Interactions between self- and solute trapping of photocarriers in Pd-doped AgCl

Abstract

EPR studies on ultraviolet-irradiated crystals of AgC1:Pd have confirmed earlier results on AgCl:Cu for the existence of an energy barrier in the self-trapping of the photohole. The height of this barrier is near 1.8 meV. Migration of the self-trapped hole was found to be athermal for temperatures below 30 K; above 35 K the self-trapped hole hops, with a diffusivity given by D =7&(10 exp( -. 61 meV/kT) cm /sec. This suggests that the value of the electron transfer integral is about 1% of the energy of the phonons involved, that the bandwidth for the self-trapped hole is of the order of 2 meV, and that the binding energy of the hole is approximately 0.1 eV. The dependence on temperature of the efficiency of photoproduction of various trapped-carrier palladium centers was determined, and was correlated with the migration of the self-trapped hole. The presence of a small amount of Fe + "tracer" served to indicate those decay processes that were due to thermal release of trapped holes.

Exciton self-trapping in AgCl nanocrystals

“…With the ͑heavy͒ hole fixed in the center and subject to an average Coulomb potential provided by the fast moving ͑light͒ electron, in a NC of radius R ϭ4 nm we calculated a depth of 230 meV which is to be compared with the 100 meV of the STH potential depth. 6 One therefore expects that at low temperatures the STH tends to tunnel towards the center of the NC. The recombination luminescence of the different STE configurations is superimposed in the spectrum and gives rise to the observed inhomogeneity of the emission band with respect to the energy dependence of the decay times ͑Fig.…”

“…3 and 4͒ It has been found that the bare hole becomes self-trapped on a Ag ϩ lattice site 5 with the trapping accompanied by a tetragonal ͑static͒ Jahn-Teller distortion of the surrounding halide ions that provides a stabilizing potential of about 100 meV. 6 In forming the STE, the hole weakly binds an electron that leaves the hole state practically unaffected. From optically detected magnetic resonance ͑ODMR͒ measurements it was concluded that the majority of the longlived 500 nm STE luminescence is from a triplet state even though the singlet STE for reasons of parity is expected to have a long lifetime, too ͑see, Ref.…”

Jahn–Teller and Non-Jahn–Teller Systems Involving CuF64– Units: Role of the Internal Electric Field in Ba2ZnF6:Cu2+ and Other Insulating Systems

“…The Jahn–Teller (JT) effect has been widely invoked in the last 50 years for explaining the properties of systems involving d 9 , d 7 , and d 4 cations under octahedral coordination. Accordingly, experimental results on Cu 2+ systems like superconducting copper oxides or copper oxyfluorides and on silver­(II) fluorides have often been analyzed assuming the existence of a JT effect. − Nevertheless, this assumption implies that there exist a high symmetry reference conformation possessing an orbitally degenerate electronic state, but this crucial condition has not been proved in all cases. , So, while it is fulfilled for the polaron in AgCl − and d 9 impurities in cubic lattices without any close defect, − doubts have been raised analyzing the experimental and theoretical results for CuF 6 4– complexes in the tetragonal K 2 ZnF 4 lattice. ,− Indeed, the ground state in K 2 ZnF 4 :Cu 2+ is certainly unusual when compared to cubic fluoride ,− or chloride lattices − doped with d 9 ions displaying a static JT effect. In the latter systems the hole is always in the b 1g (∼ x 2 – y 2 ) level while it surprisingly resides in the a 1g (∼3 z 2 – r 2 ) level for K 2 ZnF 4 :Cu 2+ . ,, Furthermore, the optical properties of CuF 6 4– complexes in K 2 ZnF 4 cannot be quantitatively explained considering only the isolated CuF 6 4– unit at the equilibrium geometry derived for K 2 ZnF 4 :Cu 2+ . , Nevertheless, in a previous study it was found that both the d–d transitions and the unusual ground state of K 2 ZnF 4 :Cu 2+ are reasonably explained merely adding in the calculation the internal electric field, E R ( r ), that the rest of ions, belonging to the tetragonal lattice, create on the complex where active electrons do reside. , These results also imply that due to the action of E R ( r ) there is a gap, Δ 0 , between b 1g (∼ x 2 – y 2 ) and a 1g (∼3 z 2 – r 2 ) levels of CuF 6 4– units in K 2 ZnF 4 when the complex is perfectly octahedral …”

Stability and Polaronic Motion of Self-Trapped Holes in Silver Halides: Insight through DFT+UCalculations

“…Indeed the resemblance of EPR data of KCl:Ag 2+ or NaCl:Ag 2+ 36-37 to those reported 19,21,23,35 for the STH in AgCl (Table 1) proves that this species can be viewed, in a first approximation, as an elongated AgCl6 4-complex with the hole located in a x 2 -y 2 -type orbital resulting from a Jahn-Teller distortion. Despite this, the hyperfine and the isotropic superhyperfine constants measured for the STH in AgCl are somewhat smaller than those for KCl:Ag 2+ or NaCl:Ag 2+ 38 (Table 1) thus suggesting that a fraction of the unpaired electron in AgCl is lying outside the AgCl6 4- Moreover, that study showed the existence of a dominant mechanism of polaronic hopping, involving an activation energy of 61  3 meV 25 , whose nature is hitherto unknown. As regards the 5 optical absorption transitions associated with the STH in AgCl, there is a band peaked at 1.2 eV measured experimentally whose origin has not been clarified yet 21,27,31 .…”

“…Our chosen model systems are diamagnetic binary lattices with simple rocksalt structure, AgCl and AgBr, that have been extensively studied due to its relevance in photographic film technology [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] . As explained below, these experimental studies open important questions on the basic properties of polarons making them ideal to study the physics of these quasi-particles in detail.…”