STRUCTURE AND PHASE-TRANSFORMATION OF LANTHANUM CHROMATE

Lanthanum chromate (LaCrO4) was synthesized as a low temperature (< 70 0 degrees C) intermediate to perovskite-type, lanthanum chromite (LaCr O3). The lattice parameters, atom positions and bond lengths determine d from X-ray powder diffraction show that LaCrO4 forms a monazite-type crystal structure. Lanthanum chromate forms a solid solution with Ca, having a solubility between in and 20 at %. Thermal analysis shows th at at ambient oxygen pressure, LaCrO4 transforms to LaCrO3 at a temper ature near 700 degrees C. It also indicates that (La, Ca)CrO4 initiall y transforms to LaCrO3 and CaCrO4 with the subsequent formation of a ( La,Ca)CrO3 solid solution. Evidence of Ca segregation to the surface o f (La, Ca) CrO3 particles is given by Auger electron spectroscopy and scanning electron microscopy.