Pressure Dependence of the Electric Field Gradient in Metallic Indium

The shift of the ${}_{}{}^{115}{}_{}{}^{}\mathrm{In}_{\left|\frac{7}{2}\right|\leftrightarrow \left|\frac{9}{2}\right|}^{}{}_{}{}^{}$ nuclear quadrupole resonance (NQR) in indium metal has been measured at 25.5°C as a function of hydrostatic pressure to 8 kbar. In addition, the linear compressibilities at 25.5°C and the linear thermal-expansion coefficients and resonance-frequency temperature dependence in the room-temperature region were measured. Since the electric field gradient is a function of three variables, these measurements are insufficient to separate the explicit dependences. The obvious third experiment, the effect of uniaxial stress on the NQR, is not feasible, as it would require detection of the broad NQR of indium in the skin depth of a single crystal. We can therefore only infer that the major contribution to the change in field gradient is due to the change in distortion parameter $\left(\frac{c}{a}\right)-1\mathrm{}$. A possible solution to this problem at one temperature is discussed.