Predicting structural, electronic, and optical properties of mixed-phase LiMg0.5X0.5PO4 (X = lanthanide) via first-principles study

Optically Stimulated Luminescence (OSL) has emerged as the preferred technique for a wide range of applications in radiation dosimetry, including personal monitoring, environmental surveillance, retrospective dosimetry, space dosimetry, and more. Over recent decades, LiMgPO4, a phosphate material, has garnered significant attention due to its superior characteristics compared to commercially available OSL dosimeters like Al2O3: C and BeO. In this paper, we systematically investigate the structural, electronic, and optical properties of mixed-phase LiMg0.5X0.5PO4 (X = Dy, Eu, Nd, Gd, Pm, Sm, Tb, and Tm) using a first-principles method based on density functional theory. Our calculations reveal that both LiMgPO4 and LiMg0.5Eu0.5PO4 exhibit insulating characteristics with direct band gap values of 5.47 eV and 4.99 eV, respectively. In contrast, for the other seven LiMg0.5X0.5PO4 (X = Dy, Nd, Gd, Pm, Sm, Tb, and Tm), their band gap values converge to zero eV, indicating a transition from insulator to metal. The reduction in the bandgap has led to the emergence of smaller peaks in the absorption coefficient at lower energy ranges while maintaining enhanced performance in optical absorption at higher energy ranges. Our computational results demonstrate consistent trends with the incorporation of various lanthanides. These materials exhibit stable structures, and the reduction in band gap enhances sensitivity and expands the energy response range. These findings offer valuable insights for future experimental and theoretical investigations in radiation dosimetry using mixed-phase phosphate materials.