Re: [Ifeffit] charge transfer by SCF in FEFF8 and simulation for charge disproportionation
Dear all, I am doing a SCF calculation by Feff8.40 for a model compound PbTiO3. However, the obtained charge transfer deviates far away from its well-known formal oxidation state. According to common electron negativity data, the oxidation state would be: Pb 2+ Ti 4+ O 2- The SCF run gave out charge transfer for those atoms as following: Pb 0.717 Ti 0.181 O -0.300 After I read through the existing threads on charge transfer in this list, I know that transfered charge FEFF gives out is just a portion of general formal oxidation state. Can we undderstand that this is just ionic portion for entire bonding? Such as for oxygen, "-0.300" is ionic bonding, while the rest of valency (-1.700) exists as covalent bonding, which doesn't have apparent charge transfer by definition. Another related question is then: is there any way in FEFF to take into account this "covalence" part? (maybe deriving from overlapping area of mufftin-potential?) I think this may be crucial for a self-consistent analysis of charge disproportionation. Any comments on this would be greatly appreciated. Attached please see"atom.inp" and "feff.inp". Thanks so much! Ying Ying Zou Research associate,Dr. Phys. Physics Department, UWM ***************atoms.inp*********************************** ! This atoms input file was generated by WebAtoms 1.8 (Atoms 3.0beta10) ! Atoms written by and copyright (c) Bruce Ravel, 1998-2001 title = Perovskite: PbTiO3 space = p 4 m m a = 3.9050 b = 3.9050 c = 4.1560 alpha = 90.0 beta = 90.0 gamma = 90.0 core = Ti edge = K rmax = 4.2950 atoms ! elem x y z tag occ. Pb 0.00000 0.00000 0.00000 Pb 1.00000 Ti 0.50000 0.50000 0.53900 Ti 1.00000 O 0.50000 0.50000 0.11400 Oapical 1.00000 O 0.50000 0.00000 0.61700 Oplanar 1.00000 ****************************************************************** ************feff.inp*************************************** * This feff.inp file generated by ATOMS, version 2.50 * ATOMS written by and copyright (c) Bruce Ravel, 1992-1999 * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * * total mu = 5194.4 cm^-1, delta mu = 761.6 cm^-1 * specific gravity = 7.942, cluster contains 25 atoms. * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * * mcmaster corrections: 0.00093 ang^2 and 0.165E-05 ang^4 * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * -- * TITLE Perovskite: PbTiO3 EDGE K S02 1.0 * pot xsph fms paths genfmt ff2chi CONTROL 1 1 1 1 1 1 PRINT 1 0 0 0 0 3 * r_scf [ l_scf n_scf ca ] SCF 3.88586 0 15 0.1 * ixc [ Vr Vi ] EXCHANGE 0 0 0 EXAFS RPATH 7.77172 * kmax [ delta_k delta_e ] *XANES 4.0 0.07 0.5 * r_fms [ l_fms ] *FMS 3.88586 ******** * *RPATH 0.10000 * emin emax resolution *LDOS -20 20 0.1 POTENTIALS * ipot z [ label l_scmt l_fms stoichiometry ] 0 22 Ti -1 -1 0 1 8 O -1 -1 3 2 82 Pb -1 -1 1 3 22 Ti -1 -1 1 ATOMS 0.00000 0.00000 0.00000 0 Ti 0.00000 0.00000 0.00000 -1.76630 1 Oapical 1.76630 1.95250 0.00000 0.32417 1 Oplanar 1.97923 0.00000 -1.95250 0.32417 1 Oplanar 1.97923 -1.95250 0.00000 0.32417 1 Oplanar 1.97923 0.00000 1.95250 0.32417 1 Oplanar 1.97923 0.00000 0.00000 2.38970 1 Oapical 2.38970 -1.95250 -1.95250 1.91592 2 Pb 3.36084 1.95250 -1.95250 1.91592 2 Pb 3.36084 -1.95250 1.95250 1.91592 2 Pb 3.36084 1.95250 1.95250 1.91592 2 Pb 3.36084 1.95250 -1.95250 -2.24008 2 Pb 3.55563 -1.95250 1.95250 -2.24008 2 Pb 3.55563 -1.95250 -1.95250 -2.24008 2 Pb 3.55563 1.95250 1.95250 -2.24008 2 Pb 3.55563 -3.90500 0.00000 0.00000 3 Ti 3.90500 0.00000 3.90500 0.00000 3 Ti 3.90500 3.90500 0.00000 0.00000 3 Ti 3.90500 0.00000 -3.90500 0.00000 3 Ti 3.90500 0.00000 0.00000 4.15600 3 Ti 4.15600 0.00000 0.00000 -4.15600 3 Ti 4.15600 3.90500 0.00000 -1.76630 1 Oapical 4.28589 0.00000 -3.90500 -1.76630 1 Oapical 4.28589 -3.90500 0.00000 -1.76630 1 Oapical 4.28589 0.00000 3.90500 -1.76630 1 Oapical 4.28589 END *******************************************************************
Hi Ying, I think maybe the problem isn't so much with FEFF as with a century of conventions in chemistry. Until fairly recently, no one really knew where electrons were sitting in real materials. Chemistry "works" pretty well if you do things like assign formal oxidation states and apply octet rules. But as a physicist, you know that the actual wave functions aren't going to be that simplistic. As both theory and experiment advanced, it became very clear that even the most stereotypically ionic substances don't actually consist of isolated ions interacting with their classically assigned charges. I guess you can say that implies a partially covalent character, but electrons are indistinguishable, so there aren't really individual "pairs" of electrons in covalent bonds either. The whole solid is actually a great big messy quantum mechanical entity. That does not mean that models like formal oxidation state, chemical bonds, and the like aren't extremely useful. Personally, I consider myself a chemical physicist, not a solid state physicist, and I much prefer the bonding description to, for example, a model based on "bands" for understanding in my own head what is happening. But the bottom line, to me, is that you aren't going to find those 1.7 electrons that you think should be involved in the bonding somehow in the FEFF SCF calculation, nor do they really belong. FEFF, as I understand it, doesn't have interactions between electrons in different atoms at all (bonding and antibonding orbitals, if you like that kind of description), and thus only needs to know the net charge transfer. "Covalent bonds" have no meaning in FEFF. OK, FEFF experts, how'd I do? :) --Scott Calvin Sarah Lawrence College On Aug 28, 2008, at 4:03 PM, Ying Zou wrote:
Dear all,
I am doing a SCF calculation by Feff8.40 for a model compound PbTiO3. However, the obtained charge transfer deviates far away from its well-known formal oxidation state. According to common electron negativity data, the oxidation state would be:
Pb 2+ Ti 4+ O 2-
The SCF run gave out charge transfer for those atoms as following:
Pb 0.717 Ti 0.181 O -0.300
After I read through the existing threads on charge transfer in this list, I know that transfered charge FEFF gives out is just a portion of general formal oxidation state. Can we undderstand that this is just ionic portion for entire bonding? Such as for oxygen, "-0.300" is ionic bonding, while the rest of valency (-1.700) exists as covalent bonding, which doesn't have apparent charge transfer by definition.
Another related question is then: is there any way in FEFF to take into account this "covalence" part? (maybe deriving from overlapping area of mufftin-potential?) I think this may be crucial for a self- consistent analysis of charge disproportionation.
Any comments on this would be greatly appreciated. Attached please see"atom.inp" and "feff.inp". Thanks so much!
participants (2)
-
Scott Calvin -
Ying Zou