Inversion and suppression of an oxygen bulk phase transition in confined geometry.

By means of susceptibility experiments on oxygen confined to the pores of a Gelsil substrate we investigated the bulk gamma- beta transition. At low fillings, the transformation is completely suppressed. At higher but still incomplete filling we observe a partial transformation from gamma to beta on cooling and a subsequent inverted transition on heating, i.e. the transformation of gamma-phase material into the beta-phase upon heating. At 100% filling this inversion disappears. We discuss our results on the basis of geometric considerations.