Xiao-Kun Chen

Concentration-dependent near-infrared quantum cutting in NaYF4:Pr3+, Yb3+ phosphor

Near-infrared quantum cutting phenomenon has been demonstrated in Pr3+–Yb3+-codoped NaYF4 microcrystals synthesized by a facile hydrothermal route, which involves the emission of two near-infrared photons from an absorbed blue photon via a cooperative downconversion process. The Pr3+–Yb3+ energy transfer mechanisms have been identified by the dependence of Yb3+ doping concentration on the visible and infrared emissions, decay lifetime, and quantum efficiency. Upon excitation of Pr3+ ions with a blue photon at 443 nm, Yb3+ ions emit two near-infrared photons at 977 nm with an optimal quantum efficiency of 133.6%. The excellent luminescence properties of NaYF4:Pr3+, Yb3+ microcrystal demonstrate its potentiality to become identified as a better candidate for application as downconversion layer on the top of crystalline silicon solar cell panels in order to reduce thermalization loss.

Magnetic enhancement across a ferroelectric–antiferroelectric phase boundary in Bi1−xNdxFeO3

The structural and magnetic properties of well-prepared Bi1−xNdxFeO3 (0 ≤ x ≤ 0.2) powders are investigated by combining x-ray diffraction, Raman scattering spectra, and magnetic measurements. Structural symmetric breaking from rhombohedral R3c to orthorhombic Pnma at the substituted Nd concentration of x = 0.125–0.15, accompanying a ferroelectric-antiferroelectric phase transition, is identified from x-ray diffraction patterns and Raman scattering spectra. The magnetization enhancement of Bi1−xNdxFeO3 is observed before approaching ferroelectric-antiferroelectric phase boundary conditions (0 ≤ x ≤ 0.15). We consider that such enhancement results from the destruction of the spin cycloid accompanying the structural transition. At x > 0.15, the magnetization decreases, which is probably attributable to further degradation of the space modulated spin structure, allowing a more perfect antiferromagnetic ordering.

Pressure effect on structural and vibrational properties of Y-substituted BiFeO3

The structural and vibrational properties of 5% Y-substituted BiFeO3 under pressure have been investigated using synchrotron x-ray diffraction (SXRD) and Raman scattering measurements. At a pressure below 30.3 GPa, distinct changes in the Raman spectra and SRXD pattern show evidence for one pressure-induced structural transition from the polar rhombohedral R3c phase to the nonpolar orthorhombic Pnma phase commencing at 3.6 and completed at 7.2 GPa, where there is a region of phase coexistence between the R3c and Pnma phases. At a higher pressure of 40.8 GPa, another phase transition from orthorhombic to cubic is observed accompanied by the insulator-metal transition. Our data do not suggest the pressure-induced re-entrance of ferroelectricity in the model multiferroic Bi0.95Fe0.05O3 in the pressure range studied.

Pressure effect on structural and vibrational properties of Sm-substituted BiFeO3

The structural and vibrational properties of 5% Sm-substituted BiFeO3 under pressure are investigated using synchrotron X-ray diffraction and Raman scattering measurements. The results yield the pressure-induced structural phase transitions from the polar R3c phase to the orthorhombic Pnma phase commencing at 3.9 and being complete at 7.6 GPa, where there is a region of the coexistence of the R3c and Pnma phases. This structural transition is companied by the ferroelectric-paraelectric transition for the Sm-substituted BiFeO3. We find that the Sm substitution leads to lower transition pressure compared to that of the pure BiFeO3 system due to the substitution-induced chemical pressure. Our results do not suggest the pressure-induced reentrance of ferroelectricity in the model multiferroic BiFeO3 in the pressure range studied.