Effect of microstructure on positron-annihilation parameters in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

The temperature dependence of positron Doppler line shape has been measured for several samples of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ (x≤0.1). Two distinct temperature dependences have been observed. In one case the line-shape parameter S increases by ≊0.5% between 15 and 100 K, remains constant between 100 and 230 K, and then decreases by ≊0.25% between 230 and 290 K. In the second case, S remains constant between 20 and 100 K, decreases by ≊1% between 100 and 200 K, and then remains constant up to 290 K. Room-temperature positron-lifetime measurements also reveal distinct differences between samples. An explanation of these differences based on micro- structural differences observed by transmission electron microscopy is suggested.