Amorphization of Ta₂O₅ under swift heavy ion irradiation

Crystalline Ta₂O₅ powder is shown to amorphize under 2.2 GeV ¹⁹⁷Au ion irradiation. Synchrotron X-ray diffraction (XRD), Raman spectroscopy, small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) were used to characterize the structural transition from crystalline to fully-amorphous. Based on Rietveld refinement of XRD data, the initial structure is orthorhombic (P2mm) with a very large unit cell (a = 6.20, b = 40.29, c = 3.89 Å; V = 971.7 Å³), ideally containing 22 Ta and 55 O atoms. At a fluence of approximately 3 × 10¹¹ ions/cm², a diffuse amorphous background becomes evident, increasing in intensity relative to diffraction maxima until full amorphization is achieved at approximately 3 × 10¹² ions/cm². An anisotropic distortion of the orthorhombic structure occurred during the amorphization process, with an approximately constant unit cell volume. The amorphous phase fraction as a function of fluence was determined, yielding a trend that is consistent with a direct-impact model for amorphization. SAXS and TEM data indicate that ion tracks exhibit a core-shell morphology. Raman data show that the amorphous phase is comprised of TaO₆ and TaO₅ coordination-polyhedra in contrast to the TaO₆ and TaO₇ units that exist in crystalline Ta₂O₅. Analysis of Raman data shows that oxygen-deficiency increases with fluence, indicating a loss of oxygen that leads to an estimated final stoichiometry of Ta₂O_4.2 at a fluence of 1 × 10¹³ ions/cm².