Effect of fluorine on the critical current density of thallium based high T_c superconductors

(Tl_0.5, Pb_0.5)Sr_1.6Ba_0.4Ca₂Cu ₃O_y/F_x high temperature superconductors are reported with transition temperatures above 125 K. Samples with different fluorine contents (0 ≤ x ≤ 3) have been prepared. The addition of fluorine has little effect on the transition temperature, but remarkably increases the width of the hysteresis loops (which is proportional to J_c). The width of the hysteresis loops at 4.2 K was found to increase with x reaching an optimum value at x = 2.2. The, maximum width of the hysteresis loops is about 3 times larger than that for fluorine free material. Upon further increases in fluorine content the width of the hysteresis loops starts decreasing.