Charge and spin state of dilute Fe in NaCl and LiF

There is an apparent mismatch between electron paramagnetic resonance and Mössbauer spectroscopy results on the charge and spin states of dilute Fe impurities in NaCl; Mössbauer spectroscopy data have been interpreted in terms of high-spin Fe$^{2+}$, while electron paramagnetic resonance studies suggest low-spin Fe$^{1+}$. In the present study, the charge and spin states of dilute substitutional Fe impurities in NaCl and LiF have been investigated with $^{57}$Mn → $^{57}$Fe emission Mössbauer spectroscopy. A scheme is proposed which takes into account the effects of nearest-neighbor distances and electronegativity difference of the host atoms on the Mössbauer isomer shift and allows for the unequivocal differentiation between high-spin Fe$^{2+}$ and high/low-spin Fe$^{1+}$ in Mössbauer spectroscopy. From these considerations, the Mössbauer results are found to be consistent with dilute Fe impurities in NaCl and LiF in a low-spin Fe$^{1+}$ state. These conclusions are supported by theoretical calculations of isomer shifts and formation energies based on the density-functional theory. The experimental results furthermore suggest that charge compensation of dilute Mn$^{2+}$ dopants in NaCl and LiF is achieved by Na vacancies and F$^-$ interstitials, respectively.