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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Chemical bonding in solids from ab-initio calculations -I

24 March 2011; V172

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Transfer of electron from electropositive ion to the electronegative ion

Coulomb interactions plays an important role

Size of the cation reduces and size of anion increases

Electro-negativity difference between ions decide the strength of ionic bonding.

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Electrons moved from ions and distributed uniformly in the interstitial regions.

The electrons in the interstitial regions move freely around the crystal.

Good conductors and ductile

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Charge Density Distribution in TiC – Mixed iono-covalent bonding

Depleted Charge at Ti site indicate the electron transfer from Ti to C – ionic bonding

Finite charge in between atoms and their nonspherical distribiution show the covalent bonding.

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Charge transfer plot

Diamond (covalent)

Nb ( metallic)

NaCl (ionic)

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

AlP (polar)

eAl (metallic)

elf=0.5

NaCl (ionic)

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Charge density

Formation of molecularlike H-Ni-H (NiH2 ) sub units

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Charge transfer

(r)=(r)comp. - (r)atomic

Ionic bonding between H and the host lattice

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

ELFThe polarization of negative charges at H towards the electropositive La and In

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Charge density Charge transfer ELF

Ravindran et al. PRL (2002)

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Chemical bonding

Charge density Charge transfer ELF

KMgH3 – example for dominant ionic bonding

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Charge density Charge transfer ELF

La3+Sc3+O36-

It is not a perfect ionic material and has finite covalency

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Crystal structure of Ca3 Co2 O6

Rhombohedral (R-3c space group) with 22 atoms/unit cell

Alternating face-shared octahedra and trigonal prisms form one- dimensional chains along c-axis

Hexagonal with 66 atoms /unit cell

Co spins are arranged along c-axis

For every two-up spin chain, there is one down-spin chain resulting in Ferrimagnetic structure

One-dimensional Oxides

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Charge density and related features in Ca3 Co2 O6

Charge density Charge transfer ELF

Very weak interaction between Co1 and Co2

Magnitude of covalent interactions between Co1-O and Co2- O are different .

There is strong covalent interaction within the chain and ionic interaction between Ca and the chains.

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Bonding study from density of states analysis

Ca is in completely ionizedstate, donating almost all itsvalence electrons to O.

Co is in two different oxidation and/or spin states, seen from difference in the topology of DOS curves.

Oxygen is energetically degenerate with Co1 and Co2 forming strong covalent bond with them.

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Partial Density Of States in KGaH4

- Well separated Ga-s and -p state

- Energetically degenerated H-s and Ga-p states

- Strong covalent bonding Ga-H

- Ionic bonding between K and H

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Charge Density Distribution in KGaH4

0.2

0.15

0.1

0.05

0.001

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Lone pair electrons in BiMnO3

Seshadri and Hill, Chemistry of Materials, 13, 2892 (2001)

Visualization of lone pairFrom ELF

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

ELF isosurface (0.7) show that Bi lone-pair electrons are responsible for ferroelectricity.

Ravindran et al. PRB (2006)

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Chemical bonding in BiFeO3 from DOS analysis

Isolated Bi-s is the lone pair electrons.

Negligible Bi electrons in the valence band indicate ionic bonding

Degenerate Fe-d and O-p states indicate the covalent bonding.

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P.Ravindran, FME-course on Ab initio Modelling of solar cell Materials 24 February 2011 Chemical bonding in solids from abinitio calculations -I

Charge density Charge Transfer ELF