| http://www.w3.org/ns/prov#value | - t one of magnesium and calcium, [0044] M??? is at least one of aluminum, gallium and indium, [0045] M?????? is at least one of sulphur, selenium and tellurium, [0046] RE is a rare earth element, especially europium and cerium, and 0<a<1. [0047] In preferred embodiments of the invention, each of M???, M??? and M?????? is a single element.
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