EPR study of hole trapping at cation vacancies in silver halides

Abstract

Hole trapping at cation vacancies in doped, irradiated silver halides is studied by means of electron paramagnetic resonance (EPR). From detailed studies of the behavior of the EPR spectra upon thermal annealing and of the effects of the concentrations of various divalent cations, it is demonstrated that in AgCl the positive hole can indeed be bound to the negative cation vacancy.The resulting two types of paramagnetic centers, which survive up to 70 and 110 K, respectively, are identified as perturbed self-trapped hole centers with a cation vacancy in either the nextnearest-neighbor or nearest-neighbor position in the equatorial plane, respectively. In addition, the perturbing cation vacancy is determined to be an isolated vacancy, free from any nearby divalent cation. In AgBr, however, no corresponding EPR effects due to the interaction between the hole and the cation vacancy have been observed.

Formation of Long-Lived Color Centers for Broadband Visible Light Emission in Low-Dimensional Layered Perovskites

“…The coincidence of the buckling of the lead iodide layers with the low-temperature broadband emission is an important indication of a possible (and likely) interplay between the crystalline structure and the measured white emission, as already pointed out in the literature. − ,, However, such coincidence does not provide a strong evidence for a structural origin of this emission and does not rule out other possible causes. Two recent works in fact suggested lattice defects as key players in the white emission, through the formation of self-localized emissive states. , This is also consistent with the extended literature on ionic defects acting as emitting F-centers in inorganic crystals. − To investigate this further, we carried out density functional theory calculations of the HA 2 PbI 4 perovskite, considering both ideal, nondefective crystalline models and defective ones.…”

Broadband Emission in Two-Dimensional Hybrid Perovskites: The Role of Structural Deformation

“…In (NBT) 2 PbI 4 , there is one frontier iodine atom per organic cation (two such iodine atoms are instead present in (EDBE)­PbI 4 ) with a consequently stronger electrostatic interaction that disfavors the vicinal iodine motion from forming I 3 – . Similar point defects, consisting in a V-center coupled to an adjacent cation vacancy, were demonstrated in LiF: These are known as V F centers and regarded as “the antimorph of the F centers”, involving the complementary electron trapping at a halogen vacancy. , …”

Defect structures in the silver halides

“…9,62 The difference in our distribution may be due to the localization problem in DFT. Pacchioni et al have recently found that the hole in an ␣-quartz Al center is incorrectly described by DFT.…”

“…7 At low temperatures the hole may become self-trapped; at elevated temperatures this species exists only if stabilized by a nearby negatively charged cation vacancy. 8,9 Recent low-temperature ENDOR studies on the isolated self-trapped hole ͑STH͒ in AgCl have suggested that it is localized over a central silver 4d x 2 −y 2 orbital and four surrounding chloride orbitals, forming a ͓AgCl 4 ͔ 2− unit. 10 Within the bulk, it is thought that the electron can become trapped at an interstitial silver ion at low temperatures, 11 which has been verified by more recent experiments, with the trap having been found to be very shallow.…”