Off-center Tl and Na dopant centers in CsI

Area of Science:

• Solid State Physics
• Materials Science
• Computational Chemistry

Background:

• Cesium Iodide (CsI) is a widely used scintillator material.
• Understanding dopant behavior is crucial for optimizing scintillator performance.
• Thallium (Tl) and Sodium (Na) are common dopants in CsI.

Purpose of the Study:

• To characterize the electronic and structural properties of Tl and Na dopant centers in CsI.
• To investigate the coupling of these dopants to the crystal lattice.
• To elucidate the role of dopant-induced distortions in CsI scintillation mechanisms.

Main Methods:

• Density Functional Theory (DFT) calculations were employed.
• Electronic structure calculations were performed.
• Lattice relaxation dynamics were simulated.

Main Results:

• Tl and Na dopant centers can accept one or two electrons.
• Dopants couple to long-range lattice relaxations, leading to significant off-center distortions.
• Multiple distinct energy minima were identified for the distorted configurations, even without triplet excitation.
• These distortions provide a mechanism for coupling to phonon modes.

Conclusions:

• The observed long-range distortions are critical for phonon-assisted polaron transport in activated CsI.
• These phenomena are expected to significantly impact the light emission properties of CsI scintillators.
• The findings provide fundamental insights into the operation of doped CsI scintillators.