272
J. P h p . Chem. 1981, 85, 272-275
111. Presumably, the deposited platinum acts as surface states,6J6making the deposition behaviors much complicated. The energetic correlation between the electrode and electrolytes is given in Figure 6. Since the energy levels of Pd2+/Pd and PtC1,2-/Pt are lower than that of the conduction band edge of Ti02,which is around -0.25 V vs. SCE in the plating bath,17the band is bent upward when the crystal and the plating bath are in equilibrium in the dark. When the front face of the crystal is illuminated and the back face is kept in contact with the solution, photogenerated positive holes and electrons moves in opposite directions because of the electric field of the space charge layer; electrons move to the bulk of the crystal, while positive holes move to the front face. The deposition occurs. when the bandbending is decreased enough to permit the escape of electrons from the back face to the metal ions. If the back face is insulated, however, electrons cannot escape from the back into the solution. Electrons are then accumulated in the bulk of the crystal, raising the energy band in the bulk. As the result, the bandbending is decreased, enabling electrons to escape from the front face. (16) B. Kraeutler and A. J. Bard, J. Am. Chem. SOC.,100,5985 (1978). (17) E. C. Dutoit, F. Cardon, and W. P. Gomes, Ber. Bunsenges. Phys. Chem., 80, 475 (1975).
Judging from published results on cathodic processes of other electrochemical reactions at Ti02 single crystal electrode^,^^^^ the deposition reaction is believed to proceed via surface states, as schematically illustrated in Figure 6. According to the morphology of the deposited metal, the nature of sites onto which the deposition takes place seems to be different between the front and the back faces if both faces are available for the deposition reaction. This may imply that there are surface states of a different nature in the crystal face. However, the morphology of deposited metal on the illuminated face in the absence of the nonilluminated face was quite similar to that on the nonilluminated face. This finding seems to suggest that surface states which play principal roles in the photodeposition have the same nature in both faces. Acknowledgment. Capacitance measurements to determine the donor concentration were carried out by Mr. Hideyuki Shiotani. This research was supported by Grants-in-Aid for Scientific Research (No. 412212 and 505008) from the Ministry of Education of Japan. ~~
~
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(18) R.Nodi, R.A. Kohl, S. N. Frank, and A. J. Bard, J. Electrochem. SOC.,125, 246 (1978). (19) J. Vandermolen, W. P. Gomes, and F. Cardon, J.Electrochem. SOC.,127,324 (1980).
Magnetic Susceptibility of Lithium-Ammonia Solutions A. Deprlester, J. Fackeure, and J. P. Lelleur’ Laboratolre des Surfaces et Interfaces, L.A.253 du CNRS, LIlb, France (Recelved: Ju& 18, 1979; In Final Form: August 8, 1980)
Magnetic susceptibilitymeasurements of lithium-ammonia solutions were performed with the Gouy technique as a function of lithium concentration (from 0.7 to 19 mol % of metal (MPM))and as a function of temperature between --70 and -10 “C. These measurements confirm previous data in Na- and Cs-NH3 systems. Diamagnetic pairing between electronic species is displayed in the low concentration range of the experimentally studied solutions. The electronic magnetic susceptibility is positive (paramagneticsusceptibility)for metal concentration larger than -3 MPM and displayed a maximum at 10 MPM.
-
Introduction Magnetic susceptibility of concentrated or moderately concentrated alkali metals in liquid ammonia solutions is a question related to the problem of the metal-to-nonmetal transition in disordered systems.ls2 In a previous work, Lelieur and Rigny3 for Na- and Cs-NH3 solutions, showed the increase of the paramagnetic susceptibility of valence electrons from the alkali metal when the concentration of the solution increases. In the present work, we report the magnetic susceptibility of Li-NH3 solutions vs. concentration (from 0.70 to 18.9 MPM (mol ’70of metal)) and vs. temperature (from - 4 0 to -10 “C). Experimental Section The magnetic susceptibility of Li-NH3 solutions has been measured by the classical Gouy technique, which uses (1) For a review of solutions of metals in liquid ammonia, see J. C. Thompson, “Electrons in Liquid Ammonia”, Clarendon Press, Oxford, England, 1976. (2) For a review of the metal-nonmtal transition in disordered systems, see N. F. Mott, R o c . Scott. Uniu. Summer Sch. Phys. 19, 149 (1978). (3) J. P. Lelieur and P. Rigny, J. Chem. Phys., 59, 1142 (1973). 0022-3854181 12085-0272$0 1.OOlO
long (-15 cm in the present work) cylindral cells with calibrated internal diameter (of -8-10 mm in the present work). The cells have been cleaned following the usual procedure! A known amount of lithium is introduced into the cell under argon in a glovebag and connected to a calibrated vacuum line, pumped for more than 12 h down to a pressure of -5 X lo+ torr. A known amount of ammonia, previously stored over sodium for drying, is then condensed over lithium. The cell is then sealed off and stored in dry ice or liquid nitrogen. The amounts of lithium and ammonia are arranged in such a manner that the upper level of the liquid solution, in each sample, at any temperature, was placed high enough so that the magnetic field in this region could be considered as negligible. In this case, the solution in the magnetic field is submitted to the force F s p 6 F = 1/2x,SHO2 (1) (4) A. Demortier, P.Chieux, and G. Lepoutre, Bull. SOC.Chim. Fr., 10, 3421 (1971). (5) L.N.Mulay in “Physical Methods of Chemistry”,Vol. I, part IV, A. Weissberger, Ed., 1972, p 431. (6) J. P. Lelieur, These, Orsay, France, 1972.
0 1981 American Chemical Society
The Journal of Physical Chemistv, Vol. 85,No. 3, 198 1 273
Magnetic Susceptibility of Li-NH, Solutions
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Results and Discussion Measurements have been made on 10 Li-NH3 samples from 0.70 to 18.9 MPM between --70 and -10 OC,except for the solutions in the liquid-liquid phase-separation range for which the measurements are limited above the critical temperature.
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[NH3]/[Li] of a given solution and XNH* is the molar susceptibility of pure ammonia. The magnetic susceptibility of 1 mol of lithium results from the contribution x ~ of Li+ cations and from the contribution xe of the valence electrons of lithium given to the solution. Therefore the electronic susceptibility x,, i.e., the magnetic susceptibility of 1 mol of valence electrons in solution, is given by eq 3. (3) Xe = XLi - XLi+ Finally, the measurements of F and the knowledge of R by the sample preparation and of x N H s (-16.45 X lo4 cgs at -40 O C ) and xLi+(-6.696 X lo-' cgs at -40 "C)allow one to determine the magnetic susceptibility of 1 mol of valence electrons given by the lithium metal to the solution. Magnetic field I& was usually 11.7 kG given by a Drusch EAF 16H electromagnet. A microbalance Setaram type MTB 1012 with a sensitivity of -5 pg is used. The relative uncertainty of the mole ratio is *2 X lo9.
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Figure 2. Volume susceptibility (106xvin cgs units) vs. temperature for a 4.69 MPM Li-NH3 solution.
where xv is the magnetic susceptibility per unit volume of solution, S the internal cross section of the sample, and Hothe magnetic field at the center of the gap. This quantity Hois determined by calibration with distilled water or benzene. From xvvalues, the specific magnetic susceptibility xg of the solution can be obtained by the relation xv = px where p is the specific mass of the solution, measurecfby Lo7for Li-NH3 solutions. The magnetic susceptibility of Li in solution is obtained if the ammonia contribution is taken into account. The contribution of ammonia is substracted, assuming that the magnetic susceptibility of ammonia in solution is the same as in pure liquid ammonia. If one uses Wiedemann's mixing rule, the magnetic susceptibilityof 1 mol of lithium in solution xLiis given by eq 2, where R is the mole ratio x L i = xg[17.031R 6.9391 - XNHJ (2)
R.E.Lo, Anorg. Allg. Chern., 230, 344 (1966).
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Examples of the experimental measurements are shown in Figures 1-3, where the volume susceptibility xvdirectly deduced from the measured magnetic force F by relation 1 is plotted against the temperature for solutions of concentration 0.70, 4.69, and 14.8 MPM, respectively. The values of the specific susceptibility xg, the electronic susceptibility xe, and the temperature coefficient (1/ x,)(dx,/dT) at -40 "C,for the different samples, are plotted against the lithium concentration in solution in Figures 4-6, respectively. It must be emphasized that the overall susceptibility of the solutions resulting from the various contributions (ammonia, Li+, electrons) is diamagnetic for all studied samples; Le., xvand xg (quantities related to the overall susceptibility) are always negative and correspond to a diamagnetic susceptibility. This can be observed for xB in Figure 4. In fact, to have information about the electronic states of valence electrons in solution, it is necessary
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Figure 5. Electronic susceptibility ( 1 0 ' ~in~cgs units) per mole of valence electrons in solutions vs. the lithium concentration (MPM) at -40 OC.
to subtract the contribution from ammonia and from Li+ to get the electronic susceptibility xe, which is displayed in Figure 5, where two concentration ranges can be distinguished. On the less concentrated side, for concentrations less than -3 MPM, xe is negative. For concentrations larger than -3 MPM, xe is positive, which means that the paramagnetic contribution of the electrons is larger than their diamagnetic one. It must be rememberedg-" that for infinitely dilute solutions xe should be positive and of the Curie type and, when the concentration increases in the very dilute region, xe decreases corresponding to a pairing of paramagnetic species. Considering our present measurements, it seems that, if they were extended to solutions less concentrated than 0.70 MPM, xewill probably increase when the metal (8)E. Huster, Ann. Phys. (Leipzig), 33, 437 (1938). (9)S.Freed and N. Sugarman, J. Chem. Phys., 11,354 (1973). (10)C.A. Hutchison and R. Pastor, Rev. Mod. Phys., 25 285 (1953). (11)C.A. Hutchison and R. Pastor, J. Chem. Phys., 21,1959 (1953).
--
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coefficient (l/Xe)(dXe/dT)is deduced from the temperature coefficient of the measured magnetic force, from the temperature coefficient of the specific mass? and from the temperature coefficient of the molar susceptibility of liquid ammonia. The values of (l/xe)(dxe/dr) at -40 "C are plotted in Figure 6 against the lithium concentration of the solution. The temperature coefficient appears to be negative for the most dilute solutions which have been studied, because for these solutions xe is negative. In fact dx,/dT is always positive, which means that the paramagnetic contribution increases when the temperature increases. The analysis of the experimental data is based upon Wiedemann's mixing rule and on the hypothesis that the contribution of ammonia in solution can be taken equal to the susceptibility of pure liquid ammonia. This last hypothesis can possibly be criticized because of the interaction between Li+ and ammonia molecules. In fact the reliability of such static susceptibility data is indicated by
J. Phys. Chem. 1981, 85, 275-280
the closeness of the static and spin susceptibility data3J1 as observed for sodium ammonia solutions. The same type of experiments should be performed in lithium-ammonia solutions to check the closeness between static and spin susceptibility.
Conclusion The magnetic susceptibility of lithium-ammonia solu-
275
tions has been measured for the metallic solutions and for solutions in the metal-nonmetal transition range for the classical technique. These measurements confiim previous data related to Na- and Cs-NH3 solutions, but the present measurements have been extended to low enough concentrations to display nonambiguously the strong diamagnetic pairing of the electronic species on the dilute side of the metal-nonmetal transition range.
Ytterblum-Ammonia Solutions R. Hagedorn and J. P. Lelleur’ Laboratoire des Surfaces et Interfaces, L.A.253 du CNRS, L i k , France (Received: JuW 17. 1979; In Final Form: August 6, 1980)
Differential thermal analysis and vapor-pressure and density measurements were performed on the ytterbium-ammonia system. Vapor-pressuremeasurements, as a function of ytterbium concentration, between -65 and 4 0 “C, show the existence of a liquid-liquid phase separation between -0.6 and 4.5 mol % of metal (MPM) and the existence of the compound Yb(NH3)=,where x is 6.5. The critical temperature of the liquid-liquid phase separation is above room temperature. Thermodynamic data are obtained from vapor-pressure measurements for concentrated solutions. The densities of ytterbium-ammonia solutions increase with ytterbium concentration, while in Li-, Na-, K-, and Ca-NH3 solutions, the density decreases with increase of metal concentration. Introduction It is often quoted that rare-earth metals ytterbium and europium are soluble in liquid ammonia. Warf and Korstl were the first to observe solutions of ytterbium and of europium in liquid ammonia and to suggest the existence of a compound between each of these metals and ammonia. However the solutions of ytterbium and of europium in liquid ammonia have not been as well studied as, for instance, solutions of alkali metals in liquid ammonia. Schroeder, Thompson, and Oerte12measured the electrical conductivity and noted a close similarity in behavior between ytterbium- and alkaline-earth-ammonia solutions and suggested that ytterbium be treated as divalent in these solutions. From electrical-conductivity measurements they established the composition (7.1 mol ?% of metal (MF’M) and temperature 183K) of the eutectic point of these solutions. Thompson, Stone, and Waugh3 investigated the composition dependence of the vapor pressure of ammonia over solutions of europium and of ytterbium in liquid ammonia at -75.9 OC. They also found that these two metals formed M(NH3), compounds where M is either europium or ytterbium, and n obtained by extrapolation of the ammonia vapor pressure to zero value was 6.3 and 6.4 for europium and ytterbium, respectively. More recently Frisbee and Senozan4measured the equilibrium pressure of ammonia over the europium- and ytterbium-ammonia compound. Therefore, it must be realized that basic physical data for ytterbium-ammonia solutions are rather sketchy. In the present paper, information about the phase diagram obtained by differential thermal analysis and vapor-pressure measurements are reported with the densities of dilute ytterbium-ammonia solutions. (1) J. C. Warf and W. Korst, J. Phys. Chem., 60,1590 (1956). (2) R. L.Schrder, J. C. Thompson, and P. L. Oertel, Phys.Reo., 178, 298 (1969). (3) D. S. Thompson, M. J. Stone, and J. S. Waugh, J. Phys. Chem., 70, 934 (1966). (4) R. H.Frisbee and N. M. Senozan, J. Chem. Phys.,57,1248 (1972).
Experimental Section Vapor-Pressure Measurements. The vapor pressure of ammonia over the ytterbium-ammonia solutions was determined in a vacuum line with calibrated volumes by using a procedure similar to that of Marshall and Hunt.s The vacuum line was connected with an ammonia reservoir, where ammonia was stored over sodium for drying, a mercury manometer, and a tube with known volume, which could be isolated from the vacuum line by means of a stopcock. The reaction cell where the solution of ytterbium in liquid ammonia is made was connected to the tube with a grinding. An ionization gauge indicated that a pressure less than 5 X lo4 torr could be attained in the system. The reaction cells, which had a cylindrical or spherical form, with a volume of 20 mL were cleaned by standard proceduree before use. The ytterbium (99.5% purity from Alpha Inorganics) was stored under argon, and before use its surface was cleaned mechanically; the ytterbium was cut into pieces, weighed with a microbalance, and introduced into the reaction cell. The reaction cell was then connected to the vacuum line and was pumped down to a pressure less than 5 X lo4 torr for more than 12 h. The vacuum line was then isolated from the vacuum pump, and ammonia from the storage reservoir was evaporated into it. The ammonia pressure in the line was determined with a cathetometer of f0.02-mm precision. The room temperature was read with a mercury thermometer, and the number of moles of ammonia in the line was calculated with the van der Waals equation, the constants of this equation for ammonia gas being taken from the “Handbook of Physics and Chemistry”. Ammonia was condensed on ytterbium at dry-ice or liquid-nitrogen temperature. An alcohol thermostated bath was then placed around the sample tube, the temperature of which was held constant within *0.05 “C at N
(5) P. R. Marschall and H. J. Hunt,J.Phys. Chem., 60, 732 (1966). (6) A. Demortier, P. Chieux, and G. Lepoutre, Bull. SOC.Chim. Fr., 10, 3421 (1971).
OO22-3654/81/2O8~-0275$01.OW0 0 1981 American Chemical Society
