The Role of Phonons and Oxygen Vacancies in Non-Cubic SrVO₃

Abstract

Combining neutron diffraction with pair distribution function analysis, we have uncovered hidden reduced symmetry in the correlated metallic d 1 perovskite, SrVO3. Specifically, we show that both the local and global structures are better described using a GdFeO3 distorted (orthorhombic) model as opposed to the ideal cubic ABO3 perovskite type. Recent reports of imaginary phonon frequencies in the density functional theory (DFT)-calculated phonon dispersion for cubic SrVO3 suggest a possible origin of this observed non-cubicity. Namely, the imaginary frequencies computed could indicate that the cubic crystal structure is unstable at T = 0 K. However, our DFT calculations provide compelling evidence that point defects in the form of oxygen vacancies, and not an observable symmetry breaking associated with calculated imaginary frequencies, primarily result in the observed non-cubicity of SrVO3. These experimental and computational results are broadly impactful because they reach into the thin-film and theoretical communities who have shown that SrVO3 is a technologically viable transparent conducting oxide material and have used SrVO3 to develop theoretical methods, respectively.

High-temperature reduction thermochemistry of SrVO3−δ

“…Unsurprisingly, no soft modes were observed in any of the BZ paths considered here. This is in line with other theoretical treatments [39,41], although we note that Hossain et al [40] did find soft modes, which we suspect to be artefactual.…”

“…Comparison of the predicted partial densities of states (PDOS) of pristine and oxygen-deficient SrVO 3 revealed that the excess electrons resulting from oxygen vacancies are redistributed to the neighboring V 4+ t 2g states, a conclusion consistent with Bader and charge density difference (CDD) analysis. Phonon dispersions and vibrational density of states (VDOS) analysis confirmed the dynamic stability of cubic SrVO 3 , which agrees with previous experimental and theoretical observations [39,41]. Comparison of the vibrational properties of pristine and defective SrVO 3 suggests that ionic contributions to the VDOS are strongly affected by the oxygen vacancies, and thereby the temperature-dependent reduction properties of SrVO 3−δ .…”

“…A recent report revealed exceptional imaginary LDA phonon frequencies at the R point for cubic SrVO 3 , attributed to a phase transition of this material [40]. However, Berry et al confirm that, similar to TiO 2 [42], the origin of these imaginary phonon frequencies is not due to a phase transition but are an artifact of insufficient k-point mesh sampling in DFT calculations [41].…”

“…In the cubic phase of SrVO 3 , the larger Sr 2+ cation is positioned in the center of the unit cell with corner-shared VO 6 octahedra, where V 4+ is located at the center of octahedral oxygen cages (figure 1(a)) [30]. Identified as a cubic perovskite in 1957 [31], SrVO 3 has been the subject of experimental [30,[32][33][34][35] and theoretical [36][37][38][39][40][41] studies for many low-and high-temperature applications. For instance, Hui and Petric experimentally investigated the conductivity and stability of SrVO 3 , reporting excellent electronic conductivity (∼1000 S cm −1 at 1073 K and p(O 2 ) ∼ 10 −20 atm) and concluding that its derivatives remain stable for p O 2 ∼ 10 −14 -10 −20 atm [32].…”