Charge transfer ${\mathrm{Cr}}^{2+}\left(d^4\right)\to {\mathrm{Cr}}^{1+}\left(d^5\right)$ induced by hydrostatic pressure in chromium-doped GaAs

Hall-effect measurements have been performed on $n$-type GaAs: Cr crystals under hydrostatic pressure up to 11 kbar at 300 and 77 K. These experiments provide evidence for a trapping process of free electrons and a nonlinear variation of their mobility. The interpretation of the results requires that the ${\mathrm{Cr}}^{1+}$ level is degenerate with the conduction band. This energy level is situated 55 meV at 77 K and 115 meV at 300 K above the bottom of the conduction band. The ${\mathrm{Cr}}^{1+}$ level behaves like a trapping center which then scatters the electrons as a double acceptor state. A quantitative fit of the variation of the mobility requires the assumption of a pairing between chromium atoms and donor impurities.