Unexpected 700 nm upconversion emission for Cs ₂ NaYbCl ₆ and its co-doped hexachloroelpasolites

Excitation of samples of the elpasolite Cs ₂ NaYbCl ₆ , prepared using high purity Yb ₂ O ₃ , by an infrared laser at temperatures below ∼100 K produces a strong, structured emission near 700 nm. The emission is also observed for samples co-doped with other rare earth ions and it exhibits the characteristic power-intensity dependence of a 2-photon process. Analysis of the vibronic structure indicates that it can be assigned to one, and not several, electronic transitions. However, the transition energy does not correspond to any of the electronic transitions associated with Yb ³⁺ or Yb ²⁺ . From evidence displayed in the electron spin resonance spectrum and from the spectral interpretation, the emission is associated with the MnCl ₆ ²⁻ ion.