The origin of low bandgap and ferroelectricity of a co-doped BaTiO₃

¹ Department of Physics and Astronomy, Molecular and Condensed Matter Physics, Uppsala University Box 516, SE-75120 Uppsala, Sweden
² Department of Solid State and Structural Chemistry Unit, Indian Institute of Science - Bengaluru 560012, India
³ Rossendorf Beamline at ESRF - The European Synchrotron - CS40220, 38043 Grenoble Cedex 9, France
⁴ Helmholtz Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology - PO Box 510119, 01314 Dresden, Germany

^(a) dibya.phuyal@physics.uu.se

Received: 31 August 2018
Accepted: 16 October 2018

Abstract

We recently demonstrated the lowest bandgap bulk ferroelectric, BaTi_1−x(Mn_1/2Nb_1/2)_xO₃, a promising candidate material for visible light absorption in optoelectronic devices. Using a combination of x-ray spectroscopies and density functional theory (DFT) calculations, we here elucidate this compound's electronic structure and the modifications induced by Mn doping. In particular, we are able to rationalize how this compound retains its ferroelectricity even through a significant reduction of the optical gap upon Mn doping. The local electronic structure and atomic coordination are investigated using x-ray absorption at the Ti K, Mn K, and O K edges, which suggests only small distortions to the parent tetragonal ferroelectric system, BaTiO₃, thereby providing a clue to the substantial retention of ferroelectricity in spite of doping. Features at the Ti K edge, which are sensitive to local symmetry and an indication of Ti off-centering within the Ti-O₆ octahedra, show modest changes with doping and strongly corroborates our measured polarization values. Resonant photoelectron spectroscopy results suggest the origin of the reduction of the bandgap in terms of newly created Mn d bands that hybridize with O 2p states. X-ray absorption spectra at the O K edge provide evidence for new states below the conduction band of the parent compound, illustrating additional contributions facilitating bandgap reduction.