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

Phase relationships and thermal expansion have been investigated in the La(Cr, Ni)O3 system by x-ray diffraction at room and at high temperatures, by differential thermal analysis, thermogravimetry, and dilatometry. special attention was paid to the thermal decomposition behavior of LaNiO3 in air and flowing oxygen. The rhombohedral perovskite LaNiO3 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) irreversibly to La2NiO4 plus NiO. LaCrO3 is orthorhombic at room temperature and transforms at 271-degrees-C into a rhombohedral high-temperature phases which persists at least until 1550-degrees-C. The La(Cr, Ni)O3 system shows complete miscibility both at room and at higher temperature. 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 phases 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 measurements. The coefficients of thermal expansion of the rhombohedral high-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).