Europium(II) oxide is a violet compound as a bulk crystal and transparent blue in thin film form. It is unstable in humid atmosphere, slowly turning into the yellow europium(II) hydroxide hydrrate and then to white europium(III) hydroxide. EuO crystallizes in a cubic sodium chloride structure with a lattice parameter a = 0.5144nm. The compound is often non-stoichiometric, containing up to 4% Eu and small amounts of elemental europium. However, since 2008 high purity crystalline EuO films can be created in ultra high vacuum conditions. These films have a crystallite size of about 4 nm.
Europium(II) oxide is ferromagnetic with a Curie Temperature of 69.3 K. With the addition of about 5-7% elemental europium, this increases to 79 K. It also displays colossal magnetoresistance, with a dramatic increase in conductivity below the Curie temperature. One more way to increase the Curie temperature is doping with gadolinium, holmium, or lanthanum.
Europium(II) oxide is a semiconductor with a band gap of 1.12 eV.