Two cations A and B in A 2 BO 4 spinels appear in precise 2:1 Daltonian ratio ("line compounds") only at very low temperature. More typically, at finite temperature, they tend to become either A rich or B rich. Here we survey the experimentally observed stoichiometry asymmetries and describe the first-principles framework for calculating these. Defect calculations based on first principles are used to calculate the enthalpies of substitution of A atom ΔH(AT d ) and B atom ΔH(BO h ) and determine their site occupancies leading to (non)-stoichiometry. In Co 2 ZnO 4 , the result of the calculation for site occupancy compares well with that measured via anomalous x-ray diffraction. Further, the calculated phase boundary also compares well with that measured via Rietveld refinement of x-ray diffraction data on bulk ceramic sintered samples of Co 2 ZnO 4 and Rh 2 ZnO 4 . These results show that Co 2 ZnO 4 is heavily Co nonstoichiometric above 500C, whereas Rh 2 ZnO 4 is slightly Zn nonstoichiometric. We found that, in general, the calculated ΔH(AT d ) is smaller than ΔH(BO h ), if the A-rich competing phase is isostructural with the A 2 BO 4 host, for example, A 2 AO 4 , whereas B-rich competing phase is not, for example, BO. This observation is used to qualitatively explain nonstoichiometry and solid solutions observed in other spinels.
|Original language||English (US)|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - Aug 22 2011|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics