Abstract: The emission spectrum of Eu²⁺ ion in Ba_1-x-yEu_xMgF₄（x＜0.10） does not show the narrow line emission of the f-f transition of Eu²⁺ .However,When Gd³⁺ and Eu²⁺ ions are codoped in BaMgF 4,not only the band emission of the d-f transition of Eu²⁺ is observed,but also the narrow line emission of the 4f（⁶P_7/2）=4f（ ⁸S_7/2） of Eu²⁺ ion is exhibited.The intensity of the narrow line emission depends on the concentration of Gd³⁺ （ y ） and that of Eu²⁺（x）.
The results show that the effect of Gd³⁺ on Eu²⁺ is not the sensitization.The Gd³⁺ ions in the system induce the production of the f-f transition of Eu²⁺. The presence of the f-f transition of Eu²⁺ in the emission spectrum is due to the increase of the coordination number of Eu²⁺ in hosts.The coordination number of Eu²⁺ in Ba_1-x-yEu_xGd_yMgF_4+y could be 10.