Review—Photoluminescence Spectroscopy of Eu2+-Activated Phosphors: From Near-UV to Deep Red Luminescence

The present review article discusses the characteristic luminescence behaviors of the Eu 2+ -activated phosphors from a spectroscopic point of view. This spectroscopic point of view makes possible to classify the Eu 2+ -activated phosphors into two groups: types Eu2-A and Eu2-B, where an energy inequality relation of the lowest excited state of type Eu2-A to be given by 4 f 7 ( 6 P 7/2 ) ∼ 4 f 6 5 d , whereas that of type Eu2-B is given by 4 f 7 ( 6 P 7/2 ) > 4 f 6 5 d . Simply saying, the 6 P 7/2 excited manifold of the 4 f 7 configuration is located just below and well above the lowest-energy 5 d level of the Eu 2+ ion in types Eu2-A and Eu2-B phosphors, respectively. While generally uncommon, in some host materials the 4 f 7 ( 6 P 7/2 ) → 4 f 7 ( 8 S 7/2 ) intra-configurational transitions of Eu 2+ occur with exhibiting the characteristic narrow-line emissions. An analysis method based on the Franck−Condon principle within the configurational-coordinate motel is proposed to exactly determine the lowest excited-state 4 f 6 5 d energy in both types of phosphors Eu2-A and Eu2-B. Thermal stability of the phosphor properties is of crucial importance and scientific interest. Therefore, key understandings of the temperature dependences of luminescence intensity and decay time in the Eu 2+ -activated phosphors are also presented as the additional and important concern in the present review article.