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Effect of microstructure on positron-annihilation parameters in YBa2Cu3O7.

The temperature dependence of positron Doppler line shape has been measured for several samples of YBa/sub 2/Cu/sub 3/O/sub 7-//sub x/ (xless than or equal to0.1). Two distinct temperature dependences have been observed. In one case the line-shape parameter S increases by approx. =0.5% between 15 and 100 K, remains constant between 100 and 230 K, and then decreases by approx. =0.25% between 230 and 290 K. In the second case, S remains constant between 20 and 100 K, decreases by approx. =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.