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A. V. Nikolaeva, P. P. Seregin, A. B. Jarkoi
USING THE 57mFe3+ MOSSBAUER PROBE TO DETERMINE THE EFG TENSOR PARAMETERS IN THE COOPER SITES TO THE LATTICES OF CuO And La2-*Sr*CuO4
Mossbauer emission spectroscopy of the 57Co(57mFe) isotope has shown that the impurity iron atoms appearing at the CuO-lattice cation sites after the decay of 57Co2+ are donors and can become stabilized in two charge states,
57mFe3+ and 57mFe2+. A satisfactory agreement between the calculated and experimental values of the quadrupole splitting in Mossbauer spectra has been obtained for the 57mFe3+ centers. This permits one to consider the results obtained in the 57Co(57mFe) Mossbauer emission spectroscopy study of cuprates as reliable experimental data on the lattice electric-field gradient ( lattice EFG) tensor parameters at copper sites. The parameters of the lattice EFG tensor at the copper sites in the La2-xSrxCuO4 lattice (the main component of the EFG tensor Vzz and the asymmetry parameter) were determined experimentally by emission Mossbauer spectroscopy with 57Co(57Fe) isotopes. A comparison of the experimental and calculated dependences of Vzz on x shows that the holes arising from the substitution of La3+ by Sr2+ are localized mainly at the oxygen sites in the Cu-O2 plane.
Keywords: Mossbauer emission spectroscopy, electric field gradient tensor.
А. В. Николаева, П. П. Серегин, А. Б. Жаркой
ИСПОЛЬЗОВАНИЕ МЕССБАУЭРОВСКОГО ЗОНДА 57mFe3+
ДЛЯ ОПРЕДЕЛЕНИЯ ПАРАМЕТРОВ ТЕНЗОРА ГЭП В УЗЛАХ МЕДИ РЕШЕТОК CuO И La2-xSrxCuO4
Методом эмиссионной мессбауэровской спектроскопии на изотопе 57Co(57m Fe) показано, что примесные атомы железа, образующиеся после распада 57Co2+ в катионных узлах решетки CuO, являются донорами и могут стабилизироваться в
\ \ 57m гр 3+ 57m тт 2+ ^ 57m т-р 3+ \
двух зарядовых состояниях re и Fe . Для центров re получено удовлетворительное согласие расчетных и экспериментальных величин квадрупольного расщепления мессбауэровских спектров. Последнее обстоятельство позволяет рассматривать данные по исследованию металлоксидов меди методом эмиссионной мессбауэровской спектроскопии на изотопе 57Co(57m Fe) в качестве надежных результатов по экспериментальному определению параметров тензора кристаллического ГЭП в узлах меди. Параметры тензора кристаллического ГЭП в узлах меди в решетке La2-xSrxCuO4 (главная компонента тензора ГЭП Vzz и параметр асимметрии) были экспериментально определены методом эмиссионной мессбау-эровской спектроскопии на изотопах 57Co(57m Fe). Сравнение экспериментальных и рассчитанных зависимостей Vzz(x) показало, что дырки, возникающие при замещении La3+ на Sr2+, локализуются преимущественно в узлах кислорода в Cu-O2 плоскости.
Ключевые слова: эмиссионная мессбауэровская спектроскопия, тензор градиента электрического поля.
1. Introduction
The Mossbauer spectroscopy is widely used for investigations of high-temperature superconductors (HTSC) [6; 8]. Because copper does not have Mossbauer isotopes, Mossbauer spectroscopy of 57Fe impurity atoms is extensively used in investigations of the copper sublattices; it is assumed that the 57Fe probe stabilizes at the copper sites of the HTSC sublattices, so that, by comparing the measured with calculated quadrupole splittings in Mossbauer spectra, one can draw conclusions on the nature of the local copper-atom environment [2].
The quadrupole splitting can be calculated reliably only for the Fe3+ probe having a spherically symmetric 3d5 outer electronic shell, where the electric-field gradient (EFG) at the iron nuclei is generated primarily by lattice ions (the crystal-field EFG). However stabilization of Fe3+ at the sites of divalent copper Cu2+ (which is the most probable copper state in most HTSCs) should give rise to the formation of centers compensating the difference in charge between the substituting and replaced atoms. The compensating centers can be located near the impurity probe and in this way affect the EFG in a non-obvious way. The validity of the above considerations was buttressed by study of the state of 57Fe impurity atoms in CuO made by Mossbauer absorption spectroscopy [8]; the Fe3+ impurity atoms were found to substitute for Cu2+ ions in the CuO lattice to form associations of the type [Fe3+-V-Fe3+] (V is the cation vacancy), and calculation of the EFG tensor parameters at 57Fe nuclei in such associations turns out to be a difficult problem because of the creation of additional sources of EFG which change appreciably the total EFG.
As will be shown later, however, these problems can be eliminated by using the emission version of Mossbauer spectroscopy [1; 3-5; 7]. We have carried out a comparison of experimental and theoretical quadrupole splittings in 57Co(57mFe) Mossbauer emission spectra of 57mFe3+ impurity ions in copper sites for the CuO and La2_xSrxCuO4 lattices.
2. Experimental and Results
2.1. Samples and spectroscopy
Copper oxide was obtained by precipitating copper hydroxide from a CuSO4 water solution with an alkali, with subsequent annealing of the sediment in an oxygen ambient. Cobalt in the form of 57CoSO4 was added to a water solution of blue vitriol, with the cobalt concentration in CuO not exceeding 1017cm-3. The experiments were made with samples of La2-xSrxCuO4:57Co (x = 0.1, 0.2, 0.3, the cobalt concentration ~ 1015 cm-3) synthesized by a conventional ceramic technology. The reference samples were monophase with a K2NiF4 structure and had Tc of 25, 37 and 27 K for x = 0,1, 0.15 and 0.2 respectively and < 4.2 K for x = 0.3. The emission Mossbauer spectra have been recorded using CuO:57Co and La2-xSrxCuO4:57Co sources and K4Fe(CN)6.3H2O absorber with a surface density of 0.1 mg cm2 of 57Fe. Isomer shifts IS are given relative to a-Fe.
The Mossbauer spectra were measured on a CM-2201 spectrometer at temperatures varied from 295 to 5 K. The absorber was K4Fe(CN)6.3H2O with a 57Fe surface density of 0.1 mg/cm2.
The lattice EFG tensors at the CuO and La2-xSrxCuO4 sites were calculated according to the point charge model. In the general case the measured quadrupole interaction constant C is a sum of two terms
eQUzz = eQ(1 - y)Vzz + eQ(1 - R)Wzz , (1)
where Vzz and Wzz are the principal components of the tensors of the crystal and valence electric field gradients, and y and R are the Sternheimer coefficients of the probe atom.
The contribution of the valence electrons to the total electric field gradient tensor can be neglected for the probe 57mFe3+. Therefore
eQUzz * eQ(1 - 7)Vzz. (2)
Thus, the experimental data obtained on the parameters of the electric field gradient tensor using the 57mFe3+ probe can be correlated with calculations of these parameters based on an ionic model of the crystal lattice (point charge model).
We calculated the tensors of the crystal electric field gradient at the copper sites of the CuO and La2-xSrCuO4 lattices based on the point-charge model. In so doing, following the X-ray crystal-lographic data [8], the lattices were represented as a superposition of the following sublattices: Cu2O2 and [La2-xSrx][Cu][O(1)]2[O(2)]2.
The components of crystal EFG tensor were calculated using the equations:
V„ = X e* X -71% -1 = X e'firrl, vn = X e* X = X , (3)
k i rki rki k k i rki k
where k is the sublattice number, i is the site number within the sublattice, p and q are the Cartesian
*
coordinates, ek is the atomic charges in the k sublattice, rki is the distance of a site with the k and i
indices from the reference point.
The lattice sums Gppk and Gpqk were calculated numerically. The summation was carried out within spheres of 30 A radius.
2.2. CuO:57Co
If the copper hydroxide was annealed within the 830-920°C interval (the anneal time was 2 h), the Mossbauer spectra of CuO:57Co samples taken at 295 K were a superposition of two quadrupole doublets (fig. 1) with isomer shifts corresponding to the ions 57mFe3+ (spectrum 1) and
57mFe2+ (spectrum 2), with the fraction of the 57mFe3+ ions being 0.79 (0.02) for the anneal temperature of 920°C, and 0.19 (0.02) for 830°C. The parameters of spectra 1 and 2 are listed in table I. A decrease of the spectrum measurement temperature below the Neel point is accompanied by a broadening of the spectra, with a gradual onset of the allowed hyperfine structure (spectrum 3 in fig. 2). It is significant that, even for a sample containing Fe3+ and Fe2+ in a ratio —1:1, the spectrum taken at 5 K was a magnetic sextet corresponding to one iron-atom state only (fig. 2). The characteristics of this spectrum are given in table I. The magnitude of the isomer shift and magnetic field at the nucleus are typical of divalent iron.
Скорость, мм/с
Fig. 1. Mossbauer emission spectra of CuO:57Co measured at 295 K samples annealed at 920 °C and 860 °C. The experimental spectra are unfolded into quadrupole doublets, and the positions of these doublets corresponding to (1) 57mFe3+ and (2) 57mFe2+ centers are shown
Table I
Parameters of the Mossbauer emission spectra of 57mFe impurity atoms in CuO
Type of spectrum T, K Ion A mm/s IS, mm/s G, mm/s B, T
1 295 Fe3+ 2.50(2) -0.25(1) 0.40(2)
2 295 Fe2+ 1.52(2) -0.80(1) 0.42(2)
3 80 Fe2+ 1.30(4) -0,81(2) 0.45(3) 25.5(2)
Note. A — Quadrupole splitting, IS — isomer shift, G — spectral linewidth, B —magnetic-field induction at the 57mFe3+ nuclei.
Скорость, мм/с
.8 -6 -4 -2 0 2 4 6 8
Скорость, мм/с
Fig. 2. Mossbauer emission spectra of CuO:57Co measured at 295 K and 5 K for a sample annealed at 870 °C (the ratio of the areas bounded by spectra 1 and 2 at 295 K —0.4:0.6). The 295 K spectrum is deconvolved into two quadrupole doublets, and the positions of these doublets corresponding to (1) 57mFe3+ and (2) 57mFe2+ centers are shown. (3) The position of the components of the Zeeman sextet corresponding to the 57mFe2+ centers at 5 K
The presence of a correlation between the transition of CuO to the antiferromagnetic state and the creation of magnetic fields at the 57mFe nuclei permits a conclusion that the impurity cobalt atoms enter copper sites in the CuO lattice, and that the daughter 57mFe iron atoms produced in the cobalt radioactive decay may reside in different charge states, at least at room temperature.
The chemical properties of cobalt and the conditions of sample preparation used suggest that cobalt impurity atoms should form Co2+ isovalent substitutional centers in the CuO lattice, so that the 57mFe daughter atoms should occupy substitutional sites.
It should be pointed out that electron capture in 57Co is accompanied by Auger-electron emission leaving the daughter atom in a multiply ionized state 57mFe”+ (n ~ 7). Such an ion is an effective trapping center for Auger electrons, and the 57mFen+ ion becomes neutralized to one of its valence-stable states in a time ~ 10-12 s. The final stabilized state of the daughter atom (57mFe2+ or 57mFe3+) depends, however, both on the nature of the electrical activity of the iron center and on the nature and concentration of electrically active native defects in the CuO lattice [1; 2; 5; 10; 11].
EPR data show undoped CuO samples to be oxygen deficient, this deficiency being the larger, the higher is the anneal temperature; oxygen deficiency can be reduced by annealing in an oxygen ambient [12]. Thus it could be expected that spectrum 1 is due to single 57mFe3+ impurity
ions produced by radioactive decay of 57Co2+ in the CuO copper sites. In other words, one of the Auger electrons is captured during the 57mFen+ neutralization by a native lattice defect.
We calculated the crystal-field EFG tensor for the cation site in the CuO lattice in the point-charge approximation (the lattice was presented in the Cu2+O2 form). The crystal-field EFG tensor was off-diagonal in the crystallographic axes, and its diagonalization yields Vzz = 0.738 e/A3 and n = 0.29, where n=(Vyy—Vxx)/Vzz is the asymmetry parameter of the crystal-field EFG tensor, and Vxx, Vyy, and Vzz are components of the crystal-field EFG tensor, with \ Vzz\ > \Vyy\> |V4
The quadrupole splitting of a 57mFe3+ Mossbauer spectrum was calculated from
Д1 eOVz7\(1 -у)
1/2
(4)
where y is the Sternheimer coefficient [for the Fe3+ ion, y = -(7.97—9.14) [8], and the expected value of Acr lies from 2.14 to 2.42 mm/s. Thus one observes a satisfactory agreement between the experimental A and calculated Acr quadrupole splittings of Mossbauer spectrum 1, if we assume it to originate from single 57mFe3+ ions occupying copper sites in the CuO lattice.
The nature of the 57mFe2+ state presented by spectrum 2 can be understood if one takes into account the experimental observation that the only state left in the emission spectra taken below the Neel temperature of samples containing comparable amounts of 57mFe2+ and 57mFe3+ is 57mFe2+. Obviously enough the transition of 57mFe3+ to 57mFe2+ entails electronic processes. In other words, the 57mFe2+ ion should be considered as a neutral donor center, whereas 57mFe3+ is a singly ionized state of this center. The fine structure of a Mossbauer emission spectrum depends on the relative length of the time required for a thermodynamic equilibrium to set in between the neutral and ionized iron centers, t, and the lifetime t0 of the 57Fe Mossbauer level, namely, if t << t0, the experimental spectrum will reflect an averaged state of iron atoms produced in a fast electron exchange between the 57mFe2+ and 57mFe3+ centers, while if t >> t0, the experimental spectrum will be a superposition of the 57mFe2+ and 57mFe3+ spectra, and the 57mFe3+ contribution to the spectrum will correspond to the fraction of the 57Co atoms having electron trapping centers within the mean free path X of the Auger electrons.
Taking into account the presence of both iron states in room-temperature spectra of CuO:57Co samples, one should conclude that the emission spectra of CuO:57Co reflect a nonequilibrium situation (t >> t0) arising in the course of neutralization of highly-charged 57mFen+ states in the CuO lattice, and that the fraction of 57mFe3+ ions contributing to the spectra corresponds to that of the 57Co2+ atoms having electron trapping centers within their close environment (within X0).
Decreasing the temperature of spectral measurements from 295 to 5 K brings about stabilization of all daughter iron atoms in the 57mFe2+ neutral state only. To interpret this observation, one should recall that undoped cupric oxide CuO is oxygen deficient and n type. The isovalent 57Co2+ impurity in the CuO lattice is electrically inactive, and its presence does not affect the Fermi level position. The concentration of electrically active 57mFe iron centers in CuO is so low that the presence of iron likewise does not influence the Fermi level. Thus the position of the Fermi level is controlled only by native defects in the CuO structure. At low temperatures, the Fermi level lies close to the conduction-band bottom, and all native-defect levels are occupied by electrons. Therefore the Auger electrons formed in the course of radioactive transformation of the parent 57Co2+ atoms become trapped by the daughter iron atoms, so that all iron atoms end up by being stabilized in the 57mFe2+ state (with no free-electron-capture centers present within a distance of X0). An increase of temperature shifts, as a rule, the Fermi level towards the midgap and,
therefore, part of the native-defect levels turn out to be free and can act as Auger-electron trapping centers. As a result, iron atoms stabilize in the form of both 57mFe2+ and 57mFe3+, with the concentration ratio of these forms depending on both the spectrum measurement temperature (the higher the temperature, the larger is the fraction of the 57mFe3+ canters) and the concentration of native defects (other conditions being equal, the native-defect concentration is the higher, the higher is the anneal temperature and, hence, the higher is the 57mFe3+ center concentration).
2.3. La2-xSrxCuO4:57Co
The emission Mossbauer spectra of the La2-xSrxCuO4:57Co samples are quadrupole doublets, which points to a non-cubic environment of the Cu sites (fig. 3).
The isomer shifts of all spectra correspond to Fe3+. The quadrupolar splitting of the 57Fe Mossbauer spectra is determined by the excited state of the nucleus (spin I = 3/2, quadrupole moment Q = 0.211 b) and is described as follows
Ґ 2\1/2
А =
1+n
(5)
V ^ y
where Uzz is the principal component of the total EFG tensor at the 57Fe nucleus; n = (Ux - Uyy)/Uzz is the asymmetry parameter of the total EFG tensor. For the tensor components the inequality Uxx\ < 11vv < \ UZZ\ should be valid.
Velocity', mm/s
Fig 3. The Mossbauer spectra of La2-xSrxCuO4:57Co recorded at 295 K
Ge3+ probe Uz
Thus, the value of A is proportional to \ Uzz\, which is determined by the lattice EFG for the (1-y) Vzz , where Vzz and y relate to the Cu sites and Fe3+ ion, respectively.
To identify the compensating centres arising from Sr doping of the La2-xSrxCuO4 lattice, we calculated the EFG tensor components at the copper and lanthanum sites in the framework of the point charge model. According to [8], the atomic positions in the unit cell were:
(La,Sr): (0, 0, 0.36046c), (0, 0, 0.63954c), (a/2, b/2, 0.13954c), (a/2, b/2, 0.86046c);
(Cu): (0, 0, 0), (a/2, b/2, c/2);
O(l): (0, 0, 0.1824c), (0, 0, 0.8176c), (a/2, 6/2, 0.3176c), (a/2, 6/2, 0.6824c);
0(2): (a/2, 0, 0), (0, b/2, 0), (a/2, 0, c/2), (0, b/2, c/2).
The lattice-sum tensors from the individual sub-lattices are diagonal in the crystalline axes and have an axial symmetry (n = 0). Their principal axes coincide with the c axis.
In addition, we calculated the principal component of the lattice EFG Vzz = Vcc for various
3+ 2+ 2+ 3+ 2_ _
combinations of La , Sr , Cu , Cu , O and O ions at the La2_xSrxCu04 lattice sites using formula (5) and taking into account the electronegativity requirement. If a sublattice included
3+ 2+ 2+ 3+ 2_ _
ions with different charges (La and Sr , Cu and Cu , O and O ), then their total charge was considered as uniformly distributed over all the sites of the sublattice, i.e., ek was taken to be equal to the average charge of the sublattice ions. This model is supported by the smooth dependence of the Mossbauer spectral parameters on x, whereas the charge localization at the sites near the Mossbauer probe would produce additional spectral lines.
The calculated values for Vzz vary from 0.55 e/A3 to 0.70 e/A3 for x = 0.1 to 0.3. The eQUzz values determined from the 57mFe3+ Mossbauer spectra are within the range of the calculated values, which seems to indicate a higher reliability of the Sternheimer coefficient for Fe3+. But the absolute value of eQUzz cannot be used to identify the compensating centres. The difference can be accounted for by the uncertainty of the Sternheimer coefficient and by the reduced ion charges as compared with the formal chemical ones (the so-called lattice charge contrast).
However, knowledge of the eQUzz value is unnecessary, because its relative variation is sufficient to make the identification by comparing the dimensionless parameters P = = \eQUzz]x/[eQUzz\x=0.1 and p = \Vzz\x/\Vzz\x=0.1. It is important that, unlike eQUzz, the experimental parameter P depends neither on the Sternheimer coefficient nor on the charge contrast of the lattice. Figure 4 shows the p(x) plots for copper sites calculated for four possible models: (A) the hole is in the copper sublattice with the average site charge (2+x)e; (B) the hole is in the 0(1) sublattice with the average 0(1) charge _2(2 _ x/2)e; (C) the hole is in the 0(2) sublattice with the average 0(2) charge _(2 _ x/2)e; (D) the hole is shared by the 0(l) and 0(2) sublattices with the oxygen charge _(2 _ x/4)e.
Fig. 4. The p - x plots for copper sites in La2-xSrxCuO4.
The calculated curves A, B and C are for a hole at the Cu, 0(1) and 0(2) sublattice and D for a hole shared between the 0(1) and 0(2) sublattices respectively. The experimental points are related to the 57mFe3+.
In all of these cases the La(Sr) ion charge was taken as (3 - x/2)e. The experimental P points plotted in fig. 4 show that the decrease of eQUzz with increasing x for 57mFe3+ can be described quantitatively only for the holes localized in the O(2) or predominantly in the O(2) sublattice. The other three models contradict the experimental data. The P values in fig. 3 calculated from the 57Fe absorption Mossbauer data for the La2-xSrxCu0.995Fe0.005O4 samples [9] show a much stronger dependence on x than predicted by the calculations. The difference in the behaviour of the 57mFe3+ and 57Fe3+ centres in the La2-xSrxCuO4 lattice can be attributed to their different origin and local symmetry. The 57mFe3+ centres arise from 57Co2+ at the regular copper sites, whereas the 57Fe3+ ones, having substituted Cu2+ in the synthesis, have some compensating centres (like cation vacancies) in their vicinity, which leads to an uncontrolled change in the resulting EFG.
3. Conclusion
By the Mossbauer spectroscopy both the isolated 57mFe3+ centers in the cation sites of the CuO lattice (emission spectroscopy). For the isolated 57mFe3+ centers a satisfactory agreement has been established between the quadrupole splitting values calculated by the point charge model on the one hand and measured in the Mossbauer spectra on the other hand. This conclusion seems to be valid for copper sublattices of high-rc superconductors, and should be taken into account in the interpretation of the Mossbauer spectra of Fe-doped HTSCs. The 57Co(57mFe) emission spectroscopy is preferable in this aspect.
The comparison of experimental and calculated dependences of the EFG at the cation sites of La2-xSrxCuO4 on the x value indicates that the compensating centres arising from the substitution of La3+ by Sr2+ are the holes totally or predominantly localized in the O(2) sublattice, i.e. at the oxygen sites in the CuO2 plane.
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9. Imbert P., Jehanno G, Hodges J. A. Mossbauer study of super and semi-conducting samples of 57Fe-doped La2-xSrxCuO4 // Hyperfine Interact. 1989. V. 50. P. 599-606.
10. Regel' A. R., Seregin P. P. Mossbauer investigations of impurity atoms in semiconductors // Soviet physics. Semiconductors 1984. V. 18. No. 7. P. 723-734.
11. Seregin P. P., Nasredinov F. S., Vasilev L. N. A study of radiation defects in solids by means of
Mossbauer spectroscopy // Physica Status Solidi (A) Applied Research. 1978. V. 45. No. 1. Р. 11-45.
12. Stewart S. J., Borzi R.A., Punte G., Mereader R. C. Phase stability and magnetic behavior of Fe-
doped CuO // Phys. Rev. B. 1998. V. 57. P. 4983-498б.
REFERENCES
1. Efimov A. A., Shipatov V T., Seregin P P Effekt Messbaujera na primesnyh atomah zheleza v So3O4 // Fizika tverdogo tela. 19б9. T. 11. S. 3032-3033.
2. Masterov V F, Nasredinov F. S., Seregin N. P., Seregin P P Ispol'zovanie messbaujerovskogo zonda 57mFe3+ dlja opredelenija parametrov tenzora GEP v kationnyh uzlah reshetki CuO // Fizika tverdogo tela. 1999. T. 41. Vyp. 8. S.1403-1406.
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4. Masterov V F, Nasredinov F. S., Seregin P P., Saidov Shch. S. Eksperimental'noe opredelenie parametrov tenzora kristallicheskogo GEP v uzlah redkozemel'nyh metallov reshetok RBa2Cu3O7 metodom emissionnoj messbauerovskoj spektroskopii na izotope 155Eu(155Gd) // Sverhprovodimost': fizika, himija,
tehnologija. 1993. T. 6. Vyp. 3. S. 563-567.
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metodom Messbaujera // Fizika tverdogo tela. 1968.T. 10. Vyp. 9. S. 2624-2627.
6. Nasredinov F. S., Seregin P P. Issledovanie sostojanija primesnyh atomov olova v okisi nikelja metodom Messbaujera // Fizika tverdogo tela. 1973. T. 15. Vyp. 2. S. 385-389.
7. Seregin P P., Bondarevskij S. I., Efimov A. A. Effekt Messbaujera na atomah 57mFe v CoS // Fizika tverdogo tela. 1970. T. 12. S. 1841-1842.
8. Bordovsky G., Marchenko A., and Seregin P Mossbauer of Negative Tsenters in Semiconductors and Superconductors. Identification, Properties, and Applicaton. Academic Publishing GmbH & Co. 2012. 499 p.
9. Imbert P., Jehanno G, Hodges J. A. Mossbauer study of super and semiconducting samples of 57Fe-doped La2-xSrxCuO4 // Hyperfine Interact. 1989. V 50. P. 599-606.
10. Regel' A. R., Seregin P. P. Mossbauer investigations of impurity atoms in semiconductors // Soviet physics. Semiconductors 1984. V 18. No. 7. P. 723-734.
11. Seregin P P., Nasredinov F. S., Vasilev L. N. A study of radiation defects in solids by means of Mossbauer spectroscopy // Physica Status Solidi (A) Applied Research. 1978. V 45. No. 1. P. 11-45.
12. Stewart S. J., Borzi R. A., Punte G., Mereader R. C. Phase stability and magnetic behavior of Fe-doped CuO // Phys. Rev. B. 1998. V 57. P. 4983-4986.
М. А. Горяев, А. П. Смирнов
ГАЛОГЕНИДЫ СЕРЕБРА КАК УНИКАЛЬНЫЕ ФОТОХИМИЧЕСКИ ЧУВСТВИТЕЛЬНЫЕ ПОЛУПРОВОДНИКИ
Рассмотрены механизмы и особенности фотохимических процессов в гало-генидах серебра, а также основные их свойства. Показано, что зонная структура полупроводника, широкие возможности его легирования, относительно высокая концентрация межузельных ионов серебра, эффективное образование собственных дефектов при электронном фотовозбуждении представляют такое сочетание свойств галогенидов серебра, которое обеспечивает уникальную фотохимическую чувствительность материалов на их основе.
Ключевые слова: бромид и хлорид серебра, зонная структура полупроводника, электрон-ионный механизм фотолиза, образование собственных дефектов.
