Electronic, optical, phonon, polaron, carrier transport and scintillation properties of Cesium Hafnium Halides Cs2HfX6, (X = CI, Br and I)

In this work, we investigated the electronic, optical, phonon, polaron, and transport properties of cesium hafnium halides Cs2HfX6, X=Cl, Br and I and estimated their upper light yield. Using First-principles calculations combined the exchange-correlation energy of the Generalized Gradient Approximation GGA with the modified Becke–Johnson exchange potential (mBJ) and the spin-orbit coupling (SOC), these materials demonstrated discrete valence and conduction bands with low band dispersion and direct band gap decreasing as Cs2HfCl6 (5.48 eV)→ Cs2HfBr6 (4.18eV)→ Cs2HfI6 (2.72 eV). These obtained values are very close to the available experimental ones. Likewise, the band edge curvature of both valence band maximum (VBM) and conduction band minimum (CBM) reflects the heavier effective mass of electron at CBM Cs2HfCl6(7.58m0),Cs2HfBr6(5.94m0),Cs2HfI6(3.92m0) compared to that of the hole at VBM Cs2HfCl6(6.12m0),Cs2HfBr6(3.72m0),Cs2HfI6(2.34m0). Moreover, these materials exhibited a high transmittance rate, in their emission range, about: 93.57 % for Cs2HfCl6 91.25% for Cs2HfBr6 and 88.1% for Cs2HfI6, which is adequate to minimize the scintillated photons loss. The phonon dispersion and the normal modes for Γ point were calculated and discussed. Only three longitudinal optical phonon modes were identified active and their contribution to the ionic dielectric constant was calculated. Having First-principles results, polaron properties, the temperature dependence of polaron mobility, and relaxation time were calculated. These materials showed moderate electron (hole)-phonon couplings αe=6.72(αh=6.04) for Cs2HfCl6,αe=8.26(αh=6.54) for Cs2HfBr6 and αe=6.16(αh=4.76) for Cs2HfI6 compared to other polar materials leading to large polaron mass, high scattering rate, small relaxation time, and low electron (hole) room-temperature mobility 0.14cm2V−1s−1 (0.23 cm2V−1s−1) for Cs2HfCl6 0.14cm2V−1s−1 (0.45 cm2V−1s−1) for Cs2HfBr6 and 0.62cm2V−1s−1 (1.81 cm2V−1s−1) for Cs2HfI6which has only a positive role via the increase of the recombination yield. While such low mobility definitely limited their use in solar cell applications. Moreover, the upper light yield of these materials was estimated for the polaron and simple phenomenological models by taking the needed material inputs to these models from First-principles calculations. The results of latter one were 58,290 photon/MeV for Cs2HfCl6 76,685photon/MeV for Cs2HfBr6 and 118,000photon/MeV for Cs2HfI6 compared to the maximum experimental values 54,000 photon/MeV for Cs2HfCl6 61,500photon/MeV for Cs2HfBr6 and 70,000photon/MeV for Cs2HfI6. These computational findings may add strong evidence for suggesting experimental improvement of the crystal growth conditions and explain the remarkable scintillation properties of these materials as new and efficient gamma- and X-rays scintillators.