Bandgap engineering of the Lu_xY_1-xPO₄ mixed crystals

Bandgap modification of the Lu_xY_1-xPO₄ mixed crystals has been studied by thermostimulated luminescence (TSL) and ab-initio calculation methods. Doping of Lu_xY_1-xPO₄ with Ce³⁺ allowed to follow up the changes of electron traps depth, caused by the modification of the bottom of conduction band. The observed gradual shift of the most intensive TSL peaks to higher temperatures with increase of x value was connected with the high-energy shift of the conduction band bottom. According to the band structure calculations the bottom of the conduction band is formed by the 5d and 4d states of Lu and Y, respectively. Therefore, substitution of one cation by another is responsible for the observed variation of the electronic and optical properties. Doping with Eu³⁺ was used to study the modification of the hole traps and the top of the valence band in Lu_xY_1-xPO₄. The independence of the TSL peaks position on x value in Lu_xY_1-xPO₄:Eu³⁺ allows to conclude that the top of the valence band is negligibly affected by the cation substitution. According to the band structure calculations the top of the valence band is formed by the O 2p electronic states, which are not affected by the cation substitution. The resulting increase of the bandgap with x value is confirmed by the data of ab-initio calculations.