electronic structure of solids - :: dst unit of · pdf fileelectronic structure of solids ......
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2007 T. Pradeep CY306 Lectures
Eigenvalue equation:
Áf(x) = af(x)
KNOWN: Á is an operator.
UNKNOWNS:
f(x) is a function (and a vector), an ‘eigenfunction’ of Á;
a is a number (scalar), the ‘eigenvalue’.
Electronic structure of solids
Ackowledgement: These slides have used materials from the following link:http://www.tyndall.ie/research/computational-modelling-group/cm4107/
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2007 T. Pradeep CY306 Lectures
Problem….
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2007 T. Pradeep CY306 Lectures
PE operator V = ? to describe system, boundary conditions
KE operator T = -( ħ2/2m) d2/dx2,where m is mass and ħ = h/2π
Hamiltonian H = T+V
Time-independent Schrödinger equation, HΨ =E Ψ
Suggest a general solution Ψ(x)
Solve for E
Apply boundary conditions , get quantum number -- n
Specific set of solutions Ψn(x), En
How
do we approach the problem
?
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2007 T. Pradeep CY306 Lectures
eix = exp(ix) = cos(x) + i sin(x) ... wavelength = 2πphase difference = π/2 between sin and cos functions
For other functionseikx = exp(ikx) = cos(kx) + i sin(kx) ... Wavelength = 2π/k
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2007 T. Pradeep CY306 Lectures
Wavefunction for a free electron
Potential energy, V(x) = 0 everywhereKinetic energy, T= -(ħ2/2m)d2/dx2
H = T+V
Schrödinger equation, HΨ=EΨ
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2007 T. Pradeep CY306 Lectures
Solutions:Ψk(x)=[1/√a].exp(ikx) = [/√a][cos(kx)+isin(kx)] are plane waves
k is the quantum number; any value of k is possible for the free electron.
Ek= ħ2k2/2m is kinetic, also the total energy, as V = 0.
Monochromatic: wavelength = 2π/kPrecisely defined momentum, px = ħk and Δpx = 0
py=pz=0, Δpy= Δpz=0
Totally delocalised position: Ψ(x)Ψ*(x) = 1/a = constant over x
ie. Δx = ∞
But Heisenberg uncertainty principle demands, Δpx . Δx ≥ ħ/2
Real beams of free electrons are somewhat localised (Δx ≠∞)It can be represented as a wave packet, with a distribution of momenta (Δpx >0).
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2007 T. Pradeep CY306 Lectures
Question:
Is free electron a good approximation for the electrons in a crystalline solid?
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2007 T. Pradeep CY306 Lectures
Hückel Theory
Aim: A model for π systems such as CnHm
1. Hückel theory is wavefunction-based.MO’s are formed by LCAO
Use the φ=C:2pz as the atomic orbital ‘basis’n φ’s are used LCAO to form n MOs ψj labeled with j=0..(n-1):
Ψj = N.Σm cjmφmsum runs over m = 1 to n, cjm are coefficients to be determined and Nis a normalization constant.
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2007 T. Pradeep CY306 Lectures
2. Hückel theory is semi-empirical.
We evaluate the electronic energy of a system in terms of two integrals.
Hii = ∫ φiHφidτHij = ∫ φiHφjdτ
The values of these integrals are used by fitting. Use experimental data or use higher levels of theory ( ”semi-empirical” method).
For φ = C:2pz we use the AO energy: α= E(C:2pz) = Hii = coulomb integral = ∫ φ1Hφ1dτ = 1050 kJ/mol > 0
Interaction between two AOs:β = Hij = exchange or overlap integral = ∫ φ1Hφ2dτ = E(pz-pz overlap) < 0
Secular determinant
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2007 T. Pradeep CY306 Lectures
Φ = Σn = 1…N cnfn
H is known
Apply variational principle to get energy
Φ = c1f1 + c2f2
∫ ΦH Φdτ = ∫(c1f1 + c2f2) H (c1f1 + c2f2) dτ= c1
2H11 + c1c2H12 + c1c2H21 + c22H22
Hij = Hji
∫ Φ*Φdτ = c12S11 + c1c2S12 + c1c2S21 + c2
2S22
Sij = Sji
E(c1,c2) = c12H11 + c1c2H12 + c1c2H21 + c2
2H22
c12S11 + c1c2S12 + c1c2S21 + c2
2S22
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2007 T. Pradeep CY306 Lectures
Rewrite the eq. Differentiate w.r.t c1 and c2 ∂E/ ∂c1 = 0
c1(H11-ES11) + c2 (H12 – ES12) = 0
Secular determinant.
H11-ES11 H12 – ES12= 0
H21-ES21 H22 – ES22
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2007 T. Pradeep CY306 Lectures
C2H4
C: 2s+2px+2py = sp2 forms σ skeleton for C-C and C-H
C: 2pz form C-C π
2 ‘free’ electrons occupy the π system.
In phase, lower energy than α
Out of phase, higher energy than α
Bonding
Antibonding
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2007 T. Pradeep CY306 Lectures
We have two atoms, two AOs (m) labeled 1 and 2Two MOs (j) labeled 0,1.
ψj=(1/√2)Σmexp[iπj(m-1)].φmEj=α+2βcos(πj)
Consider j = 0, in-phaseψ0=(1/√2)(exp[0].φ1+exp[0].φ2)=(1/√2) (cos[0].φ1+isin[0].φ1+cos[0].φ2+isin[0].φ2)=(1/√2) ([1].φ1+i[0].φ1+[1].φ2+i[0].φ2)=(1/√2) (φ1+φ2)
E0=α+2βcos(πj)= α+2β and
Consider j=1, out of phaseψ1=(1/√2) (exp[0].φ1+exp[iπ].φ2)=(1/√2) (cos[0].φ1+isin[0].φ1+cos[π].φ2+isin[π].φ2)=(1/√2) ([1].φ1+i[0].φ1+[-1].φ2+i[0].φ2)=(1/√2) (φ1 - φ2)
E1=α+2βcos(π) = α-2β
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2007 T. Pradeep CY306 Lectures
Cyclic polyenesFor a general π system of cyclic CnHnThere are n atomic orbitals φ=C:2pz
n MO ψj labeled with j=0..(n-1) formed by LCAO:
ψj=NΣm cjmφmψj=(1/√n) Σm=1..n exp[i(m-1).phase-angle].φm
Ej= α+ 2βcos(phase-angle)
where phase-angle=2πj/n
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2007 T. Pradeep CY306 Lectures
ψj=(1/√6)Σm=1..6 exp[ij(m-1).π/3].φm, j = 0..5e.g. ψ0=(1/√6)Σm=1..6 exp[i0(m-1)].φm=(1/√6)Σm=1..6 (+1)m-1.φm=(φ1+φ2+φ3+φ4+φ5+φ6)/ √6) phase = 0
j=3, ψ3, phase = πψj=(1/√6) Σm=1..6 exp[i.j(m-1).π/3].φm, j=0..5ψ3= 1/√6) Σm=1..6 exp[iπ(m-1)].φm= 1/√6) Σm=1..6 (-1)m-1.φm=(φ1-φ2+φ3-φ4+φ5-φ6 )/√6
Phase changes as 0, π/3, 2π/3,… 5π/3
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2007 T. Pradeep CY306 Lectures
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2007 T. Pradeep CY306 Lectures
For benzene:
Ej= α+ 2βcos(phase-angle)phase-angle=2πj/n
n = 6, j = 0..5
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2007 T. Pradeep CY306 Lectures
Energy as a function of phase angle
Dispersion diagram
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2007 T. Pradeep CY306 Lectures
Density of states
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2007 T. Pradeep CY306 Lectures
Total MO ψ is formed by combining
atomic wavefunctions φ . phase factor exp(iθ).
Because of periodic/cyclic boundary conditions, ψcan be labeled according to phase.
MO energies depend on phase. Because phase is limited (0..2π), the energies are bounded: in the Hückel case, α+2β ≤ E ≤ α-2β.
α is the contribution to the energy from an individual unit φ.
β is due to (i) geometrical arrangement in space; (ii) closeness in energy; (iii) shape, parity of φ.
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2007 T. Pradeep CY306 Lectures
Why are cyclic polyenes relevant for crystalline solids?
Periodicity = cyclic symmetry or translational symmetry.
Consider each C atom as a 1-D unit cell of length a.
Define ‘crystal momentum’ k = phase/a.
Wavefunctions ψk are obtained by combining the wavefunction for asingle cell with the phase factor k for the interactions between cells.
Combining energy for single cell energy (~Hückel α) with interaction betweencells (β and phase factor k) gives a ‘band’ of energies E(k).
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2007 T. Pradeep CY306 Lectures
Block Theorem
Solution of the Schrodinger equation for a periodic potential will be of the type,Ψk(r) = uk(r) exp (ik.r)
Where uk(r) has the period of the crystal lattice.
exp(ik.r) introduces correct phase, where k.r = kxx+kyy+kzz
Solutions continuous at cell boundaries will be chosen.
Ψn,k(x) and En(k) in the range of -π/a ≤ k ≤ π/a (first Billouin zone) will be of significance
The energies spanned for each n will form a band.
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2007 T. Pradeep CY306 Lectures
a) from this origin, lay out the normal to every family of parallel planes in the direct latticeb) set the length of each normal equal to 2p times the reciprocal of the interplanarspacing for its particular set of planesc) place a point at the end of each normal.
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2007 T. Pradeep CY306 Lectures
Energy bands in one dimension
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2007 T. Pradeep CY306 Lectures
Brillouin zone
A Brillouin zone is a Wigner-Seitz cell in the reciprocal lattice.
Wigner-Seitz cell
1. Draw lines to connect a lattice point to all the nearby points2. At the midpoint of these lines draw new lines, normal to these lines3. Smallest volume occupied this way is the Wigner-Seitz primitive cell. All
available space of the crystal can be filled with this.
Brillouin zone
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2007 T. Pradeep CY306 Lectures
Crystal and reciprocal lattice in 1D
a
A
k
O
k = -π/a k = π/aBrillouin zone
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2007 T. Pradeep CY306 Lectures
First Brillouin zone for fcc
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2007 T. Pradeep CY306 Lectures
Dispersion
Each solution un(x) leads to a band of energies En(k). There existforbidden regions of E where boundary conditions can not be fulfilled at any k; these are ‘band gaps’.
En(k) is called the dispersion of the band.
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2007 T. Pradeep CY306 Lectures
Dispersion diagram of pz orbitals for infinite atoms arranged on x or y direction
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2007 T. Pradeep CY306 Lectures
s orbitals
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2007 T. Pradeep CY306 Lectures
px orbitals
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2007 T. Pradeep CY306 Lectures
An arbitrary system
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2007 T. Pradeep CY306 Lectures
Crystal potential splits Ek
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2007 T. Pradeep CY306 Lectures
Symmetry points in the Brillouin zoneorigin
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2007 T. Pradeep CY306 Lectures
References:
C. Kittel, Introduction to Solid State Physics, Wiley Eastern Ltd., New Delhi, 1993.D.A. McQuarrie and J. D. Simon, Physical Chemistry A Molecular Approach, Viva Books Pvt. Ltd. New Delhi, 1998.