Comment on the elastic properties of semiconducting NbO2

Previous reported values of the elastic constants of semiconducting Nb${\mathrm{O}}_2$ were determined from ultrasonic velocities measured along unconventional axes. In order to elucidate the elastic behavior of this distorted rutile structure crystal, we have transformed these stiffness moduli into a system having conventional axes. The temperature dependences of the transformed constants are then compared with those of a number of rutile structure crystals and of Te${\mathrm{O}}_2$ which has a distorted rutile structure differing from that of semiconducting Nb${\mathrm{O}}_2$. It is found that the shear constant $C_S=\frac{(C_{11}-C_{12})}{2}$ has a much smaller anomalous temperature dependence in semiconducting Nb${\mathrm{O}}_2$ than it does in these other crystals.