ORIGIN OF THE METAL-INSULATOR-TRANSITION IN THE SUPERCONDUCTING SERIES BI2SR2CA1-XYXCU2O8+DELTA

The metal-insulator transition in the solid solution Bi2Sr2Ca1-xYxCu2O 8+delta (0 less-than-or-equal-to x less-than-or-equal-to 1) has been i nvestigated by TGA (oxygen content) and by X-ray absorption spectrosco py (Bi and Cu valence states). Resistivity and AC magnetic susceptibil ity measurements have shown that the superconducting properties and th e metallic behavior vanish for x>0.55. The oxygen content delta is lar ger than x/2 for x less-than-or-equal-to 0.3 and smaller than x/2 for x greater-than-or-equal-to 0.6. For x=0, the Cu K edge shows a shift t owards high energy with respect to the Cu(II) oxide La2CuO4; this shif t decreases with increasing x in agreement with the decrease of the do ping hole density and the variations of the physical properties. For 0 less-than-or-equal-to x less-than-or-equal-to 0.3, the Bi L3 edge sho ws a shift of 1 eV towards tow energy with respect to the Bi(III) oxid e Bi2O3 in agreement with the charge transfer between [CuO2]infinity a nd [BiO]infinity planes. This shift also decreases with increasing x, but is still present for the x=0.6 composition for which delta is smal ler than x/2. A model of the metal-insulator transition in this series is proposed based on the fact that the intercalation of excess oxygen raises the bottom of the Bi-O band with respect to the Fermi level an d decreases the contribution of the Bi-O electron pocket to the hole d ensity.