Structural Studies of TcO₂by 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) A, and beta = 121.453(1) degrees . 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) A from NPD profile analysis and 2.59 A 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.

Systematic XAS study on the reduction and uptake of Tc by magnetite and mackinawite

“…From a crystal-chemical point of view, the short-long Tc pairing scheme is based on a sensitive balance between crystal field stabilization (favoring an even distribution of Tc-Tc distances by stabilizing a more symmetric TcO 6 octahedron, t 2g 3 configuration) and valence electrons available to form a Tc-Tc metal bond (shortening one of the two Tc-Tc distances). 63,64 In contrast to the EXAFS-ITFA-derived Tc speciation in the magnetite system, which showed an effect of the initial Tc concentration, the Tc speciation of the mackinawite series established in the same way shows rather an effect of Tc surface loading: samples Mack-3 and Mack-6 with the highest loading of 900 ppm have the highest contribution of TcO 2 •xH 2 O(s), although it never exceeds 20%, while the TcS x -like phase prevails in all systems ( Table 5).…”

Insights into the electronic structure of OsO2 using soft and hard x-ray photoelectron spectroscopy in combination with density functional theory

“…In contrast to the ternary and quaternary osmates the binary oxide OsO 2 belongs to a group of transition-metal dioxides, including IrO 2 , RuO 2 , PtO 2 , TcO 2 , and ReO 2 , which crystallize in the rutile structure and show metallic behavior [7][8][9][10]. Compared to the majority of SOC-driven 5d transitionmetal oxides, the electronic structure of the rutile binaries can be explained with crystal-field arguments put forward by Goodenough [11].…”

Exceptionally strong magnetism in the 4dperovskitesRTcO3(Franchini1, Archer2, He3 et al. 2011Phys. Rev. B

“…These monotonically increase as a function of the R ionic radius due to the progressive enhancement of the volume and the associated decrease of the cooperative rotation of the TcO6 octahedra.PACS numbers: 75.47.Lx,75.50.Ee,75.30.Et,71.15.Mb Intermediately located between manganese and rhenium, technetium (5s 2 4d 5 ) shares with its isovalent neighbors the intriguing possibility to form oxides with a complex structural, electronic and magnetic phase diagram. However, the rare occurrence of natural Tc (Tc is essentially an artificial product of fission reactions) and the related radioactive risks (Tc is the lightest radioactive element, whose most abundant isotope, 99 Tc, decays with a half-life of 10 5 years), have made investigations of Tc-based oxides very sparse [1][2][3][4][5][6]. Overcoming these difficulties, Avdeev et al [5] and Rodriguez et al [6] have recently reported the successful synthesis of the first ever fabricated Tc-based perovskites, namely CaTcO 3 and SrTcO 3 .…”