Superconducting compounds with metallic square net

A charged square net in a solid-state compound is well known as a crystallographic unit that possesses unpaired electrons. Such compounds look promising as parent materials for the emergence of intriguing electronic properties such as superconductivity. For example, square nets of P/As/Bi/Sb anions, which are distorted by electronic instability such as the Jahn–Teller effect, have been well investigated theoretically. In this paper, we report results of exploratory research on several series of compounds containing the metallic square nets to develop new superconductors, i.e., LnNixBi2 (Ln=rare earth) with a Bi1− square net (Tc∼4K), doped LaCo2B2 with a Co net (Tc∼4K), and La2Sb with a La net (Tc∼5K). In addition, it is reported that Ln2O2Bi with a Bi2− square net shows a metal–insulator transition (MIT) by varying the size of the Ln3+ ion that consequently causes adjusting the size of the Bi2− net and induces chemical pressure. It is explained that the MIT is induced by increased width of the Bi 6p band around the Fermi level through the chemical pressure of the Ln3+ ion substitution.

► The strategies to look for new superconductors that contain cationic or anionic square nets.
► Several compounds including La2Sb, CeNi0.8Bi2, and LaCo2B2 exhibit superconductivity.
► Ln23+O2−2Bi2− (Ln=Rare Earth) that contains Bi2− square nets shows a metal–insulator transition.