J . Phys. Chem. 1993,97,3982-3984
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First Crystalline Electride Revisted: New Magnetic Susceptibility Studies of Cs+(18-crown-6)2eMichael J. Wagner, Rui H.Huang, and James L. Dye' Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824 Received: November 3, 1992
Samples of the electride, Cs+( 18-crown-6)ze-, with the previously published structure, are antiferromagnetic with a maximum magnetic susceptibility at about 50 K and with a stoichiometric number of trapped electrons. Allowing the temperature to increase above about 230 K changes the susceptibility to CurieWeiss behavior with a Weiss constant of only -2.3 K but with retention of a nearly stoichiometric number of trapped electrons. The overall magnetic behavior is different from that found in previous studies, which characterized this electride as a CurieWeiss paramagnet with up to about 80%of the expected unpaired spins. The difference is tentatively attributed to the presence of excess cesium in the previously studied samples. The earlier behavior could be reproduced by preparing samples by the previous technique.
Introduction
In 1983, Cs+(l8-crown-6)~e-[Cs+(l8C6)2e-], the first of an entirely new class of crystallinematerials,electrides, was reported. I Since that time, this compound has been extensively characterized by single-crystal X-ray crystallography,z NMR,' EPR? powder conductivity,' optical absorption ~pectroscopy,~.~ and magnetic s u s ~ e p t i b i l i t y .Recently, ~ ~ ~ ~ ~ new c o n d u c t i ~ i t yand ~ ~EPR ~ measurementslO,llgave significantly different results than were previously obtained. The difference was attributed to a low level of excess cesium doping in the previous samples, formally as the ceside, but the identity of species responsiblehas not been verified. These results raised the question of what effect such doping may have had on the other measured properties of this electride, a questionmade all the more important by thesignificant theoretical interest in calculating its pr~perties.'z-~~ Specifically,could such doping, rather than partial decomposition or lack of correct stoichiometry of the polycrystalline samples, be responsible for the low free spin count (never exceeding about 80% of that e x p e ~ t e d )as~ ~measured ~,~ by magnetic susceptibility?
T (K) Figure 1. Temperature dependence of the molar electronic magnetic susceptibility of Cs+(18C6)2e-that has never been heated above 230 K, showing evidence of antiferromagnetic order.
Experimental Section
Results and Discussion
To determine thereason for the low spin count, undoped samples of Cs+(18C6)ze- were required. The earlier measurements had been made with polycrystalline samples prepared from solutions that were nominally stoichiometricin both complexant and metal. Thus the equilibrium 2C~'(l8C6)~e-+ Cs+(l8C6),Cs-+ 2(18C6) could lead to a mixture of e- and Cs- in solution. For this investigation, a 10-20% excess of complexant was used to drive the reaction toward electrideformation. Samplesfor susceptibility were crystallized by utilizing slow (1-2 weeks) solvent removal at constant temperature (190 K). A crystal was selected from the synthesis, and single-crystal X-ray crystallographic measurements proved that it had the same structure as had been previously published.2 Enough large (millimeter scale) single crystals for susceptibility measurements were selected and pulverized under liquid nitrogen. Magnetic susceptibilitieswere measured with an S.H.E. computer-controlled variable-temperature SQUID susceptometer. The electronic contribution to the susceptibility was determined by subtracting the diamagnetic signal of the sample after decomposition.
* To whom correspondence should be addressed. 0022-3654/93/2097-3982f04.00/0
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Slroceptibility Measmrnents. A typical plot of the temperature dependenceof the molar susceptibility,which is field independent betwen 0.05 and 0.7 T, is shown in Figure 1. The susceptibility can be well fit by the Curie-Weiss law at temperatures above -70 K. Least-squares fitting of the high-temperature data (>70 K) to the Curie-Weiss law yielded 107(2)% unpaired electrons and a Weiss constant of -82(3) K. (Numbers in parentheses are estimated standard deviations of the last digit for a single run.) It should be noted that these numbers represent an upper limit since the high-temperature susceptibility of an antiferromagnet is expected to approach Curie-Weiss behavior asymptotically.'S Amaximumin thesusceptibilitymrredat -5OKwithagradual decrease below this temperature. At very low temperatures, what appeared to be a "Curie tail" from a very small amount of paramagnetic impurities could be seen whose magnitude was synthesisdependent. Simpleextrapolationof the low-temperature behavior to 0 K after subtraction of a 0.4% Curie tail gave an intercept of about two-thirds of the maximum susceptibility, as would be expected for a 3D antiferromagnet. The reproducibility of these data was confirmed with two other samples; a hightemperature Curie-Weiss law fit yielded 107( 1) and 106(2)% unpaired electrons and Weiss constants of -84(2) and -85(4) K, respectively. Although these new measurements clearly showed that the crystallineelectrideis an antiferromagnet,the subsequentbehavior 0 1993 American Chemical Society
First Crystalline Electride Revisited
The Journal of Physical Chemistry, Vol. 97, No. 16, 1993 3983
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T (K) Figure 2. Temperature dependence of the inverse molar electronic magnetic susceptibility of Cs+( 18C6)2e-. The uppermost plot is that of a sample that has never been heated above 230 K (diamonds). The middle plot is the behavior observed after a brief excursion above 230 K (squares); the line through the data is a linear combination of the upper data and the Curie-Weiss law fit of the lower plot. The lower plot is the behavior observed after repeatedly heating the sample above 230 K (circles); the line through the data is the least-squares CurieWeiss law fit.
was highly dependent on the thermal history of the sample. Allowing the sample to warm briefly above about 230 K caused a seemingly irreversible change in the susceptibility to behavior reminiscent of a ferrimagnet.I5 (Annealing samples either at 190 K for 16 h or at 230 K for 12 h did not result in reversion to antiferromagnetic behavior.) Repeated warming above 230 K caused further deviation from antiferromagnetism, finally resulting in behavior which can be well fit to the CurieWeiss law. Least-squares fitting of the entire temperature range (1.5240 K) yielded 90.6(2)%unpaired electrons and a Weiss constant of-2.3(2) K. The lower spin count does not necessarily represent significant decomposition due to the thermal cycling since, as previously mentioned, the high-temperature Curie-Weiss fitting of the antiferromagnetic data is expected to overestimate the number of unpaired spins. The susceptibility of samples that had not undergone a complete conversion are well fit by a linear combination of the antiferromagnetic data points and the best fit Curie-Weiss law behavior. The dependence of sample magnetic susceptibility on thermal history is illustrated in Figure 2. The nature of the change in susceptibility that occurs above about 230 K is unknown. The known low-temperature crystal structure shows the presence of two chemically equivalent but magnetically inequivalent complexed cesium cations as well as two such vacant anionic sites.2 There is a large body of evidence that theunpaired electron density is centered at thevacant anionic site.I6 The high-temperature phase shows the presence of two chemically distinct Cs+ cations in MAS NMR,3 and two major inequivalent electron trapping sites were found in recent EPR studies.' I Differential scanning calorimetry studies show no evidenceofa first-ordertransition between the twophases. Recent conductivity measurements indicate that the electride is primarily an ionic conductor in this temperature region, with Cs+ as the migrating specie^.^,^ Substantial crown ether motion has been observed in the isostructural sodide Cs+(18C6)2Na-.I7 Perhaps motion-assisted migration of Cs+ into the channels to form Cs atoms at elevated temperatures causes significant, irreversible disorder. Such disorder might distort the lattice in such a way as to create two chemically inequivalent Cs+cations and trapping sites and lead to the CurieWeiss behavior indicated by the magnetic susceptibility measurements. Comparison with hevious Results. These results are in contrast to the previous results, which had found Cs+(18C6)2e-to fit the CurieWeiss law, but with only up to about 80% of the expected
free electrons and a Weiss constant of only -1 to -2 K, indicating weak electron-electron interaction^.^,^*^ In order to confirm that the behavior depended on sample preparation, polycrystalline samples were produced by recrystallizing Cs+(18C6)2e- in the presence of a small amount of Cs+(18C6)2Cs-. The susceptibility behavior of these samples was similar to that obtained in the earlier investigations. Polycrystalline samples made in a similar fashion but with an excess of complexant present during synthesis always showed evidence of antiferromagnetic order. The previously calculated unpaired electron contact density at the Cs+ nucleus obtained from 133CsNMR spectra should be corrected in light of the new susceptibility data. It had been reported that the unpaired electron density at Cs+ was 8.75 X 1O2I ~ m - leading ~, to 0.033% atomic character at 250 K as calculated from the Knight shift measured by NMR techniques.3 Recalculating both values yields 9.6 X 1021 ~ m and - ~0.036%. While these values are somewhat different, they are still very small compared to the contact densities observed for cesium solutions in ammonia, amines, and other solvents, thus demonstrating the small occupancy of the 6s orbital of the complexed cesium cation in Cs+(18C6)2e-.3 Conclusions
This new study of Cs+(18C6)2e- has shown that the magnetic properties of samples with the published structure are significantly different from those previously ascribed to it. We found that this electride has stoichiometric trapped electrons that display antiferromagnetic order and which can undergo a gradual transition to Curie-Weiss law behavior, still retaining more than 90%of the stoichiometric spin count. This is contrary to previous studies which characterized the electride as a CurieWeiss paramagnet with never more than 80% unpaired electrons. The behavior seen in previous studies can be reproduced by rapidly harvesting polycrystalline samples from solutions that contain a slight excess of cesium. While doping with excess cesium is a likely cause of the differencein the measured propertiesof Cs+(18C6)2e-, one cannot exclude other possibilities. Poor loading technique resulting in significant decomposition as well as loss of all evidence of antiferromagnetic order is possible; however, it is highly unlikely. The measurementswere repeated several times by severaldifferent investigators who were well acquainted with the need to keep electrides as cold as possible to prevent decomposition. It is also possible that polycrystalline samples made in the presence of near stoichiometric reactants may not have the same structure as that reported for the electride. The published structure of the electride was obtained from single crystals that were grown slowly. The rapid harvesting of polycrystalline samples from near stoichiometric solutions could yield a highly disordered system which might even include some solvent in the structure. Interesting questions still remain about this electride. What is the nature of the magnetic transition above 230 K? Could the properties of the electride be so drastically altered by a small percentage of excess cesium doping and, if so, how? If one can dope the electride with cesium, does a superlattice form? These questions are currently being investigated by powder X-ray diffraction, NMR, and additional SQUID measurements. Acknowledgment. This research was supported by NSF Grant DMR 90-17292and the MSU Center for Fundamental Materials Research. The X-ray diffractometer system was provided by NSF Grant CHE 8403823. References and Notes (1) Ellaboudy, A. S.; Dye, J. L.; Smith, P.B. J . Am. Chem. SOC.1983, 105, 6490. (2) Dawes, S. B.; Ward, D.L.; Huang, R.H.; Dye, J. L. J. Am. Chem. SOC.1986,198, 3534.
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(3) Dawes, S.B.; Ellaboudy, A.; Dye, J. L. J . Am. Chem. SOC.1987,109, 3508. (4) Ellaboudy, A. S.Ph.D. Dissertation, Michigan State University, 1984. ( 5 ) Ellaboudy, A. S.; Tinkham, M. L.; Van Eck, B.; Dye, J. L.; Smith, P. B. J . Phys. Chem. 1984,88, 3852. (6) Issa, D.; Ellaboudy, A,; Janakiraman, R.; Dye, J. L. J . Phys. Chem. 1984,88, 3847. (7) Dawes, S.B. Ph.D. Dissertation, Michigan State University, 1986. (8) Moeggenborg, K.J. Ph.D. Dissertation, Michigan State University, 1990. (9) Moeggenborg, K.J.; Papaioannou, J.; Dye, J. L. Chem. Mater. 1991, 3, 514. (10) Shin, D. H. Ph.D. Dissertation, Michigan State University, 1992.
Wagner et al. (11) Shin, D. H.; Dye, J. L.; Budil, D. E.; Earle, K. A,; Freed, J. H. J . Phys. Chem. 1993, 97, 1213. (12) Rencsok, R.;Kaplan, T. A.; Harrision, J. F. J . Chem. Phys. 1990, 93, 5875. (13) Allan, B.; DeBacker, M. G.; Lannoo, M.; Lefebvre, I. Europhys. L ~ 1990, ~ 11 ~ (11, , 49, (14) Golden, S . ; Tuttle, T. R., Jr. Phys. Reu. B 1992, 45, 913. ( I 5 ) Goodenough, J. B. Magnetism and the Chemical Bond; John Wiley & Sons: New York, 1963. (16) Wagner, M. J.; Dye, J. L. Annu. Rev. Mater. Sei. 1993, 23, 223. (17) Wagner, M. J.; McMills, L. E. H.; Ellaboudy, A. S.; Eglin, J. L.; Dye, J. L.; Edwards, P. P.; Pyper, N. C. J Phys. Chem 1992, 96, 9656.
