High-spin-low-spin transition in magnesiowüstite (Mg$_{0.75}$Fe$_{0.25}$)O at high pressures under hydrostatic conditions

The spin states of Fe$^{2+}$ ions in (Mg$_{0.75}$,Fe$_{0.25}$)O magnesiowüstite crystals at hydrostatic pressures up to 90 GPa created in a diamond-anvil cell with helium as a pressure-transmitting medium have been investi-gated by transmission and synchrotron Mössbauer spectroscopy at room temperature. An electron transition from the high-spin (HS) state to the low-spin (LS) state (HS-LS crossover) has been observed in the pressure range of 55–70 GPa. The true HS-LS transition occurs in a narrow pressure range and the extension of the electron transition to $\sim$15 GPa is attributed to the effect of the nearest environment and to thermal fluctuations between the high-spin and low-spin states at finite temperatures. It has been found that the lowest pressure at which the electron HS-LS transition can occur in the Mg$_{1-x}$Fe$_x$ system is 50–55 GPa.