L. V. GREGOR A N D J. J. FRITZ
2832
VOl. 83
These attempts were successful. Figure 3 shows plots of cell dinlensions versus composition for this series. The cell size for the end product of the Ba&rTazOp conversion corresponded to the composition (Ba.&3r .,b)bTa4.Opls. No theoretical significance is attached t o the straight lines drawn through the points. A smooth curve could have been used to indicate a continuous change in cell dimensions.
Conclusion 730
E 8C
I
I
1
I
c
0
cz
Fig. 3.-Cell
O?
04
0 5
05
; 7
C B
c9
3
dimensions versus composition for ( B ~ zSr,)TarO15. L
those of pure BajTa401j. Since Sr was present, it was assunied that the product was (Bal - x Srx)jTarO15. T o test this possibility Sr5Ta4Olswas prepared, and attempts were made to obtain solid solutions of BajTaaO15 and Sr5Ta401s.
[CONTRIBUTION S o , 114 FROM
Of the various divalent cations substituted in the octahedrally coordinated positions, all except SrII, the largest ion of the series, substituted for tantalum in a random way. This result is consistent with the general observation that large size difference, as well as large charge difference, leads to ordered arrangements. In the perovskitelike compounds B a 2 ~ I r 1 ~ v V 1ordering 06, was observed regular1y.l In the Ba&PTaz09 series, on the/ 'other hand, the smaller charge difference (+2 DS. +5) leads to ordering only when combined with the large size of SrII. Acknowledgments.-n'e are grateful for assistance provided by the Sational Science Foundation, Grant ;?;SF-G6214. We also wish to thank Professor Roland brard for helpful discussions. (4) E. J. Fresia, Leiiis Katz and Roland Ward, J . A m . Ckem. S o c , 81, 4783 (1959).
THE CRYOGENIC LABORATORY, PENNSYLVANIA STATEUNIVERSITY, UNIVERSITY P A R K , P E S N S Y L V A S I A ]
Magnetic and Thermodynamic Properties of Potassium Ferricyanide at Low Temperatures Ru L. IT. GREGOR'A N D J. J. FRITZ RECEIVEDJAXUARY 9, 1961 The heat capacity and magnetic susceptibility of potassium ferricyanide have been measured froin 0.6" to B O O K . There is a small peak in the heat capacity below 2'K. whose maximum is a t 1.2', and xmT decreases sharply below 2". The susceptihility obeys a Curie-Weiss law between 20" and 2'K. The entropy removed by isothermal magnetization to 7040 oersteds at 1.245'K. was 0.122 e.u. Adiabatic demagnetization from this point lowered the temperature t o 0.52'K.
Introduction This paper reports the latest in a series of investigations by this Laboratory of the magnetic and thermodynamic properties of compounds involving the first transition series (iron group) elements. Previous have reported a variety of effects due to interactions of the magnetic ions with surrounding ions and molecules. These interactions usually result in deviations of the magnetic susceptibility from Curie or Curie-JVeiss law behavior and in heat capacity anomalies at low temperatures. In potassium ferricyanide, the Fe(CN)6-3 ion is an octahedral complex. Two of the five 3d orbitals of the ferric ion are used in the dy2sp3 bonding orbitals, leaving three de orbitals for the fi\-e remaining electrons. Two electrons each are paired in two de orbitals, leaving a single unpaired (1) Department of Chemistry, University of California, Berkeley 4, California. ( 2 ) (a) J. J. Fritz and H. L. Pinch, J . A J X .Clieiiz. S o c . , 7 8 , A228 (1956); (b) i b i d . , 79, 3644 (1957). (3) J. J . Fritz, R. V. G. Ran and S. Seki, J . P l i y ~ .chC!JL, 62, i0:i (1958). (4) J. J. Fritz and R. G. Taylor, J . A m Chcin. Soc., 8 0 , 148% (1958).
electron. The magnetic susceptibility accordingly is much closer to that corresponding to a single unpaired electron than to that of the normal 6S~/, ground state of Fef3. Although the crystal structure of K3Fe(CKj6, space group c2h5-P21,'C, is monoclinic, the angle between the c-axis and the plane of the a- and b-axis is 90°6', so that the structure is pseudo-orthorhombic.5 Crystal structure data indicate that there are two non-equivalent ferricyanide ions per unit cell6 The principal susceptibilities of K3Fe(CN)6were measured by Jackson7 from 1 4 O to 290OK. T h e susceptibility is distinctly anisotropic, and for the powdered salt is twice the amount expected for a single unpaired electron. At 14OK. the powder susceptibility is close to the value for a single unpaired electron The anisotropy observed by Jackson was explained by Howard5 in terms of a rhombic, almost axial, crystal field acting on the de orbitals. Howard's treatment succeeded in re( 5 ) Z ICrisl. Slriaktzwberichf, 2, 495 (193i). (15) 1'. Barkator a n d H. Zhdanov. .4rln Phgc Chim. ( U . S . S . R . ) , 16, 4:; ( l < l 4 2 ) ; V Barkatov, Lbiii., 46, 43 ( 1 9 4 2 ) . ( 7 ) I , . C . i a c k m n , Pvoc. R o y . .So( (Londoir), 8 1 4 0 , 6Y5 (1!)83): I ' i O , , Phyr . > C ' l . ( T d > , L , l o n j , 5 0 , i o ; (1938). ( 8 ) J. 13. Iioward, J . Chciir P h y s , 3, 814 (1985).
July 5 , 1961
MAGNETIC AXD THERMODYNAMIC PROPERTIES OF POTASSIUM FERRICYANIDE 2833
producing the observed anisotropy but gave only moderate agreement with the observed susceptibilities. More recently, McKim and Wolf9 have extended the measurements of the principal susceptibilities down to 1O K . and in addition Baker, Bleaney and BowerslO have measured the microwave absorption of both K3Fe(CN)c and a specimen in which the salt was diluted with the diamagnetic isomorph KsCo(CN)6. They found i t possible to explain the results obtained with the diluted salt using the theoretical treatment of Bleaney and O'Brien" which combined a crystal field of general symmetry with the addition of a factor to account for A binding between the iron atom and the (CN) ligands. Only general agreement was obtained with the rather diffuse results for the concentrated salt. Agreement with susceptibility data was again only fair. Baker, Bleaney and suggest that there is probably considerable exchange between neighboring ferricyanide groups. The heat capacity of K3Fe(CN)Ehas been measured from 15' to 298' by Stephenson and Morrow, n who found a broad shallow anomaly between 50' and 160'K. This anomaly is by no means large enough to account for the R In 2 entropy associated with the doubly-degenerate ground state due to the spin of the single unpaired electron and may not even be associated with magnetic phenomena. The bonding of nitrogen directly to the metal ion appears to enhance and complicate magnetic interactions in transition-metal complexes; we have observed this quite strikingly in Cu(I1) corn pound^.^^^ I n view of the rather ambiguous state of affairs with regard to K3Fe(CN)R, we have taken advantage of the opportunity to increase the available evidence by a detailed examination of the heat capacity and magnetic behavior of this salt a t low temperatures.
next as those previously described.2 Several calibration points were obtained during each set of measurements; the factors necessary for correction of observed resistances to the standard scale were determined by comparison of resistances at these calibration points. The effect of the magnetic field H on the thermometer resistance R, determined at 4', 20" and TiOK., fitted the equation
This equation was used for interpretation of resistances measured in a magnetic field. The initial molar magnetic susceptibility X Q was measured with the fluxmeter described previously.2,4 T h e average susceptibility of K3Fe(CS)s a t 77"K.? was used to obtain the fluxmeter coil constant ( L e . , turn-area of sample coils in the absence of sample, corrected for diamagnetism). Susceptibility measurements in magnetic fields (and also in zero field) were made a t 400 cycles/sec. with the modified General Radio Co. inductance bridge as b e f ~ r e . ~These measurements were corrected for the effect of the magnetic field of the measuring coils and for the slow drift with time of the geometric inductance of the coils.4 Magnetic fields up to 7050 oersteds w-ere obtained with the iron-free solenoid magnet previously described.' Heat capacity measurements were made in the manner previously described.' Above 12"K., the change of temperature with time after heating were non-linear. This was attributed t o imperfect heat transfer between the superheated thermometer and the sample during this period. Correction was made for these non-linear temperature drifts by a method similar to t h a t developed by Keesom and Kok and modified by Aston and Szasz." The details of the treatment are given elsewhere.'j
Results Heat capacities were measured from 0.6' to 21'K. in two overlapping series. The data are given in Table I. Figure 1 illustrates the data I
0.15
I
I
I
I
i
9
0.10
3
Experimental Reagent grade &Fe( CS)6 mas recrystallized and dried in accordance with procedures for primary standard reagents. As a check, a n iodine-thiosulfate titration showed t h a t the percentage of Fe+3 in the sample was within 0.05y0 of the theoretical value. The sample was ground to a fine powder and packed tightly into a n ellipsoidal Pyrex bulb 31.13 ~ m in. volume. ~ The sample weighed 34.01 g. (0.1033 gram ion Fe+3). The sample was evacuated t o 10- mm. for 11 hr. a t room temperature to remove water and adsorbed gases. The bulb was filled with a dry heliumnitrogen mixture to a pressure of 1 atmosphere and sealed off. The amount of glass in thermal equilibrium with the sample was 16.21 g. A carbon thermometer-heater'3 was placed around the outside of the sample container as described previously.2 The thermometer was initially calibrated a t 10 points in the hydrogen and helium range; the resistance was a smooth but non-linear function of log T . These data were used to construct a standard temperature scale with smooth first and second derivatives. The thermometer was more sensitive b u t not as stable from one set of measurements to the _ _ ~ . h y s . Soc. (I.ondun), 69B, (9) F. R. McKim and W P. Wolf. P Y O C P 1231 (19%). (IO) J . A I . Baker, 13. Bleaney and J D. Bowers, ibid., 69B, 1205 (1956). (11) B. Rieaney and hI. C. hr. O'Hrien, ibid, 69B, 1216 (1Y.X). (12) C. C. Stephenson and J. C Morrow, J . A n t . Chern. Soc., 7 8 , 270 (195G). (13) \V. F. Giauque, J . \V. S t o u t and C W.Clark, i b i d , 6 0 , 10>3 (1938).
0.05
0
0
1
Fig. 1.-Heat
3 4 5 Temp., OK. capacity of KsFe(CN)a below 5'K. 2
below 5'K. The uncertainty in an individual measurement below 12' is estimated a t 1%, but the data follow a smooth curve much better than this. Above 12', the maximum deviation from the smooth curve is 3%/'clprobably because of the corrections for non-linear afterdrifts and for the heat capacity of the Pyrex glass in contact with sample. The latter correction was made by using the data of Giauque and Lyon for the heat capacity of Pyrex.I6 The probable error in this range is (14) W. H. Keesom and J. Kok, Comm. P h y s . Lab., Uniu. of Leyden, No 219: (1932); J. G. Aston and G. J. Szasz, J . Chem. P h y s . , 16, 560 (1Y-17) (15) L. V. Gregor, M.S. Thesis, Pennsylvania S t a t e University.
January, 19.59. , 1047 (16) D . iY.Lyon and W.F. Giauque, J . A m . C h e m S O L . 71 (1949).
L. V. GREGORAXD J. J. FRITZ
2834
Vol. s3 TABLE I
x
HEATCAPACITY OF
8-
Temp.,
0.3
OK.
0.1
2 0
Fig. 2.--Molar
2 3 4 Temp., OK. magnetic susceptibility of KzFe(CK)B below 5OK. 1
estimated as 3%. Above 15', the data agree well with those of previous investigations.' The value a t 1.05'K. is about 50% larger than that estimated by McKim and Wolf . g I n the low-temperature region, there is a small but definite peak in the heat capacity curve with its maximum a t 1.15'. There are indications of a possible second peak at or below 0.6'K. The lattice heat capacity is given by the equation
0.67 0.84 1.06 1.283 1.614 1.628 1.905 1.942 2.204 2.213 2.535 2.844 3,074 3.137 3.570
CP >
cal./mole deg.
0.134 .088 .116
.127 ,071 ,072 ,051 ,050
,046 .043 ,043 ,045 ,048
,053 ,050
POTASSIUM
Temp., OK.
3.910 4.245 4.544 4.92 5.36 5.88
6.28 6.iO 7.24 7.89 8.61 9.48 10.40 11 41 11.84
FERRICYAKIDE
CP
CP
cal./mole deg.
Ttmp., K.
cal./mole deg.
0.056
12.51 12.65 13.46 14.23 11.27 15.02 15.38 15.78 16.06 16.37 17.04 17.09 18.15
1.63 1.77 1.85 2.27 2.14 2.5 2.7 2.9 2.7 2.9 3.3 3.2 3.5
18 40 19.90
1i
.066 ,079 .lo2 ,134 ,185 ,238 ,283 ,374 ,443 ,506 ,704 ,870 1.11 1.35
3 8
whether there was any dependence of susceptibility on frequency in the range of temperature studied. Preliminary evidence on this point was given by the fact that intercomparison of bridge and fluxmeter readings a t twelve points in the helium range were all consistent within the accuracy of the bridge observations. A more rigorous test of the lack of dependence of susceptibility on frequency c1 = CUT3 (2) (up to 400 cycles) was obtained in connection where the "constant" a rises linearly with decreas- with the magnetizations themselves. During these ing temperature from 5.9 x a t 20' to 9.0 experiments a 400-cycle field of about 10 gauss X at ' 8 and then levels off. When the lat- was imposed on the specimen itself for 20 to 30 tice heat capacity calculated from the equation minutes. In no case was there any detectable above is subtracted from the total, the remaining absorption of energy due to the alternating mag"magnetic" heat capacity above 1.2' is given by netic field, indicating that the imaginary component the expression (x") of the susceptibility a t 400 cycles must be less than 0.1% of the real component (x') and (3) thus that the susceptibility observed a t 400 cycles must be the same as that a t zero frequency. I n Fig. 1, and CI are shown as broken lines. The intensity of magnetization, I, in the iiizgThe magnetic susceptibility in zero magnetic field was measured with the fluxmeter a t 31 points netic field a t temperatures above 1.2.5'K. was between 0.52 and 20.2'K. The precision of the calculated by integration of the data of the four observations was about 0.001 unit in the molar adiabatic magnetizations. The values obtained magnetic susceptibility (xm); the accuracy is from 11 points fitted the equation estimated as 1% for susceptibilities greater than I = 0.346 - - 8.0 X 0.1. Above 4'K. the data can be represented within experimental precision by a Curie-Weiss for values of H / T from 500 to 5,000 oersteds' Law degree. This equation for I then was used to 0.368 obtain entropies of magnetization a t 1.25'K. which X m = ___ (4) T + 0.14 were then used, in conjunction with the heat Below 2', xmT falls off rapidly, and the suscepti- capacity measurements and isentropic magnetizations starting below 1.25'K., to calculate thermobility approaches a maximum a t about 0.7'K. The behavior below 5' is shown in Fig. 2 The dynamic temperatures by standard n ~ e t h o d s . ~ actual points are shown for the susceptibility, Because of the limited amount of data. the enxm; a smoothed curve is drawn in for the product tropy change accompanying each of the heat capacity determinations was divided into the corX m T. The estimation of the thermodynamic tempera- responding enthalpy change to obtain the average tures below 1'K. required knowledge of the in- temperature, which was then associated with the tensity of magnetization I as a function of H and average resistance of the thermometer. Three T , obtained by use of the adiabatic susceptibility points were obtained in this fashion, making pos( b I / b H ) s measured along the isentropes obtained sible a smooth extension of the resistance tempcrain the several adiabatic magnetizations. These ture scale to 0 3'K. The lowest temperature susceptibilities were obtained with the inductance actually attained, by demagnetization froin 7050 bridge a t 400 cycles. Before employing them in oersteds a t 1.248'. was 0.52'R.; the entropy further calculation, it was necessary to determine removed in the magnetization was 0 122 e.u.,
(3
(:)*
July 5, 1961
DIALKYLAMINO DERIVATIVES OF
.4LUMINUM
2835
about 9% of the entropy expected for a doublydegenerate ground state.
observed in the 400 cycle susceptibility indicates that any relaxation time present is substantially below a millisecond even below 1°K. This is in Discussion marked contrast to the behavior of such salts The heat capacity anomaly and the correspond- as the paramagnetic alums, whose relaxation ing decrease in xmT below 2’K. suggest that a co- times are typically above 0.01 second a t liquid operative or “exchange” interaction between the helium temperatures. l7 The existence of comagnetic ions becomes important a t these tem- operative interactions might indeed be expected peratures. Such an interaction had previously been to reduce the relaxation times. Our susceptibilities above 4’K. agree, within deduced from the paramagnetic resonance spectrum of KaFe(CN)s a t low temperatures.” The heat experimental error, with the equation given by capacity anomaly is much too small to account McKim and Wolfg for their experimental data on for the R In 2 magnetic entropy of the spin doublet potassium ferricyanide. Below 3’K. our results ground state a t high temperatures. The high are from 3 to 7% higher than theirs. The difheat capacity observed by Stephenson and Morrow ference is reflected mainly in a less rapid drop in between 50’ and 160’K. was attributed to mag- XmT for our measurements. (McKim and Wolf netic interactions, who estimated the entropy as- use an equation of different form from ours, but sociated with this anomaly as 0.09 & 0.02 e.u.12 with a correction term corresponding approximately It does not appear that their anomaly is due to to a Weiss A of 0.200, compared with our 0.140.) removal of the spin degeneracy, since even below The differences between our results and theirs 20’K. K3Fe(CN), still possesses a magnetic are of the same order of magnitude as the differences susceptibility nearly the same as that expected they observed between the susceptibility of the for a single unpaired electron. The “high-tem- powdered specimen and values calculated for the perature” anomaly may be associated with a powder from the principal susceptibilities of “quenching” of the orbital magnetic moment, crystalline specimens. McKim and Wolfg estimated the magnetic which contributes significantly to the susceptibility a t higher temperatures and may be appre- heat capacity in zero field as 0.08/T2(*20%) on ciable even a t 20°, according to the calculations of the basis of a measurement of the adiabatic suscepHoward.8 IVhatever the cause, i t is evident that tibility a t 1,000 gauss, 1.05’K. The heat capacity even the sum of the entropy decreases associated a t 1.05’K. is in fact slightly over 0.11 (their with the two regions of high heat capacity is estimate 0.07). However, this temperature is insufficient to account for all the magnetic entropy. below the maximum in the heat capacity, and the I t seems likely that there is a peak in the heat actual magnetic heat capacity in the 1/T2 region capacity below 0.6’, the beginning of which may is more than twice their estimate. Acknowledgments.-We wish to thank Profesbe the observed rise in the heat capacity curve a t sor J. G. Aston for helpful discussions, Mr. L. 0.75’. Both the heat capacity and the magnetic suscep- Sfiultz for production of the refrigerants used in tibility indicate that there are pronounced inter- this investigation and PvLr. R. G. Taylor for prepaactions between the magnetic ion and its surround- ration of the specimen. We are grateful to the ings. In the first place, the observed structure Research Corporation and Westinghouse Electric of the heat capacity curve and the hint of addi- Corporation for provision of funds for construction tional structure below 0.7’K. indicate the pres- of the iron-free solenoid magnet. ence of fairly complicated cooperative effects. (17) C. J. Gorter, “Paramagnetic Relaxation,” Elsevier Publishmg Second, the fact that no dispersion effects were C o ,New York, N. Y., 1947, pp. 77 ff.
[CONTRIBUTION FROM THE
ROHM& HAASCOMPANY, REDSTONE ARSENALRESEARCH DIVISION,HUNTSVILLE, ALABAMA]
The Preparation and Reactions of Dialkylamino Derivatives of Aluminum BY JOHN K . RUFF RECEIVED DECEMBER 19, 1960 Two different types of dialkylamino derivatives of aluminum, (R&),AlX and (R*N)BAl, were prepared. Apparel] t molecular weights of the products were determined and their structures discussed. Exchange reactions with boron trichloride w-ere interpreted in terms of reactivity of the different types of dialkylamino groups.
Dialkylamino derivatives of aluminum have been discussed only brieffy in the literature. Davidson and Brown’ first reported the preparation of dimethylamino dimethyl aluminum from dimethylamine and trimethyl aluminum. I n 1955 trisdimethylamino alane, bis-dimethylamino alane and dimethylamino a1ane2 were reported by (1) N. Davidson and H. C. Brown, J . Am. Chcm. SOL., 64, 316 (1942).
Wiberg. a Recently several other aluminum derivatives containing one dimethylamino group were ~repared.~ This paper presents further results on the preparation and reactions of aluminum derivatives containing two or more dialkylamino groups. (2) posed (3) (4)
The nomenclature used in this paper is an extension of t h a t propreviously; i b i d . , 811, 2141 (1960). E. Wiberg and A. M a y , 2 . Nolurforschg., lab, 234 (19551. J. K. RUB, J . A m . Chcm. SOL.,83, 1798 (1961).
