High-pressure synthesis and superconductivity of Sr–Ca–Cu–O–Cl compounds with 0212-type and 0223-type structure

We synthesized 02( n−1) n-type Sr 2Ca n−1 Cu n O x Cl y ( n=2, 3) with their apical sites occupied by both oxygen and chlorine by a high-pressure technique without using a precursor. The n=2 phase showed superconductivity below 83 K, while the T c for the n=3 phase was found to be 35 K. We examined the effects of oxygen partial pressure and Cl content on the phase formation by varying the nominal composition in starting materials. It was found that phases with a fewer number of CuO 2 sheets are formed, when samples are heated under higher oxygen pressure, which is similar to the tendency observed in other homologous series of cuprates. The results of thermoelectric power measurements suggest that the as-synthesized n=2 phase is located in the underdoped region and the n=3 phase is much more underdoped than the n=2 phase.