Marginally Metallic Oxides

CSIR Centre of Excellence in Chemistry Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India and Materials Research Laboratory, University of California, Santa Barbara, 

LaNii Mn,Oj is an example of an oxide system which becomes insulating as x is increased, LaNi03 (x = 0.0) being metallic. I shall discuss briefly the nature of such metal oxides to point out how the I-M transition occurs across a state which can be considered as barely metallic and yet obey the known criteria for metallicity.t 

Fig 2 Resistivities of some of the oxides exhibiting marginal metallicity. Data on metallic Nb.Sn. Pd and Cu are shown for comparison. 

Compositionally controlled metal-insulator transitions in perovskite oxides 

In Fig. 4 we show typical compositionally controlled I-M transitions in two perovskite oxide systems. In La3 — ASrl( oO , the material becomes metallic as x is increased beyond a critical composition. A similar behaviour is seen in La3 - Sr VO-j. Metallicity arises in these oxides because of fast hopping of 

FIgl Resistivities of higjily conducting (metallic) oxides 

2 Resistivities of me of the oxides exhibiting margii mmllicity. Data on metallic NbsSn, Pd and Cu ase shown for comi scm. 

Another criterion for metallicity arises from electron localization induced by disorder proposed by Anderson. Electrons diffuse when the disorder is small, but at a critical disorder they do not diffuse (giving rise to zero conductivity). A transition from the metallic to the insulating state occurs as the disorder increases. Mott proposed that there exists a minimum metallic conductivity, a j , or maximum metallic resistivity, pmax. for which the material may still be viewed as being metallic, prior to the localization of electrons due to disorder. The Omin is given by where I is the mean free path of the electrons. Mott s 

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Fig. 12 Complexity of the problem of marginal metallicity (adapted from ref. 27). The oxides discussed in this article fall somewhere in the three-dimensional space indicated here. The other factors include electron-lattice interaction, magnetic polaron and finite temperature effects.

Aluminum oxide is the only trivalent oxide that has been used to form a ceramic some heat treatment is needed. Kingery claims to have observed a setting reaction between trivalent iron oxide and phosphoric acid, but this reaction may have been caused by traces of magnetite in the trivalent oxide. Pure trivalent iron oxide such as hematite (Fe203) does not react with phosphoric acid. Overall, trivalent metal oxides have a solubility that is only marginal and falls below that of even sparsely soluble divalent oxides, while the solubility of oxides of most quadrivalent metals (zirconium is an exception) is too low to form a ceramic. 

It was pointed out in the introduction to this chapter that an experimental criterion for metallicity is the observation of a positive temperature coefficient to the electrical resistivity. The so-called bad or marginal metals are those that meet this criterion, but in which the value for the resistivity is relatively high (p > 10 flcm). Many transition metal oxides behave in this manner, while others (e.g. ReOs and RuOa) have very low electrical resistivities, similar in scale to those of conventional metals (p < 10 " O cm). Consider the Ruddlesden-Popper mthenates. Both strong Ru 4d-0 2p hybridization and weaker intrasite correlation effects compared to the 3d transition metals are... 

These plots are near straight lines since A//° and AS° are nearly constants (AF° = AH° -TAS°). There are points of slope changes at the temperatures of transformation (of metal and metal oxide) - these are not shown in the sample plot. The slope change is marginal when there is solid to liquid transformation, but is pronounced when transformation to gas occurs. 

Plasters. External applications that possess sufficient consistenco not to adhere to tho fingers when cold, but which become soft and adhesive at tho temperaturo of the human body. Plasters are chiefly composed of unctuous substances united to metallic oxides, or to powders, wax, or resin. They arc usually formed whilst warm, into 4 pound rolls about 3 or 9 inches long, and wrapped in paper. TThen required for use, a little iw melted off the roll by means of a heated iron spatula, and spread upon leather, linen, or silk. The less adhesivo piasters, when spread, arc usually surrounded with a margin of resin plaster, to cause them to adhere. lu the preparation of plasters, tho heat of a watcr-uatn, or steam, should bo alone employed,... 

X-ray diffraction analysis is used routinely by every catalyst manufacturer to determine the phase composition of the catalysts produced and the size of coherently scattering domains, and hardly needs a detailed description. An aspect that we would like to emphasize concerns the influence of the enviromnent on the oxidation state of carbon-supported metal nanoparticles. Quite often, authors try to correlate electrochemical performance with the phase composition of as-prepared samples. It has, however, been demonstrated convincingly in a number of publications by both x-ray diffraction [155] and x-ray absorption spectroscopy [156] that as-prepared fuel cell catalysts and samples stored under ambient conditions are often in the form of metal oxides but are reduced under the conditions of PEMFC or DMFC operation. The most dramatic changes are observed for samples with high metal dispersions, while larger particles are affected only marginally [17]. One should keep in mind, however, that the extent of the particle oxidation depends critically on the preparation procedure. 

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