We report on the fluorine incorporation in powder based materials for the fabrication of Al and Al/Yb co-doped silica glasses. The achieved maximum Fluorine concentration of 1.55 mol% SiF4 corresponds to a refractive index decrease of -8 x 10-3. Simultaneously, the Tg of the doped material is reduced by about 200 K compared to pure silica. Moreover, the fluorine doping is also eminently suitable for the direct refractive index adjustment in active doped silica glass materials (e.g. Al/Yb or Al/Tm). The index matching with pure silica is possible to date up to 2.7 mol% Al2O3 and 0.1 mol% Yb2O3. The additional influence on the blue shift of the UV transmission will also be discussed.