Niobium oxide layers with a thickness in the range of 1.0-1.4 mu m were deposited on silicon single crystal wafers by magnetron sputtering using substoichiometric niobium oxide target materials. After the deposition process, the layer material was completely amorphous. The primary crystallization of the hexagonal Nb2O5 phase and the subsequent transformation to the orthorhombic phase were investigated by means of in-situ high-temperature X-ray diffraction up to a temperature of 900 degrees C under a reducing N-2/H-2 atmosphere. The precise determination of the cell parameters by Rietveld refinement enabled the determination of the anisotropic thermal expansion behavior of the crystalline Nb2O5 phase. Besides, an activation energy of the primary crystallization reaction of + 460(50) kJ/mol was quantitatively determined by isothermal in-situ experiments in the temperature range of 505-545 degrees C.