Na₂MP₃O₉ (M = K, Rb): Two Potential Zero-Order Waveplate Materials with DUV Cutoff Edges

Zero-order waveplates (ZOWPs) are crucial for polarization measurements and the laser industry. In this work, two deep-ultraviolet (DUV) optical crystals, Na₂MP₃O₉ (M = K, Rb), were synthesized successfully by introducing alkali metals and isolated [P₃O₉] groups with the low polarizability. Comprehensive characterization reveals that Na₂MP₃O₉ (M = K, Rb) feature DUV cutoff edges (<200 nm) and tiny birefringence (0.0078 and 0.0087 @ 532 nm, respectively). In addition, they are found to be congruent melting compounds by X-ray diffraction (XRD) analysis. The first-principles calculations exhibit that the P-O unit is the main contributor to the bandgap and other optical performances. The investigation demonstrates that the [P₃O₉] ring is an advantageous group for exploring DUV ZOWP optical materials.