Synthesis and Structure of Oxygen Deficient Lead-Technetium Pyrochlore, the First Example of a Valence V Technetium Oxide

Abstract

The structure of lead-technetium pyrochlore has been refined in space group Fd3¯m with a = 10.36584(2) Å using a combination of synchrotron X-ray and neutron powder diffraction data and confirmed via Electron Diffraction. The oxide is found to be oxygen deficient with a stoichiometry of Pb2Tc2O7-d. Displacive disorder of the Pb cations is evident from the refinements, as has been observed in Bi2Tc2O7-d. X-ray absorption spectroscopic measurements at the Tc K-edge demonstrate the valence of the Tc is greater than 4.0 as anticipated from the refined oxygen stoichiometry. Raman spectroscopy confirms the presence of disorder leading us to conclude that this pyrochlore is the first example of a valence V technetium oxide.

Vacancy-Driven Disorder and Elevated Dielectric Response in the Pyrochlore Pb1.5Nb2O6.5

“…Such pyrochlores, including Pb 1.5 Nb 2 O 6.5 and Pb 1.5 Ta 2 O 6.5 , possess relatively electropositive B-site cations with a 5+ oxidation state. Other Pb-based cubic pyrochlores, such as Pb 2 Tc 2 O 7– x , Pb 2 Ru 2 O 6.5 , Pb 2 Rh 2 O 7– x , Pb 2 Re 2 O 7– x , Pb 2 Os 2 O 7– x , and Pb 2 Ir 2 O 6.5 , possess more electronegative B-site cations with mixed-valent 4+/5+ oxidation states and exhibit vacancies on the O′ site. When the B-site cation only exhibits a 4+ oxidation state (such as in Pb 2 Sn 2 O 6 ), the A-site is fully occupied and the O′ site is vacant to maintain charge neutrality.…”