Columbite-(Mg) Pagola S, Carbonio R E, Alonso J A, Fernandez-Diaz M T Journal of Solid State Chemistry 134 (1997) 76-84 Crystal structure refinement of Mg Nb2 O6 columbite from neutron powder diffraction data and study of the ternary system MgO-Nb2O5-NbO, with evidence of formation of new reduced pseudobrookite Mg5-xNb4+xO15-d (1.14