The local environment around Yb3+ ions in silica-based optic fibre preforms has been studied in specimens with different composition and submitted to different annealing processes. The formation upon annealing of nano-sized particles of YbPO4 has been evidenced by X-ray Absorption Spectroscopy whereas optical luminescence present Stark-split states in the anti-Stokes lines, typical of a structurally ordered phase. This evidences the preferential location of Yb in highly coordinated phosphate environment, with remarkable effects of such ordered phase on the optical response of the system. This study confirms and extends analogous observations done in previous literature on Erbium in similar glasses.