Topochemically controlled hydrogen reduction of scheelite-related rare-earth metal molybdates

Abstract

Scheelite-related α-Ln₂Mo₃O₁₂(Ln = La, Pr, Nd, Sm, Gd, Tb, or Dy) oxides are reduced by hydrogen at 780–870 K yielding molybdenum (IV) oxides of formula Ln₂Mo₃O₉. The latter crystallize in a tetragonal scheelite (ABO₄) type structure where one third of the A sites and a quarter of the anion sites are vacant: Ln_2/3(□cat)_1/3MoO₃(□an). The reaction Ln₂Mo₃O₁₂+ 3H₂→ Ln₂Mo₃O₉(□an)₃+ 3H₂O may be regarded as topochemically controlled, since both the parent and the product phases have scheelite-related structures. Infrared spectra and electrical and magnetic properties of these metastable defect scheelite phases are reported.