Substrate strain and doping effects on the crystal structure of SrNbxTi1−xO3

Abstract

Strontium titanate, SrTiO3 (STO), is an interesting material for both fundamental studies and technological applications. Modifications of the atomic and crystal structure by doping, e.g. replacing titanium with niobium atoms, and by strain, i.e. by growing STO on a different substrate such as lanthanum aluminate, LaAlO3 (LAO), have been proposed to tune the STO electronic, optical and transport properties for applications. Here we report the results of ab initio density-functional theory (DFT) simulations of both strain and Nb-doping effects, independently and joint, on the STO crystal structure. We found that the DFT energy differences among the three commonly observed STO crystal structures, Pm3m, P4∕mmm, and I4∕mcm, are very small, <2.6 × 10−4 Ry, so that the ground-state cannot be determined unambiguously at this level of theory and physics. Our calculations show that an in-plane strain, at least at the level of only − 0.4% as observed in STO on LAO, does not lead to the expected increase in c toward tetragonal symmetry, where c is the length of the cell axis perpendicular to the plane. Instead, c also is reduced and the cubic symmetry tends to be restored. Nb doping, even at the maximum experimental level of 3.7%, does not have significant effects on lattice parameters. The latter result is confirmed also under the presence of strain, so we could not find any crossed effect of strain and doping.