Introduction The defect pyrochlore ABB’O6 with a cubic crystal structure is widely used in photocatalysis, ion exchange, microwave dielectric ceramics and ion conductors. Recently, KNbTeO6 transparent ceramics with a high density and no secondary phase were prepared by a pseudo-hot isostatic pressing (PHIP) technology. The in-line transmittance of over 80% was obtained in this material, demonstrating potential applications in infrared imaging, windows, and optical lenses, etc.. The optical properties of transparent ceramics are closely related to their composition and structure. ANbTeO6 (A=K, Rb) compounds have a defect pyrochlore structure, and the alkali metal cations affect the crystal structure and optical bandgap of ANbTeO6. It is thus important to elucidate the impact of A-site ions on the structure and optical properties of the ANbTeO6 defect pyrochlore transparent ceramics. This paper was to investigate the composition, structure and optical properties of ANbTeO6 (A=K, Rb) transparent ceramics.Methods KNbTeO6 and RbNbTeO6 (abbreviated as KNT, RNT) powders were synthesized via solid-state reaction in air with K2CO3 (99.99%), Rb2CO3 (99.99%), Nb2O5 (99.99%), and TeO2 (99.99%) as raw materials in a ratio of 1:1:2. The synthesized and ball-milled powders were pressed into disks with 20 mm diameter and 3 mm thickness in a uniaxial pressure at 5 MPa, and then cold-isostatically pressed at 200 MPa for 5 min. The green bodies were sintered in air to obtain the pre-sintered bodies, and then the pre-sintered bodies were placed in a model HP D60/0 SPS apparatus (FCT Co., Germany) for PHIP processing. The obtained ceramic samples were ground and finally polished on the both sides.The phase composition of KNT and RNT powders and ceramics was examined by a model PANalytical X-ray diffractometer with Cu Kα radiation (XRD). The microstructure of powders and ceramics was determined by a model FEG250 field emission scanning electron microscope (FEI Quanta). The refractive indices of the ceramics in a wavelength range of 193?1 690 nm were measured by a model M-2 000 V elliptical polarization spectrometer. The in-line transmittance spectra were collected from the ceramics polished on the both sides with the thickness of 1 mm. The Raman vibration spectra of transparent ceramics were measured by a model LabRAM laser confocal microscopy Raman spectrometer. All the tests were conducted at room temperature.Results and discussion The XRD patterns of both powders exactly correspond to KNT and RNT with a cubic defect pyrochlore structure, respectively. The Rietveld method was used to refine the XRD patterns of the powders to obtain the information of crystal structure. Clearly, the lattice constant increases from 10.247 9 ? (KNT) to 10.258 0 ? (RNT) and the cell volume expands from 1 076.24 ?3 (KNT) to 1 079.41 ?3 (RNT) with little change in the length of (Nb/Te)-O bond. The composition and element distribution of the PHIP-ed ceramics with a few residual pores and an irregular grain boundary are not affected by sintering, according to the EDS spectra. The relative density of the both ceramics is 99.78% (KNT) and 99.55% (RNT), respectively. The grain size distribution is narrow, and the average grain size is 417 nm (KNT) and 378 nm (RNT), respectively. The optical transmission range of two transparent ceramics is 0.5-7.2 μm. The maximum in-line transmittances reach 80.25% (KNT, @ 2 677 nm) and 80.14% (RNT, @ 3 876 nm), which are similar to their theoretical values. The optical properties of KNT and RNT transparent ceramics were further discussed through valence electron band transitions, molecular electron polarizability, and lattice phonon vibrations. Compared with KNT, RNT transparent ceramic exhibits a slight red-shift in the absorption edge of visible light because of the reduction of the band gap from 2.23 eV (KNT) to 2.20 eV (RNT). The optical refractive index at 589.3 nm increases from 1.973 (KNT) to 2.013 (RNT) obtained from the fitted Sellmeier equation. The electron polarization of Rb+ is greater than that of K+, increasing the total molecular electron polarization, and K+ has a smaller radius than Rb+, expanding the lattice and increased the cell molar volume. The Abbé number of RNT (i.e., 18.86) is slightly lower than that of KNT (i.e., 19.45), indicating that the dispersion of RNT transparent ceramic is greater than that of KNT. Based on the Raman spectroscopy analysis, the high-frequency vibration in RNT shifts towards a low wavenumber, i.e., weakening in (Nb/Te)O6 octahedral vibration, which broadens the infrared cutoff wavelength of RNT by 0.3 μm.Conclusions Pure defect pyrochlores ANbTeO6 (A=K, Rb) ceramic powders were synthesized via solid-state reaction, and the transparent ceramics with the maximum in-line transmittance of 80% both were fabricated via pressure-less sintering and pseudo-hot isostatic pressing sintering. Based on the crystal structure analysis, the increase in the radius of A-site cations led to the cell expansion with a little change in the length of (Nb/Te)-O bond. Compared with KNT, RNT transparent ceramics had a red-shift in a visible light absorption edge, an increase in refractive index with a slightly higher dispersion, and a wider infrared cutoff wavelength. Defective pyrochlore ANbTeO6 (A=K, Rb) transparent ceramics had a potential application in miniaturization of mid-infrared lenses and devices with a good optical transparency and a high refractive index in the infrared area.