LiCs2La(BO3)2 and Li3K9La3(BO3)7: new mixed alkali metal lanthanum borates with three-dimensional open frameworks and short cut-off edges

Abstract

Two new mixed alkali metal lanthanum borates, LiCs₂La(BO₃)₂ and Li₃K₉La₃(BO₃)₇, were synthesized through a high-temperature solution method and millimeter-sized colorless crystals were obtained. The structures of LiCs₂La(BO₃)₂ and Li₃K₉La₃(BO₃)₇ have an intricate three-dimensional (3D) open framework with one-dimensional (1D) infinite channels surrounded by cation polyhedra and isolated BO₃ units. In particular, LiCs₂La(BO₃)₂ exhibits a short cut-off edge <190 nm and a large experimental band gap of 6.20 eV, evidenced by diffuse reflection spectroscopy. Moreover, to better understand the relationship between the electronic structures and optical properties of LiCs₂La(BO₃)₂, theoretical calculations using density functional theory were performed. The result indicated that LiCs₂La(BO₃)₂ has a calculated birefringence of 0.036@1064 nm and the direct band gap is 4.54 eV. In addition, the effects of ion substitution on structural transition and the metal ion/boron ratio on dimensions of the B–O frameworks are also discussed in detail.