Temperature Dependence of the Nuclear Quadrupole Spectrum ofNb93in Ferroelectric KNbO3

The nuclear electric quadrupole interaction of ${\mathrm{Nb}}^{93}$ in ferroelectric KNb${\mathrm{O}}_3$ has been measured as a function of temperature in the tetragonal, orthorhombic and rhombohedral phases. The values of $\left(\frac{1}{\nu }\right)\left(\frac{d\nu }{\mathrm{dT}}\right)$ for these three phases are approximately -${10}^{-3\mathrm{}}$, -${10}^{-3\mathrm{}}$, and -${10}^{-4\mathrm{}}$ per degree centigrade, respectively. In the tetragonal phase at 220°C, $\frac{e^2\mathrm{qQ}}{h}$ has been measured as 24 Mc/sec and the asymmetry parameter $\eta$ is zero. An ionic calculation of field gradients in the tetragonal phase has been made including point charge and induced dipole contributions. The results of this calculation give the Sternheimer antishielding factor ($1-{\gamma }_{\infty }$) as 15 in good agreement with its estimated value of +16. The temperature dependence of the electric field gradient at the ${\mathrm{Nb}}^{93}$ site as calculated from this ionic model is in agreement with the measured temperature dependence in the tetragonal phase.