A Metal−Insulator Transition in R₂O₂Bi with an Unusual Bi²⁻ Square Net (R = Rare Earth or Y)

A series of tetragonal ThCr2Si2-type compounds, R2O2Bi (R = rare earth or Y), are synthesized in which an unusual Bi2− anion forms a square net layer that is sandwiched between (R2O2)2+ fluorite layers. Two-dimensional (2D) electronic bands around the Fermi energy are predominantly composed of 6px6py orbitals in the Bi2− square net, which contains a positive hole per Bi2− ion. The decrease in the size of the square net caused by reducing the size of the R ion enhances the electrical conductivity because of the hole, resulting in a “chemical pressure”-induced metal−insulator transition.