Lanthanide double perovskite nanocrystals are described. The nanocrystals display high energy luminescence, making them useful in a variety of light-emitting materials and devices. Methods of preparing the lanthanide double perovskite nanocrystals using a hot injection method are also described.

Phonon engineered, lanthanide doped upconverting nanoparticles with very low phonon energies and tunable methods of synthesis that adjust OA:OM ratio and reaction temperature are provided. Low phonon energy KPb.sub.2X.sub.5 (X=Cl, Br) upconverting nanoparticles, both doped and undoped, exhibit dramatically suppressed multiphonon relaxation, enhancing upconversion emission from higher lanthanide excited states and enabling room temperature observation of avalanche like upconversion by Nd.sup.3+ ions. Intrinsic optical bistability (IOB) of the materials can provide bit level functionality to all optical computing. The IOB of Nd.sup.3+ doped nanocrystals, which are either bright or dark at the same excitation power based on power history, illustrate the functionality. High contrast switching and IOB are enabled via the photon avalanche process, which sustains population inversion between the ground and the first excited 4f.sup.N states of Nd.sup.3+ ions. The IOB of these nanocrystals can be controlled by temporal pump modulation and can store information.