FLUORINE OXIDATION OF HIGH-TEMPERATURE SUPERCONDUCTORS

Both YBa//2Cu//3O//7// minus //x and EuBa//2Cu//3O//7// minus //x starting materials were treated with fluorine gas at 350 degree C. YBa//2Cu//3O//7// minus //xF//z exhibits a 2-4 K enhancement of the superconducting critical temperature T//c for z equals 0. 04-0. 2. For 0. 2 less than z less than 0. 4 the T//c enhancement of the yttrium compound is about 2 K, and for z greater than 0. 5 the T//c and the fraction of superconducting material decrease. For EuBa//2Cu//3O//7// minus //xF//z, T//c decreases by about 2 K after fluorine treatment for z equals 0. 12-0. 54. Powder x-ray diffraction patterns for both compounds reveal no structural difference between the fluorine-treated and untreated samples for F//2 treatment yielding less than 0. 5 mole F** minus /mole compound, additional phases and destruction of the perovskite structure are evident. Heat treatment (300-950 degree C for 1-24 h) of F//2//- treated samples in an air or oxygen environment causes the samples to regain their original, untreated superconducting properties. Fluoride ion analysis of these F//2-, then O//2- or air-treatment samples shows that little or no F** minus remains in the samples.