| http://www.w3.org/ns/prov#value | - Of course, the Nb-containing solutions used in EP-A 895 809 are also suitable as niobium source. [0058] With regard to all other possible elements M2, starting compounds which are particularly suitable for the purposes of the present invention are their halides, nitrates, formates, oxalates, acetates, citrates, carbonates and/or hydroxides.
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