Energy Barrier against Self-Trapping of the Hole in Silver Chloride

Laredo, E.; Rowan, L. G.; Slifkin, Lawrence

doi:10.1103/physrevlett.47.384

Cited by 22 publications

(7 citation statements)

References 15 publications

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“…Finally the study of impurities centres can provide an insight into the nature of defects formed in pure insulators under x-irradiation [120]. For instance, the self-trapped hole formed in AgCl essentially corresponds to the AgCl 4− 6 complex [120][121][122], which is also found in chloride lattices containing Ag 2+ impurities [31,123]. Similarly, in pure PbWO 4 the electron is self trapped as a WO 3− 4 complex [124,125].…”

Section: Introductionmentioning

confidence: 99%

Microscopic insight into properties and electronic instabilities of impurities in cubic and lower symmetry insulators: the influence of pressure

Moreno

Barriuso

Aramburu

et al. 2006

J. Phys.: Condens. Matter

224

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This article reviews the microscopic origin of properties due to transition-metal (TM) impurities, M, in insulator materials. Particular attention is paid to the influence of pressure upon impurity properties. Basic concepts such as the electronic localization in an MX(N) complex, the electrostatic potential, V(R), arising from the rest of the lattice ions or the elastic coupling of the complex to the host lattice are initially exposed. The dependence of optical and magnetic parameters on the impurity-ligand distance, R, in cubic lattices is discussed in a first step. Emphasis is put on the actual origin of the R dependence of 10Dq. Examples revealing that laws for strict cubic symmetry cannot in general be transferred to lower symmetries are later given. This relevant fact is shown to come from allowed hybridizations like nd-(n+1)s as well as the influence of V(R). As a salient feature the different colour in ruby and emerald is stressed to arise from distinct V(R) potentials in Al(2)O(3) and Be(3)Si(6)Al(2)O(18). The last part of this review deals with electronic instabilities. The phenomena associated with the Jahn-Teller (JT) effect in cubic lattices, the origin of the energy barrier among equivalent minima and the existence of coherent tunnelling in systems like MgO:Cu(2+) are discussed. An increase of elastic coupling is pointed out to favour a transition from an elongated to a compressed equilibrium conformation. Interestingly the equilibrium geometry of JT ions in non-cubic lattices is shown to be controlled by mechanisms different to those in cubic systems, V(R) playing again a key role. The relevance of first principles calculations for clarifying the subtle mechanisms behind off-centre instabilities is also pointed out. Examples concern monovalent and divalent TM impurities in lattices with the CaF(2) structure. The instability due to the transition from the ground to an excited state is finally considered. For complexes with significant elastic coupling vibrational frequencies and the Stokes shift are expected to undergo bigger changes through a chemical rather than a hydrostatic pressure. The reduction of Huang-Rhys factors upon increasing the pressure is discussed as well.

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Section: Introductionmentioning

confidence: 99%

Microscopic insight into properties and electronic instabilities of impurities in cubic and lower symmetry insulators: the influence of pressure

Moreno

Barriuso

Aramburu

et al. 2006

J. Phys.: Condens. Matter

224

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“…All of these expectations were established in a determination of the temperature dependence of the number of photoholes in irradiated AgCl that become self-trapped before being captured by an added dopant ion [19,20]. In one experiment, the AgCl crystal was treated so as to contain both Cu + (to serve as stationary traps for photoholes) and Cu 2+ (to serve as a source of holes under illumination with sub-band-gap light [21]).…”

Section: The Activation Energy For the Self-trapping Transitionmentioning

confidence: 99%

Dynamics of self-trapped hole processes in AgCl

Slifkin

2001

J. Phys.: Condens. Matter

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A set of phenomena involving the self-trapped hole (STH) in AgCl have been studied in crystals doped with hole sources, traps for holes and electrons and providers of cation vacancies. An energy barrier was demonstrated in the self-trapping process; its energy was related to appropriate phonon energies, and tunnelling appeared in the temperature range predicted by Mott and Stoneham. Migration of the STH can occur either by phonon-assisted hopping or by tunnelling. From the temperature dependences, one obtains the activation energy (0.062 eV), the trap depth (0.12 eV) and the temperature exponent for quantum migration (1.3). The structures and energetics of two centres involving an STH bound to a vacancy have been deduced. Also, it was shown that low-temperature capture of a hole by substitutional Fe2+ causes production of a Frenkel pair, as well as ultimately leading to formation of the (interstitial Fe3+) - (multivacancy) defect that was first characterized by Hayes et al.

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“…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.…”

Section: Introductionmentioning

confidence: 99%

“…Interestingly, a hop like X -x 2 -z 2 makes possible that a STH which is initially placed in XY plane can reach another equivalent plane. Moreover, if the dominant mechanism for polaron motion is through Y -x 2 -y 2 and X -x 2 -z 2 hops, this means that a STH placed 26 at the beginning at (0,0,0) can easily move along the sublattice of silver sites described by  = a(mi + nj + pk) where a is the lattice parameter and m, n and p are integers. By contrast, it is more difficult for the polaron to reach silver sites corresponding to the other cubic sublattices starting in (a/2, a/2, 0), (a/2, 0, a/2) and (0, a/2, a/2).…”

Section: Final Remarksmentioning

confidence: 99%

“…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.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2016

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Polarons and their associated transport properties are a field of great current interest both in chemistry and physics. In order to further our understanding of these quasi-particles, we have carried out first-principles calculations of self-trapped holes (STH) in the model compounds AgCl and AgBr, where extensive experimental information exists. Our calculations confirm that the STH solely stabilizes in AgCl but with a binding energy of only 165 meV, an order of magnitude smaller than that found for the Vk center in KCl. Key contributions to this stabilization energy come from the local relaxation along breathing (a1g) and Jahn-Teller (eg) modes in the AgCl6 4-unit. In order to study the transfer of the STH among silver sites we (i) use first-principles calculations to obtain the hopping barrier of the STH to first and second neighbors, involving eight distinct paths, using firstprinciples and (ii) construct a simple model, based on Slater-Koster parameters, that highlights the similarity of polaron transfer with magnetic superexchange. This allows one understanding why the movement of STH to second neighbors is highly enhanced with respect to closer ones. In agreement with experimental data and the model, the present calculations prove the existence of a dominant mechanism of polaronic motion that corresponds to the displacement of the STH to the next nearest sites in the <100> direction and a small barrier of 37 meV. This mechanism is dominated by the covalency inside a AgX6 4-complex (X:Cl;Br) thus explaining why the STH is not stabilized in AgBr following the increase of covalency due to the ClBr substitution. The present calculations confirm that ~10% of the charge associated with the STH in AgCl is lying outside the AgCl6 4-complex. This fact is behind the differences between optical and magnetic properties of the STH in AgCl and those observed in KCl:Ag 2+ .2

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Energy Barrier against Self-Trapping of the Hole in Silver Chloride

Cited by 22 publications

References 15 publications

Microscopic insight into properties and electronic instabilities of impurities in cubic and lower symmetry insulators: the influence of pressure

Microscopic insight into properties and electronic instabilities of impurities in cubic and lower symmetry insulators: the influence of pressure

Dynamics of self-trapped hole processes in AgCl

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

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