Ba-free mercury-based cupric oxide superconductors "(Hg, M)Sr
2Ca
n-1Cu
nO
y" (M=Re, Cr, Mo; n=2, 3) were prepared by using high-pressure technique. The substitution of M for Hg was found to be essential to the stabilization of the Ba-free phases, which are isostructural to HgBa
2Ca
n-1Cu
nO
y As-grown Re-doped sample was over-doped, and T
c varied systematically as a function of temperature of annealing in an evacuated quartz ampoule. The maximum TT thus attained is 120 K for Re-doped n=3 phase. As-grown Cr-doped n=3 phase was under-doped superconductor, and T
c up to 110K by oxidization. As opposed to samples of HgBa
2Ca
n-1Cu
nO
y' these compounds "(Hg, M)Sr
2Ca
n-1Cu
nO
y" (M=Re, Cr, Mo; n=2, 3) are stable in air at room temperature.
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