SnHPO₄: A Layered Tin(II) Phosphate with Enhanced Birefringence

As promising optoelectronic functional materials in the short-wavelength spectral region, such as ultraviolet (UV) and deep UV, phosphates have recently received increased attention. However, phosphate materials commonly suffer from limited birefringence owing to the highly symmetrical PO₄ tetrahedra. We herein report a layered tin(II) phosphate with improved birefringence. By employing a polarizing microscope, the measured refractive index difference determined on a (010) wafer is 0.042@550 nm, which is very close to the calculated refractive index difference of 0.039@550 nm between n and n through the density functional theory (DFT) method. The largest birefringence appears on the (100) plane, which is theoretically determined to be 0.078@550 nm. Single crystals of SnHPO₄ measuring 20 × 4 × 3 mm³ can be easily grown by a hydrothermal method. In addition, SnHPO₄ is UV transparent with a short UV absorption cutoff edge of 242 nm and an optical band gap of 4.50 eV, implying that it could be a potential UV birefringent material.