Tero has a new publication about thermal behaviour of the NaYF₄ materials, check it out.

Laihinen, T., Lastusaari, M., Pihlgren, L., Rodrigues, L.C.V., and Hölsä, J., Thermal Behavior of the NaYF₄:Yb³⁺,R³⁺ Materials, J. Thermal Anal. Calorim. 121 (2015) 37-43.
Read online: http://dx.doi.org/10.1007/s10973-015-4609-x