x = 0.4 spectrum is strongly modified with respect to the x = 0 and 0.2 spectra, with a marked collapse in the intensity of the 783 nm emission line. This may reflect a switch from a regime where (Tm 3+ , Yb 3+ ) are distributed between the A-and B-sublattices, to one where they can be accommodated at the Bsublattice only (see Discussion, Section 4). At the same time, the luminescence lifetimes (measured at 780 nm) are progressively shortened as nonstoichiometry increases (Figure ). The emission mechanism does not vary with nonstoichiometry, with all compositions exhibiting a two-electron transfer process determined by measurements of luminescence intensity vs pump power (Figure ).