Sr₂CdGe₃O₉: A Potential Zero-Order Waveplate Crystal Featuring an Enlarged Bandgap and Extended Mid-Infrared Transparency

Zero-order waveplates play pivotal roles in polarization manipulation and laser technology, yet the fabrication of zero-order waveplate crystals that simultaneously exhibit small birefringence, a large bandgap, and a wide infrared (IR) transmission range remains a challenge. In this work, a novel germanate optical crystal, Sr₂CdGe₃O₉ (SCGO), featuring isolated rigid [Ge₃O₉] clusters, was identified through spontaneous crystallization. SCGO demonstrates remarkable thermal stability, with an incongruent melting point of 1054 °C. Importantly, SCGO displays a favorable low birefringence of 0.006@1064 nm, superior to that of the commercially available waveplate crystal α-SiO₂ (0.0092@532 nm). Additionally, this crystal exhibits an enlarged bandgap of 4.7 eV, a notably short UV cutoff edge at 225 nm, outperforming most germanates containing transition metals, along with an extended IR transmission window up to 6.0 μm. These exciting optical properties highlight the potential of SCGO as a zero-order waveplate crystal spanning from the UV to mid-IR wavelength range. The presence of isolated [Ge₃O₉] groups in the SCGO structure introduces a weak structural anisotropy, opening up new avenues for exploring zero-order waveplate materials in wide wavelength ranges.