Fig. 1. (a) Room-temperature transmittance and reflectance spectrum, with insert showing Mg₄Ta₂O₉ crystal wafer, and (b) absorption spectrum with insert showing variation of (α hv)² with photon energy (hv) of as-grown Mg₄Ta₂O₉ crystal.

Fig. 2. (a) Refractive index and extinction coefficient versus wavelength, and (b) dielectric constant and dielectric loss factor versus wavelength of Mg₄Ta₂O₉.

Fig. 3. Plot of n and (n² – 1)⁻¹versusE² of Mg₄Ta₂O₉.

Fig. 4. Polarized Raman spectrum in (a) Y(ZZ)-Y and (b) Z(XY)-Z geometry, with inserts showing view direction along b and c axes of Mg₄Ta₂O₉ single crystal, respectively.

Fig. 5. (a) PL spectrum of Mg₄Ta₂O₉ crystal at room temperature, (b) schematic band diagram with phonon excitation, A and B emission process and non-radiative transition, (c) temperature-dependent PL spectra of Mg₄Ta₂O₉ crystal, and (d) two temperature-dependent fitted peak centers in a range from 85 K to 605 K.