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Polycrystalline CrO2 is shown to exhibit a giant magnetoresistance (GMR) at low temperatures. A rapid decrease in the GMR with increasing temperature is correlated with a decrease in the intergranular resistance. Single-crystal CrO2 is a half-metallic ferromagnet, as the data are interpreted to reflect two types of charge carriers, crossing a grain boundary: those that tunnel between conduction bands of adjacent grains and those that hop after residing at a localized state for a time long compared to a spin relaxation time.

Fine-particle metal chromites (MCr204, where M = Mg, Ca, Mn, Fe, Co, Ni, Cu, and Zn) have been prepared by the combustion of aqueous solutions containing the respective metal nitrate, chromium(II1) nitrate, and urea in stoichiometric amounts. The mixtures, when rapidly heated to 350°C, ignite and yield voluminous chromites with surface areas ranging from 5 to 25 mZ/g. MgCr204, sintered in air at 1500°C for 5 h, has a density of 4.0 g/cm3. [

Pure and Ru-doped Li[Mn2−xRux]normalO4 ( x=0.1 and 0.25) spinel compounds are prepared by the high temperature solid-state reaction and characterized by X-ray diffraction (XRD), scanning electron microscopy, Brunauer, Emmett, and Teller surface area, density, IR, Raman spectroscopy, and X-ray absorption spectroscopy. XRD studies show single-phase compounds with the cubic spinel structure and a decrease in (Mn–O) and an increase in (Li–O) bond lengths on Ru doping. The electrochemical cycling behavior of Li[Mn2−xRux]normalO4 is examined by galvanostatic cycling and cyclic voltammetry (CV). The CV shows that the normalMn3+↔normalMn4+ couple lies in the range from ∼4.0to∼4.2V and the spinel-to-double-hexagonal (S-DH) transition at ∼4.5V in agreement with results in the literature. The normalRu4+↔normalRu5+ redox couple operates at ∼4.3V vs Li in the spinel structure. The S-DH phase transition is suppressed on Ru doping. Charge–discharge cycling, up to 25 or 40cycles , in the voltage ranges 3.5–4.3 and 3.5–4.6V vs Li at 0.14C rate gave initial charge capacities ranging from 118to139mAhnormalg−1 for various x . Compositions with x=0.1 and 0.25 showed smaller initial capacity loss and smaller capacity fading during cycling as compared to x=0 . For x=0.25 in the voltage range 3.5–4.6V and 2–25cycles , the capacity fade is only 9% thereby showing the beneficial effect of Ru doping, by way of suppression of S-DH transition, participation of Ru-redox couple, and better electronic conductivity of the compound during cycling, in comparison to pure LinormalMn2normalO4 .