COMPOUND AND SOLID-SOLUTION FORMATION, PHASE-EQUILIBRIA AND ELECTRICAL-PROPERTIES IN THE CERAMIC SYSTEM ZRO2-LA2O3-TA2O5

One new compound, ZrLaTa3O11 has been synthesized in the system ZrO2-La2O3-Ta2O5, with an orthorhombic unit cell, a = 10.890(3) angstrom, b = 12.450(3) angstrom and c = 6.282(2) angstrom. Extensive ternary solid solutions are formed by five of the binary phases: La2Zr2O7, La3TaO7, LaTaO4, LaTa3O9 and LaTa5O14. Two main solid-solution mechanisms are in evidence: La + Ta arrow pointing right over arrow pointing left 2Zr and La + 3Zr arrow pointing right over arrow pointing left 3Ta. The solid-solution phase previously described as being based on Ta2Zr6O17 has, instead, the ideal stoichiometry, TaZr2.75O8. A significant correction is made to the unit cell dimensions of La3TaO7. The subsolidus phase diagram of the ZrO2-La2O3-Ta2O5 system has been determined at 1500-degrees-C. The electrical conductivity of a selection of the phases has been determined. Most are very poor semiconductors, but one, LaTa3O9, has a high conductivity, 10(-4)-OMEGA-1 cm-1 at 400-degrees-C, which appears to bedue to oxide ions. ZrLaTa3O11 by contrast is an excellent insulator with a resistivity greater-than-or-equal-to 10(6)-OMEGA-cm at 800-degrees-C.