Conventional transparent materials mainly include glass and polymers, and the materials both are widely used. However, they have shortcomings such as low mechanical strength, strong infrared spectral absorption, and unstable chemical properties, which restrict their applications. The degradation of the service environment for materials and the increasing strength requirements have generated an urgent demand for the development of transparent materials with the superior performance. The preparation of ceramic materials with superior optical properties becomes an important aspect in the development of inorganic materials, and the structure and function of the integration of transparent ceramics is a hot spot in materials research. The existing transparent ceramics with various functions can be prepared through the continuous optimization of the ceramic preparation process and the exploration of different ceramic systems. However, enhancing their optical properties is a challenge. A₂B₂O₇ pyrochlore ceramics have excellent properties that can break through the limitations of conventional material design and broaden the application prospects. The preparation of non-stoichiometric ceramics has attracted recent attention, but research in this aspect is still in its nascent stage. In this paper, La_1-xErZr₂O_7-3x/2 transparent ceramics were synthesized via combustion sintering with subsequent combustion. The impact of La content on the phase composition, morphology, and transmittance of ceramics was investigated, and the optical properties of the ceramics under non-stoichiometric conditions were analyzed.

La(NO₃)₃·6H₂O, Er(NO₃)₃·5H₂O, Zr(NO₃)₃·5H₂O and glycine were used. The ceramic powder was prepared by a combustion method, and the powder was subsequently shaped into blocks through ball milling, freeze drying and cold isostatic pressing. After the samples were pre-sintered, they were sintered in vacuum (<10^-3 Pa) at 1 825 ℃ for 6h, and then subjected to subsequent treatment to obtain transparent ceramics.

The phase compositions of powders and ceramics were analyzed by a model X'Pert Pro X-ray diffractometer (XRD) at Cu K_α rays λ of 0.154 426 nm with a scanning step size of 0.03°. The structure and chemical composition of the sample were determined by a model InVia Raman spectrometer. The hot corrosion surface and cross-sectional morphology of ceramics were examined by a model Ultra-55 scanning electron microscope (SEM). The 200 particle sizes of their thermally eroded surfaces were labeled and statistically analyzed by a software named Nano Measurer. The ceramic transmittance was tested in the wavelength range from 200 nm to 2 500 nm by a model Solidspec-3700 solid ultraviolet absorption spectrophotometer. The fluorescence spectra of La_1-xErZr₂O₇-3_x/2 transparent ceramics were characterized by a model F-4500 fluorescence spectrophotometer. The linear transmittance of La_1-xErZr₂O_7-3x/2 transparent ceramics in the near-mid infrared band was measured by a model Spectrum Fourier transform infrared spectrometer (FT-IR). The up-conversion emission spectra of ceramics were determined by a model FLS980 steady-state transient fluorescence spectrometer.

For the prepared non-stoichiometric La_1-xErZr₂O_7-3x/2 transparent ceramics, changes in the crystal structure occur at different La contents in the A-site. For instance, the coexistence of the dual phase in the ceramic gradually transitions to single-phase when La content in the A-site decreases. The lattice distortions in the internal structure of ceramics appear as the nonstoichiometry in the A-site increases.

La_1-xErZr₂O_7-3x/2 transparent ceramics exhibit a good and stable transmittance in the infrared range of 2.5-7.0 μm. In the wavelength range of 200-1 400 nm, La_0.8ErZr₂O_6.7 transparent ceramic has the maximum transparency with a transmittance of approximately 68.8% (1.0 mm in thickness). The main reason for the high transmittance of the ceramic is due to the presence of a few pores and impurities in the grain boundaries, resulting in a lower light scattering. The appropriate reduction of La content effectively eliminates the pores in the grain boundaries of the ceramic and alters the transmittance of the material. Under laser excitation at 980 nm, La_0.8ErZr₂O_6.7 transparent ceramic exhibits three emission peaks, including two main emission peaks centered at 446 nm and 561 nm, which originate from the ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transitions of Er³⁺, respectively. Besides, theintense red emission located at 684 nm originates from the ⁴F_9/2→⁴I_15/2 leap. The fluorescence emission intensity gradually weakens as La content decreases. The possible reason is that the decrease in La content leads to the lattice distortion and vacancies in the crystal structure, resulting in a decrease in the cell parameters and the distance between Er ions.

Non-stoichiometric La_1-xErZr₂O_7-3x/2 transparent ceramics with the micron-sized particles were synthesized via combustion sintering and subsequent combustion. The results showed that the La content could have an impact on the ultimate phase composition, morphology and transmittance of the ceramics. The transmittance of ceramics in the visible light range firstly increased and then decreased as the La content decreased, and the moderate decrease of La content could favor the improvement of the optical quality. The ceramic with x of 0.2 had a superior transmittance in the visible to mid-infrared region. As the La content decreased, there existed a gradual transition from a coexistence of pyrochlore phase/defective fluorite phase to predominantly defective fluorite phase in the ceramic, and the average grain size was 96.6 μm at x of 0.5. The ceramic with the thickness of 1.0 mm and x of 0.1 in the infrared range of 2.5-7.0 μm exhibited a stable transmittance level (i.e., 74.8%).