The role of oxygen vacancies in superconducting properties of Ba
2YCu
3O
y (6.22<
y<6.96) has been investigated by EXAFS and XANES on the Cu K-edge. Fourier transform analysis of EXAFS shows that oxygen vacancies influence the local structure through (1) the disappearance of O1-Cu1-O1 chain and displacement of atomic positions along the c-axis, (2) the Cu-O vibrational properties and (3) introducing structural disorder between the CuO
2 planes. Cu XANES spectra show that Cu valency in Ba
2YCu
3O
y is essentially 2+ with some 3+ contribution. Oxygen vacancies create Cu1
+for
y<6.7 at which high T
c (90K) degrades to low T
c (50-60K) superconducting states.
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