We report the XRD observation of the YBa2Cu3O7/YBa2Cu3O8 structural is
othermal transformation at room temperature and atmospheric oxygen pre
ssure. It is shown that this structural transformation runs according
to the scheme: YBa2Cu3O7-x --> --> Y2Ba4Cu7O14+y --> YBa2Cu4O8. These
results are the first straightforward experimental evidence that the s
uperconducting orthorhombic YBa2Cu3O7-x (x approximate to 0) is metast
able under the ambient pressure at moderate temperature.