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 FeH x compounds have been studied by measuring the electrical resistance R ( T ) at high pressures (180–216 GPa) in the temperature range of ~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 ≈ 25.0 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 ~ 0.063 ± 0.001 K/GPa and 0.056 ± 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.