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Electron gain and loss centres in silver chloride
Volpme 39, number 1
CHEMICAL PHYSICS LEl-T&RS
1 April 1976
ELECTRON GAIN AND LOSS CENTRES IN SILVER CHLORIDE D. Robert BROWN and Martyn C.R. SYMONS” Departmentof Chemistry, The University, Leicester, LEI 7RH. UK Received 20 January 1976
bfixed crystals of silver chloride and sodium chloride exposed to 6oCo r-rays at 77 K gave Ag(II) as the primary electron-loss centre and an electron-gain centre, characterised by a singlet atg = 1.891, thought to be associated with Agf clusters.
Attempts to detect primary centres in photolysed or irradiated silver halide crystals by optical or ESR methods have been unsuccessful. However, when potassium exposed
chloride to
doped dilutely with Ag+ ions is
high energy radiation, Cl, and AgO (or AgCIg-) centres are formed at 77 K, the former giving Ag(iI) centres (A&-) on annealing [ I] _ Attempts to prepare more concentrated soIutions of AgCl in KC1 resulted in phase separation, but this was not the case for AgCl in NaCl, for which a wide range of homogeneous mixtures were obtained. Dilute solutions again gave Ago, with ESRspectra showing superhyperfine coupling to six equivalent chloride ligands, and Cl,, which gave Ag(I1) on annealing. As the [Ag+] was increased, the Cl, features diminished and gave way to Ag(II) features. The Ago features were also reduced in intensity and extra, poorly-defined, lines within the envelope of 19 chlorine hyperfke features were detected. When the [AgC]_reached ca. 0.15 M-F. an asymmetric feature in the g= 1.891 region was detected, which grew in intensity, shifted slightly to lower g-vaiues and lost its asymmetry as the [Agf] was increased. The [Ago] rapidly fell to zero in this M-F. region. The g = 1.891 feature was unchanged on cooling to 4.2 K, but the Ag(II) features changed ‘a the manner expected for the quenching of a dynamic JahnTeller distortion [ 11. (Four strdngly coupled chloride ligands give six less strongly coupled ligands on warm: ing to 77 K.) We suaest that the g 7 1.S9 1 species, which is reproducibiy formed in the range 0.15 -
= 0.8 M.F.
is the expected electron-gain centre, but its apparent intensity is only a small fraction of that of the electron-loss centre. This can be understood if the single line detected is really part of a complex spectrum of many lines, all others being undetectable. This could arise for two reasons; (a) the other features are dynamically broadened or (b) none of them have precise locations. The former concept seems to us improbable because the spectrum was unaltered at 4.2 K. The latter, (b), however, arises naturally from the following model. We suggest that as the [A$! increases there is a tendency for clustering. Initially, units containing two, three or four Agf ions would trap electrons to give centres resembling the Agi (j&v. = 1.989), Ags (g,y = I .973) and A$ Gp;y = 1.970) centres previously described 121. Such centres, present in relatively low concentration, would have features largely hidden by the main Ago features, and could well be present. It is noteworthy that gav shifts to low values in this series. As the number of A$ ions involved in a cluster increases, so gav is expected to reach a limiting value (as the number of periferal A$ ions becomes smzll compared with the inner ions). The hyperfine features from logAg and lo7Ag are expected to comprise a multiplet covering a range of ca. 500-600 G. For kentres containing an even number of equivalent Ag+ ions, there will always be a central line which should be narrow when the hyperfine coupling to each Ag+ is small. However, no other features from these clusters will coincide unless the clusters are equivalent. 69
--V0ium~_39,-nO~ber 1 .’ _‘. -_; : _ _. .we.expt
td -km a tide m-g
‘-
CHEMICALtiHYSICSLETTERS
df duster. sizes and,
hence, to‘&unable to’d&ect‘any but this cent&l. f feature in the limitingg-value~re&n. The~asymrnetriti brobdening on the.low-field side of the singlet in the low concentration region accords with this model. If this model is correct, we conclude that excess electrons in silver chloride are not initially trapped and localised by the formz&on of discrete Ags or .A$ units. This contrasts with the behaviour of ‘holes” in alkali-halides which are trapped as discrete hal- centres. Instead, they-are delocalised over many Agz ions, this being the conduction band in the limit of pure silver chloride. However, the electron-loss centres are not the expected Cl, species, but are A&PI) centres. These appPar to be strongly localised
_-
-rio
_.
. .’
-._..
.:;
-
1 April 1976
at one sjlvsr, rather than being d&c&s&d over many. zlzus, in this case, the Jahzz-Teller distortion is effec.tive and prevents delocalisation.
We would like to thank the S.R.C. for a C.A.S.E. award to D.R.B., and Dr. FarnelI of Kodak Limited, Harrow, for helpfti advice and assistance.
References [l] CT. Delbecq,iv. Hayes, M.C.M.O’Brien and P.H. Yusten, Proc. Roy. Sot. A (1963) 271. [2] C.E. Forbes and M.C.R. Symons, Mol. Phys. 27 (1974) 467.
CHEMICAL PHYSICS LEl-T&RS
1 April 1976
ELECTRON GAIN AND LOSS CENTRES IN SILVER CHLORIDE D. Robert BROWN and Martyn C.R. SYMONS” Departmentof Chemistry, The University, Leicester, LEI 7RH. UK Received 20 January 1976
bfixed crystals of silver chloride and sodium chloride exposed to 6oCo r-rays at 77 K gave Ag(II) as the primary electron-loss centre and an electron-gain centre, characterised by a singlet atg = 1.891, thought to be associated with Agf clusters.
Attempts to detect primary centres in photolysed or irradiated silver halide crystals by optical or ESR methods have been unsuccessful. However, when potassium exposed
chloride to
doped dilutely with Ag+ ions is
high energy radiation, Cl, and AgO (or AgCIg-) centres are formed at 77 K, the former giving Ag(iI) centres (A&-) on annealing [ I] _ Attempts to prepare more concentrated soIutions of AgCl in KC1 resulted in phase separation, but this was not the case for AgCl in NaCl, for which a wide range of homogeneous mixtures were obtained. Dilute solutions again gave Ago, with ESRspectra showing superhyperfine coupling to six equivalent chloride ligands, and Cl,, which gave Ag(I1) on annealing. As the [Ag+] was increased, the Cl, features diminished and gave way to Ag(II) features. The Ago features were also reduced in intensity and extra, poorly-defined, lines within the envelope of 19 chlorine hyperfke features were detected. When the [AgC]_reached ca. 0.15 M-F. an asymmetric feature in the g= 1.891 region was detected, which grew in intensity, shifted slightly to lower g-vaiues and lost its asymmetry as the [Agf] was increased. The [Ago] rapidly fell to zero in this M-F. region. The g = 1.891 feature was unchanged on cooling to 4.2 K, but the Ag(II) features changed ‘a the manner expected for the quenching of a dynamic JahnTeller distortion [ 11. (Four strdngly coupled chloride ligands give six less strongly coupled ligands on warm: ing to 77 K.) We suaest that the g 7 1.S9 1 species, which is reproducibiy formed in the range 0.15 -
= 0.8 M.F.
is the expected electron-gain centre, but its apparent intensity is only a small fraction of that of the electron-loss centre. This can be understood if the single line detected is really part of a complex spectrum of many lines, all others being undetectable. This could arise for two reasons; (a) the other features are dynamically broadened or (b) none of them have precise locations. The former concept seems to us improbable because the spectrum was unaltered at 4.2 K. The latter, (b), however, arises naturally from the following model. We suggest that as the [A$! increases there is a tendency for clustering. Initially, units containing two, three or four Agf ions would trap electrons to give centres resembling the Agi (j&v. = 1.989), Ags (g,y = I .973) and A$ Gp;y = 1.970) centres previously described 121. Such centres, present in relatively low concentration, would have features largely hidden by the main Ago features, and could well be present. It is noteworthy that gav shifts to low values in this series. As the number of A$ ions involved in a cluster increases, so gav is expected to reach a limiting value (as the number of periferal A$ ions becomes smzll compared with the inner ions). The hyperfine features from logAg and lo7Ag are expected to comprise a multiplet covering a range of ca. 500-600 G. For kentres containing an even number of equivalent Ag+ ions, there will always be a central line which should be narrow when the hyperfine coupling to each Ag+ is small. However, no other features from these clusters will coincide unless the clusters are equivalent. 69
--V0ium~_39,-nO~ber 1 .’ _‘. -_; : _ _. .we.expt
td -km a tide m-g
‘-
CHEMICALtiHYSICSLETTERS
df duster. sizes and,
hence, to‘&unable to’d&ect‘any but this cent&l. f feature in the limitingg-value~re&n. The~asymrnetriti brobdening on the.low-field side of the singlet in the low concentration region accords with this model. If this model is correct, we conclude that excess electrons in silver chloride are not initially trapped and localised by the formz&on of discrete Ags or .A$ units. This contrasts with the behaviour of ‘holes” in alkali-halides which are trapped as discrete hal- centres. Instead, they-are delocalised over many Agz ions, this being the conduction band in the limit of pure silver chloride. However, the electron-loss centres are not the expected Cl, species, but are A&PI) centres. These appPar to be strongly localised
_-
-rio
_.
. .’
-._..
.:;
-
1 April 1976
at one sjlvsr, rather than being d&c&s&d over many. zlzus, in this case, the Jahzz-Teller distortion is effec.tive and prevents delocalisation.
We would like to thank the S.R.C. for a C.A.S.E. award to D.R.B., and Dr. FarnelI of Kodak Limited, Harrow, for helpfti advice and assistance.
References [l] CT. Delbecq,iv. Hayes, M.C.M.O’Brien and P.H. Yusten, Proc. Roy. Sot. A (1963) 271. [2] C.E. Forbes and M.C.R. Symons, Mol. Phys. 27 (1974) 467.