"Structural defects within Ta2O5 monoclinic structures were obtained by crystallisation at 850 C in atmosphere O2–H2O of amorphous thin films deposited by injection metal-organic chemical vapour deposition (i-MOCVD) on Si substrates. These defects, observed by high resolution transmission electron microscopy (HRTEM), contain alignments of Ta5+ vacancies along a crystallographic direction . H2O gas of different partial pressures was obtained by bubbling O2 gas through deionised water or Na2CO3–K2CO3 aqueous solution maintained at different temperatures in a range of 25–45 C. The amount of structural defects increases dramatically with H2O partial pressure and annealing time from HRTEM observations and X-ray diffraction results. Such defects are thermally stable, at least up to 850 C. Using Na2CO3–K2CO3 aqueous solution, similar structural defects including mainly K+ species were formed. Na+ species contributes mostly to the formation of another phase of composition close to (Na0.8K0.2)2Ta4O11 of natrotantite type structure. Assuming that Ta5+ vacancies are occupied by H+ in a form of bridging hydroxyl groups Ta–OH–Ta in order to satisfy electrical neutrality, the presence of significant levels of hydrogen atoms within Ta2O5 concordant with H enrichment at defects was identified by time-of-flight secondary ion mass spectrometry (ToF-SIMS)."