ESR Detection of Self-Trapped Holes in AgCl

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

“…Indeed, for AgCl 6 4− complexes in alkali chlorides, as well as in AgCl, the elongated geometry with a hole in the x 2 −y 2 orbital is energetically favorable with respect to a compressed one with the hole in 3z 2 −r 2 (Figure 2a) as demonstrated by EPR data. 19,21,23,36,37 Along this line, theoretical calculations carried out on KCl:Ag 2+ and NaCl:Ag 2+ reveal that the energy difference between compressed and elongated conformations is equal to 127 and 62 meV, respectively. 53 These relevant facts, whose actual origin is discussed in refs 43, 51, and 53, support that in irradiated AgCl the hole is in a x 2 −y 2 -type orbital.…”

“…The results on section 4 and Table 1 have underlined the similarities between the spin Hamiltonian parameters of the STH in AgCl with those coming from Ag 2+ impurities formed in NaCl and KCl host lattices. Accordingly, the comparison of optical absorption data of the STH in AgCl 21,23,27,31 with those for KCl:Ag 2+36 can shed light on the actual origin of the broad band peaked at 1.2 eV observed for the former system. 21,27,31 As shown in Table 9, the three crystal-field transitions have been observed for the AgCl 6 4− complex formed in KCl.…”

“…Moreover, they are consistent with experimental EPR and ENDOR data on this compound, proving the formation of elongated AgCl 6 4− complexes with the hole placed in the antibonding x 2 −y 2 orbital. 19,23,25,27,35 Upon hole localization, the Ag 2+ −Cl − R 0 distance is found to be reduced by about 0.13 Å with respect to the Ag + −Cl − bond length in pristine AgCl material, R L = 2.76 Å (Table 6). We observed that the R 0 values for elongation and compression are similar, whereas the ε value for elongation (ε = 0.119 Å) is about twice that of compression.…”

“…Concerning silver halides, EPR, ENDOR, and cyclotron resonance data proved the formation of a STH in AgCl, but not in AgBr. [17][18][19]21,23,[25][26][27][28][29][30][31]35 As a salient feature, the nature of the self-trapped species in the cubic AgCl crystal, formed after ultraviolet irradiation, is rather different from that found in alkali halides as the top of the valence band involves a strong admixture of 3p(Cl) and 4d(Ag) wave functions. 29 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 AgCl 6 4− complex with the hole located in a x 2 −y 2 -type orbital resulting from a Jahn−Teller distortion.…”

“…[17][18][19]21,23,[25][26][27][28][29][30][31]35 As a salient feature, the nature of the self-trapped species in the cubic AgCl crystal, formed after ultraviolet irradiation, is rather different from that found in alkali halides as the top of the valence band involves a strong admixture of 3p(Cl) and 4d(Ag) wave functions. 29 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 AgCl 6 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+ (ref 38; Table 1), thus suggesting that a fraction of the unpaired electron in AgCl is lying outside the AgCl 6 4− complex.…”