Transcript Document

The Electronic Structure of the Ti4O7 Magneli Phase
Leandro Liborio
Giuseppe Mallia
Nicholas Harrison
Computational Materials Science Group
Magneli Phases
1)
2)
Metal nets in antiphase. (121)r
Cristallographic shear plane.
TnO2n-1 composition. For n between 3 and 9 shear planes are the (121).
Ti4O7 is a semiconductor at T<120K semicond. with 0.25eV band gap (1).
T4O7 Metal-semicond. transition at ~150K, semicond-semicond. trans. at T~135K.
(1) D. Kaplan et al., Philosophical Mag., Vol. 36, pp. 1275, 1977.
(2) P. Waldner and G. Eriksson, Calphad Vol. 23, No. 2, pp. 189-218, 1999.
Ti4O7 Magneli Phase: Electric and Magnetic properties
1)
2)
• Ti4O7 conductivity is higher than the graphite one.
Conductivity of Ti4O7 single crystals
• 3 well-differentiated phases.
• semicond-semicond and semicond-metal transitions.
Material
Resistivity (-cm)
• Exp. Evidence suggests: Charge localization on the
Ti atoms changes at every phase.
Copper
1.7
Ti4O7
500
Graphite
1375
Table (1)
1) J. R. Smith et al, J. Appl. Electroch., 28, pp 1021, (1998).
2) S. Lakkis et al, PRB., 14, pp 1429, (1976).
3) L. N. Mulay et al, J. of Appl. Phys., 41, pp 877 , (1970).
Ti4O7 Magneli Phase: The Bipolaron Model
1)
3)
2)
Charge distribution at
H.T.P.
I.T.P.
L.T.P.
low and intermediate T
Ti-Ti pairs: small on-site localised bipolarons, which are bound states of two Ti+3 ions stabilised by a lattice
distortion.
 In the low T phase the Ti 3d electrons forming the bipolarons were paired in non-magnetic bonds. The
bipolarons were ordered.
 In the intermediate temperature phase the bipolarons disordered.
 In the high temperature phase the bipolarons dissociated and the 3d electrons delocalized.
1) M. Marezio et al, J. Solid. St. Chem., 6, pp 213, (1973).
2) S. Lakkis et al, PRB., 14, pp 1429, (1976).
Ti4O7 Magneli Phase: The Bipolaron Model drawbacks
• No intrinsic EPR signal in the bipolaron model.
1)
Intensity of the EPR signal as a function of T.
• New model for the 140K phase.
This structure shows long range order: the bipolarons are still present, but they are
not disordered (Reference (1)).
1) Y. Le Page et al , J. Solid St. Chem., 53, pp 13, (1984).
2) S. Lakkis et al, PRB., 14, pp 1429, (1976).
Ab inito calculations
CRYSTAL
Hybrid density functional: B3LYP,
GGA Exchange
GGA Correlation
20% Exact Exchange
Local basis functions: atom centred Gaussian type functions.
Ti: 27 atomic orbitals, O: 18 atomic orbitals
Supercell approach.
Ti4O7 structures at the low, intermediate and high temperature
phases taken from M. Marezio et al, J. Solid. St. Chem., 6, pp
213, (1973).
Ti4O7 structure at the different phases taken from Y. Le Page
et al , J. Solid St. Chem., 53, pp 13, (1984).
Ti4O7 low, intermediate and high T structures from Marezio et al (1).
298 K
140 K
120 K
Ti atoms in Ti4O7. View of a (1x2x2) supercell along the tetragonal Ti4O7 unit cell a axis. Spin +/- 0.05e/bohr3
isodensity surfaces.
• At 120K the spin localises in Ti+3 t2g-like orbitals which are antiferromagnetically coupled forming dimmers.
• The 140K and 298K structures are essentially equivalent.
• At 298K the electrons delocalise.
(1) M. Marezio et al, J. Solid. St. Chem., 6, pp 213, (1973).
Ti4O7 low, intermediate and high T structures from Marezio et al (1).
298 K
140 K
Ti atom
120 K
µ = nα-nβ
298 K
140 K
120 K
1
-0.4491
-0.4417
-0.8722
3
0.4199
0.6719
-0.0259
5
0.3311
-0.3796
0.8748
7
-0.3027
0.1263
0.0387
Ti t2g-like spin population in bohr magnetons
Ti4O7 new intermediate T structure from Le Page et al (2)
Ti atom
µ = nα-nβ
120 K
140 K
298 K
Ti ↑
0.8748
0.788
0.4199
Ti ↓
-0.8722
-0.751
-0.4491
• At 140 K the spin is localised in Ti+3 t2g-like orbitals. Only a subset of these Ti+3 ions form
antiferromagnetically bonded pairs.
• The 140 K electronic structure is not a bipolaronic state: there is a mixture of polarons and
bipolarons.
(2) Y. Le Page et al , J. Solid St. Chem., 53, pp 13, (1984).
Interpretation of magnetic measurements
Susceptibility measurements
a)
Ferromagetic.
b)
Flip the spin of half of
the elctrons forming
bipolarons.
c)
Flip the spin of half of
the electron forming
polarons.
Lowest energy for change:
0.1 eV per Ti4O7 unit.
EPR measurements
1)
Ti atom
µ = nα-nβ
120 K
140 K
298 K
Ti ↑
0.8748
0.788
0.4199
Ti ↓
-0.8722
-0.751
-0.4491
Intensity of the EPR signal as a function of T.
Conclusions
• We propose an alternative interpretation of the Ti4O7 electronic structure.
•The Ti4O7 120K phase is an antiferromagnetic charge-ordered semiconducting
state.
• The Ti4O7 120K phase is a bipolaronic state, but the bipolarons are NOT
covalently bonded: the spin localises in t2g-like orbitals belonging to Ti+3 ions, and
these ions are antiferromagnetically coupled.
• According to our calculations, in the new ordered structure for the 140 K phase,
spin localises in Ti+3 t2g-like orbitals. But the 140K state is not a bipolaronic state:
there is a mixture of polarons and bipolarons.
• In the 298K phase electrons delocalise and spin moments decrease their value.
•Our results provide a sensible explanation for the behaviour of the magnetic
susceptibility and EPR measurements with temperature.
Ti4O7 low, intermediate and high T DOS.
298 K
140K
(1) M. Marezio et al, J. Solid. St. Chem., 6, pp 213, (1973).
120 K
Optical Properties
1)
3)
2)
DOS for T<120K
Is the theoretical band gap reasonable?
Some proposed absorption mechanisms:
• Singlet-Triplet mechanism by Lakkis.
• Spin flipping: Antiferro-Ferro
E(120K)Ferro-E(120K)Antiferro = 0.3 eV
• Infrared active phonon modes
1) D. Kaplan et al, Phil. Mag., 36, pp 1275, (1977).
2) S. Lakkis et al, PRB., 14, pp 1429, (1976).
Ti4O7 ground state
2)
• Charge-ordered semiconductor
• 1.25 eV band gap
• Antiferromagnetic state.
• Ti+3 pairs with opposite spin on
each Ti+3: bipolaron.
• Bipolaronic ground state.
Ti atoms in Ti4O7. View of a (1x2x2)
supercell along the tetragonal Ti4O7
unit cell a axis. Temperature T<120K.
3)
Ti pairs
DOS for T<120K
Ti-Ti distances (A)
298K
140K
120K
8-3
3.067
3.101
3.104
8-5
2.812
2.806
2.837
5-1
3.019
2.990
2.802
Ti4O7 high and intermediate temperature phase
298 K
140 K
Lattice parameters (A)
298K
140K
120K
a
5.593
5.590
5.591
b
7.125
7.128
7.131
c
12.456
12.483
12.487
• Semiconductor with charge
distributed homogeneously
• 0.4 eV band gap
• Antiferromagnetic state (smaller
spin values).
• Bipolarons are gone
M. Marezio et al, J. Solid. St. Chem., 6, pp 213, (1973).
Y. Le Page et al , J. Solid St. Chem., 53, pp 13, (1984).
Ti4O7 Magneli Phase: Electric and Magnetic properties
Charge distribution at
low and high T
View of Ti4O7 along it’s a axis. Ti are yellow and
oxygen are red. Only some oxygen octahedra are
plotted.
Ti4O7 Magneli Phase: Alternative Magnetic measurements
2)
Cp/T. The Debye temp is 493. Inset
shows Debye temp as a function od T.
Ti4O7 Magneli Phase: On the controversial semiconductor-metal transition
A. D. Inglis et al, J. Phys. C: Solid State Phys., 16, pp 317, (317).
Ti4O7 Magneli Phase: Optical Properties
Magneli Phases at surfaces
1)
2)
•TiO2(110) wafers were annealed in
UHV for 24 h at 800ºC.
•Sample from yellowish to blue.
•STM at room temperature.
STM image of TiO2(110) surface. Ubias=2V,
IT=0.2nA
3)
4)
Step to step distance is 17.6 Å along [-111]
Periodicity along [-11-1] is 8.8 Å
H. Norenberg et al, Surf. Sci., 402-404, pp 738, (1998).