•Novel poly(imide-benzoxazole)s were synthesized by one-pot method using polyphosphoric acid as solvent.•Tds of poly(imide-benzoxazole)s are 570–595 °C in N2.•Tensile strengths of poly(imide-benzoxazole) fibers were 2.65–3.12 GPa.A series of copolyimides containing benzoxazole moieties in the main chain are synthesized through a one-pot method with aromatic dianhydrides and benzoxazole diamines in polyphosphoric acid. The benzoxazole diamines are prepared in advance and react with aromatic dianhydrides without separating polyphosphoric acid. The feasibility of this one-pot method for polymerization is confirmed by a model reaction between benzoxazole diamines and phthalic anhydride. The resulting compounds and copolymers are characterized by Fourier Translation Infrared Spectroscopy (FT-IR), 1H nuclear magnetic resonance (1H NMR) and thermogravimetric analyses (TGA). All copolymer were obtained at high yields. The high molecular weight was also obtained with the inherent viscosities (ηinh) ranging from 5.60 to 6.98 dL/g. Fibers spun from the copolymers solution are strong, and the tensile strengths are up to 2.45–3.12 GPa. TGA results show that the thermal degradation of the copolyimides takes place at around 570 °C in nitrogen. It confirms that the introduction of the benzoxazole moieties has improved the thermal stability and the mechanical property of the copolymers. The copolyimides containing benzoxazole moieties are ideal candidates for high performance fibers.

Under laser excitation of 976 nm, the green and red UC emissions of CaTiO3: Er3+ nanocrystals were consistent with 2H11/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transition of Er3+, respectively. The green and red UC luminescence of CaTiO3: Er3+/Yb3+ nanocrystals were significantly stronger than the counterpart of CaTiO3: Er3+ nanocrystals, which was resulted from energy transfer between Yb3+ and Er3+. The blue and red UC emissions of CaTiO3: Tm3+/Yb3+ nanocrystals near 480 and 650 nm were observed due to the 1G4 → 3H6 and 1G4→3F4 transition of Tm3+, respectively. The overall and relative UC luminescence intensities of Er3+/Tm3+/Yb3+ tridoped CaTiO3 nanocrystals were found to be depended highly on the concentration of Yb3+ and Tm3+, for which the involved mechanisms were demonstrated. By adjusting the concentration of Yb3+ and Tm3+, CaTiO3: Er3+/Tm3+/Yb3+ nanocrystals with the CIE coordinates close to (0.33, 0.33) were obtained.Graphical abstractCIE x–y chromaticity diagram displaying the chromaticity coordinate of the UC fluorescence from the CaTiO3: 0.5%Er3+/0.7%Tm3+/7%Yb3+ nanocrystals.Highlights► Rare-earth doped CaTiO3 nanocrystals were synthesized. ► Multicolor upconversion luminescence was realized. ► Yb3+ and Tm3+ contents influence UC luminescence. ► White light was observed in CaTiO3: 0.5%Er3+/0.7%Tm3+/7%Yb3+ nanocrystals.

Er3+/Yb3+/Li+-tridoped BaTiO3 nanocrystals were prepared by a sol–gel method to improve the upconversion (UC) luminescence of rare-earth doped BaTiO3 nanoparticles. Effects of Li+ ion on the UC emission properties of the Er3+/Yb3+/Li+-tridoped BaTiO3 nanocrystals were investigated. The results indicated that tridoping with Li+ ion enhanced the visible green and red UC emissions of Er3+/Yb3+-codoped BaTiO3 nanocrystals under the excitation of a 976 nm laser diode. X-ray diffraction and decay time of the UC luminescence were studied to explain the reasons of the enhancement of UC emission intensity. X-ray diffraction results gave evidence that tridoping with Li+ ion decreased the local symmetry of crystal field around Er3+, which increased the intra-4f transitions of Er3+ ion. Moreover, lifetimes in the intermediate 4 S3/2 and 4I11/2 (Er) states were enhanced by Li+ ion incorporation in the lattice. Therefore, it can be concluded that Li+ ion in rare-earth doped nanocrystals is effective in enhancing the UC emission intensity.Research Highlights► Er3+/Yb3+-codoped BaTiO3 nanocrystals emit green and red upconversion luminescence. ► Increasing Yb3+ content increases red upconversion luminescence. ► Increasing Li+ content increases green and red upconversion luminescence.