CRYSTAL-CHEMISTRY AND THERMAL-BEHAVIOR IN THE LA(CR, NI)O3 PEROVSKITESYSTEM

Phase relationships and thermal expansion have been investigated in th e La(Cr, Ni)O3 system by x-ray diffraction at room and at high tempera tures, by differential thermal analysis, thermogravimetry, and dilatom etry. special attention was paid to the thermal decomposition behavior of LaNiO3 in air and flowing oxygen. The rhombohedral perovskite LaNi O3 decomposes above 825-degrees-C both in air and oxygen to La4Ni3O10 plus NiO and above 1196-degrees-C in air (1291-degrees-C in oxygen) ir reversibly to La2NiO4 plus NiO. LaCrO3 is orthorhombic at room tempera ture and transforms at 271-degrees-C into a rhombohedral high-temperat ure phases which persists at least until 1550-degrees-C. The La(Cr, Ni )O3 system shows complete miscibility both at room and at higher tempe rature. In orthorhombic LaCrO3, up to 60% of the Cr can be replaced by Ni at room temperature, while samples containing 70 mole percent (m/o ) LaNiO3 or more are rhombohedral. The orthorhombic-rhombohedral phase s transition temperature LaCrO3 increases with the incorporation of Ni to a maximum value of 346-degrees-C at 20-30 m/o LaNiO3 content. The transition temperatures then decrease, as observed by dilatometer meas urements. The coefficients of thermal expansion of the rhombohedral hi gh-temperature phases are always higher than those of the orthorhombic low-temperature phases (around 10 . 10(-6)-degrees-C-1 and 6 . 10(-6) -degrees-C-1, respectively).