This paper describes the structure of Er(3+)-doped SiO(2)-HfO(2) waveguides containing nanocrystals of HfO(2). Pure and 1 mol% Er(3+)-doped 70SiO(2)-30HfO(2) films were deposited by the sol-gel method on amorphous SiO(2) substrates using the dip-coating technique. Each waveguide has experienced a single thermal treatment at temperatures ranging from 900 to 1200 degrees C, for either short (30 min) or long (24 h) durations. Crystallization and microstructure were studied by x-ray diffraction (XRD). The local environments of hafnium and erbium ions were determined, respectively, from Hf and Er L(3)-edges extended x-ray absorption fine structure (EXAFS) experiments. Both XRD and EXAFS results demonstrate the substitution of Hf(4+) by Er(3+) ions in the crystalline structure. XRD shows the nucleation of tetragonal HfO(2) nanocrystals after heat treatment at 1000 degrees C for 30 min in the pure waveguide, and at 900 degrees C for 24 h in the waveguide doped with Er(3+). In both series, partial transformation from tetragonal to monoclinic HfO(2) nanocrystals starts after heat treatment at 1100 degrees C for 24 h. The average crystallite size and size distribution can be controlled by thermal annealing temperature and duration, respectively, with brief treatment yielding a more homogeneous nanocrystal size.