Prediction of new ground-state crystal structure of $\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$

Tantalum pentoxide $\left(\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5\right)$ 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 $\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$ is not yet uniquely determined. Based on first-principles calculations in combination with evolutionary algorithm, we identify a triclinic phase of $\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$, which is energetically much more stable than any phases or structural models reported previously. Characterization of the static and dynamical properties of the phase reveals the common features shared with previous metastable phases of $\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$. In particular, we show that the $d$ spacing of $\sim 3.8\AA$ found in the x-ray diffraction patterns of many previous experimental works is actually the radius of the second Ta-Ta coordination shell as defined by radial distribution functions.

Figure 1

Schematic diagram for the structural models of $\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$: (a) ${\beta }_{\mathrm{AL}}$; (b) $\delta$; (c) $L_{\mathrm{SR}}$; (d) $B$; (e) $\lambda$; and (f) $\gamma -\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$.

Figure 2

Total energies of the $B,\lambda$, and $\gamma -\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$ as a function of volumes normalized to the equilibrium volume of $\gamma -\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$ at $P=0\mathrm{GPa}$. The dashed lines are common tangent along which phase transformation between two phases occurs.

Figure 3

(a) Simulated XRD pattern for $\gamma -\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$, using the $\mathrm{Cu}K\alpha$ radiation (wavelength $\sim 1.541\AA )$. (b), (c) RDFs $g_{\mathrm{TaO}}$ (b) and $g_{\mathrm{TaTa}}$ (c) of $B-\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$, calculated by averaging over the atomic configurations obtained in MD simulations at 300 K.

Figure 4

Ball-and-stick representation of $\mathrm{Ta}{\mathrm{O}}_6$ octahedra (left panels), their spatial distribution around one central $\mathrm{Ta}{\mathrm{O}}_6$ (middle panels), and the local bonding structures of ${\mathrm{O}}_{3\mathrm{c}}$ (right panels) in $B$ (a), (d), (g); $\lambda$ (b), (e), (h); and $\gamma -\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$ (c), (f), (i).

Figure 5

(a)–(c) Calculated phonon dispersions of $\gamma -\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$ along the $\mathrm{\Gamma }X$ (a) and $\mathrm{\Gamma }L$ (b) lines, and the VDOS, panel (c). (d) Polarization vectors of the vibrational modes of $\gamma -\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$ at $\mathrm{\Gamma }$ point with the wave numbers $\tilde{\nu }=1007\mathrm{c}{\mathrm{m}}^{-1}$. The vibrations of ${\mathrm{O}}_{3\mathrm{c}}$ and ${\mathrm{O}}_{2\mathrm{c}}$ are represented by sky-blue and deep-blue arrows, respectively.