Crystal-field effects in Er3+ - and Yb3+ -doped hexagonal NaYF4 nanoparticles

Since the up-conversion phenomenon in rare-earths (REs) doped ${\mathrm{NaYF}}₄$ is strongly affected by the crystal electric field (CF), determining the CF parameters, wave functions, and scheme of the energy levels of the RE $J$ multiplets could be crucial to improve and tune the up-conversion efficiency. In this work, the temperature and magnetic field dependent magnetization of ${\mathrm{NaY}}_{1\ensuremath{-}x}{[\mathrm{Er}(\mathrm{Yb})]}_x{\mathrm{F}}₄$ hexagonal nanoparticles (NPs) is reported. The data were best fit using the appropriated CF Hamiltonian for the $J=15/2\phantom{\rule{0.16em}{0ex}}(J=7/2)$ ground state multiplet of ${\mathrm{Er}}³⁺\phantom{\rule{0.16em}{0ex}}({\mathrm{Yb}}³⁺$) ions. The ${B}₂⁰, {B}₄⁰, {B}₆⁰$, and ${B}₆⁶$ CF parameters were considered in the Hamiltonian for RE ions located at the hexagonal ${\mathrm{C}}_3h$ point symmetry site of the ${\mathrm{NaYF}}₄$ host lattice. These results allowed us to predict an overall CF splitting of $\ensuremath{\sim}214$ ($\ensuremath{\sim}356$ K) for ${\mathrm{Er}}³⁺$ (${\mathrm{Yb}}³⁺$) and the wave functions and their energy levels for the $J=15/2$ ($J=7/2$) ground state multiplet which are in good agreement with the low temperature electron spin resonance experiments. Besides, our measurements allowed us to calculate all the excited CF $J$ multiplets that yield to a good estimation of the up-conversion light emission linewidth. The nonlinear optical light emission of the studied ${\mathrm{NaY}}_{1\ensuremath{-}x}{[\mathrm{Er}(\mathrm{Yb})]}_x{\mathrm{F}}₄$ hexagonal NPs was also compared with the most efficient up-conversion codoped ${\mathrm{NaY}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Er}}_x{\mathrm{Yb}}_y{\mathrm{F}}₄$ hexagonal NPs.