Phase transition and electrical conductivity in the system YMnO3-CaMnO3

Solid solution formation between the ferroelectric hexagonal compound YMnO3 and the semiconducting perovskite compound CaMnO3 has been studied in the Y-rich region of the pseudobinary system. The materials were prepared by reaction of the corresponding oxides and the calcium carbonate for obtaining the corresponding phases and subsequent sintering of the pressed powders in the 1400-1525 degrees C-temperature range. Their ceramic characteristics were established. Samples with relative density >98% D-th were obtained. The hexagonal symmetry of the pure yttrium manganite changes to an orthorhombic one, isostructural with that of the GdMnO3 compound, for similar to 32 mot % CaMnO3. The incorporation of the Ca cation causes an increasing in the compactness of the lattice cell, promoting a rise in the apparent density, despite of the smaller atomic weight of the Ca cation against that of the Y one, until the transition to the orthorhombic lattice occurs. Subsequent increase in the Ca amount lead to a decrease in the theoretical density. The electrical conductivity of the solid solutions increases strongly from 100 to the 90/10 Y/Ca composition, and more moderately for higher Ca amounts. Conductivity values as high as 10(-2) S . cm(-1) were measured on the denser samples. The activation energy for conduction suffers a corresponding decrease when the Ca amount is increased. (C) 1999 Kluwer Academic Publishers.