THE JOURNAL OF
PHYSICAL CHEMISTRY (Registered in U. S. Patent Office)
VOLUME 61
(0Copyright, 1960, by the American Clieiiiical Society)
MARCH 29, 1960
XUMBER 3
THE HIGH TEMPERATURE THERMODYNAMIC FUNCTIONS OF CERIUM, NEODY&lIUhl AND SAMARIUM’ B Y F. H. SPEDDING, J. J. MCKEOWN AND A. H. DAANE Contribution No. 618 from the Institute for Atomic Research and Department of Che?tListry, Iowa State College. Work waa performed i n the Ames Laboratory of the U . S . Atomic Energy Cotwuisszon Received March 19. 1959
The high temperalare enthalpies of cerium, neodymium and samarium were measured from 0 to 1 1 0 0 O using a Bunsen ice calorimeter. The hieat ca acity and related thermodynamic quantities of the metals are tabulated at 5Megree intervals. The heats of transitSon aiicffusion were determined for these metals, with cerium having the lowest values and samarium the highest. Corre1atio:n has beeii made between the results of this study and calculated quantities contributing to the heat capacity of metals.
I. Introduction As the rare earth metals have become available in quantity and in high purity, many of their physical properties are being determined. In studies in this Laboratory, ithermal analyses to determine the melting points of some of these metals2 showed the presence of a solid state transformation a t temperatures near the melting points. The sizable heat effects associated with this transformation indicated a fundamental change in the nature of these metals, and this was confirmed by Barson, Legvold and Speddingaain dilatometric studies and by Spedding, Daane and Herrmannab in electrical resistivity studies. To examine this phenomenon in these metals and to determine their heat capacities, and heats of transition and fusion, a high temperature calorimetric study of the rare earth metals was begun. This paper describes the procedures used and the results obtained in a study uf cerium, neodymium and samarium metals. Equipment and Materials
assembly was found to function satisfactorily instead of a paraffin-packed, soft iron needle valve. Grease for lubrication was not necessary and thus there was no contamination of the mercury from this source. To reduce thermal conduction out of the calorimeter along the central tube, the halves of the gate housing were separated by a rubber “0” ring, and screws holding the gate housing together passed through thermal insulators. The outer glass flask was attached to the metal head of the calorimeter by means of a collar seal. The open end of the flask had a ground glass flange which seated on a Neoprene gasket. A rubber “0” ring fitted on the outside of the flask on the top of the ground glass flange. A circular brass collar screwed on the brass head and compressed the “0”ring and flask against the Neoprene gasket. A vacuum was drawn between the chambers before the brass collar waa screwed tightly into place. The inner glass flask was sealed to the head by means of a Kovar seal. The calorimeter was submerged in a constant temperature bath contained in a 30-gallon barrel; this bath contained crushed ice and a very dilute solution of methanol in water. It was found that by adding a few milliliters of methanol to the bath each morning before packing wlth ice, the heat leak could be adjusted to a low constant value for a period of 16 hours without any repacking of the ice. The space between the calorimeter chamber and the outer flask could be evacuated continuously, thus eliminating the task of evacuating this chamber and filling it with a dry insulating gas each day. A new technique waa employed for forming the ice mantle. Calorimeter.-A modification of the Bunsen ice calorimeter employed by Furukawa and others‘ waa used in this Helium, reviously cooled by passing through a copper coil investigation. A Teflon stopcock in the mercury accounting immersefin liquid nitrogen, was passed down a tube inserted into the calorimeter well. Two different types of heliumdis ersin tips were employed on this tube; one had an open (1) This paper is bawd in part on a Ph.D. Tbesia preaented to Iowa e n f whict localized freezing on the bottom plate while the State College, February, 1958, by J. J. McKeown. other tube had a closed end with horizontal ’eta located near (2) F. H. Spedding and A. H. Daane, in “Progresr in Nuclear the bottom. By adjusting the height of t i e inserted tube Energy,” aer. 5 , Vol. 1. H. M. Finniston and J. P. Howe, Eds., Pergaand the rate of flow of helium, an ice mantle of the desired mon Press, London, 1956, p. 413. (3) (a) F. Bamon, 8. Legvold and F. H. Speddinn, Phy8. Rsr., 101, shape could be frozen a t the desired rate. The shape of the mantle during freezing was observed by means of a simple 418 (1957); (b) F . H. Spedding. A. H. Daane and K. W. Henmann, glass periscope designed for this purpose. The usual preJ . M c k b , 9, 895 (1957). cautions were observed in introducing outgassed conductivity (4) G. T. Furukawa, T. 8. Douglas, R. E. McCoskey and D . C. water and mercury into the calorimeter. Ginnings, Bur. Slandora!s J . Rsacarch, BT, 67 (1956).
289
F. H. SPEDDING, J. J. MCKEOWN AND A. H. DAANE
290
The furnace employed was designed to determine the heats of fusion of the rare earth metals melting below 1200'. The main windings of the furnace consisted of #18 B and S gauge platinum (10% rhodium) wire. The power to these windings was riupplied by an electronic temperature controller designed by Svec, Reade and Hiker.' The power to auxiliary windings on each end of the furnace tube was regulated by a voltage stabilizer and adjusted with a variable transformer. These end windings were used to minimize the thermal gradient in the center of the furnace where the sample was locateid. The dropping mechanism waa a modification of the one described by Southard.6 To attain thermal equilibrium in the furnace, the sample wm held in position in the furnace by a string attached to the soft iron piston. The sample was dropped by releasing the soft iron piston from the core of a solenoid coil, after releasing the string. Reproducible dropping times were achieved by adjusting the weight of the falling assembly by addition of lead granules to the hollow 13oftiron piston. An empty platinum crucible of the same weight used to contain the corundum sample was dropped at 100" intervals as a blank; for the rare earth metal samples, the blank waa an empty platinum crucible containing an empty tantalum crucible, both of the same weight used to contain the sample. The temperature of the sample waa measured with a platinum ( 10y9rhodiumtplatinum thermocouple whose hot junction was within a dummy platinum capsule inserted in the sample's position in the furnace. The calibration of this second thermocouple was checked frequently with the freezing point of samples certified by the National Bureau of Standards. Materials.--To standardize the calorimeter a fused aAlto3 (corundum) sample 1" long and 1/2" in diameter weighing 12.31.36 g. was used; this was enclosed in a drawn platinum crucible. Spectrographic analyses indicated only a faint trace of silicon as an impurity in the corundum. The rare earth metal samples were prepared from salts separated and purified by ion-exchange methods described by Spedding and Powell.' The anhydrous fluorides of cerium and neodymium were prepared by heating a mixture of the respective oxide with ammonium bifluoride to 400" in a stream of dry air. The metals were prepared by reducing these fluorideis with calcium by methods described by Spedding and Daane.2 The more volatile samarium metal was prepared by reducing its oxide with lanthanum metal i n vacuo a t 1400" as described by Daane, Dennison and Spedding.8 The cerium and neodymium were melted down in their crucibles in a high vacuum at temperatures 100" above their molting points. Because of its high vapor pressure at its melting point, the samarium sample was melted into its crucible under an atmosphere of helium. Lids were welded on the tantalum crucibles under an atmosphere of helium; these were in turn forced into snugly fitting, drawn platinum crucibles which were welded closed in a helium atmosphere. 'The weights of the cerium, neodymium and samarium samples were 21.9943, 21.1556 and 22.5138 g., respectively. Table I indicates the results of spectrographic analyses of the metals.
TABLE I SPEC!TROQRAPHIC ANALYSIS OF SAMPLES Sample Cerium Neodymium Samarium
Impurities,' % Si Ca La 50.05 50.01 50.02 Pr Sm Si Ta Ca Fe -0.1 50.01 5 0 . 0 4 5 0 . 0 3 5 0 . 0 2 50.02 Fe La Mq Si Ca Eu < 0 . 0 5 5 0 . 2 -0.01 98.!1~0-(F2~~H~02) T
T
0.0 16.64 16.64 6.76 16.64 16.68 0.042 6.80 16.73 17.79 1.064 7.56 16.93 18.84 1.914 8.15 2.634 17.20 19 83 8.62 3.252 17.51 9.00 20.76 3,790 17.84 9.32 21.63 22.45 4.263 18.19 9.60 23.23 4.683 18.55 9.84 10.05 23.97 5.059 18.91 24.67 5.399 19.27 10 25 5.707 19.G3 25.31 10.42 25.97 5.98!1 19.98 10.58 20 33 26.58 6.248 10.73 6.487 20.67 10.86 27.16 6.709 21.01 10.99 27.72 28.26 6,916 21.34 11.11 7.109 21.67 28.78 11.22 29.28 7.290 21.99 11.33 11.41 29.67 7.428 22.24 8.055 22.24 11.22 30.30 22.31 30.39 8.079 11.22 30 85 8.204 22.65 11.22 8.320 22.97 31.29 11.22 8.417 23.25 31.67 11.22 14.04 33.20 9.950 23.25 9.964 23.29 14.04 33.25 23.63 33.74 10.105 14.04 (H'298.15 - H I ) / T = 6.07 cal. degree-' mole-' unpublished value according to Jennings, Hill and Spedding. 298.15 300 350 400 450 500 550 600 650 700 750 800 850 900 050 1000 1050 1100 1150 1190.15 1190.15 1200 1250 1300 1345.15 1345.15 1350 1398.15
( 1 1 ) L. Jenninaa, E. Hill and F. H. Spedding, "The Low Temperature Heat Capacit.y of Samarium." to be nubliahed. Iowa State College, Ames. Iowa (1957).
RInrch, 1960
HIGHTEMPERATURE THERMODYNAMIC FUNCTIONS OF CE, N D AND SM
by Spedding and Dame2to be 80-1, 1024 and 1052', respectively, from thermal analyses of these metals; the transition in samarium was observed a t 917'. In the present study, samarium was found to melt a t 1072 f 5' by dropping the sample a t 5' increments above 1052'. This higher value for the melting point of samarium is believed to be due to the O.2y0europium in the samarium used by Speddiiig and Dame. The transition temperature was found t o be 917' in both studies. It should be noted that the last value of the enthalpy reported for neodymium in the transition region was a t 981j0, 40 degrees below the reported melting point. Above 985' the enthalpy deviated positively from the linear curve generated by the previous three points in this region. Through a comparison of the transition range for the other rare enrths studied, it was assumed that the enthalpy varies linearly with temperature and the curve for neodymium generated by the three previous points was extrapolated to the melting point. The anamolous behavior between 985 and 1024' may be due to a premelting phenomenon although spectrographic analysis did not indicate any abnorninlly high metallic impurities. Jaeger, Bottenia and R ~ s e n b o h r n ~determined *.~~ the heat capacity of cerium but reported sporadic results and gave an equation only between 380480' to express their data. They found the value of Cp a t 400' to be 7.43 cal. per degree per mole. They reported a melting point of 635' which is considerably lower than the presently accepted value. We found C, to be 7.74 cal. per degree per mole at 400'. Stull and Sinkel* and Brewer16 estimated the heat of fusion to be 2.2 kcal. per mole; both had ,msigned too small a value for the heat of transition. The experimental values from this work are 7010 and 1238 cal. per mole, respectively, for the heats of transition and fusion. Jaeger, Bottenna and Rosenbohm18f16measured the heat capacity of neodymium and gave two empirical equaticins for expressing the data in two temperature regions. Evaluating the appropriate equation a t 400°,,the value of 10.45 cal. per degree per mole was obtained. Although their sample was initially quite pure, they observed a reaction with their platinum container a t about 600'. The heat capacity of neodymium was found to be 8.03 cal. per degree per mole in this study. Stull and Sinke14estimated the heats of transition and fusion to be 340 and 2600 csl. per mole, while our experimental values are 713 and 1705 cal. per mole. The low tempierature calorimetric group of this Laborat,ory" has determined the heat capacity of samarium and found the value a t 340'K. to be 7.35 cal. per degree per mole. The value of 7.42 cal. (12) F . hl. Jaeger, J . A. Bottema and E . Rosenbohm, Proc. Acad. Sci. Anislerdam,, 39, 91'2 (1936). (13) F. A I . Jaeger, 1. A. Bottema and E. Rosenbohm, Rec. trau. chim., 67, 1137 (1938). (14) D. R. Stull and G. C. Sinke, "Thermodynamic Properties of
the Elenients." American Chemical Society, Washington, D. C., 1956. (15)
L. Brewer, in "The Cheniistry and hletallurgy of hliscellaneL. L. 'Qiiill, Ed., McGraw-Hill Book Co., Inc., New York, N. Y . , 1950. P. 13.
o w hlaterials,"
( I S ) F. M . Jaeger, J . A . Bottema and E . Rosenbohm, Proc. Acod. Sci. Amslcrdatn. 41, 120 (1938).
293
per degree per mole a t 340'K. from this work compares favorably with the low temperature work, as only a point a t 100' was measured in this range. Stull and Sinke's14 estimates of the heats of transition and fusion are 3G0 and 2650 cal. per mole, whereas our work shows these values to be 744 and 20G1 cal. per mole. Since the metals employed in this study were purer than those used by previous investigators and the calorimeter was found to function properly according to the comparison of the data on aA1203 with the National Bureau of Standards, these data are considered to be more accurate. If one considers the heat capacity to be the sum of various terms, one may write Cp
Cvci,
+ Cecu + C-W + K'
(13)
where C, is the total measured heat capacity, Cv(,) is the lattice contribution to the heat capacity from a Debye treatment, Cec,,is the inner electronic (4f) contribution, Cq0) is the outer or conduction electronic term and 6C is the dilatation term. To obtain a qualitative comparison between total measured and estimated heat capacity, the various contributions were evaluated at some arbitrary temperature which, for convenience, was chosen to be 1000'K. The lattice contributions a t this temperature can be taken as the upper limit of the Debye equation, 3R. The contributions of the 4f electrons by promotion to the next higher energy state within a multiplet can be approximated. The metals are looked upon as being trivalent, ions which, of course, is not strictly true. The ground states of the trivalent species following Hund's rules are: cerium (zF1/J,neodymium (411s/,) and samarium (6Ha,,). The higher states of the multiplet in each case are denoted by unit increments of J from J m i n = L S to J,,, = L S. The energy differences between states in the multiplet can be approximated according to the method outlined by Van Vleck." Utilizing the energy differences, the partition function can be calculated. Employing the partition function's relationship to the heat capacity, the inner electronic contribution5 to the total heat capacity can be evaluated. At 1000'K. the promotion energy contributions were calculated to be : cerium (0.91), neodymium (1.22) and samarium (1.84) cal. per degree per mole. Gerstein, el al.,'8 through a private communication with Boorse, reported the temperature coefficient for the conduction electronic contribution (cal. degree-* of lanthanum to be 2.4 X mole-'). This value is used for evaluating the electronic contribution for neodymium. Clausius and Franeosini'Q measured the low temperature heat capacity of thorium and reported the coefficient equal to 1.6 X (cal. degree-2 mole-'). Because of the electronic and structural similarities between cerium and thorium, this value is used for the temperature coefficient for cerium. Jennings, Hill and Spedding" calculated the temperature co-
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(17) J. H. Van Vleck, "The Theory of Electric and hlagnetic Susceptibilities." Great Britain, Oxford at the Clarendon Press. (18) B. C. Gerstein, M. Griffel, L. D. Jennings. R. E. hliller. R. E . Skochdopole and F. H. Spedding, J . Chem. Phus., 27, 394 (1957). (19) IC. Clausius and P. Franzosini, F. Naturforsch.. 11, 957 (1956).
G. W. MELLORS AND S. SENDEROFF
294
efficient for samarium from the low temperature work of Robertsz0and found it to be 3.1 X (cal. degree-2 mole-'). The conduction electronic contributions to the heat capacity at 1OOOOK. are: cerium (1.6), neodymium (2.4) and samarium (3.1) cal. per degree per mole. The dilatation differences were computed a t room temperature by using the usual thermodynamic formula. The values for the compressibilities were obtained from the data of Bridgman.21 The coefficients of expansion of cerium, neodymium and samarium were taken from the data of Barson, Legvold and Spedding.2s22The values of the X-ray density necessary t'o compute the molar volumes were obtained from the review of Spedding and Daane.2 The value of Cp/C, can be computed a t any other temperature by means of the approxima.te equ a t'ion -
c,/c,
= 1
+ Aa,T
(14)
(20) L. M . Roberts, P r m . P h y s . Soc. London, B70, 471 (1957). (21) P. W . Bridgman, Proc. Am. Acad. Sei., 83, 1 (1954). (22) F. Barson, unpublished work on the thermal expansion of aamariuni, I o a a State College, Allies, Iowa, 1957.
Vol. 64
where A is a constant and crl is the linear coefficient of expansion. The dilatation corrections a t 1000'K. are: cerium (0.09), neodymium (0.13) and samarium (0.11) cal. per degree per mole. The total theoretical contributions and the experimental values are, respectively: cerium (8.54 and 9.12), neodymium (9.49 and 10.03) and samarium (10.99 and 10.96). Because of the qualitative nature of the calculations, little can be said regarding the discrepancies. The agreement for samarium is quite good and may be fortuitous. Factors which may contribute to these discrepancies are: anharmonicity of lattice vibrations, neglect of crystal field splitting, magnetic effects, analogies used to determine the outer electronic contributions for cerium and neodymium, and the treatment of the inner electronic promotion energies as trivalent species. Acknowledgment.-The authors wish to express their thanks to J. E. Powell and coworkers for the pure rare earth oxides and G. F. Wakefield and C. E. Haberman for preparing the metals.
THE DENSITY AKD ELECTRIC CONDUCTANCE OF THE MOLTEN SYSTEM CERIUM-CERIULM CHLORIDE BY G. W. MELLORS AND S. SENDEROFF Research Laboratories, Nalional Carbon Company, Division of Union Carbide Corporation, Cleveland 1, Ohio Received Mav 9,1969
The density and electric conductance of the Ce-CeCls system have been measured from the pure salt to the almost satu&& solution ( 9 mole % ' Ce) from just above the meltin point to 950'. The density of ure CeC18 is 3.216, g./cc. a t 850'. It decreases by about 1.5% on addition of 0.3 mole % 8 e metal after which a n almost enear increase to 3.2875g./cc. at 8 mole % Ce is observed. The specific and equivalent conductances of pure CeC4 are 0.9600 ohm-' crn.-' and 24.54 ohm-' em*., respectively, at 850'. Large increases in these values (43 and 46%, respectively) are observed on the initial addition of Ce metal corresponding to a concentration of 0.63 mole 70. Beyond this initial addition the conductivity rises slowly to values of 1.8900ohm-' cm.-' and 48.26 ohm-' cm.2 a t 7.75 mole % Ce. The large effects of low concentration of Ce on the density and conductivity of the system are explained by postulating the existence of comparatively free electrons resulting from the reaction CeO + Cel+ e-, some of which are trapped by the reaction Ce3+ 2e F! Cel+. The comparatively free electrons in the systems dilute in Ce metal have large "molar volumes" and high mobilities as observed in Na-liquid ammonia systems. The electronic "molar volume" and mobility are reduced rapidly with increasing amounts of added cerium so that t>heconductivity mechanism a t higher concentration becomes uncertain, although it is likely that even a t the highest roncentracion there is still some component of electronic conductivity.
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Introduction Measurements of density and electric conductance have been but rarely reported for molten metal-metal halide systems, though such information exists on the one hand concerning molten metals and 011 the other molten halides. Aten' reported the density and conductance of the CdCdClz system, and the conductance and viscosity of Bi-BiC13 meltsJ2 the density of which has been reported by Keneshea and Cubi~ciotti.~I n both these systems the specific conductance decreases with increasing metal content, an effect which was observed also by Cubicciotti4 for the molten Ca-CaClz system. Aten attributed the decreasing conductance to the formation of the (Cds)2+ cations (analogous to Hg2+) the existence of which (1) A. H . W. Aten, Z . phyrzk. Chcm., 73, 578 (1910). (2) A. H. W. Aten, ibad., 66, 641 (1909). (3) F. J. Keneshea, Jr., and D . D. Cubicciotti, THIS JOCRNAL, 63. 843 (1958). (4) D . D . Cubiociotti, h'IDDC-1058, 1940-49, USAEC.
+
was postulated by Grjotheim, Gronwold and Krogh-Moe on the basis of magnetic susceptibility measurements.6 Bronstein and Bredigs measured conduct,ances of alkali metal-alka.li halide melts, but they did not measure densities and for the purpose of calculating equivalent conductances assumed additivity of equivalent volumes. Greenwood and Worrall' measured the conductance and density of fused gallium dichloride and dibromide and found them to be typical molten salts with a conductance (and viscosity) characteristic of compounds in which the cation is considerably smaller than the anion. It had been shown earlier8 on the basis of the Raman spectrum ( 5 ) K. Grjotheim, F. Gronwold and J. Krogh-AMoe,J . Am. Chcm. S O C .71, , 5824 (1955). (6) H. R . Bronstein and M. A. Bredig, i b i d . , 80, 2077 (1958). (7) N. N. Greenwood and I . J. Worrall, J . Chcm. Soc., 1680 (1958). (8) L. A. Woodward. G. Garton and H. L. Roberta, i b i d . , 3723 (1956).
