The hyperfine interactions at In and Sn sites of In2O3∶Sn (ITO) were measured through time-differential perturbed angular correlations and Mossbauer spectroscopy, respectively. Polycrystalline samples prepared by co-precipitation with nominal 0.025, 1 and 5 at.% Sn were studied. They all showed the cubic bixbyite structure characteristic of In2O3 after annealings at 200 °C. The quadrupole interaction at In sites appears nearly independent of Sn concentration being the main result of the presence of Sn in the lattice, the gradual disappearence of the dynamic perturbation caused by after-effects. The Mossbauer data demonstrate that Sn ions are in a 4+ state with ΔQ=0.606 mm/s andδ=0.226 mm/s relative to SnO3Ca at room temperature.Facultad de Ciencias Exacta

Bibiloni, Aníbal Guillermo

Desimoni, Judith

López García, Alberto Raúl

Massolo, Claudia Patricia

Mendoza Zélis, Luis Alejandro

Mercader, Roberto Carlos

Sánchez, Francisco Homero

Terminiello, Laura Adriana

Hyperfine Interactions

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Hyperf'me Interactions 23 (1985)211 - 220 211 H Y P E R F I N E  C H A R A C T E R I Z A T I O N  O F  T I N - D O P E D  I N D I U M  S E S Q U I O X I D E  R.C. MERCADER, F.H. SANCHEZ, L.A. MENDOZA-ZI~LIS, L. TERMINIELLO, A.G. BIBILONI, C.P. MASSOLO, J. DESIMONI and A.R. L6PEZ-GARCiA Departmento de Fisica, Facultad de Ciencias Exactas, Universidad Universidad Nacional de La Plata, Calle 49 y 115, 1900 La Plata, Argentina Received 10 January 1985 (Revised 27 February 1985) The hyperfine interactions at In and Sn sites of In2 03 :Sn (ITO) were measured through time-differential perturbed angular correlations and M6ssbauer spectro- scopy, respectively. Polycrystalline samples prepared by co-precipitation with nominal 0.025, 1 and 5 at.% Sn were studied. They all showed the cubic bixbyite structure characteristic of In203 after armealings at 200 ~ The quadrupole interaction at In sites appears nearly independent of Sn concentration being the main result of the presence of Sn in the lattice, the gradual disappearence of the dynamic perturbation caused by after-effects. The M6ssbauer data demonstrate that Sn ions are in a 4+ state with AQ = 0.606 mm/s and 6 = 0.226 mm/s relative to SnO3Ca at room temperature. 1. I n t r o d u c t i o n  Films of  transparent semiconducting oxides (e.g. SnO 2 :Sb, CdO:Sn, In20 3 :Sn and Cd2SnO4) show a singular combination of  electrical and optical properties: high concentration of  nearly free electrons, U.V. absorption and high I.R. reflectance. Con- sequently, these oxides are of  high technological importance and pose very interesting problems to the solid state physicist. They have been intensively studied by several techniques and diverse preparation methods have been developed. Nevertheless, the physical properties of  these complex systems are not well understood, since they are greatly dependent on preparation methods [ 1 ] .  Perhaps the most interesting of  this family of  oxides is the indium tin oxide (ITO), In20  3 :Sn. Moreover, it is one of  the few systems that presents naturally in its composition two elements that are good probes for two different hyperfine tech- niques: In for TDPAC and Sn for M6ssbauer spectroscopy (ME). In spite of  this advantage, a systematic investigation of  its quadrupole parameters has not yet been 9 J.C. Baltzer A.G., Scientific Publishing Company 212 R. C. Mercader et al., Hyperfine characterization attempted, let alone their relation to other physical properties or dependence on the production method. Only one M6ssbauer measurement on ITO films has been reported [2] ,  but no parameter was quoted. With this purpose in mind, we have undertaken a study of  In203 doped with different concentrations o f  Sn, making use o f  the above mentioned techniques. In order to attain a good characterization of  the compounds, the present research has been restricted to polycrystalline samples. We report in this paper results obtained on samples with three concentrations of  Sn, namely 0.025, 1 and 5 atomic percent, which are discussed in connection to our previous measurements on undoped In203 [3]. 2.  E x p e r i m e n t a l  We obtained polycrystaUine In2Oa:Sn by the co-precipitation method de- scribed by Frank et al. [4].  This method assures a quick attainment of  thermo- dynamic equilibrium due to the intimate contact of  indium and tin during oxidation, but may lead to the formation of  a hexagonal modification of  the structure (corundum) apart from the normal cubic one described below. Solutions of  SnC14 and InC1 a (containing some 111INC13 in the samples to be analyzed by TDPAC) were mixed in adequate proportions to obtain 0.025, 1 and 5 at.% Sn. The precipitation temperature was 60 ~ and the resulting gel was dried in air at 200 ~ The powders so obtained were annealed in air at temperatures ranging from 550 to 1300 ~ (see table 1). X-ray diffractograms were performed on each sample after each preparation stage. Table 1 Nominal composition and thermal treatments of the studied samples (* indicates the experimental technique used) Sample Sn concentration Annealings X-ray TDPAC ME number (at.%) t(h) X T(~ I 0.025 15 • 200 + 10 • 1050 , , 2 1.0 15 x 200 + 10 x 1000 * * * 3 1.0 2•  150+ 8x 950 * 4 1.0 2 x 150 +21 X 1040 * * 5 5.0 15x 200+16x 530 * + 10 x 1000 * + 2 X 1300 * * * 6 5.0 15 X 200 * * + 9 X 560  * * + 10 X 1000 * * 7 5.0 15 X 200 * * + 16 X 1300 * * R.C. Mercader et at, Hyperflne characterization 213 The M6ssbauer data, obtained with transmission geometry in constant accelera- tion mode, were accumulated simultaneously in two halves of 256 channels each, in a multiscaler and waveform generator developed and built at this Physics Department. A CallgmSnO3 source of = 5 mCi activity was used. A 50 ~um Pd filter removed the undesirable Sn X-rays. The detector was a 2 mm thick NaI(TI) crystal. Room tempera- ture was kept constant within -+ 1 ~ at 20 ~ The two halves of the spectra were fitted independently with lorentzian curves on a parabolic baseline with a non-linear least-squares program with constraints. The velocity calibration was obtained by means of a 10 mCi STCoRh source and a 6/am thick natural Fe foil as absorber. Velocity non- linearity was fitted to a third degree polynomial. The TDPAC measurements were performed using the 173-247  keV 3 ' - 7  cascade following the electron-capture decay of the 111In activity. The 11~Cd inter- mediate 5/2 + level has a lifetime of 85 ns and a quadrupole moment of + 0.7712 b. The angular correlation was measured using a conventional automated fast-slow coincidence setup including a 4.5 • 4.5 cm 2 CsF fixed detector and a 5 • 5 cm 2 NaI(T1) mobile one. Time resolution was 2.4 ns (FWHM). Experimental asymmetry ratios R(t) were evaluated and fitted with theoretical functions A 2 G2(t) folded with the measured time-resolution curve. Further details of the electronic equipment and data analysis can be found in ref. [5]. 3. Results The X-ray diffractograms of all the samples showed the pattern belonging to In20 a with the cubic bixbyite structure (spatial group Ia3). None of the lines corre- sponding to the corundum-type modification (spatial group R3c) nor the InO(OH) peaks reported by Frank et al. [4] could be observed even in samples just annealed at 200 ~ A sharpening of the lines occurred with increasing temperature of annealing due to the greater particle size. In some of the 5 at.% Sn samples, it was possible to notice two lines that denoted the presence of small quantifies of SnO 2 only after annealings at 1000 ~ or higher temperatures. Figure 1 shows TDPAC spectra obtained on samples with different concentra- tions of tin and annealed at temperatures >/ 1000 ~ Two static quadrupole inter- actions were necessary to fit the data and the resulting parameters, shown in table 2, are temperature independent between 15 and 800 K and very close to those of pure In203 [3]. These interactions can still be associated with the two non-equivalent sites of In in the crystalline structure of In203 that are in the relative population ratio of 3:1. Like undoped In203, the 0.025 at.% sample also showed a dynamic interaction that disappeared when measurements were taken at higher temperatures. This is discussed elsewhere [6]. A 5 at.% sample was also measured after an annealing at 530 ~ before the 1000 ~ treatment. It displayed a TDPAC spectrum clearly different from the others 214 R.C. Mercader et aL, Hyperfine characterization R ( t )  - 0 . 0 5  - 0.00 - 0 . 1 0  -0.00 - 0 . 10  - 0.05 - 0 . 1 0  - 0.05 r i i i i | i ! i i i 20 40 60 t ( n f , )  Fig. 1. TDPAC spectra at RT of In203 :Sn annealed at T ~ 1000 ~ containing nominally: (a) 0.0, (b) 0.025, (c) 1.0, and (d) 5.0 at.% Sn. The solid lines show the least-squ~res fit to the data. R.C. Mercader e t  al., Hyperf ine  characterization 215 Table 2 Static quadrupole parameters, ~Q = eQVzz2n/40 h and n = ( Vxx - Vw)/Vzz, ob- tained from TDPAC at room temperature, for Ina Oa :Sn after annealing(it 1000 ~ The relative FWHM, 6, of a lorentzian frequency distribution around ~Q is also quoted at.% Sn ~oQ1 (Mrad/s) ?21 61 t~Q2 (Mrad/s) "~2 62 0.0 18.33 0.722 0.001 24.36 0.122 0.001 0.025 17.93 0.722 0.011 24.13 0.143 0.001 1.0 18.32 0.731 0.04t 23.92 0.182 0.011 5.0 18.32 0.692 0.061 24.22 0.264 0.021 R ( t )  -0.05 -0.00 -0.I0 -0.05 , , , i r , , , 20 40 60 t (ns)  Fig. 2. TDPAC spectra at RT of In203 :Sn contain- ing nominally 5 at.% Sn annealed at: (a) 530 ~ and (b) 1000 ~ The solid lines show the least-squares fit to the data. 216 R. C. Mercader et al., Hyperfine characterizan'on (fig. 2) that could only be fitted allowing for a highly distributed quadrupole inter- action on top of the already mentioned ones. This additional contribution accounted for 78% of the measured attenuation and was characterized by the following para- meters: ~OQ = 202 Mrad/s, r~ = 0.52,8 = 0.286. e- o m Itl In w E tn e- o II,,. 4a;  O GI I k .  G b -4 -2 0 2 4 V (mm/s) Fig. 3.M6ssbauer spectraat RTofln~O~ :Sn annealed at T ;~ I000 ~ containing nominally: (a) 1.0 and (b) 5.0 at.% Sn. The solid lines show the least-squares fit to the data. Figure 3 shows typical M6ssbauer spectra of  1 and 5 at.% Sn samples (it was not feasible to obtain M6ssbauer spectra of the 0.025 at.% Sn samples). All of the data exhibit isomer shifts characteristic of tin ions in the 4+ state and are composed of unresolved quadrupole doublets. Since the X-ray diffractograms of the 5 at.% Sn samples annealed at 1000 ~ denoted the existence of SnO2, the fittings of these samples were performed with two quadrupole interactions, one with initial fitting parameters close to those of SnO2 and the other with values similar to those of 1 at.% Sn samples, where almost totally dissolved Sn in In203 is expected. This was con- R. C Mercader et aL, Hyperfine characterization 217 sidered to also be the case even for the specimens annealed at low temperature that did not show X-ray lines corresponding to Sn02 because of the very small particle size. In should be emphasized, however, that a neat result was not expected, since the ~ 9 Sn has a broad M6ssbauer line and the two species, Sn02 and tin dissolved in ITO, have apparently very similar parameters (almost identical A Q and isomer shifts differ- ing at most by about 1 [4 th of the natural line width). These results are shown in table 3. Table 3 M6ssbauer quadrupole parameters at ~19Sn in In203 :Sn at room temperature. The results shown are averages over 10 measurements of 5 at.% and 8 of 1 at.% samples. The third and fourth columns are the averages of the fitted values corresponding to SnO2. The isomer shifts are referred to CaSnO3 at room temperature at.% Sn A Q (ram/s) 6 (ram/s) :, Q t (minis) ~i' (minis) 1.0 0.572 0.182 5.0 0.646 0.276 Adopted values 0.606 0.226 0.616 - 0.024 4. Discussion In20 a crystallizes in the cubic bixbyite structure, closely related to that of CaF a, from which it can be derived by removing orderly 1/4 of the anions and then allowing for relaxation [7]. The oxygen retains its 4-coordination, while two non- equivalent sites arise for the 6-coordinated In atoms. Twenty-five percent of indiums are occupying the centers of trigonally distorted oxygen octahedra (site b). The re- maining 75% are located at the centers of more distorted octahedra of lowest sym- metry (site cl). Electric field gradients arising from a pure ionic lattice account for the ob- served quadrupole interactions at ~1 ~ Cd and grant a reliable assignment of the fre- quencies and asymmetry parameters to the two nonequivalent In sites. Indeed, point- charge calculations, summing over 30 nearest neighbours reproduce the observed EFGs within experimental errors (see table 4). A summation over the whole lattice, using the de-Wette method, yields similar results. In ITO, indium atoms are substituted by tin up to a small percentage that de- pends on the preparation method. This doping produces a broadening of the TDPAC frequency distributions and a slight increase in the value of the asymmetry parameter 77 z with concentration. In order to interpret these results, we made point-charge calcu- 218 R. C Mercader et al., Hyperflne characterization Table 4 Measured and calculated Vzz and r/of the electric field gradient at sites d and b of pure and tin-doped IntO3 ( Vzz in units of 10 ~7 V/crn 2) (I -7=) P'zz d 7/d (1 - 3'o.) Vzz b b Vzz/Vz d b Pure In203 6.39 0.722 8.31. 2 0.122 (experimental) Pure In2 O3 5.0 0.54 6.8 0.00 (calculated) Sn 4§ substituted at one of the 6 possible 4.9 0.56 5.6 0.61 n n  In 3+ sites Sn 4§ substituted at one of the 6 poss~le 5.3 0.62 7.7 0.09 n n n  In 3+ sites 1.33 1.36 lations with substitution of one Sn 4+ ion for one In 3+ at a site of subsequent indium neighbour shells. From the values presented in table 4 it can be seen that in spite of the crude model used (which ignores any shielding effect), the general trends are well- reproduced. With increasing Sn concentration, the probability for In probes to have one Sn 4§ in its neighbourhood will grow. Then the measured 7/2 value, resulting from the various possible r; b, will rise too. This agreement is better if preferential substi- tution of In at site b is proposed. The frequencies themselves are concentration independent, but the distribu- tions observed are mainly caused by the increasing presence of tin substituents. Fre- quency distributions can also originate on surface effects and, due to smaller particle sizes, these could be particularly important for samples not treated at higher tempera- tures. Indeed, the broadly distributed component found in the sample annealed at 530 ~ can be attributed to the greater relative proportion of probes near to or at the particle surface. The X-ray diffractograms of this sample also show wider diffraction peaks that, in the framework of Bragg law [8], denote a particle diameter smaller than 10 nm. For this particle size, the relative number of  bulk to surface atoms is consistent with the measured contribution of the broad frequency to the perturbation factor when atoms further than seven interatomic distances away from the surface are con- sidered to be no longer perturbed by surface effects. The progressive dilution of Sn in the In203 lattice causes a gradual disap- pearance of the time-dependent perturbation. This is originated in the after-effects produced by the electron capture in 111In [6]. Its contribution lessens and finally vanishes because of the donation of the extra electrons from tin impurities to the R. C. Mercader et al., Hyperfine characterization 219 conduction band. This is also proof of the effective dilution of tin in the lattice. Since the measured EFG is neither dependent on tin concentration nor on the measurement temperature, it can be inferred that the conduction band has predominantly s- character. This is coherent with the schematic energy diagram proposed by Fan and Goodenough [9]. The values shown in table 3 are the averages obtained from the fittings of the M6ssbauer spectra for the 1 at.% Sn samples with only one quadrupole interaction and 5 at.% Sn samples fitted against one component belonging to SnO 2 and the other to Sn in ITO. Significantly, better X 2 values were obtained when these two quadrupole doublets were used for the theoretical fitting functions solely in the case of 5 at.% Sn samples. This was not so for 1 at.% Sn samples, where fits with two interactions gave poorer • values. However, it cannot be discarded that a very small quantity of SnO 2 may contribute to the signal. The values of AQ and 8 adopted for Sn dissolved in ITO are the averages over those obtained for both concentrations (see table 3). These values correspond defini- tively to an Sn 4§ ion and rule out another possible charge state of the tin ion. As already mentioned, the tin ions in ITO axe substituting the indium ions of indium sesquioxide. Since TDPAC measurements reveal two dearly different quadrupole interactions, through the values of COQ and 77, it might be expected that M6ssbauer spectra also showed these two non-equivalent situations. However, since the 119Sn M6ssbauer transition is between levels of spins 3[2 and 1/2, the contributions of Vz~ and r /cannot be disentangled from the resulting quadrupole interaction. The quadru- pole splittings corresponding to the two sites might differ at most by 23%, too small a difference to be observed through the broad 1~9Sn M6ssbauer line. Besides, the presence of SnO 2 in the samples makes the case even more difficult because of the very similar quadrupole parameters of both species. For these reasons, it is not possible to discern an eventual preferential substitution of Sn for sites b or d in the lattice. The Vzz measured through ME on Sn is about five times greater than either of the two values that TDPAC yields for nominally the same site of In, taking Qcd = 0.77 b, Qsn = - 0.08 b, (1 - 7**)ca = 30.3 and (1 - ~/**)sn = 11. An explanation of this may be attempted on considering that the Sn 4§ ion has a AZ = + 1 respective to the In ion it substitutes. This charge excess may be compensated by an increased local density of free electrons that would tend to occupy the lowest empty tin orbitals, probably hybridized to fulfill the requirements of the local symmetry, thus increasing the local EFG. 5. Conc lus ions  The static quadrupole parameters of ITO at indium sites are very similar to those of  pure In203. Only an increase of the value of ~ at site b is observed. The main consequence of the dilution of tin in the In203 lattice is its in- fluence on the extinction of the dynamic interaction through the donation of Sn 220 R . C  Mercader et al., Hyperfine characterization electrons to the conduction band. The static EFGs are not altered by this augmented concentration because of  the predominant s-character of  these electrons. The M6ssbauer parameters of  ITO at tin sites are ~ = 0.22 +-0.06 mm/s and AQ = 0 .60+0.06  mm/s, indicating a 4+ state for Sn ions. The derived EFG is greater than those measured by TDPAC, probably due to local contributions. A bixbyite-type structure o f lTO was formed at temperatures as low as 200 ~ Further annealings only contributed to enlarge the particle size. No corundum-type structure was observed for any tin concentration or annealing temperature. Acknowledgements The authors are grateful to CONICET and CICPBA, Argentina, and Inter- nationales Beziehungen, Kemforschungszentrum Karlsruhe GmbH, West Germany for partial economic support. The provision of  the X-ray diffractograms by CETMIC, La Plata, Argentina, and the X-ray laboratory at this Physics Department is kindly acknowledged. R.C.M., F.H.S., L.A.M.Z., A.G.B., C.P.M. and A.R.L.G. are members of  Carrera del Investigador Cientifico CONICET, L.T. is a fellow of  CICPBA, and J.D. is a fellow of  CONICET. References [1] Z.M. Jarzebski, Phys. Stat. Sol. (a) 71(1982)13. [2] H. K6stlin, R. Jost and W. Lems, Phys. Stat. Sol. (a) 29(1975)87. [3] J. Desimoni, A.G. Bibiloni, L.A. Mendoza-Z~lis, A.F. Pasquevich, F.H. S~nchez and A.R, L~pez-Garcia, Phys. Rev. B28(1983)5739; A.G. Bibiloni, J. Desimoni, C.P. Massolo, L.A. Mendoza-Z~lis, A.F. Pasquevich, F.H. S~nchez and A.R. L~pez-Garcia, Phys. Rev. B29(1984)1109. [4] G. Frank, R. Olazcuaga and A. Rabenau, Inorg. Chem. 16(1977)1251. s [5] A.F. Pasquevich, F.H. S~nchez, A.G. Bibiloni, J. Desimoni and A.R. L6pez-Garcia, Phys. Rev. B27(1983)963. [6] A.G. Bibiloni,C.P. Massolo, J. Des'maoni, L.A. Mendoza-Z~lis, F.H. S~nchez, A.F. Pasquevich, L. Damonte and A.R. L~pez-Garcia, Phys. Rev. B (to be published). [71 M. Marezio, Acta Cryst. 20(1966)723. [8] B.K. Vainshtein, in: Modern Crystallography I, Springer Series in Solid State Sciences, Vol. 15, ed. H.J. Queiser (Springer Verlag, Berlin, 1981) p. 238. [9] J.C.C. Fan and J.B. Goodenough, J. Appl. Phys. 48(1977)3524. 

Hyperfine characterization of tin-doped indium sesquioxide

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Hyperfine characterization of tin-doped indium sesquioxide

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