Os⁴⁺ Instability in the Pyrochlore Structure: Tl_2–xBi_xOs₂O_7–y

Abstract

Osmium-containing oxides are rare due to the difficulty in stabilizing complex structures with a fixed stoichiometry and metastability of the phases. Bismuth-substituted thallium osmate pyrochlore samples, Tl2–x Bi x Os2O7–y , were synthesized using solid-state reactions where the solubility limit was found to be approximately x = 1.4. Members of this solid solution were characterized by their structural, electronic, magnetic, and thermal properties to understand the influence of Bi3+ substitution on the ground state. The Os-containing pyrochlores crystallize in the ideal cubic pyrochlore structure (Fd m), and the lattice parameter a was found to slightly increase as a function of Bi content. A possible interplay between structure and cation valence states was explored using both neutron powder diffraction and X-ray absorption spectroscopy, suggesting that a combination of Os4+/Os5+ and Tl1+/Tl3+ mixed valency throughout the solid solution allows for the stabilization of the pyrochlore structure. The system is metallic for the entire solid solution and predominantly exhibits temperature-independent paramagnetism. Specific heat measurements show an enhanced Sommerfeld coefficient, a possible flat-band signature. This system gave insight into the bonding preferences of Os, indicating a dependence on high oxidation states and mixed valence for the stability of complex structures.

Quantum materials with strong spin–orbit coupling: challenges and opportunities for materials chemists

“…Iridium and osmium both form pyrochlores with a range of A-site species such as Cd, Hg, main-group and rare-earth elements. [82][83][84][85] However, as one of the seven oxygens in the pyrochlore formula unit is crystallographically distinct from the other six and is not involved in the MO 6 coordination octahedra, this makes pyrochlore materials susceptible to oxygen non-stoichiometry, as in for example Pb 2 Ir 2 O 6.5 . 86…”

“…High-pressure (multi-GPa) or high-oxygen-pressure methods are two approaches that have been used to achieve this and access the higher oxidation states of osmium, such as Os 8+ without significant production of volatile and highly toxic OsO 4 (which most osmates are unstable towards in an oxidising environment, and many even on storage in ambient air). 82,136,137 The stabilisation of single (or multiple controlled) oxidation states and further elucidating the chemistry of osmium in the solid state is a focus of current developments. 82…”

“…82,136,137 The stabilisation of single (or multiple controlled) oxidation states and further elucidating the chemistry of osmium in the solid state is a focus of current developments. 82…”