PHYSICAL REVIEW B 101, 075134 (2020) 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.

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