The Jahn-Teller (JT) theorem constitutes one of the most fundamental concepts in chemistry.Intransition-element chemistry,t he 3d 4 and 3d 9 configurations in octahedral complexes are particularly illustrative,w here ad istortion in local geometry is associated with ar eduction of the electronic energy.However,there has been alasting debate about the fact that the octahedra are found to exclusively elongate.I n contrast, for Na 9 Bi 5 Os 3 O 24 ,t he octahedron around Os 6+ (5d 2 ) is heavily

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... mpressed, lifting the degeneracy of the t 2g set of 5d orbitals such that in the sense of aJ Tc ompression ad iamagnetic ground state results.This effect is not forced by structural constraints,t he structure offers sufficient space for osmium to shift the apical oxygen atoms to as tandardd istance.T he relevance of these findings is far reaching, since they provide new insights in the hierarchy of perturbations defining ground states of open shell electronic systems. Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.