TY - JOUR
T1 - Asymmetric cation nonstoichiometry in spinels
T2 - Site occupancy in Co
2
ZnO
4
and Rh
2
ZnO
4
AU - Paudel, Tula R.
AU - Lany, Stephan
AU - D'Avezac, Mayeul
AU - Zunger, Alex
AU - Perry, Nicola H.
AU - Nagaraja, Arpun R.
AU - Mason, Thomas O
AU - Bettinger, Joanna S.
AU - Shi, Yezhou
AU - Toney, Michael F.
PY - 2011/8/22
Y1 - 2011/8/22
N2 -
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.
AB -
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.
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U2 - 10.1103/PhysRevB.84.064109
DO - 10.1103/PhysRevB.84.064109
M3 - Article
AN - SCOPUS:80052360173
SN - 1098-0121
VL - 84
JO - Physical Review B - Condensed Matter and Materials Physics
JF - Physical Review B - Condensed Matter and Materials Physics
IS - 6
M1 - 064109
ER -