Soft modes in strained and unstrained rutile TiO2
2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 13, 134303- p.Article in journal (Refereed) Published
We have studied the lattice dynamics of crystalline TiO2 using density-functional perturbation theory and the local-density approximation in a plane-wave pseudopotential formalism at equilibrium and uniaxially strained geometries. We present well-converged calculations of the dispersion curves, which sample a more complete volume of the Brillouin zone than in previous studies. We find an anomalously soft TA mode in a region of reciprocal-space previously unexplored either by any previous calculation or experiment. This is quite separate from the A2u mode which becomes soft at the point and is responsible for the incipient ferroelectric behavior. The harmonic frequency of the soft TA mode around q = (1/2, 1/2, 1/4) decreases to zero under an isotropic expansion with a strain slightly above 0.5% and we suggest that it may be possible to observe anomalous diffuse inelastic scattering corresponding to a dynamical instability using neutron scattering. In addition to the softening under isotropic strain, the frequency of this mode goes to zero under uniaxial strain along the  direction in both compression and expansion (at close to -0.5% and + 1.0%, respectively which offers new possibilities for experimental tests of softening under compressional strain. We further suggest that the soft TA mode may help explain the anomalously long-ranged convergence observed in previous calculations on slab models of the TiO2 (110) surface by providing a mechanism for small changes in bonding at the surface to propagate deep into the bulk. The behavior of other modes under strain was also studied. The ferroelectric A(2u) mode frequency is nearly independent of  strain, which contrasts with the behavior in response to  strain reported in the literature of a strong dependence. However, the frequency of the Raman-active B-2g mode does decrease to zero frequency under 1.3% strain, which should be observable using Raman spectroscopy.
Place, publisher, year, edition, pages
2010. Vol. 81, no 13, 134303- p.
Condensed Matter Physics
IdentifiersURN: urn:nbn:se:kth:diva-27929DOI: 10.1103/PhysRevB.81.134303ISI: 000277207900033ScopusID: 2-s2.0-77955195052OAI: oai:DiVA.org:kth-27929DiVA: diva2:383126
QC 201101042011-01-042011-01-032011-02-15Bibliographically approved