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Calcium fluoride nanoparticles with various amounts of erbium ion dopants were prepared by CTAB/C4H9OH/C7H16/H2O reverse micro-emulsion method. The nanoparticles were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FTIR), absorption and fluorescence spectra. The XRD patterns indicate a typical cubic fluorite structure and no other impurities. TEM results show the synthesized particles having uniform grain size and without agglomeration. FTIR spectra reveal that there are some amounts of -OH, NO − 3 and other organic functional groups on the particle surfaces before the annealing process. Many absorption peaks and bands are present in the absorption spectra, corresponding to the rich energy levels of erbium ion. The Red-Shift of absorption bands and Blue-Shift of fluorescence peaks can be attributed to the weakened energy level split as a result of the decrease in crystal field strength.

The effects of different sintering addictives on the preparation of CaF 2 transparent ceramics were studied. Transparent CaF 2 ceramics were fabricated by vacuum sintering and hot isostatic pressing (HIP) method, using CaF 2 nanopowders synthesized by chemical precipitation method as raw materials. The nanopowders and transparent ceramics were studied using X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and spectrophotometer. The experimental results indicated that the obtained nanopowders presented normal distribution with grain size about 30 nm; transmittance of CaF 2 transparent ceramics was 39% and 26% at 1100 nm for LiF and NaF as sintering addictives, respectively, with corresponding mean grain size 188 μm and 44 μm. Loss of transmission could be attributed to the residual closed porosity. Sintering mechanism was liquid-phase sintering at pre-stage, then solid-phase sintering at later stage, as well as solid solution of lithium ions and sodium ions in the CaF 2 lattice structure.

Highly-transparent trivalent erbium ion doped calcium fluoride (5 mol % Er:CaF 2 ) ceramics were fabricated by a hotpressing (HP) method using high-purity Er:CaF 2 nanoparticles, which were synthesized by co-precipitation method. The mean grain size of the nanoparticles was about 24.7 nm. The nanoparticles were sintered at 600 • C, 700 • C, 800 • C and 900 • C, respectively, for 30 min under a uniaxial pressure of 30 MPa and vacuum of 10 −3 Pa with 1 mol % lithium fluoride (LiF) as sintering additive. The 5 mol % Er:CaF 2 ceramics sintered at 800 • C exhibits high density and pore-free microstructure with an average grain size of about 8 µm. The optical transmittance of the transparent ceramics is close to 85 % at visible and nearinfrared wavelengths. The strong and broad absorptions peaks corresponding to characteristic absorption of trivalent erbium ions make the ceramics a potential candidate for infrared and upconversion laser operating.