Pioneering sulfophosphite crystals: Co-assembly of phosphite and sulfate units in novel inorganic architectures

Abstract

Compounds [Ln(HPO₃)[S_0.5(OH)₄] (Ln = La, Ce) and La[HPO₂(OH)]SO₄·H₂O represent the first reported examples of sulfophosphite materials constructed through the co-assembly of HPO₃ and SO₄ building units. The structures of Ln(HPO₃)[S_0.5(OH)₄] (Ln = La, Ce) feature 2D anionic Ln–P–O layers, composed of 1D Ln–O chains bridged by HPO₃ units, which are further linked by S(OH)₄ tetrahedra to form its 3D framework. La[HPO₂(OH)]SO₄·H₂O consists of 1D La–S–O chains formed by La₂O₁₄ dimers and SO₄ tetrahedra, which are further extended into a three-dimensional network via HPO₂(OH) linkers. The partial occupancy of sulfur sites in Ln(HPO₃)[S_0.5(OH)₄] (Ln = La, Ce) was validated by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. Optical properties investigated through diffuse reflectance spectroscopy, combined with Urbach tail fitting and Tauc plot analysis, revealed that all three compounds exhibit direct band gaps of 4.53, 4.55, and 4.49 eV, respectively. This work not only broadens the structural and compositional diversity of phosphite-based materials but also provides a novel design strategy for the development of functional sulfophosphites with tunable properties.