Introduction Transparent ceramics have been developed in the past decades due to their superior performance in optics, lenses, and armor. For their effective use, transparent ceramics exhibit a high optical transmittance from the ultraviolet-visible to the infrared range as well as superior mechanical and thermal properties. Metastable gamma-type aluminum oxide (γ-Al2O3) is one of the transition γ-Al2O3 crystals with a face-centered cubic crystal structure. γ-Al2O3 crystal can theoretically transmit s light over a substantial range of wavelengths (from deep ultraviolet to mid-infrared) and exhibit optical qualities comparable to those of single crystal sapphire. However, little work on this ceramic compound as a transparent bulk material has been conducted. Materials and method γ-Al2O3 powder was synthesized by a homogeneous-precipitation method. A solution containing Al3+ ions was prepared via dissolving aluminum nitrate nonahydrate (Al(NO3)3·9H2O, 99.99%) in distilled water. A precipitant solution was prepared with 1.5 mol/L ammonium hydroxide (NH4HCO3, analytical grade) and 0.5 mol/L ammonium hydroxide (NH4·H2O, analytical grade) in ethanol and distilled water (a molar ratio of 1:3). A nitrate salt solution was added in dropwise into the precipitant solution by a peristaltic pump at 2.5 mL/min under stirring at 18 ℃. After 24 h, the suspension at room temperature was filtered, and the resulting precipitate was washed with distilled water and ethanol. The precipitates were dried in a vacuum drier at 100 ℃ for 24 h, sieved through a 200-mesh screen, and then calcined in ambient air at 500-1 200 ℃ for 4 h. We carried out a series of high-pressure experiments at a temperature range from room temperature to 700 ℃ and at a pressure range from 3 GPa to 6 GPa. High-pressure sintering experiments were performed by a model mavo press LPR 1 000-400/50 Kawai-type apparatus with a Walker-module (Max Voggenreiter GmbH, Germany) and a model DS6&14 MN cubic press apparatus (China). The Kawai-type apparatus was used to explore the synthesis condition for the γ-Al2O3 transparent ceramics, and the cubic press apparatus was employed to acquire the large-size sample. The γ-Al2O3 powders prepared as described above were uniaxially dry-pressed at 20 MPa and finally the green bodies were cold-isostatically pressed at 200 MPa. The green bodies were then cold compressed at 3-6 GPa and subsequently heat-treated at 300-700 ℃ at a heating rate of 20 ℃/min and a cooling rate of 10 ℃/min. After maintaining this temperature for 20 min, the temperature decreased to room temperature at the same rate and then the pressure was released. The sample chamber temperature was measured directly by a thermocouple (PtRh 6%-PtRh 30%), and the pressure in the sample chamber was calibrated using the phase transitions of Bi, Tl, and Ba at a high pressure.Results and discussion Cubic gamma-alumina oxide transparent ceramic materials were prepared via high pressure sintering at a pressure of a certain GPa and a lower temperature. The result shows that the optimum optical properties of γ-Al2O3 transparent ceramic material with an average grain size of 20 nm sinterred at 5 GPa and 300 ℃ are achieved. The maximum transmittance of 86% can be obtained in the range of 0.6-1.2 μm, compared to that of single-crystal sapphire. The Vickers hardness of cubic γ-Al2O3 transparent ceramic is 17 GPa, which is similar to that of a conventional sapphire single crystal. In addition, the dielectric constant (i.e., 9.46) and dielectric loss (i.e., 0.00 11) are also comparable to those of sapphire in the c-axis direction. For γ-Al2O3 doped with Eu3+ ions, trivalent Eu3+ can be self-reduced to bivalent Eu2+. The most intense Eu2+ emission intensity is acquired in γ-Al2O3:Eu3+ materials heated at 300 ℃ and gradually decreases to a minimum value when further improving at 1 200 ℃, accompanied by the increase of trivalent Eu3+ ions emission intensity. The cubic alumina matrix has a structural advantage for self-reduction response, and hence is suitable for the incorporation of variable valence ions for acquirement of cubic aluminum oxide functional transparent ceramic materials.Conclusions The optimum optical properties of γ-Al2O3 transparent ceramic sinterred at 5 GPa and 300 ℃ were achieved. The optical transmittance of high quality γ-Al2O3 transparent ceramic reached 86% in visible and near-infrared wavelength region. The average grain size was 20 nm. The Vickers hardness measured at room temperature was 17 GPa. The dielectric constant and the dielectric loss were 9.46 and 0.001 1, respectively. When heated in air, γ-Al2O3 evolved into tetragonal δ-Al2O3 phase at 900 ℃, θ-Al2O3 phase at 1 000 ℃, and α-Al2O3 phase at 1 100 ℃, respectively. However, γ-Al2O3 phase started to transform into the mixed phase including metastable phase γ-AlOOH, Al2O3.H2O and stable α-Al2O3 phase after sintering at 5 GPa and 600 ℃. When the trivalent Eu3+ was added into γ-Al2O3 matrix, the trivalent Eu3+ could self-reduce to the bivalent Eu2+. Also, the most intense Eu2+ emission was obtained after heating at 300 ℃. However, the emission intensity of Eu2+ decreased with the increase of heating temperature to 500 ℃. After further heated at 1 200 ℃, the emission of Eu2+ in γ-Al2O3 matrix disappeared, accompanied by the increase in the emission intensity of trivalent Eu3+ ions. These results indicated that the cubic alumina matrix could be suitable for the incorporation of variable valence ions, which was conducive to the preparation of cubic aluminum oxide functional transparent ceramic materials.