Pressure dependences of the elastic constants of semiconducting NbO2at 296 K

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 $C_{Q{S}^{\prime }}$ 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.