  Defect pyrochlore oxides have attracted a great interest as promising
luminescent materials due to their flexible composition and high electron/hole
mobility. In this work, we investigate the structural and electronic properties
of lanthanide-doped (Ln) defect pyrochlore oxides \ce{KNbWO6}:0.125Ln$^{3+}$ by
using first-principles calculations. We perform structural optimizations of
various defect pyrochlore models and calculate their electronic structures,
revealing that hydration has a significant influence on both local symmetry
around Eu$^{3+}$ ion and band structures with an alteration of their
luminescent behaviour. In the hydrated compounds, the electric-dipole
$^5$D$_0-^7$F$_2$ transition is found to be partially suppressed by the raised
local symmetry, and the water molecules in the compounds can mediate the
non-radiative energy transfer between the activator Eu$^{3+}$ ions and the
host, resulting in the quenching effect. It turns out that the oxygen vacancies
are detrimental to luminescence as they reduce the Eu$^{3+}$ ion in its
vicinity to Eu$^{2+}$ ion and also serve as traps for conduction electrons
excited by incident light. Our calculations for \ce{KNbWO6}:0.125Ln$^{3+}$ (Ln
= Ce, Pr, Nd, Pm, Sm) support that defect pyrochlore oxide \ce{KNbWO6} can also
be used as luminescence host for Ln$^{3+}$ ion doping, giving a valuable
insight into a variation trend in luminescent properties of these materials at
atomic level.
