Trends in bandgap of epitaxial A2B2O7 (A = Sn, Pb; B = Nb, Ta) films fabricated by pulsed laser deposition

Pyrochlore oxides A2B2O7 have been a fruitful playground for condensed matter physics because of the unique geometry in the crystal structure. Especially focusing on the A-site tetrahedral sub-lattice, in particular, pyrochlore oxides A2B2O7 (A = Sn, Pb and B = Nb, Ta), recent theoretical studies predict the emergence of the “quasi-flat band” structure as a result of the strong hybridization between filled A-ns and O-2p orbitals. In this work, we have established the growth conditions of Sn2Nb2O7, Sn2Ta2O7, Pb2Nb2O7, and Pb2Ta2O7 films by pulsed laser deposition on Y-stabilized ZrO2 (111) substrates to elucidate their optical properties. Absorption-edge energies, both for direct and indirect bandgaps, increase in the order of Sn2Nb2O7, Sn2Ta2O7, Pb2Nb2O7, and Pb2Ta2O7. This tendency can be well explained by considering the energy level of the constituent elements. A comparison of the difference between direct and indirect bandgaps reveals that Pb2B2O7 tends to have a less dispersive valence band than Sn2B2O7. Our findings are consistent with the theoretical predictions and are suggestive of the common existence of the hybridized states in this class of compounds.