Secondly because the phase being stable at xA K disappears

To further reveal the phonon I-BET-762 with respect to P, five phonon dispersion curves are shown in Fig. 4. The reciprocal lattice points X1X1 (0, 0, 0.16) and X2X2 (0, 0, 0.05) near the zone center are shown in panels (b) and (d), respectively, due to the interesting behaviors. The great phonon bends observed in spectrums are attributed to the strong coupling effect between the lattice wave and free electrons, named as “Kohn anomaly”. Our investigations state clearly that the three phases can remain dynamically stable for a broad P   range until the acoustic modes at X1X1 and X2X2 become softening at phase transitions, consistent with the enthalpy results. These anomalous behaviors of the phonon modes confirm the sequent phase transitions and further reveal the dynamical-unstability-driving nature. Fig. 5 shows the homologous eigenvectors of Zr and O atoms at ΓΓ (0, 0, 0), corresponding to the phonon spectrums plotted in Fig. 4. When P = 0, the M-phase eigenvectors are along the [0 0 1¯] direction for Zr and O atoms, as shown in panel (a). As P increases, the eigenvectors begin to rotate in the (0 1 0) plane until reaching the [1¯ 0 0] direction for P = 16 GPa, as shown in panel (b). Unexpectedly, when P >> 16 GPa, the vectors rotate in the (0 0 1) plane [See panel (c)], which is responsible for that the structure becomes dynamically unstable near the first-order phase transition. Panel (d) shows that the metastable-T-phase eigenvectors are along the [1 0 0] direction. As P further increases, the O atoms come to display the different directions: the [1 0 0] direction for the (0 1 0) plane and the [1¯ 0 0] direction for the (0 0 1) plane. This leads to the dynamical instability, which is responsible for the second-order transition [See panel (e)]. Finally, for metastable C   phase [See panel (f)], two perpendicular eigenvectors of O atoms are observed in the (101) plane: two atoms along the [1¯ 1¯ 1] direction and two along the [1 1¯1¯] direction; the similar results are obtained for the (1¯ 0 1) plane. Therefore, the direction rotation of eigenvectors in the (0 0 1) plane leads to the first-order M-to-T phase transition, the same-to-opposite change of direction of eigenvectors of O atoms results in the second-order T-to-C transition.