Acknowledgment.-It is a pleasure to acknowledge the help given by Dr. Li'arren DeSorbo and Mrs. E. L. Fontanella of the General Electric Research Laboratory and N r . Emiel Palmer in the construction and operation of the apparatus, by
[ C'OYTRIi3LTIOS
Urs. A. J. King, F. X. Kanda and 1-irginia Russell in the preparation of the borides, and by Dr. H. 12. n'irth for helpful discussions during the course o f the research. SYRACUSE,
N. y
S O . !%FROM T H E C R Y O G E h - I C LABORATORY OF T H E COI,I.EGE OF c H E M I S T R Y AS11 P H Y S I C S , T H E pESS5YI~\'.I>.I.\ STATE ~ ~ S I V E R S I T Y ]
Heat Capacity and Magnetic Susceptibility of Copper(I1) Tetrammine Sulfate Monohydrate from 1.3 to 24°K. BY J. J. FRITZAND H. L. PINCH XRCEIVEDJASUARY 24, 1957 The heat capacity and magnetic susceptibility of copper tetrainmine sulfate have been investigated lietxvecn 1.3 and 29" K , The results indicate strong interactions between copper ions, leading to a transition, of order higher than tlie second, \vith ~ a t 3 . 0 ° K . The magnetic susceptibility is nearly constant ixtiveeli maximum in the heat capacity of 0.75 cal. ~ n o l e -deg.-' 1 and L I O E ; . , arid thereafter falls gradually; the mnximum valuc of tlie n i d a r susceptibilit>- i, 0.04. I t i i believed t h a t tlic interaction is directly related to tlie coiirdination with ammoni:i.
Introduction The magnetic behavior of crystalline salts of cupric ion varies considerably from salt to salt. The Tutton salt, CuS04.K2S04.GH20,is one of the most ideal of all paramagnetic salts, and has been used in the attainment of temperatures far below 1 OK.' The ordinary hydrated sulfate, CuS04. 5H20, has a magnetic transition? in the vicinity of 1OK., the exact nature of which is not clear. The hydrated chloride, CuC12.2H20, has a lambda-type transition a t about 4.2"K. below which the salt becomes anti-ferromagnetic.B The acetate, Cu(CrHaO?)2.He0, becomes diamagnetic a t about 50 K. 1l;ith the exception of the acetate, the properties of all the salts described above may be explained in terms of the effect of crystalline environment on a substantially free copper ion. The ground state of free cupric ion is ?Dj,,?. At temperatures above the magnetic transitions. the typical copper salts behave almost as ?S states; their magnetic susceptibilities are but slightly higher than the "spin-only" values. Copper tetramniine sulfate differs formally from CuS04.5H20only in the replacement of four of the water molecules by ammonia molecules. In the crystal the basic unit is Cu(TH3)4++,rather than C U ( H ? O ) ~ + +due ; to differences in crystal structure. the less near neighbors of the copper ion are different in the two salts. 9 t the outset of this research, it was expected that the ammine salt would behave in the same general fashion as the hydrate. so that comparison of the two would show the effect of the ammonia ancl in addition shed further light on the behavior of the ordinary hydrate. Experimental The experimental procedures were substantially the same as those employed by Fritz a n d Pinch in the investigation (1) J. Ashmead, Y a t i t r ~ 143, , 853 (193.9). (2) T. H . Geballe and W. P. Giaurjue, THISJ O T R N A L . 7 4 , 3,715 (1952). (3) S Friedberg, P h y s i r o . 18, 714 ( 1 9 5 2 ) ; J. v a n den Handel, H. I f . Gijsman and A* S f'oulis, ibid, 18,8fi2 (19521. (4) €3 C. Guha. P ~ o r Roy. .COG. ( L o n d o u ) , A206, 333 (19.51).
of rnnndiurn ammonium alum,5 and will not be drscrilictl i n detail. T h e ellipsoidal sample container had a n internal ancl contained 32.64 g. of the s a l t . volume of 31.13 T h e weight of t h e container was 19.5O g. Tlir carbon thernioineter had :t resistance a t room temperature of :ihout 6000 ohms. .At loiv temperatures its sensitivity v a s decidedly higher than t h a t of the thermometer previously rlescriljcd , G but it was also consideralily less stable. The specimen used in the investigation \\-:is prep;irctl l)y addition of C.P. ammonium hydroxide to ;I iolution of C . r . copper sulfate according to the method of \Valton.6 ?'lie crystalline salt was obtained bj. cooling of the resulting hot solution. The crJ-stals used were ground to a po\vtler; the portion selected for use passed througli a 20-171esIiscrcc'ii, but failed to pa53 through a 100-mesh qcrecn. Tlie irrcgular particles were stored for a time under ;I wturatetl solutioii O f the salt and then dried manually. h sample for analysis \vas witlidran.n during tlir filling of the sample tube. Its copper content i i - a s determined electrol>-tically to be 25.927, (theoretical 25.877,); tlie :tninioni:i-copper ratio \vas determined by titration to be 3.92 to I . Measurements of heat cxpacitj- a n d magnetic suscqitihility w r e m a d e in t h e nianner previously tlescril)etl.' For tlic low temperature susceptibility meaSurcniciits, the coil constants ivcrc obtained with the specimen nc:u 7 0 ° K . , :tfter the coil5 1i:td been cooled by liquid Iieliuin or I i ~ ~ d r o g e n . niple a t 70°K., upon tlic coil> i v a i tletere experiment in wliich the spccimen \v;ii cooled from room temperature t o 70'K. T h e xuiccptibilities were thus hasetl upon the room tcmperaturr suiccptihilit>-of the salt. The molar susceptibility a t 290°K. i i I .10 X according to Bliatnage, Lesslieim a n d Klliitiiia.' :in independent check on our specimen by the C;ou!- iiietliotl gave :i result of 1.35 X (For t h e purposc of tile m:rrection this susceptibility is required only to about 211%. 8'
Results The heat capacity of the salt was measured beThe observed measurements tween l ..? and "OK. were corrected for the heat capacity of 19.Xi g. of Pyrex as previously described.j A t the highest temperatures, this represented ','4 of the observed heat capacity. In view of the unusual behavior of the heat capacity between 12 and 20°K.. this region was investigated in three separate sets of experiments; the results of the several sets were consistent. The heat capacity measurements are given J . J F r i t z a n d H . I< Pinch, THISJ O T - K Y A L , 78, 022.I ( 1 H. F. n'alton, "Inorganic Preparations." Prentice--€I I'ork, h-. Y , 1918. I' 79. 17) S S Bhatnage. 13 1,essheim and If. 1.. Khanna. J Ii>c! C/>civ .So 8, 137 (1955). (10) J. F. Niekirk a n d F. R . L. Schoening, ibid, 6 , 227 (1453). (11) We t h a n k Dr. Julian Eisenstein for suggesting this possibility, 112) T. Okamura a n d 3 f . D a t e , Phrs. Reo., 94, 314 ( 1 9 2 ) .
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J. J. FRITZA N D H. L. PINCH
served for the magnetic susceptibility. Detailed theoretical treatment should aid in a more exact description. Above 10°K. the low lying magnetic "transition" contributes only a small fraction of the total heat capacity. The heat capacity between 10 and :!O"K. does not show the steep rise characteristic of heat capacity due t o lattice vibrations. (For example, see ref. 5.) Instead. it is a nearly linear function of the temperature between !I: and 24°K. lIoreover, the values of the molal heat capacity are coiisiderably higher than those giveii by Duyckaerts'" for the analogous salt. CuS04.3H20. Coriiparison of the data for the two salts shows that the difference between the heat capacities reaches a maximum near 20°K. and diminishes a t higher temperatures. Any other reasonable assignment for the lattice heat capacity gives a similar broad maximum in the heat capacity due to other causes. It is possible t h a t the excess heat capacity is due to L: second magnetic "transition." There is no evidence of a sharp rise in the heat capacity such as might be associated with a coiiperative transition. Complete examination of this "transition" will rrquire additional measurements above 20°K. Magnetic Behavior of Copper Tetrammine Sulfate.-The unquestionably large coupling bctween copper ions in the tetrammine sulfate was surprising, particularly in view of the observed crystal structure. I t is possible t h a t this coupling is an accident of the structure, but we think this quite unlikely in view of the rather large distance between copper ioris. I n the tetrariiininc, the smallest distance-between copper ions is 5;:3 A. compared with 5 . i .A; iii CuSOdGH?O, 3.5 -1. in CuC12.2Hr0, and 2.6 A. in C U ( C ~ H ~ O ? ) ~ . HThe ?O. chloride displays large exchange interactions, leading to the previously cited transition to antiferromagnetism a t 4 2 ° K . However, the minimum CuCu distance is much smaller than in the tetrammine, and there are in fac: two other pairs of copper ions within only 5.3 A. of each ion. The exchange interactions in the tetrammine are much larger than would be expected by comparison. 11-e believe t h a t the (comparatively) large exchange interactions are no accident, b u t are probably enhanced greatly by the presence of the ammonia molecules. Ammonia is well known to co(131 G. Duyckaerts, S O L .R o y Sci.
Yol. +i!l
ordinate with copper (and some other iron group ions) much more strongly than does water or the sulfate ion. (Copper acetate is a special case, in that in crystalline copper acetate adjacent copper ions in a pair are a t a distance almost equal to the Cu-Cu separation in metallic copper."' Thus i t is almost certain t h a t t h e very strong exchange interactions in t h e acetate occur because of directly coupling between adjacent copper ions.') T o our knowledge, t h e present case is the first where extensive low teriiperature investigation has been inade of a cupric salt in which the copper was coordinated with species other than water, halide ions or oxygenated anions. The latter species are notoriously poor coordinating agents for copper ion, and it is not surprising t h a t the introduction of stronger codrdinating agents might enhance interactions between the cupric ions. Conclusions The heat capacity and magnetic susceptibility o f copper tetrammine sulfate below 20°K. indicate t h a t there is strong interaction between cupric ions. This coupling may be simply of the exchange variety, or may occur as a result of a low temperature crystal transition. I t will be highly desirable to carry out both X-ray and microwave investigations on this salt below 90°K. There is evidence of transition (not first order) in the salt starting about 10°K. 1L-e propose to investigate this transition further by other means, and plan for the future a complete Third Law study of the salt. I n addition we strongly recoimneiid investigations of copper salts containing strong coordinating and chelating agents. One such investigation is already in progress. Acknowledgments. -15-e wish to thank Dr. S. Seki and N r . R. G. Taylor for assistance with the experimental measurements, and Nt-. L. F. Shultr, for aid in production of refrigerants. 1Ve thank E. I. du Pont de Nemours and Company and the Office of Naval Research for financial aid in the early stages of the experiments, and t h e Sational Science Foundation for aid during their completion. L5-e gratefully acknowledge the provision of funds for the iron-free solenoid magnet by the Research Corporation and the IVestinghouse Electric Conipany. LTXIVERSITY P A R K , P A .
