R.J.D. Tilley

An electron microscope study of perovskite-related oxides in the SrTiO system

Abstract In order to investigate the relationship between nonstoichiometry and the static dielectric constant, phases in the SrOTiO2 pseudo-binary system have been studied by X-ray diffraction and electron microscopy. Three ternary oxides were found in the temperature interval 1373–1673°K, Sr2TiO4, Sr3Ti2O7, and SrTiO3. In all of these oxides, nonstoichiometry seems to be taken up by coherent intergrowth of lamellae of various homologous oxides Srn+1TinO3n+1 The results are discussed in terms of the crystal chemistry of the perovskite oxides and factors which influence the magnitude of the static dielectric constant of solids.

Examination of substoichiometric WO3−x crystals by electron microscopy☆

The defect structure of high purity crystals of tungsten oxides with compositions between WO3.00 and WO2.99 has been investigated by transmission electron microscopy. The oxygen deficiency appears to be accommodated by Wadsley defects, disordered crystallographic shear planes, parallel to {120}R (R = ReO3-type parent lattice). The planes tend to collect into bands which cause some regions of the crystal to have compositions far removed from the bulk. An observed maximum in carrier mobility is attributed to the destruction of conduction band states in the defect-rich regions. A modified form of the dislocation model for shear plane formation is proposed.

Notes on phases occurring in the binary tungsten-oxygen system

Abstract The phases occurring in the binary tungsten-oxygen system in the composition region WO 3 WO 2 have been clarified by electron microscopy and powder X-ray diffraction in the temperature range from 723 to 1373 K. There are five structure types in the binary system, besides WO 3 , viz., the {102} CS structures, the {103} CS structures, W 24 O 68 , W 18 O 49 , and WO 2 . The {102} and {103} CS structures, and W 24 O 68 structures, were always disordered and true equilibrium was not achieved even after 5 months of heating at 1373 K. The lowest temperature for the formation of the CS phases was of the order of 873 K, and the disordered W 24 O 68 structure formed at somewhat higher temperatures. The formation of the latter phase was also slower than the formation of the CS phases. The results suggest that elastic strain energy is of importance in controlling the microstructures found in the nonstoichiometric regions.

An electron microscope study of some nonstoichiometric tungsten oxides

Large crystals of WO3 have been reduced to a composition of approximately WO2.91 at 3 different temperatures, 950, 1000, and 1070°C. After reduction the crystals were examined by optical microscopy and transmission electron microscopy. The crystals were faulted in a variety of ways and rarely perfectly ordered. Large crystals heated at 1070°C supported oxygen loss by formation of {103} CS planes while crystals heated at 950°C contained {102} CS planes. At 1000°C {102} and {103} CS planes coexisted. It was found that the way in which the WO3 structure accommodated oxygen loss was a function of composition and of temperature. In all experiments, some vapour transport also took place, resulting in the growth of needle shaped crystals. These were always members of the WxO3n−2 homologous series of oxides, and contained {103} CS planes, irrespective of the formation temperature.

Etude du système SbMoO à 500°C: Mise en évidence de deux nouveaux oxydes de molybdène-antimoine

Abstract The ternary system SbMoO has been investigated by reduction using H 2 H 2 O gas mixtures and direct synthesis under vacuum. The products were studied by X-ray diffraction, electron diffraction, and high-resolution electron microscopy. Two new compounds were characterized: Sb 0.2 MoO 3.1 , which is orthorhombic, with an average oxidation state for molybdenum of 5,5; and Sb 0.4 MoO 3.1 , which is hexagonal and in which the average oxidation state of molybdenum is 5. These two phases can be considered in some ways as molybdenum bronzes, but without possessing all the related bronze characteristics.

An electron microscope study of tungsten oxides in the composition range WO2.90WO2.72

Abstract The structures of tungsten oxides occurring in the composition range WO 2.90 WO 2.72 have been studied by transmission electron microscopy. In the oxygen-rich part of the phase range W n O 3 n −2 CS phases exist. The lowest value of n found for extensive areas of ordered material was 16. At the oxygen-poor end of the phase range, W 18 O 49 exists. No evidence was found to indicate that this phase has an appreciable composition range. Between the W n O 3 n −2 oxides and W 18 O 49 a previously unreported oxide was found. Its structure has been partly elucidated by high-resolution electron microscopy and it has been shown to bear more resemblance to a tunnel structure than to a CS phase.

Phase relations in the pseudobinary system TiO2Ga2O3

Abstract Phase relations and microstructures in the TiO2-rich part of the TiO2Ga2O3 pseudobinary system have been determined at temperatures between 1373 and 1623°K using X-ray diffraction and electron and optical microscopy. The phases occurring in the system are TiO2 (rutile), β-Ga2O3, a series of oxides Ga4Tim−4O2m−2 (m odd) which exist above 1463°K, and Ga2TiO5, which exists above 1598°K. The width of the phase region occupied by the Ga4Tim−4O2m−2 phases varies with temperature. At 1473°K it is narrow, and has limits of Ga4Ti25O56 to Ga4Ti21O48 while at higher temperatures it broadens to limits of from Ga4Ti27O60 to Ga4Ti11O28 at 1623°K. These phases are often disordered and crystals frequently contain partially ordered intergrowths of oxides with various values of m. On the TiO2-rich side of the phase region there is a continuous change in texture from rutile to the end members of the Ga4Tim−4O2m−2 structures. The findings are summarized on a phase diagram.

An electron microscope study of tin dioxide and antimony-doped tin dioxide

Crystals of tin dioxide, SnO2, have been grown pure or doped with a few percent of antimony using vapor growth methods in order to investigate the microstructures of reduced and oxidized SnO2. They were examined by X-ray diffraction and by optical and electron microscopy. SnO2 crystals were found to contain few faults, but the antimony-doped crystals were extensively twinned in some regions. Reduction of SnO2 crystals to yield CS phases was unsuccessful. These results are discussed in terms of the known crystal chemistry of the oxides involved.

Ternary tungsten oxides with the Mo5O14 structure

Following the discovery of a ternary GeW oxide with the Mo5O14 structure, a large number of ternary MWO systems were surveyed to investigate the frequency of occurrence of this structure type. Samples were prepared by heating tungsten oxides and the appropriate ternary element or a suitable compound of the ternary element in evacuated silica ampoules at 1373°K for 1 week. The compositions investigated were close to M0.02W0.98O2.80. Oxides with the Mo5O14 structure were found in many systems across the whole of the periodic table, from Li to Bi. Some aspects of the formation of these phases and the way in which they could affect the course of reduction of WO3 to W metal are discussed.

An electron microscope study of the rhombohedral phase occurring in the Bi2O3BaO system

Abstract The rhombohedral phase which occurs at about 20 mole% BaO in the Bi2O3BaO system has been studied by electron microscopy. While some material seems identical to that found by X-ray diffraction, many crystals show the existence of a superlattice on the electron diffraction patterns. This superlattice is believed to arise as a result of ordering of the Bi and Ba atoms in the structure. Electron micrographs suggest that the ordered regions are small and can be regarded as microdomains within the parent crystal matrix.