Transparent Gd₂O₃ ceramics with a cubic crystal structure were successfully synthesized for the first time. The Gd₂O₃ ceramics subjected to hot isostatic press (HIP) at 1070 ℃ consisted of uniform grains of 2–3 μm, and only a few residual pores could be detected inside the materials. X-ray diffraction (XRD) shows that the obtained polycrystalline materials have a cubic crystal structure, and the transmission polarized light observation shows that there is no birefringence inside the materials, indicating inherent optical properties of the cubic crystal structure. The optical absorption edge is 640 nm (optical band gap: 2.1 eV), and the transmittance in the visible to near-infrared region is 72%, which is close to the theoretical transmittance calculated from Fresnel loss. Verdet constant of this material is almost wavelength-independent, and it is diamagnetic since it has the opposite polarity to that of common paramagnetic materials.

Excellent mechanical and isotropic optical properties are achieved simultaneously from fully dense polycrystalline advanced alumina ceramics with a hexagonal crystal structure, which are optically anisotropic. A small amount (240 ppm) of ZrO₂ additive is used to synthesize the transparent alumina ceramics with an average grain size of 0.7 µm, and locally detected optical distortion (birefringence) is extremely minimized, less than 20 nm/pass. Total transmittance (86%) of the alumina ceramics (thickness = 0.3 mm) in ultraviolet (UV) to infrared (IR) regions is similar to that of commercial c-axis sapphire single crystals produced by Czochralski (CZ) method, while the extinction ratio in the visible wavelength area is over 25 dB. Like glass and the c-axis sapphire single crystals, the alumina ceramics can clearly display texts and images on liquid crystal display (LCD) screens. A grain boundary phase of nano-sized Al₂O₃–ZrO₂ composition (amorphous) is formed at the grain boundary of the advanced alumina ceramics, which enhances four-point bending strength and fracture toughness (K_IC) simultaneously to 921 MPa and 6.8 MPa·m^0.5, respectively, and hence the mechanical properties are superior to those of the sapphire single crystals.