perovskit trends

8/8/2019 Perovskit trends

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JOURNAL CLUB


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Perovskite Oxides (AMO3)

Not many theory papers discussing formation energies on this subject?

Problems with describing strongly correlated systems has prevented its

use for estimation of properties such as band gaps and electron

localization-delocalization for oxides.

Formula: A2B4O3


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Method details

All DFT calculations performed with the plane wave code DACAPO using RPBE-GGA exchange-correlation functional.

DACAPO uses ultrasoft pseudopotentials to represent the ion-electron interaction.

Atomic relaxations were done with the quasi-Newton minimization scheme

until a maximum force below 0.05 eV1 between atoms was reached.

Spin-polarized calculations were carried out when needed.


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Gibbs energy of O2 is are not at all well reproduced by standart DFT methods.

It is indirectly obtained from the tabulated Gibbs energy of formation of water

and from DFT energies of H2and H2O

In the calculation of the Gibbs energies of formation we neglect the entropy(S) and zero point energy effects (ZPE) in the bulk crystals (Gcrystal

EDFTcrystal),and consider only those of gas-phase molecules.

Energy of formation:


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Excellent agreement between experiment and theory in this case.

Speculate that the localized nature of d-electrons of 3d metals is not well

captured at this level of theory due to their self interaction. This probably

explains a constant shift of approx 0.75 eV


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In fact, all families of perovskites exhibit alinear scaling between their energetics

and the atomic number of M.

Energies of formation are known to

scale with M in La perovskites at

1273K (experimental)

The slope of the lines is determined

by the oxidation state of A and M.

Can be divided into two groups:

1)Perovskites with same oxidation state for A

and M (Y,La) .

2)Perovskites in which their oxidation states

differ.(Ca/Sr/Ba)


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Origin of Stability

Solution:

Formation energy of perovskites Vs d-energy

Thus the slope should approx. be -0.5

Approx: -0.5 for M4+ and -0.44 for M3+ from figure

Independent of d-orbital

of metal! Makes little

sense!


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Term in brackets:A in an A2O3 lattice replaced by M to form the perovskitestructure.

Formation of A2O3 Lattice deformation Swapping between A and M

Origin of the differences in stability of AMO3 (A=M3+)

Known experimentally for La and Y

3.3 eV for La2O3, calculated as the energy difference in space groups

P3m1 and Pm3m with a perovskite lattice constant of 3.97 .

Approximately 4.1 eV for Y2O3, calculated as the change from

space group Ia3 to Pm3m with a lattice constant of 3.88 .

Interchange between A and d-block metal atoms implies breaking

and creation of bonds, so this can be estimated as the difference

in the d-band centers of these atoms.


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A constant that collects formation energiesof sesqui-oxides and its deformation is around -13.6 eV for La2O3and -13.7 eV for Y2O3

The differences in energies among these perovskites can be attributed to the

relative ease of swapping atoms, whereby Ti is the easiest and Cu the hardest along

the 3d metal series


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Conclusion

DFT gives sufficient atomic-scale insight into perovskites to study their formation

energies, both qualitatively and quantitatively.

Important trends are found.

Such analysis could be extended to perovskites with 4d and 5d constituents,

alkaline earth elements and ScMO3

Pourbaix diagrams for perovskites can be constructed, which are not yet

available in the literature.

This and other stability considerations are of paramount importance in any

application,especially if perovskites are to be used in alkaline or proton-exchange

membrane fuel cells.

THANK YOU!!

perovskit trends

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