RE(SO₄)[B(OH)₄](H₂O), RE(SO₄)[B(OH)₄](H₂O)₂, and RE(SO₄)[B(OH)₄](H₂O)·H₂O: Rare-Earth Borate-Sulfates Featuring Three Types of Layered Structures

Using hydrothermal reactions, three series of rare-earth borate-sulfates, namely, RE(SO₄)­[B­(OH)₄]­(H₂O) (RE = La (1), Sm (2), Eu (3)), RE(SO₄)­[B­(OH)₄]­(H₂O)₂ (RE = Pr (4), Nd (5), Sm (6), Eu (7), Gd (8)), and RE(SO₄)­[B­(OH)₄]­(H₂O)·H₂O (RE = Tb (9), Dy (10), Ho (11), Er (12), Tm (13), Yb (14), Lu (15), Y (16)), have been synthesized, which represent the first rare-earth borate-sulfate mixed-anion compounds. All these compounds possess the same fundamental building anionic units of SO₄ and B­(OH)₄ tetrahedra; however, they exhibit three different types of two-dimensional (2D) layered structures composed of 1D RE–B–O and RE–S–O chains. The rare-earth borate chains are similar in all compounds, while the rare-earth sulfate chains differ in each type of compound due to the various coordination modes of sulfate groups. On the basis of the measured UV–vis diffuse reflectance spectra, the optical band gaps of compounds 2, 3, 6, and 7 are estimated to be 4.66, 4.53, 4.62, and 4.50 eV, respectively. Luminescence studies show that compounds 2, 3, 6, and 7 exhibit strong emission in the orange or red regions. Furthermore, thermal analysis and magnetic susceptibility measurements for these four representative compounds have also been performed.