The spectroscopic properties of Pr;Yb:YLiF4 and Pr,Yb:YAlO3 are presented. Whereas Pr;Yb:YLiF4 exhibits an avalanche-type excitation mechanism under infrared pumping, no such behavior is observed for Pr,Yb:YAlO3. A time dependent rate equation model was developed to describe these behaviors. The absence of an avalanche-type excitation mechanism in Pr,Yb:YAlO3 is mainly caused by a better spectral overlap between the excited state absorption in the Pr and the ground state absorption of Yb, as well as by higher transfer rates between Yb and Pr and within the Pr in this material.

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