Abstract: The present disclosure relates crystals capable of cooling upon illumination. In certain embodiments, the crystals include yttrium-fluoride doped with a trivalent rare earth ion. Exemplary crystals include yttrium-lithium-fluoride crystals and yttrium-sodium-fluoride crystals, doped with Yb3+, Er3+, or a combination of both. Methods of producing the crystals hydrothermally and methods of cooling a solution are also provided. Further methods include use of the crystals for therapeutic hypothermia. Finally, a theranostic is provided that includes the crystals conjugated to a targeting moiety capable of selectively binding to a target.