A theoretical insight into the microstructure and electronic properties of Ho3+-doped potassium gadolinium tungstate

Abstract We have systematically studied the structural evolutions of trivalent holmium ion (Ho3+) doped potassium gadolinium tungstate (KGd(WO4)2) crystals by means of the CALYPSO structure search method coupled with first-principles calculations. The unbiased global minimum structure search is successful in reproducing the standardized monoclinic structure with C2/c phase for KGd(WO4)2. For the first time, the lowest energy structure of Ho3+-doped KGd(WO4)2 system is identified, which possesses a significantly different monoclinic configuration with P2 space group. It is found that the impurity Ho3+ ions can substitute the sites of Gd3+ ions, forming the [HoO8]13- local units with C2 site symmetry. Our electronic calculations show that the energy band gap of Ho3+-doped KGd(WO4)2 is equal to 3.03 eV. The impurity of Ho3+ ion remains the insulating character of Ho:KGW system. These results could provide important information for understanding the structural evolution and electronic properties of other rare-earth-doped materials.