Novel antimony phosphates with enlarged birefringence induced by lone pair cations

Phosphates, whose obvious disadvantage is the relatively small birefringence, can be overcome by the introduction of post-transition metal cations containing stereochemically active lone-pair electrons. In this paper, two new compounds were successfully explored in the A-Sb-P-O system, i.e. Cs₂Sb₃O(PO₄)₃ (CsSbPO) and (NH₄)₂Sb₄O₂(H₂O)(PO₄)₂[PO₃(OH)]₂ (NH₄SbPOH). Transmission spectra show that CsSbPO has a surprising transmission range with a UV cutoff edge of 213 nm. First-principles calculations show that both compounds have a wide band gap (5.02 eV for CsSbPO and 5.30 eV for NH₄SbPOH) and enlarged birefringence (Δn = 0.034@1064 nm for CsSbPO and Δn = 0.045@1064 nm for NH₄SbPOH). The results of real-space atom-cutting investigations show that the distorted [SbO] polyhedra originating from the asymmetric lone pair electrons give the main contribution to the total birefringence and overcome the disadvantage of small birefringence of phosphates but maintain wide transition windows.