Metal Oxide Crystals Research Articles

Structure determination of inorganic structures by HREM, Cip, and electron diffraction

High resolution electron microscopy (HREM) combined with crystallographic image processing (CIP) is becoming a powerful technique for solving inorganic structures. With the image processing systems CRISP and ELD, running on a personal computer, this technique can be easily established in other laboratories. HREM images and electron diffraction patterns are digitized by a CCD camera and transferred into a PC. Phases and amplitudes are extracted from the Fourier transform of the HREM images. For thin crystals of metal oxides, the phases obtained by HREM and CIP inside the Scherzer resolution of the microscope are identical to the x-ray structure factor phases.Electron diffraction extends to much higher resolution than EM images (beyond 1 Å). The quality of the amplitudes is also higher than that from images, since ED data is not affected by the contract transfer function (CTF). Amplitudes extracted by ELD are close to x-ray diffraction amplitudes (within 30%).

金属酸化物中の拡散 エレクトロニクス材料を中心として ２ 金属酸化物結晶中の拡散

金属酸化物中の拡散 エレクトロニクス材料を中心として ２ 金属酸化物結晶中の拡散

New routes in the synthesis of metal oxides, II

Abstract Single crystals of numerous new metal oxides “rich in cations” are prepared by using methods such as dis- or conproportionation, thermal decomposition of higher valence oxides, or oxidation of metals and intermetallic compounds (“reactions with the wall”). Exchange reactions allow growth of single crystals even outside of thermodynamic equilibrium. The role of vacancies in structural chemistry as well as in “tailor-made” syntheses is emphasized and illustrated. Molecular aspects of solid state chemistry are demonstrated by cutting chains or rings and by dimerization of small entities. Many examples are provided.

New routes in the synthesis of metal oxides, I

Single crystals of numerous new metal oxides “rich in cations” are prepared by using methods such as dis- or conproportionation, thermal decomposition of higher valence oxides, or oxidation of metals and intermetallic compounds (“reactions with the wall”). Exchange reactions allow growth of single crystals even outside of thermodynamic equilibrium. The role of vacancies in structural chemistry as well as in “tailor-made” syntheses is emphasized and illustrated. Molecular aspects of solid state chemistry are demonstrated by cutting chains or rings and by dimerization of small entities. Many examples are provided.

Design and operation of the arc transfer furnace

The arc transfer method of crystal growth is a relatively simple technique for obtaining large single crystals of transition metal and rare earth oxides. A design for the arc transfer furnace is presented followed by a discussion concerning arc stability during the growth process. The growth and crystal quality of NiO is discussed as an example.

Mesures de Hall AC dans les Oxydes Métalliques

An apparatus has been developed for AC measurements of the Hall constant in high impedance and low mobility materials. For such systems, our method gives better results than the DC method almost universally used. Reasons for this are given in the paper.The apparatus has been used for a detailed study of Hall mobility, at room temperature, of four metal oxide crystals with widely different charge carrier densities. Contrary to expectation, the mobility is found to decrease with increasing density.A polaron mass (in the low charge density limit) is calculated for titanium dioxide at room temperature, using the measured mobility.

Growth of Single Crystals of Cuprous Oxide in Silica Gels at Near Ambient Temperatures

CRYSTAL growth in gels is becoming increasingly recognized as a promising technique for the preparation of single crystals of substances which are slightly soluble in water and which cannot be conveniently grown from the melt or from the vapour1. The growth of single crystals of many metal oxides is of considerable interest, particularly for basic studies of their electronic properties and other pertinent characteristics. Single crystal growth in gels at near ambient temperatures offers a possibility of obtaining specimens free from thermal strains and the impurities frequently associated with high temperature preparative methods. The preparation of single crystals of metal oxides in gels at low temperatures has not previously been successfully achieved, probably because of a strong tendency of metal oxides to form hydrated polycrystalline or amorphous precipitates in such media. We report the growth of single crystals of cuprous oxide in silica gels at near ambient temperatures by the controlled reduction of cupric sulphate with selected mild reducing agents such as hydroxylamine hydrochloride and hypophosphorous acid.

Growth of single crystals of transition metal oxides by the arc-transfer method

A new method for preparing single crystals of refractory oxides by the direct transport of material across the gap of a d.c. arc has recently been described by Drabble and Palmer. We report here observations on the growth characteristics of NiO, CoO, Fe 3 O 4 and Mn 3 O 4 crystals and other oxides by this method including the influence of the ambient atmosphere and Li doping. CoO and Fe 3 O 4 have been examined in detail in an attempt to understand the growth process. The transfer mechanism remains obscure, but over a limited range of arc-currents the variation of ingot diameter can be explained by assuming that the heat losses are radiation limited. The linear growth rates are generally small (≁ 4 mm/hour) and independent of current over most of the lower range. Some practical difficulties and limitations of the method are also discussed.

Growth of transition metal oxide crystals by halide vapour hydrolysis

Single crystals of NiO, CoO, FeO, Co3O4, and Fe3O4 have been epitaxially deposited on MgO single-crystal substrates by the hydrolysis of anhydrous metal bromides in the gaseous phase. All of the crystals were grown at temperatures below 700° C and at a total pressure of less than 25 torr. The deposition was usually complete after approximately 30 min, and the crystals were of uniform thickness with a glassy-clear, smooth surface. Depending on the number of depositions, these crystals can be formed in various thicknesses ranging from approximately 5 μm to 1 mm. The crystals were found to deposit in parallel alignment with the (100) cleavage plane of MgO. A substantial increase in chemical purity is observed, and only 103 to 105 dislocations/cm2 were observed in all NiO crystals. A reaction mechanism is proposed which involves: (i) the formation of a hydroxybromide transport phase with fluid properties; (ii) adsorption of the transport phase on the MgO substrate; followed by (iii), the chemical disproportionation-crystallisation of the intermediate phase to the oriented, solid oxide phase.

An Interpretation of the Selective Photo-Electric Effect from Two-Component Cathodes

Evidence is produced to support the view that photoelectrically selective, two component cathodic surfaces are crystalline in nature. Then, assuming that Fowler's equation for the energy of electrons selectively transmitted through a single potential valley [$W=(\frac{{n}^{2}{h}^{2}}{8m{d}^{2}})$] is equally valid for the energy of electrons selectively transmitted through the periodic sequence of valleys characteristic of the potential field within a crystal, and that all of the energy of photoelectrons is acquired from the incident light quanta, the wave-lengths of light to which such a surface should respond selectively can be computed. Such computations have been made with $d$ equal to the internuclear distance between electro-positive ions in the lattice structure of alkali metal hydride, oxide and sulphide crystals. The hydride crystals belong to the sodium chloride type and the oxide and sulphide crystals are supposedly of the calcium-fluoride type. The correlation between these computed values and the positions of the observed selective maxima is exceptionally good. Moreover, the fact that the alkali metal hydrides exhibit but one selective maximum and the oxides two or three maxima is in keeping with the geometry of their respective crystalline types.