Structure and phase transformation of lanthanum chromate

Lanthanum chromate (LaCrO4) was synthesized as a low temperature (< 700 degrees C) intermediate to perovskite-type, lanthanum chromite (LaCrO3). The lattice parameters, atom positions and bond lengths determined 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 that at ambient oxygen pressure, LaCrO4 transforms to LaCrO3 at a temperature near 700 degrees C. It also indicates that (La, Ca)CrO4 initially transforms to LaCrO3 and CaCrO4 with the subsequent formation of a (La,Ca)CrO3 solid solution. Evidence of Ca segregation to the surface of (La, Ca) CrO3 particles is given by Auger electron spectroscopy and scanning electron microscopy.