A₂B₂O₇ system compounds, which usually present three phase structures mainly based on the ionic radius ratios of r_A and r_B (r_A/r_B), have been studied for potential applications in many fields, such as thermal barrier coatings, luminescence powders, fast-ion conductors, photocatalysts, and matrices for immobilization of highly active radionuclides. Since 2005, La₂Hf₂O₇ was fabricated into transparent ceramics and much more attentions were paid on A₂B₂O₇ transparent ceramics for new applications. In this review, the development of A₂B₂O₇ system transparent ceramics was described. The structure characteristics, powder synthesis method, and sintering techniques of the final A₂B₂O₇ transparent ceramics were summarized. After that, the mostly reported A₂Hf₂O₇, A₂Zr₂O₇, and A₂Ti₂O₇ system transparent ceramics were systematically introduced. The potential application fields and future development trends were also discussed, focusing on scintillators, optical elements, and other luminescent materials

Aluminum nitride (AlN) is considered one of the most desirable materials for integrated circuits and electronic packaging substrates. However, raw AlN powder reacts easily with water, forming Al(OH)₃ or AlOOH on the surface and hindering the development of aqueous tape casting process for preparing AlN ceramic substrates. In this study, hydrolyzed polymaleic anhydride (HPMA) was used to modify AlN powder, which has good water solubility and dispersibility. The AlN powder modified with 5 wt.% HPMA remained stable in water for at least 90 h under magnetic stirring condition and 24 h under ball milling condition, indicating that the HPMA-modified AlN powder has good resistance to hydrolysis. The action mechanism of HPMA is revealed. Firstly, the -COOH of the HPMA polymer and the oxide layer on the surface of the AlN powder underwent a dehydration condensation reaction to form a compound. Secondly, the long chains of the polymer further coated the surface of the AlN powder, forming an anti-hydration layer with a thickness of about 7.0 nm on the surface of the AlN particles. In addition, AlN green sheets were successfully prepared by aqueous tape casting using the HPMA-modified AlN powder without additional dispersants. Subsequently, AlN ceramic substrates were obtained by sintering at 1750 ℃ for 4 h under N₂ atmosphere with a pressure of 0.2 MPa. The relative density and thermal conductivity were tested to be 97.3 % and 122 W·m^-1·K^-1, respectively.

Multiwall boron nitride (BN) nanotubes were synthesized by a novel physical vapor deposition (PVD) method, in which the BN nanotubes grow on a compact substrate composed of AlN, γ-Al₂O₃, Y₂O₃, and carbon powders. The obtained BN nanotubes assemble in an orderly manner with a typical length of over one millimeter and a diameter of one-hundred nanometers. The hollow multiwall tubes have a spherical tip, which is presumed to be a liquid drop at the synthesis temperature, indicating the vapor–liquid–solid (VLS) growth mechanism.

Highly transparent Ho:Y₂O₃ ceramics for laser diode lighting were prepared using the vacuum sintering method with 0.3 at.% Nb₂O₅ as a sintering additive. The microstructures, transmittance, and luminescence properties of the Ho:Y₂O₃ ceramic samples were investigated in detail. The transmittance levels of all samples with various Ho³⁺ concentrations reached ~81.5% (2 mm thick) at 1100 nm. Under the excitation of 363 nm (ultraviolet) or 448 nm (blue) light, Ho:Y₂O₃ transparent ceramic samples showed that green emission peaked at 550 nm. The emission intensity was strongly affected by the concentration of Ho³⁺ ions, reaching its highest level in the sample doped with 1 at.% Ho³⁺. The CIE coordinates of the luminescence were in the green region (i.e., the CIE coordinates of the sample doped with 1 at.% Ho³⁺ were [0.27, 0.53] and [0.30, 0.69], under the excitation of 363 nm and 448 nm light, respectively). The possibility of its application as laser diode lighting was reported. Under the excitation of 450 nm blue laser, the sample doped with 0.5 at.% Ho³⁺ had the best performance: the saturated luminous flux, lumen efficiency, and the luminescence saturation power densities were 800 lm, 57.7 lm/W, and 17.6 W/mm², respectively. Furthermore, the materials have high thermal conductivity and mechanical strength due to their host of rare-earth sesquioxide. Thus, Ho:Y₂O₃ transparent ceramics are expected to be a promising candidate for green-light-emitting devices for solid-state lighting, such as laser diode lighting.

Highly transparent Er:Y₂O₃ ceramics (1–9 at.% Er) were fabricated by hot pressing sintering with ZrO₂ as the sintering additive. The microstructures, transmittance, luminescent properties, thermal conductivity, and mechanical properties of the Er:Y₂O₃ ceramic samples were investigated in detail. The samples all exhibited dense and fine grain microstructures; the average grain sizes were about 0.8 μm. The transmittance levels of the samples with various Er concentrations (2 mm thick) at the wavelengths of 600 and 2700 nm were ~74 and ~83%, respectively. As the Er doping concentration increased from 1 to 9 at.%, the up-conversion luminescence of the samples gradually changed from green to red, with the intensity ratio of red/green light increasing from 0.28 to 2.01. Meanwhile, the down-conversion luminescence properties of the specimens were also studied. When the samples were under 980 nm excitation, the emission bands were detected at 1552, 1573, 1639, and 1661 nm. The thermal conductivity of the samples was found to decrease from 8.72 to 5.81 W/(m·K) with an increase of the Er concentration from 1 to 9 at.%. Moreover, the microhardness and fracture toughness of the samples with 1 at.% Er concentration were ~8.51 GPa and ~1.03 MPa·m^1/2, respectively.