Structural studies of TcO₂ by neutron powder diffraction and first-principles calculations

Abstract

Crystalline technetium dioxide was prepared and for the first time its crystal structure determined by neutron powder diffraction. In addition, electronic structure calculations using density functional theory were performed to further elucidate the bonding mechanisms in TcO₂. The crystal structure determined by Rietveld analysis with the NPD data is of a distorted rutile type, similar to that of MoO₂; space group P2₁/c, a=5.6891(1), b=4.7546(1), c=5.5195(1) Å, and β=121.453(1)°. The NPD analysis also establishes a new neutron scattering length of 6.00(3) fm for ⁹⁹Tc. Our results clearly show metal-metal bonding between Tc pairs along the edge-sharing chains of TcO₆ octahedra. The Tc-Tc bond was found to be 2.622(1) Å from NPD profile analysis and 2.59 Å from first-principles DFT calculations. The bond is somewhat longer than expected from earlier predictions, suggesting that the nature of the Tc-Tc interaction is weaker than anticipated for the Tc(IV) cation with three outer electrons. The NPD results supported by the DFT calculations suggest that the filling of antibonding orbitals and the influence of the crystal field stabilization of the d³ Tc cations lead to more regular TcO₆ octahedra and diminish the metal-metal bond strength compared with closely related oxides such MoO₂ and WO₂.