See Fig. -S28 and Method for details). Fig. shows the powder pattern which was modelled in a similar way to the "collapsed" oxysulphide (Fig. ) with collapse of the interlayer spacing between Sr2MnO2Se2 slabs from 8.97 Å 39 in Sr2MnO2Cu1.5Se2 to 8.34 Å and the presence of stacking disorder which was modelled as described above for the oxysulfide (Fig ). The Se-Se distance (Fig. ) of 2.43 Å is in the typical range for polyselenides (dSe-Se = 2.35-2.55 Å) , and oxidation of Se 2-anions into anionic (Se2) 2-dimers was confirmed by the pre-edge feature at 12657 eV and shift of the main peak by ca. 3.4 eV in the Se K-edge XANES (Fig. ). The Mn K-edge XANES hardly shifted compared with Sr2MnO2Cu1.5Se2 (Fig. ) and the effective moment μeff = 5.32(1) μB given by magnetometry (Fig. ) remained in between the spin-only values for Mn 2+ and Mn 3+ . A Raman peak at 282 cm -1 (Fig. ), was comparable to Se-Se stretching mode of pyrite-type MnSe2 (267 cm -1 with dSe-Se = 2.38 Å) . However, like the oxysulfide the "collapsed" oxyselenide was not Cu/Li-free;