Abstract
The pressure dependences of the elastic-stiffiness moduli of Nb${\mathrm{O}}_{2}$ at 296 K have been obtained by measuring the velocities of ultrasonic waves as a function of hydrostatic pressure to 5 kbar. Most modes stiffen with pressure. However, those involving ${C}_{Q{S}^{\ensuremath{'}}}$ and ${C}_{66}$ soften. Using the pressure dependences of the elastic constants of Nb${\mathrm{O}}_{2}$, we find anomalous contributions to the temperature dependences of ${C}_{44}$ and ${C}_{66}$ due to dilatational effects. The behavior of acoustic modes in semiconducting Nb${\mathrm{O}}_{2}$ is compared with that of corresponding modes in pseudoisomorphic crystals. These results suggest the possibility of a high-pressure monoclinic phase of Nb${\mathrm{O}}_{2}$ and, more generally, that structures with edge-shared anion octahedra may be intrinsically unstable.
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