Variation of Refractive Index of MgO with Pressure to 7 kbar

Abstract

All the photoelastic investigations to date on single crystals have been limited to a maximum pressure of only 10-500 bar, since in all these studies uniaxial stress systems have been employed with consequent development of plastic deformation at higher stress levels. However, by adopting the hydrostatic pressure system the upper limit can be raised by one to two orders of magnitude. The variation of refractive index of MgO with pressure to 7 kbar has been determined, in the visible region of the spectrum, from the shift of the localized interference fringes across the crystal kept in the fluid pressure medium. Due allowance for the change in the thickness of the crystal was made with the help of Murnaghan's finite-strain theory and the third-order elastic constants. The refractive index decreases linearly with pressure in the range investigated with a slope of 1.58×${10}^{-4\mathrm{}}$/kbar for $\lambda 5893$. The strain-optical constants $p_{11}$ and $p_{12}$ have been evaluated from these data and have the values ${-0.25\mathrm{}}_9$ and ${-0.01\mathrm{}}_1$, respectively. Mueller's strain-polarizability constant ${\Lambda }_0$ increases from a value of 1.23 at atmospheric pressure to 1.27 at 7 kbar. The decrease in the electronic polarizability of the oxygen ions with decrease in the interatomic distance is so large that one observes a net negative value of $\frac{\mathrm{dn}}{d\rho }$ in spite of the positive contribution from the increase in density $\rho$. From the theory of Yamashita and Kurosawa it is found that the interaction energy due to overlap in all crystals crystallizing in the NaCl structure is very nearly the same fraction of their total energy even though the magnitude of the energy itself may vary considerably from crystal to crystal.