Percolative phase separation induced by nonuniformly distributed excess oxygens

The zero-field $^{139}$La and $^{55}$Mn nuclear magnetic resonances were studied in $\rm La_{0.8}Ca_{0.2}MnO_{3+\delta}$ with different oxygen stoichiometry $\delta$. The signal intensity, peak frequency and line broadening of the $^{139}$La NMR spectrum show that excess oxygens have a tendency to concentrate and establish local ferromagnetic ordering around themselves. These connect the previously existed ferromagnetic clusters embedded in the antiferromagnetic host, resulting in percolative conduction paths. This phase separation is not a charge segregation type, but a electroneutral type. The magnetoresistance peak at the temperature where percolative paths start to form provides a direct evidence that phase separation is one source of colossal magnetoresistance effect.Comment: 4 pages, 5 figure

Exchange coupling in CaMnO3_3 and LaMnO3_3: configuration interaction and the coupling mechanism

The equilibrium structure and exchange constants of CaMnO$_3$ and LaMnO$_3$ have been investigated using total energy unrestricted Hartree-Fock (UHF) and localised orbital configuration interaction (CI) calculations on the bulk compounds and Mn$_2$O$_{11}^{14-}$ and Mn$_2$O$_{11}^{16-}$ clusters. The predicted structure and exchange constants for CaMnO$_3$ are in reasonable agreement with estimates based on its N\'eel temperature. A series of calculations on LaMnO$_3$ in the cubic perovskite structure shows that a Hamiltonian with independent orbital ordering and exchange terms accounts for the total energies of cubic LaMnO$_3$ with various spin and orbital orderings. Computed exchange constants depend on orbital ordering. UHF calculations tend to underestimate exchange constants in LaMnO$_3$, but have the correct sign when compared with values obtained by neutron scattering; exchange constants obtained from CI calculations are in good agreement with neutron scattering data provided the Madelung potential of the cluster is appropriate. Cluster CI calculations reveal a strong dependence of exchange constants on Mn d e$_g$ orbital populations in both compounds. CI wave functions are analysed in order to determine which exchange processes are important in exchange coupling in CaMnO$_3$ and LaMnO$_3$.Comment: 25 pages and 9 postscript figure

Materials for energy conservation and storage

Some areas of energy use where improved materials could lead to energy savings are surveyed. Current energy use patterns and their likely evolution to the end of the century are also reviewed so that particular applications may be seen in perspective against overall energy consumption and other energy conservation measures. Energy conservation is viewed both from the point of view of energy inputs to materials and how process improvements might permit savings, and also by considering general modes of energy use--transport and combustion in particular--and assessing some general ways in which conservation could be achieved. Areas such as improved engine-management of internal combustion engined vehicles and improved batteries for electric vehicles, where progress depends very much on materials developments, are surveyed in more detail. The need for improved sensors to measure exhaust gas composition in vehicles is one particular aspect of a general need for improved monitoring to permit better quality control and hence optimal fuel input for industrial processes requiring heat. Energy storage procedures are surveyed, together with a discussion of electricity generating methods and future electricity use. Attention is given to current fuel cell programmes, which offer the potential of increased conversion efficiency of fossil fuel to electricity compared with conventional power stations but where materials problems are critical in determining commercial success, to heat storage materials, and to water electrolysis and hydrogen production as a means of energy storage. The utilisation of electrochemical processes in solids or at surfaces or interfaces is a general theme underlying many of the technologies discussed.

Anhydrous deuterium beta alumina Powder diffraction studies at 4.2K, 298K, 573K and 720K

SIGLEAvailable from British Library Document Supply Centre- DSC:9091.9(MPD/NBS--279) / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Thermodynamic assessment of the La-Fe-O system

ISSN:1547-7037ISSN:1863-734