Polaron-induced phonon localization and stiffening in rutile TiO2

Abstract

Small polaron formation in transition metal oxides, like the prototypical material rutile TiO 2 , remains a puzzle and a challenge to simple theoretical treatment. In our combined experimental and theoretical study, we examine this problem using Raman spectroscopy of photo-excited samples and real-time time-dependent density functional theory (RT-TDDFT), which employs Ehrenfest dynamics to couple the electronic and ionic subsystems. We observe experimentally the unexpected stiffening of the A 1g phonon mode under UV illumination and provide a theoretical explanation for this effect. Our analysis also reveals a possible reason for the observed anomalous temperaturedependence of the Hall mobility. Small polaron formation in rutile TiO 2 is a strongly non-adiabatic process and is adequately described by Ehrenfest dynamics at time scales of polaron formation. * corresponding author 1 arXiv:1612.05679v2 [cond-mat.mtrl-sci] 1 Sep 2017

Optimum in the thermoelectric efficiency of nanostructured Nb-doped TiO2ceramics: from polarons to Nb–Nb dimers

“…5,6,[38][39][40][41][42][43][44][45] However, Nb doping turns the metallic character of TiO 2 into a thermally-activated electrical conduction which is commonly assumed to originate from the polaronic nature of the Ti 3+ centers, the transport of which transport is characterized by effective masses as large as a hundred times the free electron mass. 19,26,[45][46][47][48][49][50] On the other side of the solid solution, NbO 2 adopts a distorted rutile structure which transforms into an undistorted rutile phase at about 1000 K. [51][52][53] Simultaneously, a large increase in electrical conductivity is observed suggestive of a non-metal to metal transition. [54][55][56] The mechanism of the structural/electronic transition is best understood in terms of metal-metal bonding between Nb ions, which paired along the c-axis as a result of a Peierls instability.…”