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ФИЗИКА
А. Nikolaeva, A. Marchenko
TEMPERATURE DEPENDENCE OF THE FREQUENCY OF TWO-ELECTRON EXCHANGE BETWEEN IMPURITY NEGATIVE-U TIN-CENTERS IN LEAD SULFIDE
А Fast twoelectron exchange between neutral Sn2+ and doubly ionized Sn4+ impurity negative_U tin centers in partially compensated Pbo,98Sno,o2Nao,o1Tlo,o1S solid solutions has been found by emission Mossbauer spectroscopy on 119mmSn(119mSn) isotope; the lifetime of the Sn2+ and Sn4+ states changes from ~ б*Ю— to ~ 8*W-9 s with a change in temperature from 295 to 9oo K.
Keywords: Mossbauer spectroscopy, two electron exchange, U-centers.
А. В. Николаева, А. В. Марченко* Победитель конкурса поддержки публикационной активности молодых исследователей (проект 3.1.2, ПСР РГПУ им. А. И. Герцена)
ТЕМПЕРАТУРНАЯ ЗАВИСИМОСТЬ ЧАСТОТЫ ДВУХЭЛЕКТРОННОГО ОБМЕНА МЕЖДУ ПРИМЕСНЫМИ U-МИНУС-ЦЕНТРАМИ ОЛОВА В СУЛЬФИДЕ СВИНЦА
л ж \ - 119mm& A19mc \
Методом эмиссионной мессбауэровскои спектроскопии на изотопе Ъп( Ъп) обнаружен процесс быстрого двухэлектронного обмена между нейтральными Sn2+ и двукратно ионизованными Sn4+ примесными U-минус-центрами олова в частично компенсированных твердых растворах Pbo.98Sno.o2Nao.o1Tlo.o1S, причем время жизни состояний Sn2+ и Sn4+ меняется от ~ б' Ю~4 с до ~ 8' Ю~9 c при изменении температуры от 295 до 9oo K.
Ключевые слова: мессбауэровская спектроскопия, двухэлектронный обмен, U-минус-центры.
The notion of negative-U centers was used for the first time by Anderson [1] to explain the electric and magnetic properties of glassy semiconductors. Mossbauer spectroscopy on 119Sn isotope was found to be the most efficient method for identifying negative-U centers in glassy and crystalline semiconductors, as well as superconductors [2, 3]. In particular, it was shown that impurity tin atoms in lead chalcogenides (PbS and PbSe) are located in regular sites of cationic sublattice and form two-electron donor centers with negative correlation energy; the parameters of the microscopic model of negative-U centers were also determined. However, the question of the existence of two-electron exchange between neutral and ionized negative-U centers in semiconductors remains open. In this article, we report on the uncovering of this process and determining the temperature dependence of the frequency of two-electron exchange between neutral and ionized tin centers in partially compensated solid solutions based on lead sulfide by emission Mossbauer spectroscopy on 119mmSn(119mSn) isotope in the temperature range of 80-900 K.
Pb0.98Sn0.02Na0.01Tl0.01S solid solutions were prepared by alloying the initial components of semiconductor purity grade in evacuated quartz ampoules with subsequent annealing ingots and then pressed powders at 650°C for 120 h. All samples were single-phase and had a NaCl-type structure. The samples had p-type conductivity and were nondegenerate (the hole concentration varied from ~ 5 x 1013 at 80 K to ~ 1017 cm-3 at 295 K). Pb0.98119mmSn0 02Na0.01Tl0 01S Mossbauer sources were prepared using a 118Sn preparation irradiated by a neutron flux of ~ 1015 cm-2 s-1 for 6 months.
In figure 1 Emission Mossbauer spectra of 119mmSn(119mSn) isotope in Pb0.98Sn002Na0.01Tl001S solid solutions recorded at 80 and 295 K are shown. They are superpositions of two lines of instrumental width (G 2+ = G 4+ = 0.80(1) mm/sm), isomeric shifts 5 of
Sn Sn
which correspond to the Sn2+ (^Sn2+ = 3.69(1) mm/s) and Sn4+ (^Sn4+ = 1.26(1) mm/s) centers.
Fig. 1. Emission Mossbauer spectra r 119mmo /119mo \ • ±
of Sn( Sn) isotope in
Pb0. 98Sn0.02Na0.01Tl0.01S solid solutions recorded
by a resonant detector at source temperatures of 80,
295, 400, 500, and 900 K.
The lines corresponding to the Sn2+
and Sn4+ centers are shown.
The spectrum recorded at 900 K corresponds
to the averaged state of tin centers
Velocity, mm/s
An increase in temperature from 80 to 295 K is accompanied by a decrease in relative intensity R of the Sn2+ line (R = 0.49(1) at 80 K and R = 0.41(2) at 295 K), which can be explained by a sharper temperature dependence of the fraction of recoilless processes for divalent tin compounds, as compared with four-valence tin compounds. The Sn2+ and Sn4+ lines are somewhat
broadened (GSn2+ = GSn4+ = 0.90(1) mm/s) and located closer to each other (SSn2+ = 3.62(1) mm/s)
Sn
Sn2
and Sn4+ (^Sn4+ = 1.36(1) mm/s).
At 500 K (figure 1), the relative intensity of the Sn2+ line further decreases (R = 0.31(3)) and the Sn2+ and Sn4+ lines are significantly broadened (GSn2+ = GSn4+ = 1.6(1) mm/s) and even
closer to each other (£Sn2+ = 1.5(1) mm/s and £4+ = 3.2(1) mm/s). Finally, at 900 K (figure 1), the
lines are merged into one broadened line (G = 2.4(1) mm/s), the isomeric shift of which (5 = 2.0(2) mm/s) is intermediate between those for the Sn2+ and Sn4+ centers.
The spectra in figure 1 illustrate a typical process of electron exchange between two Sn2+ and Sn4+ states; the frequency of electron exchange increases with an increase in temperature. The spectrum recorded at 900 K corresponds to the averaged state of tin centers, which arises due to the fast electron exchange between Sn2+ and Sn4+. The absence of the intermediate charge state of tin centers (Sn3+) in the spectra indicates that the exchange occurs via simultaneous transfer of two electrons.
To determine the frequency of electron exchange, the experimental spectra were processed by the least-squares method on the assumption that the shape of the spectral line is determined by the relation
w(V) = - AC+,
2+
C2 + D2
where:
A = JAG,
B = JSn*
C =(
D = (
J
+ r 4+ + r 2
Sn4+ Sn2
Sn
+ r
- У) + ГІ*) ■
-r-1 r
Sn2+ Sn4 + ’
and Jn4+ — are the amplitudes of the Sn and Sn lines, respectively; and ts 2+ and
2+ 4+
Sn2+---- “ Sn4+ --- r---------------- "n and Sn -’ -■” - 'Sn
t 4+ are the lifetimes of the Sn2+ and Sn4+ centers, respectively (we assume that t 2+ = t 4+ = t
Sn A J Sn Sn
because of the lack of experimental parameters).
The temperature dependence of the frequency of electron exchange v = t-1 between the centers of the Sn2+ and Sn4+ is shown in figure 2.
ц Hz
Fig. 2. Temperature dependence of the frequency of the electron exchange between the centers of Sn2+ and Sn4+
1000/7; K1
The activation energy of exchange is 0.11(2) eV. This corresponds to the distance of the Fermi level from the valence band to the hole partially-compensated solid solutions Pb1-x-
ySnx(Na,Tl)yS [2], and indicates that the electron exchange between the centers of Sn2+ and Sn4+ is implemented using state of the valence band. In favor of such a mechanism is evidenced by the fact that the exchange is observed at low concentrations of tin, when the process can not be a direct exchange of electrons between the centers of the tin.
REFERENCES
1. Anderson P. W. Model for electronic structure of amorphous semiconductors // Physical Review Letters. 1975. V. 34. No 15. R 953-955.
2. Bordovsky G., Marchenko A. and Seregin P Mossbauer of Negative U Centers in Semiconductors and Superconductors. Identification, Properties, and Applicaton. Academic Publishing GmbH & Co. 2012. 499 p.
3. Bordovskii G A., Castro R. A., Marchenko A.V., Seregin P P Thermal stability of tin charge states in the structure of the (As2Se3)0.4(SnSe)0.3(GeSe)0.3 glass // Glass Physics and Chemistry. 2007. V. 33. No. 5. R. 467-470.
А. Shaldenkova, P Seregin
CORRELATIONS OF THE 63Cu NMR DATA WITH THE 67Cu (67Zn)
AND THE 61Cu (61Ni) EMISSION MOSSBAUER DATA FOR CERAMIC SUPERCONDUCTORS
A linear correlation between the quadrupole coupling constant ССи measured by the 63Cu NMR technique on the one hand and the quadrupole coupling constants С2п and CNi measured by the 67Cuf7Zn) and 6ICu(6INi) emission Mossbauer spectroscopy on the other hand has been found for ceramic superconductors.
Keywords: Mossbauer spectroscopy, NMR, electric field gradient.
А. В. Шалденкова, П. П. Серегин* Победитель конкурса поддержки публикационной активности молодых исследователей (проект 3.1.2, ПСР РГПУ им. А. И. Герцена)
КОРРЕЛЯЦИОННЫЕ СООТНОШЕНИЯ МЕЖДУ ДАННЫМИ ЯМР 63Cu И ЭМИССИОННОЙ МЕССБАУЭРОВСКОЙ СПЕКТРОСКОПИИ 67Cu(67Zn) И 61Cu(61Ni)
ДЛЯ КЕРАМИЧЕСКИХ СВЕРХПРОВОДНИКОВ
Установлена линейная корреляция между постоянной квадрупольного расщепления CCu, измеренной методом ЯМР 63Cu и постоянными квадрупольного расщепления CZn и CNi,, измеренные методом эмиссионной мессбауэровской спектроскопии 67Cuf7Zn) и 6ICu(6INi) для керамических сверхпроводников.
Ключевые слова: мессбауэровская спектроскопия, ЯКР, градиент электрического
поля.
1. Introduction
One of the main problems in physics of high-temperature (high-Tc) superconductors is the determination of the spatial distribution of electronic defects in the lattices of copper metal oxides. A potentially effective method to solve this problem is to compare the experimentally determined and calculated parameters of the electric field gradient (EFG) tensor for specific lattice sites [2]. Copper sites are of utmost interest in such work because these atoms are found in the
