  Superconductive H3S synthesized from H2S is very poorly crystallized, and has
excess sulfur as impurity (3H2S -> 2H3S + S). The phase transition process
undergoes in sulfur excess condition, which might cause hydrogen deficiency.
The influence of hydrogen deficiency is not clear. Therefore investigation on
the superconductivity in H3S with no hydrogen deficiency is demanded. Two
groups performed synthesis of H3S from elemental sulfur and hydrogen (direct
synthesis) and their results have shown that no hydrogen deficiency is caused
when the direct synthesis is performed under hydrogen excessive condition.
However, no measurements of superconductivity has been carried out because of
the major technical difficulties in hydrogen experiments in diamond anvil cells
(DACs). Here, we report the first electrical resistance measurements in
superconductive H3S synthesized from elemental sulfur and hydrogen (3H2 + 2S ->
2H3S). Our powder X-ray diffraction (PXRD) using a synchrotron X-ray revealed
that synthesized H3S has much improved crystalline quality and no sulfur as
reported in previous works. We observed a superconducting transition with a
sharp drop of the resistance at Tonset = 200 K and we obtained the highest
Toffset of 186 K in S-H system.
