PROPERTIES OF CESIUM IODIDEWITH INTRINSIC POINT DEFECTS : A DENSITY FUNCTIONAL THEORY STUDY

Abstract

Cesium Iodide is one of the fastest scintillators with a very short decay time of 16ns. This has made the material to attract a lot of attention in research and imaging industry especially regarding how to make it a better scintillator as well as a detector. In spite of its obvious uses, some of its properties are not well understood. In this study, the structural and electronic properties have been studied using DFT and GW methods. Calculations using ab-initio methods together with defect formation energies and migration energies have been done for both cationic and anionic defects. The calculated lattice parameter of 4.551 °A agree well with experimental value of 4.567 °A which is a deviation of only -0.35 %. A direct band gap through gamma high symmetry direction of pristine CsI was calculated as 3.71 eV which is an underestimation of 40% of the experimental value of 6.2 eV.GWmethod resulted in an improvement of the band gap to a value of 5.5 eV which is 11.29% below the experimental value. The presence of lattice defects in CsI crystal led to a significant change in the bonding environment of the crystal. However, this led to a downward bending of the CB edge by 0.02 eV which is insignificant, resulting to an effective band gap of 3.69 eV. Defect formation energies were studied too. It was found out that the interstitial formation energies for cation and anion were 0.4325 eV and 0.30 eV, respectively. Vacancy formation energies were 0.25 eV and 0.16 eV for the cation and anion, respectively. While Frenkel defect energies were also determined at infinite distances as 0.21 eV and 0.16 eV for cation and anion, respectively. The migration paths for vacancies have also been studied. In particular, anion migration energy is easiest in the VI < 100 > at -1.5050 eV while in the other low index directions VI < 110 > and VI < 111 > were calculated as -1.5048 eV and -1.5049 eV, respectively. Cation migration energy was also easiest in the VI < 100 > at -1.5254 eV and in the other low index directions VI < 110 > and VI < 111 > were calculated as -1.5253 eV and -1.5252 eV, respectively.