Lattice instabilities in single-phase polycrystalline Y1-xCaxBa2Cu4O8 (x=0, 0.005, 0.1)

Abstract

Ultrasonic and specific heat measurements have been performed on single-phase polycrystalline Y1-xCaxBa2Cu4O8 (x=0, 0.05, 0.1 denoted as Y124, Y124Ca0.05, and Y124Ca0.1 respectively) samples in the temperature region 65 to 320 K. It is found that partial replacement of Y by Ca may enhance the interatomic couplings of the lattice, leading to a higher Debye temperature in Ca-substituted samples. A Ca-induced phase transition at 150 K is revealed in Ca-doped samples. A pronounced attenuation peak near 260 K accompanied by a simultaneous change of slope in velocity is observed in pure and Ca-doped Y124 samples. For pure Y124 sample, the corresponding specific heat anomaly is weak and may be clearly seen only after substracting an appropriate background. While for Ca-doped samples, the corresponding anomaly in specific heat is difficult to isolate, due to the influence of the phase transition at 150 K. It is suggested that the anomalies near 260 K are caused by the formation of an antiferroelectric (AFE) or ferroelectric (FE) state associated with the ordering of the oxygen atoms in some off-centre positions of the structure.