Rare earth ion-doped oxyfluoride glasses in the system 50SiO(2)-(50 - x)PbO-xPbF(2)-mLa(2)O(3)-nEr(2)O(3) were prepared by the melt-quenching method. PbF2 and LaF3 were selectively crystallized by subsequent heat treatment in these glasses. The promotion and suppression effects of La3+ ions on the crystallization of PbF2 were observed in this selective crystallization. We used advanced solid-state nuclear magnetic resonance (NMR) and X-ray diffraction techniques to probe the structural evolution in the atomic level during these crystallizations. Based on the in-depth structure study, it can be inferred that both the different effects arise from the stronger ability of La3+ ions in attracting F- ions than that of Pb2+ ions. Intensive upconversion visible luminescence was observed in these glasses. The NMR and the upconversion luminescence results imply that most Er3+ ions are dissolved in the LaF3 crystal rather than in the PbF2 crystal and the glass phases even when the content of the LaF3 crystal is as small as less than 1 mol %. The results show that the past expectation that rare earth ions are homogenously incorporated into bivalent metal fluoride crystals might not be right and prove the validity of the strategy that multiple rare earth ions are codoped to increase the upconversion luminescent efficiency.