Abstract
Metal-insulator transition near oxygen content parameter δ ≃ 0.018 was observed for electron-doped Pr 1.85 Ce 0.15 CuO 4+δ (-0.003 ≤ δ ≤ 0.03 cuprates. Cu K -edge X-ray absorption near-edge structure (XANES) studies with nearly identical threshold edge energy E o of 8980.8 eV indicate a Cu formal valence smaller than 2 for all samples, which is consistent with the estimated Cu valence of ∼ 1.84 for 20.5 K superconductor Pr 1.85 Ce 0.15 CuO 3.997 and ∼ 1.91 for Pr 1.85 Ce 0.15 CuO 4.03 insulator. The XANES spectrum reflects the Cu 3d n character where low energy peak A 1 reflects the 3d 10 configuration of Cu(I) oxidation state and A 2 peak reflects the 3d 10 L (L for a oxygen ligand hole) configuration for Cu(II) oxidation state. The variation of energy separation Δ E (A 2 -A 1 ) is consistent with the observed metal-insulator transition, increases sharply from 2.42 e V for Pr 1.85 Ce 0.15 CuO 4.018 insulator to 2.74 e V for 15 K underdoped superconductor Pr 1.85 Ce 0.15 CuO 4.015 .