The structure of a newly stabilized superconducting phase ErgaSrCu2.9(PO4)0.1O(y) was studied by means of electron microscopy and powder neutron diffraction. The electron microscopy study on ErBaSrCu2.9(PO4)0.1O(y) revealed the existence of a tweed-like structure. Rietveld refinement was carried out on ErBaSrCu(3-x)(PO4)(x)O(y) (x = 0.0, 0.10, and 0.20) with the orthorhombic space group Pmmm. Substitution of the phosphate group at the copper site in ErBaSrCu(3-x)(PO4)(x)O(y) influenced Cu(2)-O planes strongly. (C) 2000 Academic Press.