  Tantalum pentoxide (Ta2O5) is a wide-gap semiconductor which has important
technological applications. Despite the enormous efforts from both experimental
and theoretical studies, the ground state crystal structure of Ta2O5 is not yet
uniquely determined. Based on first-principles calculations in combination with
evolutionary algorithm, we identify a triclinic phase of Ta2O5, which is
energetically much more stable than any phases or structural models reported
previously. Characterization of the static and dynamical properties of the new
phase reveals the common features shared with previous metastable phases of
Ta2O5. In particular, we show that the d-spacing of ~ 3.8 {\AA} found in the
X-ray diffraction (XRD) patterns of many previous experimental works, is
actually the radius of the second Ta-Ta coordination shell as defined by radial
distribution functions.
