Closure of the charge-transfer energy gap and metallization ofNiI2under pressure

Abstract
We have performed resistivity and near-ir absorption measurements on NiI2 under pressure that strongly suggests that closure of the charge-transfer energy gap is the fundamental cause of the insulator-to-metal transition in NiI2. Our findings support the framework proposed by Zaanen, Sawatsky, and Allen outlining the electronic structure of transition-metal compounds.