Positron and positronium annihilation as probes to identify cation substitution effects in CoCr_xFe_(2-x)O_4

摘要

CoCr_xFe_(2-x)O_4 samples with several values of x between 0.0 and 2.0 were prepared by air oxidation of aqueous suspension containing Co~(2+), Cr~(3+) and Fe~(3+)ions and characterized by X-ray diffraction. Positron lifetime and coincidence Doppler broadening (CDB) measurements were carried out to identify the defects at the different stages of substitution. Initially the vacancy-type defects at the A-sites (tetrahedral) trapped positrons but, with the substitution of Fe~(3+)by Cr3* ions in low concentrations (x ≤ 0.7), positrons are trapped by defects at the B-sites (octahedral). The cationic distributions at B-sites are stoichiometry-dependent and, till x = 0.7, the deficiency of Fe~(3+)(B) ions was compensated by interchange of Fe~(3+)(A) ions with Co~(2+)(B) ions. Between x = 0.9 and 1.7, the substitution resulted in continued decrease of Fe~(3+)(B) ions and the structure got fully transformed into a normal spinel configuration during x = 1.8 to 2.0.