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The properties of cadmium stannates investigated by EPR and high-resolution solid-state 113Cd NMR spectroscopy
J. Phys. Chm. .W& Vol. 48, No. IO, pp. F’tintcd in Great Britain.
OOZZ-369718733.00 + 0.00 8 1987 Pcr8amon Joumalr Ltd.
881-885, 1987
THE PROPERTIES OF CADMIUM STANNATES INVESTIGATED BY EPR AND HIGH-RESOLUTION SOLID-STATE ‘13Cd NMR SPECTROSCOPY C. M. CARDILE, R. H. MEINHOLD and K. J. D. MACKENZIE Chemistry Division, DSIR, Private Bag, Petone, New Zealand (Received 15 December 1986; accepted 12 March 1987) Ah&net-Yellow and green forms of dicadmium stannate, Cd,SnO,, prepared under oxidising and reducing conditions, respectively, are crystallographically very similar, but have different electronic and spectroscopic properties. The g-values and peak widths of the EPR conduction electron resonances, and the “%d NMR chemical shifts, are systematically larger in the green materials than the yellow, consistent with a higher concentration of conduction electrons in the former. In contrast, the electrical conductances of the green samples are lower than the yellow, probably due to scattering by the high donor concentrations in the green materials. Dark green Cd, SnO,, prepared by electrochemical treatment of yellow material, has spectroscopic properties similar to the other green samples, but a higher electrical conductance. The spectroscopic properties of CdSnO, are also reported. Keywords: Cadmium stannate, “‘Cd MAS NMR, EPR, electric conductance.
INTRODUCTION
EXPERIMENTAL
together, either CdSnO, or Cd, SnO, (or a mixture of the two) can be formed, depending on the reaction conditions and stoichiometry [1,2]. Although the physical properties of CdSnO, appear to be relatively insensitive to the reaction atmosphere, the properties of Cd,SnO, are much more atmosphere-sensitive; when prepared in oxidising conditions, Cd,SnO, is bright yellow and exhibits a sharp, intense EPR spectrum, whereas samples produced under non-oxidising conditions or reduced pressure are various shades of green, have different electrical conductance properties to the oxidised samples, and have a much less intense EPR spectrum [3]. Recently it has proved possible to reversibly convert yellow Cd2 SnO, to a green form by electrochemical means [4], but the relationship between this and the green chemically-produced cadmium stannates has not yet been established. Polycrystalline pellets of CdSnO, and Cd,SnO, have both been shown to exhibit photoelectrolytic behaviour [ 11,but their bandgap is not ideal for solar energy conversion. However, their relatively high electrical conductivities and transparency to solar spectral wavelengths suggests other possible uses as conducting electrodes for solar cells. To provide further insight into the semiconductor properties of these materials, a new series of samples was fabricated under a variety of strictly controlled reaction conditions, using both chemical and electrochemical techniques. The EPR spectra were re-investigated, and augmented with high-resolution solid-state “‘Cd NMR spectroscopy with magic-angle spinning, and electrical conductance measurements. When Cd0
and SnO, are heated
881
Samples of both CdSnO, and Cd,SnO, were prepared by solid-state reaction of mixtures of Sn02 and CdO, the latter pre-calcined at 500°C for 0.5 h to remove hydroxide and carbonate impurities. After being weighed out in the appropriate molar ratios, the powders were intimately mixed by hand grinding and reacted at 1050°C for 6 h under the following conditions: (a) In flowing atmospheres (0.2 1mini) of dry air, oxygen-free nitrogen and hydrogen-nitrogen (5:95%); (b) In sealed silica tubes containing these atmospheres at atmospheric pressure or under reduced pressure ( 10m4torr); (c) By electrochemical treatment of yellow Cd,SnO, prepared in air atmosphere at 105O”C, then pressed into a 10 mm dia. pellet at 5.5 MPa and lightly sintered in air at 800°C for 18 h. The resulting electrode was attached by its rim to a platinum wire, using silver paint and epoxy resin, then placed in a potentiostat cell containing 0.5 M (AR) NaOH solution and platinum counter-electrodes. A potential of +0.3 V (SCE) was applied for 12 h, using a Princeton Applied Research potentiostat model 173 with a model 276 interface unit. Further details of the electrochemical experiment are reported elsewhere [4]. The progress of the reaction position of the products were powder diffraction using a Philips PW 1700 diffractometer and a graphite monochromator.
and the phase commonitored by X-ray computer-controlled with CoKa radiation The unit cell par-
C. M. CARDILE et al.
882
ameters of the monophase products were also determined from careful measurements of a number of their X-ray reflections, using Si powder as the internal angular calibrant. The X-band EPR spectra of the powdered samples were obtained at room tincture using a Varian EPR spectrometer. The solid-state ‘13Cd NMR spectra were obtained using a Varian XL-200 spectrometer at sample spinning speeds of 2-2.5 kHz. Attempts to detect the l19Sn NMR resonance were unsuccessful. Conductivity measurements were made on 4.2 mm dia. pellets pressed from 68 mg of sample at 1.4 MPa pressure, with silver electrodes painted on. A Hewlett-Packard Model 4192A low-frequency impedance analyser was used. Since measurements made over the frequency range 5 Hz-13 MHz showed no frequency dependence between 100 Hz and 100 kHz, a working frequency of 1592 Hz was adopted, to coincide with the frequency of the Wayne-Kerr model B201 bridge which was used to determine the temperature dependence of the conductivity up to 150°C. Although care was taken to reproducibly press the pellets to ensure internal consistency in the results, the absolute values of the conductances may depend on the way in which fabrication parameters influence grain boundary behaviour. RESULTS AND DLSCUSSION The Cd,SnO, samples prepared as above ranged in colour from bright yellow (fired in oxidising conditions) to green (fired in non-oxidising conditions). Samples fired in flowing oxygen-free nitrogen were yellow, and showed all the characteristics of oxidised materials, suggesting that oxygen was not fully excluded under dynamic nitrogen atmospheres. In contrast, the sample fired under oxygen-free nitrogen in a sealed tube was green. The electrochemically treated pellets were dark green. The properties of all the stannates are summarised in Table 1, which shows that the unit-cell volumes of the yellow Cd,SnO, samples were all 175.6-175.7 A3 (cf. 175.6A3 calculated from the cell constants of Cd2 SnO, listed in the JCPDS powder diffraction data file [S]). By comparison, all the green samples except
the electrochemically produced samples showed a slight but consistent expansion of the cell volume to 175.8 A3, resulting from an increase in all three cell dimensions. This may reflect the reduction of some Sni’ to the larger Sn” in the green samples, as previously suggested on the basis of ESCA results [4], but if so, it is not obvious why a similar or greater expansion is not observed in the electrochemically prepared green samples. The only successful preparation of monophase CdSnO, was in the sealed-tube experiment under reduced pressure. The product is off-white, with a measured cell volume of 239.8 A3, in good agreement with the value of 239.7 A3 calculated from the JCPDS cell parameters [6]. The room-temperature electrical conductances of the yellow CdrSnO., samples fall in the range of 2.0-3.8 x lo-‘S cm’, the conductances of the green samples being lower (0.9-1.9 x tow3 S cm-‘). The conductances of the electrochemically reduced samples (0.6-4.7 x lOwi S cm-‘) were significantly greater than those of the yellow or green Cd,SnO, samples, whereas the CdSnO, was the least conducting of all the samples (1.5 x lo-‘Scn-‘). The temperature dependence of the conductances, measured up to about lSO”C, showed that the conductance of all samples increased with increasing temperature. The strongest temperature dependence (estimated from the slope of In u vs - 1/2kT to be 0.49 eV) was shown by the yellow samples, the green being less temperature dependent (0.28 eV). The EPR spectra of all these stannates contain a major resonance at g = 1.85-1.90, arising from the conduction electrons. In some samples, other very weak resonances were occasionally found, as reported previously [3]; the random occurrence of these signals and their wide range of g-values suggests them to be due to paramagnetic impurities. The g-values of the yellow samples (1.859-1.861) were significantly smaller than those of the green samples (1.885-l ,910). The electrochemically-produced samples had the largest g-values of all the samples (2.0), whereas the g-value of CdSnO, (1.863) is similar to that of yellow CdzSn04. The widths of the EPR conduction electron resonances follow analogous trends to the g-values; the yellow samples show very
Table 1. Properties of the cadmium stannates Cd,SnO, (yellow) Cell volume (it’) Electrical conductance (Scln_‘) EPR g-value EPR peak iKidth (gauss) ‘13CdNMR chemical shift 6 (Ppm) Wd NMR peak width (Ppm)
cd,Sn04 lelectrochemical)
CdSnO,
175.6-175.7 2-3.8 x IO-*
Cd,SnO, (green) 175.8 0.9-1.9 x 10-S
175.7 0.647 x 10-l
239.8 1.5 x lo-’
1.859-1.861 12-20 272-340
1.885-1.910 7&85 581M20
2.0 350 760
1.863 23 340
185196
23&262
u 320
232
Properties of cadmium stannates I 9’
-
1.90 -
1.69 -
x 5b
1.66 -
5 Y
1.65
I& 0
, , , , , , , , 10
20
30
40
50
EPR peak width
60
70
60
90
(Goursl
Fig. 1. Relationship between the g-values and peak widths of the EPR conduction electron signal in cadmium stannates. 0, Cd,SnO, formed in dynamic gas atmospheres; 0, Cd,SnO, formed in sealed tube experiments; X, CdSnO, formed in scaled tube.
sharp peaks (12-20 gauss wide), the resonances of the green samples being much broader (70-U gauss) with the electrochemically treated sample being the broadest (350 gauss). The width of the CdSnO, signal is intermediate between that of yellow and green CdzSnO, (30 gauss). The g-values of the conduction electron peaks were found to be essentially independent of the intensities of the resonances, which were estimated from the product of the peak height and the square of the peak width, with allowance also being made for the instrumental conditions under which the spectra were recorded. This may be due to a combination of factors such as instrumental effects (skin effects and cavity detuning) or electron relaxation effects. In this respect, the EPR spectra of the cadmium stannates differ from those of cadmium oxide samples heated to various temperatures, in which the g-values of the conduction electron bands are smaller (g = 1.77-1.83) and vary linearly with the resonance intensity [7]. The widths of the conduction electron peaks in the cadmium stannates may reflect the electron spin relaxation rates (and thus may be related to the electron mobilities). As was found for heated cadmium oxides, the EPR peak widths of the cadmium stannates vary linearly with the g-values (Fig. l), with a better regression coefficient than that reported for the oxides (0.78) [7]. If the EPR conduction electron peak widths reflect the electron mobilities in the materials, they should also be inversely related to the electrical conductivity. Figure 2 shows an inverse linear relationship between In u and peak width for the yellow and green dicadmium stannates. The electrochemically-prepared Cd,SnO, samples and CdSnO, do not fall on this line, their conduc-
883
tivities being unexpectedly high and low, respectively. The high conductivity of the electrochemical samples could be due to the presence of aliovalent dopants such as Sn” (resulting from electrochemical reduction) or Na+ (from the potentiostat electrolyte); the latter seems less likely, since Na could not be detected by energy-dispersive X-ray analysis. Similarly, electrochemical incorporation of OH- from the electrolyte could not be detected by I.R. spectroscopy. Alternatively, the enhanced conductance may reflect the fact that the electrochemical samples were originally presintered pellets in which the grain-boundary effects would have been diminished. All the cadmium stannates have similar i13Cd NMR spectra, with chemical shifts 6 (measured with respect to Cd(H,O):+) in the range of 272-340 ppm for the yellow samples and 580-62Oppm for the green. The chemical shift (6) of the electrochemically treated sample (760ppm) is of similar order to the shifts of the other green samples; a smaller, broad peak at 460 ppm is also found in the spectrum of the electrochemical sample, which shows similarities to the spectra of samples doped with S-10% Sb (to be published). The chemical shift of CdSnO, (340 ppm) is similar to those of the yellow Cd,SnO, samples, and the range of a-values for all the cadmium stannates is similar to that found for the cadmium oxides (250-700 ppm) [7]. The chemical shifts of these compounds are composed of a constant component S, which reflects structural influences arising from covalency effects, on which is superimposed the Knight shift (K). The latter arises from hyperfine interactions of the conduction electrons with the nucleus and is proportional to the spin susceptibility and to PF, the electron density at the nucleus arising from the unpaired Fermi level electrons averaged over all states on the Fermi surface [7, 12, 131. The spin susceptibility is related to the number (N) of conduction electrons in the system and changes in N should effect both NMR
5 -* -2.5
I
r - 0.98
. . -3
. IO
20
30
EPR
40
50
peak width
60
70
80
90
(GaussI
Fig. 2. Relationship between electical conductance and the EPR peak width for cadmium stannates. Key to symbols as in Fig. I.
C.
M.
CARDILE ef al.
600 -
I 200
I 300 113-Cd
I 400 Chemical
I 500 shift
I 600 ippml
Fig. 3. Relationship between NMR ‘Wd chemical shifts and the g-value of the conduction electron signal in cadmium stannates. Key to symbols as in Fig. 1.
Fig. 4. Relationship between the electrical conductance and the NMR “‘Cd chemical shifts for cadmium stannates. Key to symbols as in Fig. 1.
and EPR parameters (particularly the EPR signal intensity.) Although the EPR intensity in Cd0 was found to correlate with the NMR and other EPR parameters [A, no such correlations were found here; instead, a linear relationship was found between 6 and g for all the cadmium stannates (Fig. 3). Since the g-tensor is not necessarily directly related to N, this experimental result is unexpected. If the electron behaviour is more characteristic of a conduction band than an isolated impurity band in which electrons are localised at impurity centres, then PF is likely to show relatively small changes with donor concentration, as was postulated in Cd0 [7J In such a case, 6 is more likely to be dependent on the number of conduction electrons N than on PF. Thus, 6 should also increase as the electrical conductance of the sample increases. However, an inverse linear relationship is found to hold for the dicadmium stannates, which does not extend to monocadmium stannate or the electrochemically treated sample (Fig. 4). The larger d-values of the green samples indicate a greater number of electrons in the conduction band, which in turn could be due either to a greater concentration of donors, or to a smaller bandgap in the green samples. The latter- is less likely, since approximate bandgap estimates from the diffuse reflectance spectra of yellow and green Cd2Sn04 samples [8] indicate slightly larger bandgaps in the green samples than in the yellow (2.55-2.88 eV and 2.29-2.43 eV, respectively). The presence of a higher concentration of donors in the green materials provides an explanation of their lower conductances, which are in apparent contradiction with their larger chemical shifts. If the donors (e.g. oxygen vacancies, Cd or Sn2+) give rise to electron scattering, the electron mobility will be decreased, resulting in decreased electrical conductivity. Where electron scat-
tering is significant, the temperature dependence of the conductivity ur may be of the form: ur= (Ae- M’ti’)/[l + a(T - T,)].
(1)
The exponential term describes the production of conduction electrons by thermal excitation, whereas the term in a (the temperature coefficient of resistivity due to electron scattering) reflects the influence of scattering on electron mobility, and decreases with increasing temperature. Assuming an arbitrary value of AE = 0.44 eV, similar to Cd0 [9] and substituting the measured conductivity values for the yellow and green Cd2 SnO, samples, the values of a are found to be ca zero and 0.012 for the yellow and green
600
Fig. 5. Relationship between the NMR “‘Cd chemical shift and the NMR relaxation rate for cadmium stannates. Key to symbols as in Fig. 1.
Properties of cadmium starmates
materials, respectively, i.e. scattering occurs more in the green samples than in the yellow. Further, assuming AE = 0.44eV, the calculated values of the pre-exponential term A are 0.02 and 0.55 in the yellow and green samples, respectively; since A reflects the number of donors, this implies higher donor concentrations in the green samples, consistent with the NMR results. Thus, the observed conductances and their temperature dependences in the Cd,SnO, samples can be explained in terms of an electron scattering process. Similarly, since the intensity of the EPR conduction electron resonance (I) is a function of the number of conduction electrons N, a relationship was also sought between I and S. Although such a relationship was found for cadmium oxides [I, it does not appear to hold for the cadmium stannates. The widths of the ‘13Cd NMR signals probably reflect a dist~bution of Knight shifts, possibly arising from the presence in the sample of regions or domains of different chemical environment or donor concentration. As was found for the EPR resonance widths, the NMR widths of the yellow samples are narrow (18.5-196 ppm) by comparison with the green samples (23~262ppm). The NMR tinewidth of the electrochemically treated sample is difficult to measure because of the presence of two broad overlapping bands; the width of the major band at 760 ppm is C?L 320 ppm, while that of the minor band at 460 ppm is N 200 ppm. For cadmium oxides, the NMR peak widths were found to vary linearly with the chemical shift 6, but this linear relations~p does not appear to hold for all the cadmium stannates. Measurements of the ‘Wd relaxation rate (RI) made at different points along the resonance peak were found to be essentially independent of the position at which they were measured, unlike Cd0 [;1. The relaxation rates for the yellow samples (2.64.5s-‘) are significantly smaller than those of the green samples (22.2-28.6 s-l). From the Korringa relationship for degenerate electrons, the Knight shift is proportional to R 1”. i.e. a plot of the chemical shift 6 vs R!i2 should be linear, with an intercept corre-
885
sponding to the constant chemical shift component 6,. Such a plot for the cadmium stannates is found to be linear (Fig. S), with a regression coefficient of 0.99 and an intercept (S,) of 133.3ppm. This value of S, is not unlike the chemical shiA of an additional narrow resonance observed in one of the yellow samples (S = 103.5 ppm), suggesting that the latter is the zero-Knight-shift peak. The value of S, for Cd,SnO, is significantly smaller than that reported for Cd0 (234ppm [q), consistent with a longer mean Cd-O bond length of 2.43 A in Cd,SnO, [1 11, where the Cd is seven-coordinate, compared with 2.35 w in six-coordinate Cd0 [lo]. Acknowledgements-We are indebted to Dr. I. W. M. Brown for the X-ray measurements, M. J. Ryan for assistance with the electrical measurements, Mrs. K. Card for the EDAX analyses, Dr. J. F. Clare for the use of the EPR spectrometer, Dr. P. T. Wilson for the use of the potentiostat, and Miss S. M. Ward for technical assistance. Some of the samples were kindly supplied by Dr. F. Golestani-fard, Materials and Energy Research Centre, Tehran, Iran.
REFEXENCl?S
1. MacKenzie K. J. D., Gerrard W. A. and Golestani-fard F., Silicates Ind. 45, 97 (1979). 2. Golestani-fard F., Hashemi T., MacKenzie K. J. D. and Hogarth C. A. J. Muter Sci. 18, 3679 (1983). 3. Golestani-fard F. and MacKenzie K. J. D. J. Muter. Sci. Lerr. 3, 403 (1984). 4. Hashemi T., Golestani-fard F. and MacKenzie K. J. D., High-Tech Ceramics (Edited by P. Vi~i), p. 2203. Elsevier, Amsterdam (1987). 5. Go&h E., JCPDS Card No. 31-242 (1979). 6. Shannon R. D., Gillson J. L. and Bouchard R. J., J. Phys. Chem. Soli& 38, 817 (1977). 7. Meinhold R. H., J. Phys. Chem. Solids (in press). 8. Golestani-fard F., Hogarth C. A. and Waters D. N., J. Muter Sci. Lett. 2, 505 (1983). 9. Cboi J. S., Kang Y. H. and Kim K. H., J. p&s. Chem. 81, 2208 (1977). 10. Swanson H. E. and Fuvat R. K.. NBS Circular 539.27 (1953). 11. Tromel M., Z. anorg. Alfg. Chem. 371, 237 (1969). 12. Korringa I., Physicu 16, 601 (1950). 13. Look D. C., Phys. Rev. 184, 705 (1969).
OOZZ-369718733.00 + 0.00 8 1987 Pcr8amon Joumalr Ltd.
881-885, 1987
THE PROPERTIES OF CADMIUM STANNATES INVESTIGATED BY EPR AND HIGH-RESOLUTION SOLID-STATE ‘13Cd NMR SPECTROSCOPY C. M. CARDILE, R. H. MEINHOLD and K. J. D. MACKENZIE Chemistry Division, DSIR, Private Bag, Petone, New Zealand (Received 15 December 1986; accepted 12 March 1987) Ah&net-Yellow and green forms of dicadmium stannate, Cd,SnO,, prepared under oxidising and reducing conditions, respectively, are crystallographically very similar, but have different electronic and spectroscopic properties. The g-values and peak widths of the EPR conduction electron resonances, and the “%d NMR chemical shifts, are systematically larger in the green materials than the yellow, consistent with a higher concentration of conduction electrons in the former. In contrast, the electrical conductances of the green samples are lower than the yellow, probably due to scattering by the high donor concentrations in the green materials. Dark green Cd, SnO,, prepared by electrochemical treatment of yellow material, has spectroscopic properties similar to the other green samples, but a higher electrical conductance. The spectroscopic properties of CdSnO, are also reported. Keywords: Cadmium stannate, “‘Cd MAS NMR, EPR, electric conductance.
INTRODUCTION
EXPERIMENTAL
together, either CdSnO, or Cd, SnO, (or a mixture of the two) can be formed, depending on the reaction conditions and stoichiometry [1,2]. Although the physical properties of CdSnO, appear to be relatively insensitive to the reaction atmosphere, the properties of Cd,SnO, are much more atmosphere-sensitive; when prepared in oxidising conditions, Cd,SnO, is bright yellow and exhibits a sharp, intense EPR spectrum, whereas samples produced under non-oxidising conditions or reduced pressure are various shades of green, have different electrical conductance properties to the oxidised samples, and have a much less intense EPR spectrum [3]. Recently it has proved possible to reversibly convert yellow Cd2 SnO, to a green form by electrochemical means [4], but the relationship between this and the green chemically-produced cadmium stannates has not yet been established. Polycrystalline pellets of CdSnO, and Cd,SnO, have both been shown to exhibit photoelectrolytic behaviour [ 11,but their bandgap is not ideal for solar energy conversion. However, their relatively high electrical conductivities and transparency to solar spectral wavelengths suggests other possible uses as conducting electrodes for solar cells. To provide further insight into the semiconductor properties of these materials, a new series of samples was fabricated under a variety of strictly controlled reaction conditions, using both chemical and electrochemical techniques. The EPR spectra were re-investigated, and augmented with high-resolution solid-state “‘Cd NMR spectroscopy with magic-angle spinning, and electrical conductance measurements. When Cd0
and SnO, are heated
881
Samples of both CdSnO, and Cd,SnO, were prepared by solid-state reaction of mixtures of Sn02 and CdO, the latter pre-calcined at 500°C for 0.5 h to remove hydroxide and carbonate impurities. After being weighed out in the appropriate molar ratios, the powders were intimately mixed by hand grinding and reacted at 1050°C for 6 h under the following conditions: (a) In flowing atmospheres (0.2 1mini) of dry air, oxygen-free nitrogen and hydrogen-nitrogen (5:95%); (b) In sealed silica tubes containing these atmospheres at atmospheric pressure or under reduced pressure ( 10m4torr); (c) By electrochemical treatment of yellow Cd,SnO, prepared in air atmosphere at 105O”C, then pressed into a 10 mm dia. pellet at 5.5 MPa and lightly sintered in air at 800°C for 18 h. The resulting electrode was attached by its rim to a platinum wire, using silver paint and epoxy resin, then placed in a potentiostat cell containing 0.5 M (AR) NaOH solution and platinum counter-electrodes. A potential of +0.3 V (SCE) was applied for 12 h, using a Princeton Applied Research potentiostat model 173 with a model 276 interface unit. Further details of the electrochemical experiment are reported elsewhere [4]. The progress of the reaction position of the products were powder diffraction using a Philips PW 1700 diffractometer and a graphite monochromator.
and the phase commonitored by X-ray computer-controlled with CoKa radiation The unit cell par-
C. M. CARDILE et al.
882
ameters of the monophase products were also determined from careful measurements of a number of their X-ray reflections, using Si powder as the internal angular calibrant. The X-band EPR spectra of the powdered samples were obtained at room tincture using a Varian EPR spectrometer. The solid-state ‘13Cd NMR spectra were obtained using a Varian XL-200 spectrometer at sample spinning speeds of 2-2.5 kHz. Attempts to detect the l19Sn NMR resonance were unsuccessful. Conductivity measurements were made on 4.2 mm dia. pellets pressed from 68 mg of sample at 1.4 MPa pressure, with silver electrodes painted on. A Hewlett-Packard Model 4192A low-frequency impedance analyser was used. Since measurements made over the frequency range 5 Hz-13 MHz showed no frequency dependence between 100 Hz and 100 kHz, a working frequency of 1592 Hz was adopted, to coincide with the frequency of the Wayne-Kerr model B201 bridge which was used to determine the temperature dependence of the conductivity up to 150°C. Although care was taken to reproducibly press the pellets to ensure internal consistency in the results, the absolute values of the conductances may depend on the way in which fabrication parameters influence grain boundary behaviour. RESULTS AND DLSCUSSION The Cd,SnO, samples prepared as above ranged in colour from bright yellow (fired in oxidising conditions) to green (fired in non-oxidising conditions). Samples fired in flowing oxygen-free nitrogen were yellow, and showed all the characteristics of oxidised materials, suggesting that oxygen was not fully excluded under dynamic nitrogen atmospheres. In contrast, the sample fired under oxygen-free nitrogen in a sealed tube was green. The electrochemically treated pellets were dark green. The properties of all the stannates are summarised in Table 1, which shows that the unit-cell volumes of the yellow Cd,SnO, samples were all 175.6-175.7 A3 (cf. 175.6A3 calculated from the cell constants of Cd2 SnO, listed in the JCPDS powder diffraction data file [S]). By comparison, all the green samples except
the electrochemically produced samples showed a slight but consistent expansion of the cell volume to 175.8 A3, resulting from an increase in all three cell dimensions. This may reflect the reduction of some Sni’ to the larger Sn” in the green samples, as previously suggested on the basis of ESCA results [4], but if so, it is not obvious why a similar or greater expansion is not observed in the electrochemically prepared green samples. The only successful preparation of monophase CdSnO, was in the sealed-tube experiment under reduced pressure. The product is off-white, with a measured cell volume of 239.8 A3, in good agreement with the value of 239.7 A3 calculated from the JCPDS cell parameters [6]. The room-temperature electrical conductances of the yellow CdrSnO., samples fall in the range of 2.0-3.8 x lo-‘S cm’, the conductances of the green samples being lower (0.9-1.9 x tow3 S cm-‘). The conductances of the electrochemically reduced samples (0.6-4.7 x lOwi S cm-‘) were significantly greater than those of the yellow or green Cd,SnO, samples, whereas the CdSnO, was the least conducting of all the samples (1.5 x lo-‘Scn-‘). The temperature dependence of the conductances, measured up to about lSO”C, showed that the conductance of all samples increased with increasing temperature. The strongest temperature dependence (estimated from the slope of In u vs - 1/2kT to be 0.49 eV) was shown by the yellow samples, the green being less temperature dependent (0.28 eV). The EPR spectra of all these stannates contain a major resonance at g = 1.85-1.90, arising from the conduction electrons. In some samples, other very weak resonances were occasionally found, as reported previously [3]; the random occurrence of these signals and their wide range of g-values suggests them to be due to paramagnetic impurities. The g-values of the yellow samples (1.859-1.861) were significantly smaller than those of the green samples (1.885-l ,910). The electrochemically-produced samples had the largest g-values of all the samples (2.0), whereas the g-value of CdSnO, (1.863) is similar to that of yellow CdzSn04. The widths of the EPR conduction electron resonances follow analogous trends to the g-values; the yellow samples show very
Table 1. Properties of the cadmium stannates Cd,SnO, (yellow) Cell volume (it’) Electrical conductance (Scln_‘) EPR g-value EPR peak iKidth (gauss) ‘13CdNMR chemical shift 6 (Ppm) Wd NMR peak width (Ppm)
cd,Sn04 lelectrochemical)
CdSnO,
175.6-175.7 2-3.8 x IO-*
Cd,SnO, (green) 175.8 0.9-1.9 x 10-S
175.7 0.647 x 10-l
239.8 1.5 x lo-’
1.859-1.861 12-20 272-340
1.885-1.910 7&85 581M20
2.0 350 760
1.863 23 340
185196
23&262
u 320
232
Properties of cadmium stannates I 9’
-
1.90 -
1.69 -
x 5b
1.66 -
5 Y
1.65
I& 0
, , , , , , , , 10
20
30
40
50
EPR peak width
60
70
60
90
(Goursl
Fig. 1. Relationship between the g-values and peak widths of the EPR conduction electron signal in cadmium stannates. 0, Cd,SnO, formed in dynamic gas atmospheres; 0, Cd,SnO, formed in sealed tube experiments; X, CdSnO, formed in scaled tube.
sharp peaks (12-20 gauss wide), the resonances of the green samples being much broader (70-U gauss) with the electrochemically treated sample being the broadest (350 gauss). The width of the CdSnO, signal is intermediate between that of yellow and green CdzSnO, (30 gauss). The g-values of the conduction electron peaks were found to be essentially independent of the intensities of the resonances, which were estimated from the product of the peak height and the square of the peak width, with allowance also being made for the instrumental conditions under which the spectra were recorded. This may be due to a combination of factors such as instrumental effects (skin effects and cavity detuning) or electron relaxation effects. In this respect, the EPR spectra of the cadmium stannates differ from those of cadmium oxide samples heated to various temperatures, in which the g-values of the conduction electron bands are smaller (g = 1.77-1.83) and vary linearly with the resonance intensity [7]. The widths of the conduction electron peaks in the cadmium stannates may reflect the electron spin relaxation rates (and thus may be related to the electron mobilities). As was found for heated cadmium oxides, the EPR peak widths of the cadmium stannates vary linearly with the g-values (Fig. l), with a better regression coefficient than that reported for the oxides (0.78) [7]. If the EPR conduction electron peak widths reflect the electron mobilities in the materials, they should also be inversely related to the electrical conductivity. Figure 2 shows an inverse linear relationship between In u and peak width for the yellow and green dicadmium stannates. The electrochemically-prepared Cd,SnO, samples and CdSnO, do not fall on this line, their conduc-
883
tivities being unexpectedly high and low, respectively. The high conductivity of the electrochemical samples could be due to the presence of aliovalent dopants such as Sn” (resulting from electrochemical reduction) or Na+ (from the potentiostat electrolyte); the latter seems less likely, since Na could not be detected by energy-dispersive X-ray analysis. Similarly, electrochemical incorporation of OH- from the electrolyte could not be detected by I.R. spectroscopy. Alternatively, the enhanced conductance may reflect the fact that the electrochemical samples were originally presintered pellets in which the grain-boundary effects would have been diminished. All the cadmium stannates have similar i13Cd NMR spectra, with chemical shifts 6 (measured with respect to Cd(H,O):+) in the range of 272-340 ppm for the yellow samples and 580-62Oppm for the green. The chemical shift (6) of the electrochemically treated sample (760ppm) is of similar order to the shifts of the other green samples; a smaller, broad peak at 460 ppm is also found in the spectrum of the electrochemical sample, which shows similarities to the spectra of samples doped with S-10% Sb (to be published). The chemical shift of CdSnO, (340 ppm) is similar to those of the yellow Cd,SnO, samples, and the range of a-values for all the cadmium stannates is similar to that found for the cadmium oxides (250-700 ppm) [7]. The chemical shifts of these compounds are composed of a constant component S, which reflects structural influences arising from covalency effects, on which is superimposed the Knight shift (K). The latter arises from hyperfine interactions of the conduction electrons with the nucleus and is proportional to the spin susceptibility and to PF, the electron density at the nucleus arising from the unpaired Fermi level electrons averaged over all states on the Fermi surface [7, 12, 131. The spin susceptibility is related to the number (N) of conduction electrons in the system and changes in N should effect both NMR
5 -* -2.5
I
r - 0.98
. . -3
. IO
20
30
EPR
40
50
peak width
60
70
80
90
(GaussI
Fig. 2. Relationship between electical conductance and the EPR peak width for cadmium stannates. Key to symbols as in Fig. I.
C.
M.
CARDILE ef al.
600 -
I 200
I 300 113-Cd
I 400 Chemical
I 500 shift
I 600 ippml
Fig. 3. Relationship between NMR ‘Wd chemical shifts and the g-value of the conduction electron signal in cadmium stannates. Key to symbols as in Fig. 1.
Fig. 4. Relationship between the electrical conductance and the NMR “‘Cd chemical shifts for cadmium stannates. Key to symbols as in Fig. 1.
and EPR parameters (particularly the EPR signal intensity.) Although the EPR intensity in Cd0 was found to correlate with the NMR and other EPR parameters [A, no such correlations were found here; instead, a linear relationship was found between 6 and g for all the cadmium stannates (Fig. 3). Since the g-tensor is not necessarily directly related to N, this experimental result is unexpected. If the electron behaviour is more characteristic of a conduction band than an isolated impurity band in which electrons are localised at impurity centres, then PF is likely to show relatively small changes with donor concentration, as was postulated in Cd0 [7J In such a case, 6 is more likely to be dependent on the number of conduction electrons N than on PF. Thus, 6 should also increase as the electrical conductance of the sample increases. However, an inverse linear relationship is found to hold for the dicadmium stannates, which does not extend to monocadmium stannate or the electrochemically treated sample (Fig. 4). The larger d-values of the green samples indicate a greater number of electrons in the conduction band, which in turn could be due either to a greater concentration of donors, or to a smaller bandgap in the green samples. The latter- is less likely, since approximate bandgap estimates from the diffuse reflectance spectra of yellow and green Cd2Sn04 samples [8] indicate slightly larger bandgaps in the green samples than in the yellow (2.55-2.88 eV and 2.29-2.43 eV, respectively). The presence of a higher concentration of donors in the green materials provides an explanation of their lower conductances, which are in apparent contradiction with their larger chemical shifts. If the donors (e.g. oxygen vacancies, Cd or Sn2+) give rise to electron scattering, the electron mobility will be decreased, resulting in decreased electrical conductivity. Where electron scat-
tering is significant, the temperature dependence of the conductivity ur may be of the form: ur= (Ae- M’ti’)/[l + a(T - T,)].
(1)
The exponential term describes the production of conduction electrons by thermal excitation, whereas the term in a (the temperature coefficient of resistivity due to electron scattering) reflects the influence of scattering on electron mobility, and decreases with increasing temperature. Assuming an arbitrary value of AE = 0.44 eV, similar to Cd0 [9] and substituting the measured conductivity values for the yellow and green Cd2 SnO, samples, the values of a are found to be ca zero and 0.012 for the yellow and green
600
Fig. 5. Relationship between the NMR “‘Cd chemical shift and the NMR relaxation rate for cadmium stannates. Key to symbols as in Fig. 1.
Properties of cadmium starmates
materials, respectively, i.e. scattering occurs more in the green samples than in the yellow. Further, assuming AE = 0.44eV, the calculated values of the pre-exponential term A are 0.02 and 0.55 in the yellow and green samples, respectively; since A reflects the number of donors, this implies higher donor concentrations in the green samples, consistent with the NMR results. Thus, the observed conductances and their temperature dependences in the Cd,SnO, samples can be explained in terms of an electron scattering process. Similarly, since the intensity of the EPR conduction electron resonance (I) is a function of the number of conduction electrons N, a relationship was also sought between I and S. Although such a relationship was found for cadmium oxides [I, it does not appear to hold for the cadmium stannates. The widths of the ‘13Cd NMR signals probably reflect a dist~bution of Knight shifts, possibly arising from the presence in the sample of regions or domains of different chemical environment or donor concentration. As was found for the EPR resonance widths, the NMR widths of the yellow samples are narrow (18.5-196 ppm) by comparison with the green samples (23~262ppm). The NMR tinewidth of the electrochemically treated sample is difficult to measure because of the presence of two broad overlapping bands; the width of the major band at 760 ppm is C?L 320 ppm, while that of the minor band at 460 ppm is N 200 ppm. For cadmium oxides, the NMR peak widths were found to vary linearly with the chemical shift 6, but this linear relations~p does not appear to hold for all the cadmium stannates. Measurements of the ‘Wd relaxation rate (RI) made at different points along the resonance peak were found to be essentially independent of the position at which they were measured, unlike Cd0 [;1. The relaxation rates for the yellow samples (2.64.5s-‘) are significantly smaller than those of the green samples (22.2-28.6 s-l). From the Korringa relationship for degenerate electrons, the Knight shift is proportional to R 1”. i.e. a plot of the chemical shift 6 vs R!i2 should be linear, with an intercept corre-
885
sponding to the constant chemical shift component 6,. Such a plot for the cadmium stannates is found to be linear (Fig. S), with a regression coefficient of 0.99 and an intercept (S,) of 133.3ppm. This value of S, is not unlike the chemical shiA of an additional narrow resonance observed in one of the yellow samples (S = 103.5 ppm), suggesting that the latter is the zero-Knight-shift peak. The value of S, for Cd,SnO, is significantly smaller than that reported for Cd0 (234ppm [q), consistent with a longer mean Cd-O bond length of 2.43 A in Cd,SnO, [1 11, where the Cd is seven-coordinate, compared with 2.35 w in six-coordinate Cd0 [lo]. Acknowledgements-We are indebted to Dr. I. W. M. Brown for the X-ray measurements, M. J. Ryan for assistance with the electrical measurements, Mrs. K. Card for the EDAX analyses, Dr. J. F. Clare for the use of the EPR spectrometer, Dr. P. T. Wilson for the use of the potentiostat, and Miss S. M. Ward for technical assistance. Some of the samples were kindly supplied by Dr. F. Golestani-fard, Materials and Energy Research Centre, Tehran, Iran.
REFEXENCl?S
1. MacKenzie K. J. D., Gerrard W. A. and Golestani-fard F., Silicates Ind. 45, 97 (1979). 2. Golestani-fard F., Hashemi T., MacKenzie K. J. D. and Hogarth C. A. J. Muter Sci. 18, 3679 (1983). 3. Golestani-fard F. and MacKenzie K. J. D. J. Muter. Sci. Lerr. 3, 403 (1984). 4. Hashemi T., Golestani-fard F. and MacKenzie K. J. D., High-Tech Ceramics (Edited by P. Vi~i), p. 2203. Elsevier, Amsterdam (1987). 5. Go&h E., JCPDS Card No. 31-242 (1979). 6. Shannon R. D., Gillson J. L. and Bouchard R. J., J. Phys. Chem. Soli& 38, 817 (1977). 7. Meinhold R. H., J. Phys. Chem. Solids (in press). 8. Golestani-fard F., Hogarth C. A. and Waters D. N., J. Muter Sci. Lett. 2, 505 (1983). 9. Cboi J. S., Kang Y. H. and Kim K. H., J. p&s. Chem. 81, 2208 (1977). 10. Swanson H. E. and Fuvat R. K.. NBS Circular 539.27 (1953). 11. Tromel M., Z. anorg. Alfg. Chem. 371, 237 (1969). 12. Korringa I., Physicu 16, 601 (1950). 13. Look D. C., Phys. Rev. 184, 705 (1969).