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 nx and nz 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.