Synthesis and superconducting properties of some phases of iron polyhydrides at high pressures

Experimental syntheses of two iron polyhydrides FeH$_x$(I) and FeH$_x$(II) have been carried out in diamond anvil cells by laser heating of metallic iron to temperatures of about 700 and 2000 K at pressures of 178 and 195 GPa, respectively. The initial sample is an iron plate enriched in the Fe-57 Mössbauer isotope placed in ammonia borane (BH$_3$NH$_3$). The electronic properties of FeHx compounds have been studied by measuring the electrical resistance $R(T)$ at high pressures ($180$–$216$ GPa) in the temperature range of $\sim 8$–$300$ K. Based on the obtained $R(T)$ data, two superconducting phases of FeH$_x$ compounds with the maximum critical transition temperatures $T_C\approx25.0$ and $27.7$ K have been identified. It has been found that with increasing pressure, the temperature $T_c$ in both hydrides increases linearly with the coefficients $dT_C/dP\sim0.063\pm0.001$ K/GPa and $0.056\pm 0.003$ K/GPa for the FeH$_x$(I) and FeH$_x$(II) phases, respectively. Superconductivity in iron hydrides revealed by the measured resistance $R(T)$ has been confirmed by a number of additional methods.