Rarer than a new element: Recently Wang et al. produced the [(Ir+IX)O4]+ ion by a pulsed laser vaporization/thermal expansion method and studied it by IR photodissociation spectroscopy. The oxidation state of +IX had been unknown previously. The picture shows the mass spectra for the argon-tagged iridium oxide species.
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Pyykkö et al. (2014) studied this question.
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