J. Phys. Chem. C 2008, 112, 13037–13042
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Selective Synthesis of FeS and FeS2 Nanosheet Films on Iron Substrates as Novel Photocathodes for Tandem Dye-Sensitized Solar Cells Yan Hu,† Zhi Zheng,‡ Huimin Jia,‡ Yiwen Tang,§ and Lizhi Zhang*,† Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China, Institute of Surface Micro and Nano Materials, Xuchang UniVersity, Xuchang 461000, People’s Republic of China, and Institute of Nano-science and Technology, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China ReceiVed: April 29, 2008; ReVised Manuscript ReceiVed: June 9, 2008
FeS and FeS2 nanosheet films were selectively synthesized on iron substrates through one-step hydrothermal treatment of iron foil and sulfur powder in the presence or absence of hydrazine. The resulting FeSx (x ) 1, 2) nanosheet films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) and used as novel photocathodes in tandem solar cells with dyesensitized TiO2 nanorod films as the corresponding photoanode. The photovoltaic properties of the tandem dye-sensitized solar cells were carefully studied. We found that the performance of the FeS nanosheet film photocathode was better than that of the FeS2 one in the tandem dye-sensitized solar cells. In the case of the FeS nanosheet film photocathode, a short circuit photocurrent (Isc) of 2.53 mA/cm2, an open circuit photovoltage (Voc) of 0.60 V, a fill factor (FF) of 0.31, and conversion efficiency (η) of 1.32% were obtained under an illumination of 100 mW/cm2. This study suggests that these iron sulfide nanosheet films are attractive photocathodes for tandem dye-sensitized solar cells. 1. Introduction Solar energy is one of the most promising future energy resources. The direct conversion of sunlight into electric power via solar cells is of particular interest because it has many advantages over the current electrical power generation methods. Until now, solar cells have been dominated by solid-state junction devices made of silicon. However, they are now being challenged by the emergence of a new generation of photovoltaic cells based on nanocrystalline materials1–4 and conducting polymer films.5,6 These new photovoltaic cells offer the considerable prospects of cheap fabrication, flexibility, and so on.7 The impressive progress in fabricating and characterizing nanocrystalline materials has opened up great opportunity for the new photovoltaic cells. They are totally different from the classical solid-state junction device by replacing the phase in contact with the semiconductor by an electrolyte (liquid, gel, or organic solid) to form photoelectrochemical devices.8 Among these photoelectrochemical devices, the dye-sensitized solar cells (DSSCs) appear to be low-cost and environmentally friendly ones with good efficiencies comparable to those of silicon cells. The anode of a DSSC usually consists of a nanoporous TiO2 film deposited on the transparent conductive oxide (TCO) substrate that serves as the electron acceptor and a thin layer of sensitizer-dye molecules on the TiO2 to absorb light and then inject the electron into the TiO2 conduction band. A traditional DSSC is often composed of a dye-sensitized TiO2 photoanode, a liquid electrolyte, and a platinum cathode. The liquid electrolyte generally consists of organic solvents such as acetonitrile and a redox couple I-/I3- that serves as a redox * To whom correspondence should be addressed. E-mail: zhanglz@ mail.ccnu.edu.cn. Tel/Fax: +86-27-6786 7535. † College of Chemistry, Central China Normal University. ‡ Xuchang University. § Institute of Nano-science and Technology, Central China Normal University.
agent. Upon illumination, electrons are injected from the photoexcited dye molecules into the TiO2 and move toward the TCO substrate, while the electrolyte reduces the oxidized dye and transports the positive charges to the Pt cathode.9 In order to improve the conversion efficiency of dye-sensitized solar cells, tandem systems could be used. In the tandem systems, photoactive cathodes replace the Pt electrode to absorb more solar energy. The theoretical upper limit for tandem dyesensitized solar cells is around 43%, much higher than the 30% of traditional DSSCs with one photoactive dye-sensitized electrode.10 Therefore, the tandem DSSCs could be attractive devices for the conversion of light to electricity.11,12 Pyrite (FeS2) is an alternative absorber material for solar cells because of its very high optical absorption coefficient of 5 × 105 cm-1 in the case of λ e 700 nm and desirable band gap of 0.95 eV for a solar spectrum as well as the low cost. Its optical absorption coefficient is two orders of magnitude higher than that of crystalline silicon.13 Therefore, only a thin layer (less than 100 nm thickness) is required in the fabrication of cells based on this material.14 The photoelectrochemical cell using a pyrite single crystal delivered a solar-to-electrical conversion efficiency of 2.8% with an open circuit voltage (Voc) of 187 mV, a short circuit current (Isc) of 42 mA/cm2, and a fill factor (FF) of 0.5.13,15 Meanwhile, pyrrhotite (FeS) has attracted considerable attention since the 1950s due to its electrical and magnetic properties and phase transitions.16–21 Pyrite thin films were prepared by some techniques, including sulfurization of an electrodeposited or evaporated iron layer, ion beam magnetron sputtering, spray pyrolysis, electrodeposition, MOCVD, and magnetron sputtering.22–29 In this study, we report that FeS and FeS2 nanosheet films could be selectively grown on iron substrates through the reaction of iron foil and sulfur powder by a one-step hydrothermal method. Furthermore, we utilize them as novel photocathodes instead of Pt cathodes in traditional DSSCs to construct
10.1021/jp803726c CCC: $40.75 2008 American Chemical Society Published on Web 07/29/2008
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SCHEME 1: Schematic Illustration of the Sandwich-Type Double-Photoelectrode Dye-Sensitized Solar Cells
novel tandem dye-sensitized solar cells and carefully study their photoelectrical properties. 2. Experimental Section 2.1. Preparation of the Photocathodes. The FeS and FeS2 nanosheet films were synthesized via a general method for oriented nanostructured films reported previously.30 In a typical procedure, a piece of Fe foil (Aldrich, purity: 99.99%; thickness: 0.127 mm; 1.5 cm × 0.5 cm) and 1 mmol of sulfur powder were placed in a 20 mL Teflon-lined autoclave, and then, 15 mL of deionized water (or 15 mL of 6.6 v/v% aqueous hydrazine solution) was added. Before used, the Fe foil was cleaned by ultrasonication in deionized water and ethanol. The autoclave was heated at 160 °C for 12 h and then air-cooled to room temperature. The Fe foil was taken out of solution, washed with ethanol, and finally air-dried for characterization and use as the photocathode. 2.2. Preparation of the Photoanode. The TiO2 nanorods powder was prepared by a general soft interface method reported in our previous work.31 For the preparation of the photoanode, 0.2 g of TiO2 was added into a mortar, and then, 0.02 g of ethyl cellulose and 2.0 mL of terpineol were mixed with TiO2 with grinding until a meringue-like gel was formed. The indiumdoped tin oxide (ITO) substrates were cleaned by ultrasonication in distilled water and ethanol. Both edges of the conducting glass substrates were covered with adhesive tape. A drop of the paste was added to one of the bare edges of an ITO substrate and flattened with a glass rod by sliding over the tape-covered edges,32 followed by drying at 60 °C for about 6 h. Finally, the TiO2 thin films were obtained after annealing at 400 °C for 30 min in an air atmosphere. The prepared TiO2 films were dipped in a 0.5 mM ethanol solution of cis-bis(isothiocyanato)-bis(2,2bipyridyl-4,4-dicarboxylato)-ruthenium(II) (N3, Solaronix S.A., Switzerland) for more than 24 h. The N3-coated TiO2 films were taken out of solution and air-dried for use. 2.3. Characterization. The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-RB diffractometer with monochromatized Cu KR radiation (λ ) 1.5418). The morphology, structures, and compositions of the films were investigated by scanning electron microscopy (SEM, JSM-5600). A transmission electron microscopy (TEM) study was carried out on a Philips CM-120 electron microscopy instrument. The samples for TEM were prepared by dispersing the powders scraped from the final films in ethanol, and then, the dispersion was dropped on carbon-copper grids and air-dried. 2.4. Photoelechemical Property Measurements. The photoelectrochemical experiments were performed in a standard two-electrode system (sandwich-type), as shown in Scheme 1.
Figure 1. XRD patterns of the resulting FeSx (x ) 1, 2) films on iron substrates. (a) FeS; (b) FeS2.
The N3-coated TiO2 film was used as the photoanode, and the FeSx (x ) 1, 2) nanosheet films, Fe substrate, or the Pt/FTO glass were used as the photocathode. The 0.03 mol/L I2 and 0.3 mol/L LiI propylene carbonate solution were attracted into the interelectrode space by capillary forces. The N3-coated film was illuminated through the conducting glass support with an AM 1.5 solar simulator (American Oriel, 100 mW/cm2) as the light source. A series of I-V curves were monitored and recorded using a computerized Keithley Model 2400 source measure unit. The active electrode area was typically 0.5 cm2. The results were not corrected with respect to transmission losses in the conducting substrate. 3. Results and Discussion 3.1. XRD Analysis. The compositions of the iron sulfide products were characterized by X-ray diffractometry (XRD). Figure 1a shows the XRD pattern of the sample prepared in the presence of hydrazine. The pattern matches quite well with the standard diffraction data of the FeS (Pyrrhotite, JCPDS File No. 02-1241). In Figure 1a, the peaks at 2θ values of 30.0, 33.9, 43.9, and 53.2 can be indexed to the (100), (101), (102), and (110) planes of FeS, respectively. The XRD pattern of the sample prepared in the absence of hydrazine in Figure 1b can be assigned to FeS2 (Pyrite, JCPDS File No. 71-0053). In Figure 1b, the peaks at 2θ values of 28.5 and 33.0 can be attributed to the (111) and (200) planes for FeS2, respectively. The peaks at 2θ values of 44.7 and 65.0 in Figure 1 arise from to the iron substrate (JCPDS File No. 06-0696). Therefore, XRD results reveal that FeS and FeS2 films could be selectively grown on iron substrates via the hydrothermal reactions of iron foil and sulfur powder in the presence or absence of hydrazine. This selective formation of FeS and FeS2 could be attributed to the complex formed between hydrazine and Fe2+ and the enhanced dissolution and reduction of sulfur.30 3.2. SEM and TEM Images of the FeSx Nanosheet Films. The morphologies of FeS and FeS2 films were investigated by scanning electron microscopy. Figure 2a shows that the FeS film consists of plenty of nanosheets. Most of the nanosheets have sizes of about 500 nm in diameter and 80 nm in thickness. There are also some nanosheets with larger sizes of 1-2 µm in diameter and 100-300 nm in thickness. Figure 2b reveals that thin nanosheets of FeS2 assemble into flower-like superstructures. The thicknesses of FeS2 nanosheets are about 30 nm, much thinner than those of FeS nanosheets.
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Figure 2. SEM images of the as-prepared FeSx (x ) 1, 2). (a) FeS; (b) FeS2.
The FeS and FeS2 nanostructures were further investigated by transition electron microscopy (Figure 3). Figure 3a confirms the plate-like morphology of FeS. According to Figure 3a, lots of smaller particles assemble into FeS nanosheets. The nanoflowers composed of FeS2 nanosheets were observed in Figure 3b. The FeS2 nanosheets are about 30-50 nm in thickness. All of the TEM observations are consistent with the SEM results. Because of the instability of these FeSx nanostructures under the electron beam of the high-resolution TEM (HRTEM) machine, we failed to obtain their HRTEM images. 3.3. Photovoltaic Properties of FeSx Nanosheet-FilmBased Tandem DSSCs. We constructed tandem solar cells with the FeSx nanosheet films as the photocathodes and the dyesensitized TiO2 nanorod films as the corresponding photoanode. The photovoltaic properties of the resulting double-photoelectrode DSSCs were studied and compared with those of the traditional DSSC with a single photoactive anode and a Pt cathode. Figure 4 compares the currentsvoltage characteristics of dye-sensitized TiO2 solar cell with different photocathodes including Pt (labeled as TiO2sPt), Fe (labeled as TiO2sFe), an FeS film (labeled as TiO2sFeS), and an FeS2 film (labeled as TiO2sFeS2). The Voc values of the TiO2sPt, TiO2sFe, TiO2sFeS, and TiO2sFeS2 solar cells are 0.50, 0.32, 0.60, and 0.38 V, respectively. The Voc’s of the TiO2sFeS and TiO2sFeS2 solar cells are both higher than that of TiO2sFe, confirming the significant contribution of FeSx nanosheet film photocathodes in the solar cells. It is interesting to find the Voc of tandem DSSC TiO2sFeS is even higher than that of traditional DSSC TiO2sPt. This suggests that the FeS nanosheet film photocathode is very attractive for the tandem DSSCs. The short circuit currents Isc of the TiO2sPt, TiO2sFe, TiO2sFeS, and TiO2sFeS2 solar cells are 4.08, 0.98, 2.51, and 2.04 mA/cm2,
Figure 3. TEM images of the as-prepared FeSx (x ) 1, 2). (a) FeS; (b) FeS2.
Figure 4. Current-voltage characteristics of dye-sensitized TiO2 solar cell with different photocathodes: (a) Pt; (b) FeS; (c) FeS2; (d) Fe.
respectively. Obviously, the short circuit currents of the TiO2sFeS and TiO2sFeS2 solar cells are much higher than that of TiO2sFe. This current enhancement may be attributed to the cathode current produced on FeSx nanosheet films upon illumination. The lower Isc of the TiO2sFeS tandem solar cell
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TABLE 1: Photovoltaic Properties of the Different Dye-Sensitized Solar Cells photocathode
Isc (mA/cm2)
Voc (V)
FF
η
Pt Fe FeS FeS2
4.08 0.98 2.51 2.04
0.50 0.32 0.60 0.38
0.38 0.21 0.31 0.40
2.19% 0.07% 1.32% 0.93%
SCHEME 2: Schematic Showing the Change of the Electronic Energy Levels at the Interface between an n-Type (a) and p-Type (b) Semiconductor before and after Illumination
than that of TiO2sPt is believed to be attributed to the much lower conductivity of the FeS films than that of Pt. The Isc, Voc, the fill factors, and the photovoltaic efficiencies of the TiO2sPt, TiO2sFe, TiO2sFeS, and TiO2sFeS2 solar cells are summarized in Table 1. The fill factors of the TiO2sPt, TiO2sFe, TiO2sFeS, and TiO2sFeS2 solar cells are 0.38, 0.21, 0.31, and 0.40, respectively. The photovoltaic efficiencies of TiO2sPt, TiO2sFe, TiO2sFeS, and TiO2sFeS2 are 2.19, 0.07, 1.32, and 0.93%, respectively. The fill factor (FF) is calculated using the following equation33
FF ) ImaxVmax/IscVoc
(1)
The photovoltaic efficiency (η) is calculated using the following equation34
η(%) ) ImaxVmax/Ph
(2)
where Isc is the short circuit current and Voc is the open circuit voltage, Imax and Vmax are the current and voltage obtained at the maximum power point on the photovoltaic power output curve, respectively, and Ph is the power density of the incident radiation. The higher Voc of the TiO2sFeS solar cell than that of TiO2sFeS2 may be attributed to the different semiconductor types of FeS2 and FeS. FeS2 is always reported as an n-type semiconductor.14,15,35,36 The presence of hydrazine in the reaction system could make more Fe cooperate into the crystalline framework composed of Fe and S. This may lead to the transformation of an n-type semiconductor (FeS2) to a p-type semiconductor (FeS) (Supporting Information). The different energy levels of the n-type and p-type semiconductors account for the different Voc values of tandem solar cells with FeSx nanosheet film photocathodes. Scheme 2 shows the change of the electronic energy levels at the interface between an n-type and p-type semiconductor before and after the illumination. In the case of the n-type semiconductor in solution containing a reversible redox system, as shown in Scheme 2a, the equilibrium potential of the semiconductor surface becomes more positive than the flat band potential in the bulk. As a result, there is an energy band bending from bulk to surface to form an electric field. After the semiconductor is illuminated, photogenerated electron-hole pairs are generated. These photogenerated electrons are exited to the conduction band, leaving the holes in valence band. The photogenerated electrons and holes move in mutually opposite directions in the presence of the preformed electric field. In our case, the electrons move into the bulk, and the holes move toward the interface. This separation of electrons and holes will produce a new electric field to partially compensate the previous one. This process results in the unbending of the energy bands. Meanwhile, the energy levels of the flat bands increase, and the Fermi level F is pulled up to photoexcited Fermi level Fp by the unbending of the energy bands. On the contrary, as shown in Scheme 2b, in the case of the p-type semiconductor in the electrolyte, the equilibrium potential of the semiconductor surface becomes more negative than the flat band potential. The photogenerated electrons move toward the interface, and the
holes move into the bulk; the energy levels decreases, and the Fermi level F falls down to Fp upon illumination. The different shift of the Fermi level F between the n-type and p-ype semiconductor could explain the difference between the Voc of the solar cells with different FeSx nanosheet film photocathodes as follows. Scheme 3 shows the band structure alignment and charge transfer in tandem dye-sensitized solar cells with FeS and FeS2 nanosheet film photocathodes. This scheme can help to understand the formation of the Voc of TiO2sFeS and TiO2sFeS2 tandem DSSCs. The Voc approximately equals the difference of the Fermi levels of the photoanode and the photocathode. It is known that the Fermi level F is close to the energy level of the conduction band (ECB) in the n-type semiconductor but approximately equals the energy level of valence band (EVB) in the p-type semiconductor. For the convenience of the following discussion, the Fermi level F is approximately set as ECB and EVB in n-type and p-type semiconductors, respectively. TiO2sFeS2 is a tandem solar cell with the dye-sensitized n-type TiO2 nanorod film as the photoanode and the n-type FeS2 nanosheet- film as the photocathode. The band structure alignment and charge transfer in TiO2sFeS2 are schematically illustrated in Scheme 3a. Upon illumination, the electrons that exited from dye are injected into the conduction band (CB) of TiO2, and holes are created in the HOMO orbital of the dye. In the electric field, the electrons transfer from the redox agent to the HOMO level of the dye. This process creates a potential difference between the ECB of the TiO2 photoanode and the potential of the redox agent (Ured/ox), which is labeled as U1. At the same time, electron-hole pairs also form in FeS2, and as well, a potential, U2, is establish between Ured/ox and the CB of the FeS2 photocathode. As a result, the maximum open circuit voltage produced in TiO2sFeS2 will be
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Ua ) U1 - U2 ) (ECB,a - Ured/ox) - (ECB,c - Ured/ox) ) ECB,a - ECB,c (3) where ECB,a and ECB,c are the energy levels of conduction bands of the TiO2 photoanode and the FeS2 photocathode, respectively. While in TiO2sPt, U2 is almost zero, the maximum open circuit voltage should be
U ) U1)ECB,a - Ured/ox
(4)
Because the ECB,c of FeS2 is higher than the Ured/ox of I-/I3-,15,35 Ua is smaller than U. This can explain why the Voc of TiO2sFeS2 is lower than that of TiO2sPt. Differently, TiO2sFeS2 is a tandem solar cell with the dyesensitized n-type TiO2 nanorod film as the photoanode and the p-type FeS nanosheet film as the photocathode. Scheme 3b displays the band structure alignment and charge transfer in TiO2sFeS. In this case, U1 is also formed in a similar way with TiO2sFeS2. However, U2 should be formed between Ured/ox and VB of the photocathode because the Fermi level F equals approximately to the energy level of EVB in the p-type semiconductor. Therefore, the maximum open circuit voltage offered by the TiO2sFeS will be
Ub ) U1 + U2 ) (ECB,a - Ured/ox) + (Ured/ox - EVB,c) ) ECB,a - EVB,c (5) where ECB,a and EVB,c are the energy levels of conduction band of the TiO2 photoanode and FeS photocathode, respectively. Because the ECB,c of FeS2 is higher than the EVB,c of FeS, it is SCHEME 3: Schematic Illustration of the Band Structure Alignment and Charge Transfer in the Photovoltaic Cell with Different Photocathodes, (a) n-type and (b) p-type, Under Illumination
easy to obtain Ub > Ua, which is why the Voc of the TiO2sFeS solar cell is higher than that of TiO2sFeS2. 4. Conclusions In summary, we selectively synthesized FeS and FeS2 nanosheet films on iron substrates through a one-step hydrothermal treatment of iron foil and sulfur powder in the presence or absence of hydrazine in solution. Two novel tandem dyesensitized solar cells were constructed with TiO2 nanorod films as the photoanode and FeSx films as the photocathodes. We compared the photovoltaic properties of the two doublephotoelectrode dye-sensitized solar cells with that of the traditional DSSC with a Pt cathode as well as an Fe cathode. It was found that the Voc of the tandem DSSC with FeS nanosheet film photocathodes was higher than the traditional DSSC with a metal cathode or the tandem DSSC with FeS2 nanosheet film photocathodes. This study suggests that these iron sulfide nanosheet films are attractive photocathodes for tandem dyesensitized solar cells in view of the much lower cost of FeS than that of Pt. Acknowledgment. This work was supported by the National Basic Research Program of China (973 Program) (Grant 2007CB613301), the National Science Foundation of China (Grants 20503009 and 20777026), the Program for New Century Excellent Talents in University (Grant NCET-07-0352), and the Key Project of Ministry of Education of China (Grant 108097). Supporting Information Available: I-V curves of the FeS nanosheet film electrode as the photocathode with Pt as the counterelectrode in the I-/I3- electrolyte in the dark and under illumination. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Hilgendorff, M.; Spanhel, L.; Rothenha¨suler, C.; Mu¨ller, G. J. Electrochem. Soc. 1998, 145, 3632. (3) Jia, H. M.; Xu, H.; Hu, Y.; Tang, Y. W.; Zhang, L. Z. Electrochem. Commun. 2007, 9, 354. (4) Jia, H. M.; Hu, Y.; Tang, Y. W.; Zhang, L. Z. Electrochem. Commun. 2006, 8, 1381. (5) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (6) Halls, J. J. M.; Pickler, K.; Friend, R. H.; Morati, S. C.; Holmes, A. B. Nature 1995, 376, 498. (7) Halme, J.; Saarinen, J.; Lund, P. Sol. Energy Mater. Sol. Cells 2006, 90, 887. (8) Gra¨tzel, M. Nature 2001, 414, 338. (9) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gra¨tzel, M. J. Am. Chem. Soc. 2001, 123, 1613. (10) He, J. J.; Lindstrom, H.; Hagfeldt, A.; Lindquist, S. E. Sol. Energy Mater. Sol. Cells 2000, 62, 265. (11) Bolts, J. M.; Ellis, A. B.; Legg, K. D.; Wrighton, M. S. J. Am. Chem. Soc. 1977, 99, 4826. (12) Durr, M.; Bamedi, A.; Yasuda, A.; Nelles, G. Appl. Phys. Lett. 2004, 84, 3397. (13) Ennaoui, A.; Fiechter, S.; Pettenkofer, C.; Alonso-Vante, N.; Bu¨ker, K.; Ha¨pfner, C.; Tributsch, H. Sol. Energy Mater. Sol. Cells 1993, 29, 289. (14) Oertel, J.; Ellmer, K.; Bohne, W.; Ro¨hrich, J.; Tributsch, H. J. Cryst. Growth 1999, 198/199, 1205. (15) Ennaoui, A.; Fiechter, S.; Jaegermann, W.; Tributsch, H. J. Electrochem. Soc. 1986, 133, 97. (16) Rohrbach, A.; Hafner, J.; Kresse, G. J. Phys.: Condens. Matter 2003, 15, 979. (17) Hirahara, E.; Muratami, M. J. Phys. Chem. Solids 1958, 7, 281. (18) Horwood, J. L.; Townsend, M. G.; Webster, A. H. J. Solid State Chem. 1976, 17, 35. (19) Gosselin, J. R.; Townsend, M. G.; Tremblay, R. J. Solid State Commun. 1976, 19, 799.
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