Mössbauer Effect in Iron under Very High Pressure

Abstract

The M\"ossbauer spectrum of ${\mathrm{Fe}}^{57}$ in iron metal has been measured at pressures up to 240 kbar at room temperature. Below 130 kbar the spectrum consists of the normal six lines characteristic of ferromagnetic, body-centered-cubic iron. The internal magnetic field $H$ at the nucleus decreases linearly with volume; $\frac{\ensuremath{\partial}(\frac{H}{{H}_{o}})}{\ensuremath{\partial}(\frac{V}{{V}_{o}})}=0.34\ifmmode\pm\else\textpm\fi{}0.01$, where ${H}_{o}$ and ${V}_{o}$ are the field and volume at atmospheric pressure. The center of gravity of the spectrum shifts with pressure, indicating an increase in $s$-electron density at the nucleus. The initial variation, -8.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$ cm ${\mathrm{sec}}^{\ensuremath{-}1}$ ${\mathrm{kbar}}^{\ensuremath{-}1}$, is consistent with scaling the $4s$ wave function with volume, while at higher pressures the variation is slower. Above 130 kbar a seventh line appears near the center of the spectrum due to the transformation of part of the iron source to the hexagonal-close-packed high-pressure phase. With increasing pressure this line becomes more intense and the split spectrum disappears, although the transformation is sluggish. From the absence of splitting and from the observed linewidth we conclude that the internal field in the hexagonal phase is 0\ifmmode\pm\else\textpm\fi{}3 kg. There may be a small broadening due to electric quadrupole interactions in the hexagonal lattice. The isomer shift of the hexagonal phase relative to the cubic phase is -0.017 cm/sec, indicating that the $s$-electron density is greater in the hexagonal phase. The pressure dependence of the shift in the hexagonal phase is very slight and is not consistent with scaling the $4s$ wave function with volume. Apparatus and experimental techniques that were developed for measuring the M\"ossbauer spectra of sources under pressure are described.