The following solid exists in the form of ions

Spray pyrolysis was used to build thin films of ${\mathrm{CaB}}_2{\mathrm{O}}_4$:Eu on glass substrates. At growth temperatures between 300 and 500°C, the anomalous decrease of Eu3+ to Eu2+ in as-grown thin films was discovered. By annealing films in air, blue cathodoluminescence was discovered, coinciding with additional anomalous Eu ion reduction and compound crystallisation.