We have synthesized two iron-pnictide/chalcogenide materials, CuFeTe2 and Fe2As, which share crystallographic features with known iron-based superconductors, and carried out high-pressure electrical resistivity measurements on these materials to pres
sures in excess of 30 GPa. Both compounds crystallize in the Cu2Sb-type crystal structure that is characteristic of LiFeAs (with CuFeTe2 exhibiting a disordered variant). At ambient pressure, CuFeTe2 is a semiconductor and has been suggested to exhibit a spin-density-wave transition, while Fe2As is a metallic antiferromagnet. The electrical resistivity of CuFeTe2, measured at 4 K, decreases by almost two orders of magnitude between ambient pressure and 2.4 GPa. At 34 GPa, the electrical resistivity decreases upon cooling the sample below 150 K, suggesting the proximity of the compound to a metal-insulator transition. Neither CuFeTe2 nor Fe2As superconduct above 1.1 K throughout the measured pressure range.
The CeFeAsO and CeFePO iron pnictide compounds were studied via electrical transport measurements under high-pressure. In CeFeAsO polycrystals, the magnetic phases involving the Fe and Ce ions coexist up to 15 GPa, with no signs of pressure-induced s
uperconductivity up to 50 GPa. For the CeFePO single crystals, pressure stabilizes the Kondo screening of the Ce 4f-electron magnetic moments.
Studies of the effect of high pressure on superconductivity began in 1925 with the seminal work of Sizoo and Onnes on Sn to 0.03 GPa and have continued up to the present day to pressures in the 200 - 300 GPa range. Such enormous pressures cause profo
und changes in all condensed matter properties, including superconductivity. In high pressure experiments metallic elements, Tc values have been elevated to temperatures as high as 20 K for Y at 115 GPa and 25 K for Ca at 160 GPa. These pressures are sufficient to turn many insulators into metals and magnetics into superconductors. The changes will be particularly dramatic when the pressure is sufficient to break up one or more atomic shells. Recent results in superconductivity to Mbar pressures wll be discussed which exemplify the progress made in this field over the past 82 years.
