Mat. Res. Bull., Vol. 29, pp. 89-95, 1994. Printed in the USA. 0025-5408/94 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd. INFLUENCE OF MOLYBDENUM - SUBSTITUTION ON SUPERCONDUCTIVITY OF YTTRIUM BARIUM CUPRATE J.A.Bhalodia,I.A.Shalkh,D.G.Kuberkar,G.J.Baldha and R.G.Kulkarni. Department of Physics, Saurashtra University RaJkot - 360 005 INDIA (Received March Ii, 1993; Communicated by J. Van Landuyt) ABSTRACT : Mo-substituted YBa2(CUl_xMOx)307_ 6 superconductors have been synthesized,under identical conditions and characte- rized according to resistivity and a.c.susceptibility measurements. The Mo ions suppress the superconductivity very effectively,wlth an average rate of 9 K per at % of Mo which is explained on the basis of disordering of the CuO 2 planes and decreasing free carrier concentration. MATI~I~d_~ INDEX : Cuprates, molybdenum, yttrium, barium. As molybdenum exists in multi-valent state 6+,4+,it is of interest to investigate whether the valence state of Cu remains unaffected on Mo-substitution for Cu in YBa2Cu307 8 and also to examine the effect of such substitution on crystal-structure and superconductivity. The substitution of M = Zn,Ni,Co,Fe etc for Cu in the system YBa2(CUl_xMx)307_ 6 (i-4) reduces the T ~while rare earth substitution for tHe~Y-slte shows almost no e~fect on the superconductivity (5). No significant change in T~ of the Nb-doped 1-2-3 compounds has been observed (6) inspite of ~b existing in 5 + state. In this paper, we report X-ray diffraction, resistivity and a.c.susceptibility measurements on the system YBa2(CUl_xMox)307_8, for x = 0.01,0.02,0.03 and 0.04 as no measurements have been made on Mo-substituted 1-2-3. The Mo ions suppress the superconducti- vity effectlvely,with an average rate of 9 K per at ~ of Mo, while the orthorhombicity is decreased by nearly 31% for x = 0.04 compared to the pure Y-123 (x = 0). 89