Structural Properties of Ditechnetium Heptoxide, Tc{sub 2}O{sub 7}
Group 7 elements form unique binary oxides with the stoichiometry M{sub 2}O{sub 7}. The heptoxides of manganese and technetium crystallize as molecular solids while rhenium forms an extended solid with mixed valances, 4 and 6. Unlike manganese and rhenium heptoxides which are used in the chemical industry technetium heptoxide has found very few applications due to its radioactive nature; it has been primarily use as a chemical precursor. While both of the molecular solids, Mn{sub 2}O{sub 7}{sup 3} and Tc{sub 2}O{sub 7}{sup 4} can be described by two corner-sharing MO{sub 4} tetrahedron. The most distinctive difference is that the manganese's compound has a metal-oxygen-metal bond angle of 120.7 deg. while the technetium species is 180 deg.. Additionally, Tc{sub 2}O{sub 7} has the largest molar volume of the heptoxides which suggest that it will be more susceptible to external stimuli of pressure and temperature than the others heptoxides. A pressure induced twisting has been predicted by Fang et al. Anomalous behavior is seen in the c-axis where below 2 GPa the Van der Waals interactions between the Tc{sub 2}O{sub 7} molecules is over come and compression is observed but at higher pressures the twisted distortion forces the lattice to expand. The terminology diagonal-double tetrahedrons (DDTs) is introduced to discuss the molecules of Tc{sub 2}O{sub 7}. The rigid nature of the DDTs has been attributed to the cause of this twisted distortion mechanism. Recently, the solid state and gas phase behavior of technetium heptoxide was revisited by Childs et al. in light of {sup 99}Tc being an important dose contributor in the Hanford waste and vitrification being proposed for immobilization of nuclear waste. Technetium heptoxide is one of the proposed volatilized species of technetium found in the vitrification process. In this study the crystal structure of Tc{sub 2}O{sub 7} was reported at 100 K and compared to the structure first determined by Krebs at room temperature. Inspection of the metal-oxygen distances revealed the 100 K structure had longer bond lengths for the bridging oxygen than at high temperature. It is suggested that transverse thermal vibrational modes can be used to explain this negative thermal expansion behavior. This describes the average bond length of a central atom (i.e. the bridging oxygen atom) being measured as shorter at higher temperatures due to the larger thermal motion. Diffraction methods that measure the long range average structure are susceptible to observing this shorter average length as opposed to the true bond length, atom position at oscillation node that a method like Pair-Distribution Functional PDF Analysis would yield. In the case of Tc{sub 2}O{sub 7}, the magnitude of the transverse thermal vibrational mode is at the limit of being detectable by SCXRD and likely not observable by PDF analysis. Here we report the preliminary results of our study on the nature of the thermal expansion behavior of Tc{sub 2}O{sub 7}. The transverse thermal vibration mode compression mechanism is considered and compared to the pressure induced twisted distortion mechanism predicted at high pressure. (authors)