A study of tantalum pentoxide Ta2O5 structures up to 28 GPa

Abstract

Tantalum pentoxide Ta 2 O 5 with the orthorhombic L-Ta 2 O 5 structure has been experimentally studied under high pressure using synchrotron angle-dispersive powder

Phase comparison and equation of state for Ta2O5

“…Our observed powder XRD patterns closely match observations from previous studies on bulk Ta 2 O 5 [17,18,55] and Ta 2 O 5 thin films [39,56]. Figure 1 compares an integrated pattern collected at ambient conditions with calculated profiles of the proposed Ta 2 O 5 phases.…”

“…While none of the plausible phases provide an obviously superior match to experimental data, visual inspection suggests that L SR reproduces the observed XRD features (figure 1), especially low intensity ones, slightly better than the other structures. Indeed, Stavrou et al [18] observed XRD patterns almost identical to ours and concluded that L SR was the correct structure based on this correspondence. However, our Rietveld refinements do not quantitatively support this interpretation.…”

“…Upon compression (figure 3), we observed the previously reported merging of the Ta 2 O 5 doublet peaks (i.e. the features which begin at Q ≈ 2.0 and 2.6 Å −1 ) at ≈3 GPa, followed by a transition to an amorphous phase starting at ≈25 GPa [17,18]. We did not observe any phase changes upon further compression to 60 GPa, and decompressing the amorphous phase did not result in Ta 2 O 5 recrystallization.…”

“…peaks after the 'transition' and the broadening visible in other peaks, we consider it more likely that the merging is due to P-induced broadening of adjacent peaks. This phenomenon has been previously attributed to local disorder among competing Ta coordination polyhedra [18,57] and may be the source of the residual stress buildup interpreted to lead to the amorphous transition [17,58]. We tried fitting a δ phase EoS to only those patterns at P above the peak merger, but the results did not differ from the δ EoS parameters fit over the full range of patterns.…”

“…There have been two experimental crystalline Ta 2 O 5 EoS studies, but each indexed their data to a different crystal structure. Additionally, one of these studies did not determine the P-derivative of the bulk modulus (K ′ 0 ) [17], and the other reported an unrealistically low K ′ 0 value (≈ 0.1) [18], making a direct comparison of their results difficult.…”