Thermoelectric Properties of Fe0.2Co3.8Sb12-xTex ... - Physics

Study of Phonon anharmonic effects in pyrochlores
P K Verma ∗ , U V Waghmare † , A K Sood ‡ , H R Krishnamurthy §
Deparment of Physics, Indian Institute of Science, Bangalore, India
Jawaharlal Nehru Center for Advanced Scientific Research, Bangalore, India
Structural and electronic properties of ideal pyrochlores with the composition Y 2 B 2 O 7
(where B = Ti, Zr, and Ir) were studied using the first-principles calculations. Both Y 2 Ti 2 O 7
and Y 2 Zr 2 O 7 are insulators while Y 2 Ir 2 O 7 is metal. A large anomalous Born effective charge
was observed for Ti, attributing to the hybridization between the occupied 2p states of the
oxygen and unoccupied d states of the B cation. Density functional perturbation theory
calculations were performed to obtain the phonon properties. While all the phonon frequencies
of Y 2 Zr 2 O 7 and Y 2 Ir 2 O 7 were found to be real valued as one would expect, Y 2 Ti 2 O 7
showed instabilities with respect to some optical distortions, in that 6 of the frequencies
were found to be imaginary. This is likely to underlie the anomalous temperature dependent
of the phonons that have been seen in other titanate pyrochlores. In a study as to how to
stabilize Y 2 Ti 2 O 7 , we found that the system becomes stable at 7GPa at the Γ point, but
remains unstable at other k-points in the Brillouin Zone; it becomes stable at all k-points
at 12GPa. Small distortions of the atomic positions inside the unit cell can also stabilize
the structure at ambient pressure. Phonon-phonon anharmonic effects are very important in
titanate pyrochlores as have been experimentally seen in other titanates. We have treated
the effects of these upto third order anharmonic corrections. We found phonon anomalies
for some of the phonon modes of Y 2 Ti 2 O 7 , in qualitative agreement with experiments.
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Electronin address: pramod@physics.iisc.ernet.in
Electronin address: waghmare@jncasr.ac.in
Electronin address: asood@physics.iisc.ernet.in
Electronin address: hrkrish@physics.iisc.ernet.in