic gadolinia phase is stable at room temperature, single phase monoclinic gadolinia was obtained as a result of combustion synthesis using fuel lean and stoichiometric precursor compositions. This powder was subjected to calcination treatment and ceria doping to study the stability of phases and the rate of phase transformation from monoclinic to cubic gadolinia. It was found that monoclinic gadolinia transforms to cubic gadolinia upon calcination at temperatures less than 1200 degrees C. It was also found that rate of phase transformation is more for powder produced using fuel lean compositions; and the rate is enhanced upon ceria doping. (C) 2015 Elsevier Ltd. All rights reserved.

Anomalous strains in the cubic-phase GaN films grown on GaAs (001) by metalorganic chemical vapor deposition

Strains in cubic GaN films grown on GaAs (001) were measured by a triple-axis x-ray diffraction method. Residual strains in the as-grown epitaxial films were in compression, contrary to the predicted tensile strains caused by large lattice mismatch between epilayers and GaAs substrates (20%). It was also found that the relief of strains in the GaN films has a complicated dependence on the growth conditions. We interpreted this as the interaction between the lattice mismatch and thermal mismatch stresses. The fully relaxed lattice constants of cubic GaN are determined to be 4.5038 +/- 0.0009 Angstrom, which is in excellent agreement with the theoretical prediction of 4.503 Angstrom. (C) 2000 American Institute of Physics. [S0021-8979(00)07918-4].

Cubic-phase GaN light-emitting diodes

The feasibility of growing device-quality cubic GaN/GaAs(001) films by metal organic chemical vapor deposition has been demonstrated. The optical quality of the GaN films was characterized by room-temperature photoluminescence measurements, which shows a full width at half maximum of 46 meV. The structural quality of the films was investigated by transmission electron microscopy. There are submicron-size grains free from threading dislocations and stacking faults. More importantly, a cubic-phase GaN blue light-emitting diode has been fabricated. The device process, which is very simple and compatible with current GaAs technology, indicates a promising future for the blue light-emitting diode. (C) 1999 American Institute of Physics. [S0003-6951(99)01416-3].

Structural identification of a cubic phase in hexagonal GaN films grown on sapphire by gas-source molecular beam epitaxy

The structural characteristics of gallium nitride (GaN) films grown on sapphire(0001) substrates by gas source molecular beam epitaxy (GSMBE) have been investigated using high-resolution synchrotron irradiation X-ray diffraction and cathodoluminescence with a variable energy electron beam. Besides the well-known GaN hexagonal structure, a small portion of cubic phase GaN was observed. The X-ray measurements provide an essential means for the structural identification of the GaN layers. Arising from the variable penetration depth of the electron beam in the cathodoluminescence measurements, it was found that the fraction of the GaN cubic-phase typically increased as the probing depth was increased. The results suggest that the GaN cubic phase is mostly located near the interface between the substrate and GaN layer due to the initial nucleation.

A highly oriented cubic phase formed by lipids under shear

We demonstrate the formation of a macroscopically oriented inverse bicontinuous cubic (QII) lipid phase from a sponge (L3) phase by controlled hydration during shear flow. The L3 phase was the monoolein/ butanediol/water system; the addition of water reduces the butanediol concentration, inducing the formation of a diamond (QIID) cubic phase, which is oriented by the shear flow. The phenomenon was reproduced in both capillary and Couette geometries, indicating that this represents a robust general route for the production of highly aligned bulkQII samples, with applications in nanomaterial templating and protein research.

Preparation of films of a highly aligned lipid cubic phase

We demonstrate a method by which we can produce an oriented film of an inverse bicontinuous cubic phase (QII D) formed by the lipid monoolein (MO). By starting with the lipid as a disordered precursor (the L3 phase) in the presence of butanediol, we can obtain a film of the QII D phase showing a high degree of in-plane orientation by controlled dilution of the sample under shear within a linear flow cell. We demonstrate that the direction of orientation of the film is different from that found in the oriented bulk material that we have reported previously; therefore, we can now reproducibly form QII D samples oriented with either the [110] or the [100] axis aligned in the flow direction depending on the method of preparation. The deposition of MO as a film, via a moving fluid− air interface that leaves a coating of MO in the L3 phase on the capillary wall, leads to a sample in the [110] orientation. This contrasts with the bulk material that we have previously demonstrated to be oriented in the [100] direction, arising from flow producing an oriented bulk slug of material within the capillary tube. The bulk sample contains significant amounts of residual butanediol, which can be estimated from the lattice parameter of the QII D phase obtained. The sample orientation and lattice parameters are determined from synchrotron small-angle X-ray scattering patterns and confirmed by simulations. This has potential applications in the production of template materials and the growth of protein crystals for crystallography as well as deepening our understanding of the mechanisms underlying the behavior of lyotropic liquid-crystal phases.

Predicting the orientation of lipid cubic phase films

Lipid cubic phase films are of increasingly widespread importance, both in the analysis of the cubic phases themselves by techniques including microscopy and X-ray scattering, and in their applications, especially as electrode coatings for electrochemical sensors and for templates for the electrodeposition of nanostructured metal. In this work we demonstrate that the crystallographic orientation adopted by these films is governed by minimization of interfacial energy. This is shown by the agreement between experimental data obtained using grazing-incidence small-angle X-ray scattering (GI-SAXS), and the predicted lowest energy orientation determined using a theoretical approach we have recently developed. GI-SAXS data show a high degree of orientation for films of both the double diamond phase and the gyroid phase, with the [111] and [110] directions respectively perpendicular to the planar substrate. In each case, this matches the lowest energy facet calculated for that particular phase.

Experimental evidence for proposed transformation pathway from the inverse hexagonal to inverse diamond cubic phase from oriented lipid samples

A macroscopically oriented inverse hexagonal phase (HII) of the lipid phytantriol in water is converted to an oriented inverse double diamond bicontinuous cubic phase (QIID). The initial HII phase is uniaxially oriented about the long axis of a capillary with the cylinders parallel to the capillary axis. The HII phase is converted by cooling to a QII D phase which is also highly oriented, where the cylindrical axis of the former phase has been converted to a ⟨110⟩ axis in the latter, as demonstrated by small-angle X-ray scattering. This epitaxial relationship allows us to discriminate between two competing proposed geometric pathways to convert HII to QIID. Our findings also suggest a new route to highly oriented cubic phase coatings, with applications as nanomaterial templates.

Aligned platinum nanowire networks from surface-oriented lipid cubic phase templates

Mesoporous metal structures featuring a bicontinuous cubic morphology have a wide range of potential applications and novel opto-electronic properties, often orientation-dependent. We describe the production of nanostructured metal films 1–2 microns thick featuring 3D-periodic ‘single diamond’ morphology that show high out-of-plane alignment, with the (111) plane oriented parallel to the substrate. These are produced by electrodeposition of platinum through a lipid cubic phase (QII) template. Further investigation into the mechanism for the orientation revealed the surprising result that the QII template, which is tens of microns thick, is polydomain with no overall orientation. When thicker platinum films are grown, they also show increased orientational disorder. These results suggest that polydomain QII samples display a region of uniaxial orientation at the lipid/substrate interface up to approximately 2.8 ± 0.3 μm away from the solid surface. Our approach gives previously unavailable information on the arrangement of cubic phases at solid interfaces, which is important for many applications of QII phases. Most significantly, we have produced a previously unreported class of oriented nanomaterial, with potential applications including metamaterials and lithographic masks.

Tetragonal-cubic phase boundary in nanocrystalline ZrO(2)-Y(2)O(3) solid solutions synthesized by gel-combustion

By means of synchrotron X-ray powder diffraction (SXPD) and Raman spectroscopy, we have detected, in a series of nanocrystalline and compositionally homogeneous ZrO(2)-Y(2)O(3) solid solutions, the presence at room temperature of three different phases depending on Y(2)O(3) content, namely two tetragonal forms and the cubic phase. The studied materials, with average crystallite sizes within the range 7-10 nm, were synthesized by a nitrate-citrate gel-combustion process. The crystal structure of these phases was also investigated by SXPD. The results presented here indicate that the studied nanocrystalline ZrO(2)-Y(2)O(3) solid solutions exhibit the same phases reported in the literature for compositionally homogeneous materials containing larger (micro)crystals. The compositional boundaries between both tetragonal forms and between tetragonal and cubic phases were also determined. (C) 2011 Elsevier B.V. All rights reserved.

Annealing behavior of hexagonal phase content in cubic GaN thin films grown on GaAs (001) by MOCVD

The annealing behavior of the hexagonal phase content in cubic GaN (c-GaN) thin films grown on GaAs (001) by MOCVD is reported. C-GaN thin films are grown on GaAs (001) substrates by metalorganic chemical vapor deposition (MOCVD). High temperature annealing is employed to treat the as-grown c-GaN thin films. The characterization of the c-GaN films is investigated by photoluminescence (PL) and Raman scattering spectroscopy. The change conditions of the hexagonal phase content in the metastable c-GaN are reported. There is a boundary layer existing in the c-GaN/GaAs film. When being annealed at high temperature, the intensity of the TOB and LOB phonon modes from the boundary layer weakens while that of the E-2 phonon mode from the hexagonal phase increases. The content change of hexagonal phase has closer relationship with annealing temperature than with annealing time period.

Cubic sinusoidal phase mask: Another choice to extend the depth of field of incoherent imaging system

Wave-front coding is a well known technique used to extend the depth of field of incoherent imaging system. The core of this technique lies in the design of suitable phase masks, among which the most important one is the cubic phase mask suggested by Dowski and Cathey (1995) [1]. In this paper, we propose a new type called cubic sinusoidal phase mask which is generated by combing the cubic one and another component having the sinusoidal form. Numerical evaluations and real experimental results demonstrate that the composite phase mask is superior to the original cubic phase mask with parameters optimized and provides another choice to achieve the goal of depth extension. (C) 2009 Elsevier Ltd. All rights reserved.

Structural phase transition study of the morphotropic phase boundary compositions of Na0.5Bi0.5TiO3-PbTiO3

Neutron, synchrotron x-ray powder diffraction and dielectric studies have been performed for morphotropic phase boundary (MPB) compositions of the (1 - x )Na1/2Bi1/2TiO3-xPbTiO(3) system. At room temperature, the MPB compositions (0.10 < x <= 0.15) consist of a mixture of rhombohedral (space group R3c) and tetragonal ( space group P4mm) structures with the fraction of tetragonal phase increasing with increasing PbTiO3 content. On heating, while the rhombohedral phase just outside the MPB region, i.e. x = 0.10, transforms directly to a cubic phase, the rhombohedral phase of the MPB compositions transforms gradually to a tetragonal phase, until interrupted by a rhombohedral-cubic phase transition. The correspondence of the dielectric anomalies with the structural transitions of the different compositions has been examined and compared with earlier reports.