(Naxk1−x)2Ta4O11(x≈0.93) piezoelectric phase from the transformation of Ta2O5 thin films of monoclinic structure

A phase of trigonal structure has been reported in previous articles to be piezoelectric with a very high longitudinal d33 coefficient of several thousand pm/V (J. Solid State Chem. 184 (2011) 2023, 2033). It was observed in Ta2O5 thin films and assumed to be the result of a reversible monoclinic–trigonal phase transition. However, new investigations are reported because the cell parameters of this phase are actually very close to those of the natrotantite mineral Na2Ta4O11. From analyses by X-ray energy dispersive spectroscopy (XEDS), the chemical composition of this trigonal phase corresponds to (NaxK1−x)2Ta4O11NaxK1−x)2Ta4O11 with x≈0.93x≈0.93. The origin of sodium, potassium, iron and oxygen atoms is found to be due to a pollution coming from alumina crucibles used for thermal treatments. Knowing both atomic structures Ta2O5 and Na2Ta4O11 and their structural relationships, observed by high resolution transmission electron microscopy (HRTEM), the mechanism of the reaction of transformation is re-examined and discussed. This mechanism implies that Ta5+ vacancies likely exist in monoclinic structures of tantalum oxide and that electrical neutrality could be due to incorporated proton H+ instead of O2− vacancies or Ta5+ interstitials.

(a, b  ) TEM images of interfaces between Ta2O5Ta2O5 monoclinic and (Na0.93K0.07)2Ta4O11Na0.93K0.07)2Ta4O11 trigonal phases where it appears that the ratio of periods for the transformation monoclinic-to-trigonal is 3/2 (c, d) corresponding schema of the reaction of transformation, (a, c  ) viewed along the zone axes [010]o∥[1¯21¯]R and (b, d  ) along the zone axes [100]o∥[101¯]R.Figure optionsDownload as PowerPoint slideHighlights
► The formation of a piezoelectric phase in Ta2O5Ta2O5 thin films, reported in previous articles, is re-examined.
► Its composition is actually (Nax,K1−x)2Ta4O11Nax,K1−x)2Ta4O11 with x  =0.93, instead of Ta2O5Ta2O5.
► Ta5+Ta5+ vacancies, compensated by protons within tantalumoxide, are involved in a new analysis of the transformation.