Preparation and characterization of superconducting Ba_1-xCs _x Ti₂Sb₂O, and its pressure dependence of superconductivity

A new superconducting sample, Ba_1-xCs _x Ti₂Sb₂O, was prepared and characterized in a wide pressure range. The maximum value of superconducting transition temperature, T _c, of Ba_1-xCs _x Ti₂Sb₂O was 4.4 K for x = 0.25 at ambient pressure. The crystal structure was determined to be tetragonal [space group of P4/mmm (No. 123)]. The charge density wave/spin density wave transition was observed at 44 K for Ba_1-xCs _x Ti₂Sb₂O (nominal x = 0.25) at ambient pressure, but the transition was suppressed by applying pressure. The pressure dependent X-ray diffraction showed no structural phase transition up to 23.4 GPa, but very interesting T _c-pressure (p) behavior was observed, i.e. the T _c decreases with an increase in pressure up to 4.0 GPa, but it increased above 4.0 GPa, suggestive of non-BCS type behavior. Thus, the systematic study on new pnictide superconductor, Ba_1-xCs _x Ti₂Sb₂O, was achieved, and the fascinating behavior of superconductivity against pressure was discovered.