Unusual electronic state of Sn in AgSnSe2

AgSnSe2, by formal electron count, should have Sn in a highly unusual 3+ valence state and was therefore suggested to be a valence-skipping compound with potential for negative-U centers and local electron pairing. It has been proposed that the latter may be the mechanism beyond seemingly conventional superconductivity in this compound. We report NMR measurements and first-principles calculations that agree with each other perfectly, and both indicate that valence skipping does not take place and the highly unusual Sn3+ state is realized instead, likely because of geometrical constraint prohibiting a breathing distortion that could screen the on-site Coulomb repulsion.