Ca-induced phase transition at 150 K inY1−xCaxBa2Cu4O8single-phase high-Tcsuperconducting material

Abstract

Elastic and specific-heat measurements have been performed on single-phase polycrystalline ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ (x=0, 0.025, 0.05, 0.1) in the temperature region from 100 to 180 K. Anomalies are detected in elastic and specific-heat measurements at 150 K for Ca-doped specimens. A reversible steplike change in sound velocity accompanied by an attenuation peak is observed in material with x=0.1 doping at 150 K on cooling and heating, for both longitudinal and transverse modes. These elastic anomalies have a clearly observable counterpart in the specific heat. The anomalies in elastic and specific-heat measurements show dominantly the character of a mean-field-type phase transition. We observe similar anomalies at the same temperature even for x=0.025 and 0.05, while for pure ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ samples no anomalies are found in this temperature region. Thus we find a Ca-induced, but concentration-independent, transition temperature in the available range of x values. No magnetic anomaly is found by ac magnetic susceptibility. The transition is proposed to be of a structural type although x-ray powder diffraction did not reveal the origin.