1303
Standards for Magnetic Measurements
Standards for Magnetic Measurements. A Comparison and a Proposal for the Use of Tetramethylethylenediammonium Tetrachlorocuprate(I1) . David B. Brown,la Van H. Crawford,lb James W. Hall,lb and William E. Hatfield*lb Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 275 14 and Department of Chemistry, University of Vermont, Burlington. Vermont 0540 1 (Received January 14, 1976)
The magnetic properties of three compounds which are appropriate as magnetic susceptibilitystandards have been studied in detail. A new value of -1.86 f 0.01 K has been determined for the Weiss constant of H~CO(NCS)~, and the zero-field splitting parameter, D, has been found to be +19 f 3 cm-’ with g being 2.22 f 0.03. The magnetic data for bis(2,2’-bipyridine)thioureacopper(II) perchlorate may be reproduced accurately from the expression M = NgPJBJ(X)where B J ( X )= [(2J + 1)/2Jl coth ([(2J + 1)/2JlXJ- (1/25) coth (X/2J) and X = gPJH/kT using g = 2.1175, the average of the single crystal EPR g values. The magnetic data for [(CH3)zNHCH2CHzNH(CH3)2]CuC14 may also be reproduced by these expressions using g = 2.15. This latter compouqd is suggested to be a convenient magnetic susceptibility standard.
Introduction Magnetic susceptibilities are measured over wide ranges of temperature and magnetic fields, and these ranges have been expanding as technology has progressed. Precise measurements, which are necessary for adequate characterization of materials, require standards that have Well-known values under the conditions that they are used. A number of materials have been used as magnetic susceptibility standards, including water, mercury, Cu(S04).5Hz0,N i ( e r ~ ) ~ ( S ~nickel, o ~ ) , and H ~ C O ( N C S )This ~. last complex has enjoyed wide popularity in recent years with systems designed for the study of the susceptibilities of transition metal c o m p l e x e ~ . ~ ’ ~ In view of the 4% d i s c r e p a n ~ yin~the ~ ~ reported values of the magnetic susceptibility of HgCo(NCS), based on the Curie-Weiss law fits, it is clear that additional measurements are required. This compound as well as the previously reported bis (2,2’-bipyridine)thioureacopper (11) perchlorate, [ C ~ ( b p y ) ~ ( t [C1041z, u)] were further investigated a t low temperatures and high magnetic fields and the results of those studies are reported herein. There are disadvantages to the use of both of these materials. We have found that tetramethylethylenediammonium tetrachlorocuprate satisfies all the requirements of a general susceptibility standard, and we report its preparation and properties here.
temperature range 1.5-100 K, Princeton Applied Research Models 155/150A and 155/151 vibrating sample magnetometers (VSM) were used. Each VSM was calibrated with a sphere of very pure nickel metal.5 Temperatures were measured using a Ga/As diode driven by a 100-yA constant current source; the voltage across the diode junction was monitored with a Dana Model 4700 digital multimeter. The working diode, located directly above the sample on the drive rod,6 was calibrated vs. a calibrated diode supplied by Lake Shore Cryotronics, Inc. A continuous relationship between voltage and temperature was generated using a Chebyshev polynomial. The accuracy of the temperatures which can be measured with the diode in the field independent range is estimated to be better than 0.1 % .
Results and Discussion There are several factors which must be considered in the selection of a standard for susceptibility measurements. The material must be easily prepared (or obtained) in pure form, should be stable for long periods of time in all environments to which it will be subjected, and should have well-known magnetic properties as a function of temperature and magnetic fields. There may be other requirements which are dictated by the experimental method to be employed, and these requirements may limit the choice of a standard. Regarding the common calibrants Experimental Section for magnetic susceptibility, the two liquids, mercury and The compounds HgCo(NCS)? and C ~ ( b p y ) ~ t u ( C l O ~ )water, ~ ~ are diamagnetic with quite small values3of x and, were prepared by literature methods. consequently, they are only useful for work witE dia[ (CHB)zNHCH2CH2NH(CH3)2]CuC/4. Four grams of magnetic materials. Hydrated salts, such as Cu(SO4)*5H20, tetramethylethylenediaminewere neutralized and acidified suffer the disadvantage of losing waters of hydration when by the careful addition of 12.0 mL of concentrated HCl. finely ground or exposed to vacuum. The resulting solution was added to a solution containing Although impractical for use with the Guoy or Faraday 12.0 g of CuC12.2H20in 15 mL of 3 M HCl. After stirring balance, pure nickel metal is quite satisfactory for califor 0.5 h, this solution was added slowly to 300 mL of bration purposes when the required saturation field is acetone with vigorous stirring. The resulting yellow easily obtained, and this material has the advantage that precipitate was filtered, washed extensively with acetone, the susceptibility is not highly temperature dependent. and dried under vacuum: yield, 8.4 g. This latter property is a disadvantage for use in deterAnal. Calcd for C6N2Hl8CuCl4:C, 22.27; H, 5.61; Cu, mining temperatures from susceptibilities. 19.64. Found (two independent preparations): C, 22.23, H ~ C O ( N C S )The ~ . compound HgCo(NCS), has been 22.33; H, 5.61,5.41; Cu, 19.09,19.66. Analyses for carbon used frequently as a magnetic susceptibility standard for and hydrogen were performed by Integral Microanalytical Gouy and Faraday techniques because of its properties of Laboratories, Inc., Raleigh, N.C. being easily prepared in pure form, stable, and convenient Magnetic susceptibilities were measured a t room to handle under the experimental conditions of these temperature by the Faraday method using a Cahn RG-100 techniques, e.g., it easily packs in Gouy tubes. The results electrobalance and a Varian V4004 magnet. In the of Figgis and Nyholm2had been accepted for many years, The Journal of Physical Chemistry, Vol. 81, No. 13, 1977
Hatfield et al.
1304 22
t 'x
1
I B=-1.86 "K
0
1
Hg C o ( N C S l 4
t
1
15000
A
1
,
,
,
,
,
,
,
,
0
,
II
Temperature
12
0
50
H I T
( O K )
Flgure 1. Inverse mohr magnetic susceptibilii data (0)for HgCo(NCS).+ Curie-Weiss law line calculated with C = 2.351 and 0 = -1.86 K.
but recently Rade3remeasured the magnetic properties of H ~ C O ( N C Sand ) ~ obtained different results. The 4% difference between the values obtained by these workers is significantly greater than the precision which can be obtained in the magnetic susceptibility measurement using modern equipment. Thus, it was of real importance to reexamine the properties of this compound and to obtain reliable values for this commonly used standard. The magnetic susceptibilities of five independently prepared samples were measured at room temperature on the Faraday balance. The ratios of sample weights to changes in weight due to the magnetic field gradient have the same ratio with an acceptable 1% experimental error. The data indicate that the results are reproducible for different preparations. The fact that one sample was over 8 years old, while three samples were prepared recently, certainly indicates good shelf-life stability. Data were collected from 1.7 to 50 K on two samples, and the results are plotted as inverse susceptibility vs. temperature in Figure 1. Using the Curie-Weiss law in molar units, XM = C/(T- e), the best fit line is also shown in the figure, where the parameters are C = 2.351 f 0.002 and 0 = -1.86 f 0.01 K. It is important to note that these values are obtained using no corrections to the data to account for diamagnetism or temperature independent paramagnetism of the compound; therefore, they would be the appropriate values for calibrating an experimental magnetic susceptibility apparatus. I t is interesting to compare the results obtained here with those previously reported. For the am susceptibility at 20 "C, we calculate xp = 16.20 X 10-i?cgsu. Figgis and cgsu for N y h 0 1 m ~and ~ ~ Rade3 both obtained 16.44 X xg at 20 "C, but Figgis and Nyholm7 specify using a diamagnetic correction of 137 X lo4 cgsu/mol. When this quantity is added to our value, the agreement is excellent. Rade does not specify the details of the calculation so this comparison cannot be made with his data. It is the low temperature region of the Curie-Weiss line which frequently attracts the most attention, and here we find some discrepancies, the three 0 values being +2,3 and -1.86 K (this work). The -10 K value was determined from an extrapolation of a data set collected only down to 80 K, and the +2 K value, although obtained from a data set in the region 5.8-293 K, was derived from a measurement using standards which have very small values for magnetic susceptibility. Also, the calibration apparently was carried out at a temperature almost 300 K away from the temperature axis intercept of the Curie-Weiss plot. In order to more thoroughly investigate the magnetic properties of HgCo(NCS)4,particularly in hopes of finding a convenient material to correlate high magnetic fields and The Journal of Physical Chemistry, Vol. 8 1 , No. 13, 1977
( KGauss
/ O K )
Flgure 2. Magnetization data (0)at 4.2 K for HgCo(NCS),. Broken line calculated from eq 1 with J = 312 and g = 1.90: solid line calculated from eq 2 with J = 1.23 and g = 2.24.
low temperatures, a magnetization study was undertaken. The data collected at liquid helium temperature (4.2 K), where temperature measurement and control are not field dependent, are shown in Figure 2. The corrections applied to the data are as follows: diamagnetic correction of the atoms -190 X lo4 cgsu/mol, and temperature independent cgsu/mol. These are very paramagnetism of 400 X small numbers compared to the values of susceptibilities at this temperature. The best fit to the data shown as the broken line in Figure 2 was calculated from eq 1,' where J was taken to be 3/2 for Co2+(the spin-only value) and g was found to be 1.90. The appropriate equations are where
2J+ 1 2J+ 1 B j ( X ) = -c o t h ( 7 X ) 25
&
X coth 25
and
X = g@JH/kT The symbols in these equations have their usual meanings. This value of g, being less than g,, is unreasonable for a d7 ion. Another calculation of g, from the Curie constant, eq 2,8 yields g = 2.24. The best fit for this value is shown 112
]
C( 3k/NP2) g = [ S(S 1)
+
in Figure 2 as the solid line. Here J (or S') is 1.23, a value less than 3/2, which is consistent with the finite value for 0 from the Curie-Weiss fit. Since the environment of the cobalt ion is not exactly zero field splitting of the 4A ground state is possible as a second-ordereffect arising from spin-orbit coupling. Figgis et al."?l2discuss zero-field splitting effects, but unfortunately the expressions given in their paper for the magnetic susceptibilityll are incorrect. The correct expressions are
(3Y 12) 3 3 +cosh ( y ) e z - -e-ze-x 2x 2x ez cosh ( y ) + e-ze-x
sinh ( y ) Xl=-
kT
L
1 J
1305
Standards for Magnetic Measurements
1
2 5 ,
5913
I
Hg C o ( N C S ) 4
C
1
0
'X
m N - L 3 A 4 0 1
(cgsu)
9-2.21 D.19 cm-'
i 1
H 4 0 KGauss
0
px
I
01
i
50
0
0
T e m p e r a t u r e (OK)
Flgure 3. Inverse molar magnetic susceptibilii data (0)for HyCo(NCS),. Solid line calculated from eq 3 with g = 2.21, D = 19 cm- , and H = 10000 G. .5 4 2 "K
t
Hg C o ( N C 5 ) 4
0
50 Field
/
(KGauss)
Flgure 4. 4.2 K molar magnetic susceptibility data (0)for HgCo(NCS),. Solid line calculated from eq 3 with D = 19 cm-', dashed line calculated with D = -19 cm-', g = 2.21 for both lines.
where
x = D/kT Y = gPH/kT
(3) The corrected data are plotted in Figure 3 as inverse susceptibility vs. temperature and the solid line was calculated from eq 3 with g = 2.21 and D = 19 cm-'. This g value is slightly smaller than the 2.24 obtained from the Curie-Weiss law and D is quite large. It is not possible to compare the D value obtained here with the 10 cm-' value reported earlier'' because of the problem with the formulas. The magnetic susceptibilities in eq 3 are field dependent and a t the lowest temperatures the susceptibilities are dependent on the sign of D. In Figure 4 the 4.2 K data are plotted as susceptibility vs. field along with the curves calculated for D = f19 cm-l. While the fit for D = +19 cm-' is not perfect, it is clear that the negative sign is not appropriate. The calculated curves are sensitive to small changes in the parameters and the experimental error in the measured data, noticeable at 10 kG where data were obtained on different samples, does not provide the precision in the fit noted in earlier cases. However, the data convincingly show that there is a significant zero field splitting and that the sign of D may be determined from the magnetic susceptibility. For HgCo(NW4,values of
30 rl / T
( A G a u s s 1°K)
Figure 5. Magnetization data for [Cu(bpy)2(tu)](C104)2 at 4.2 (0)and 1.95 K (0).SolM line calculated from eq 1 with J = 1/2 and g = 2.1175.
D = 19 f 3 cm-' and g = 2.22 f 0.03 describe the magnetic properties very well. The negative 8 value of the CurieWeiss fit apparently arises from zero-field splitting, and there is no direct evidence of exchange interactions. The existence of zero-field splitting implies that the temperature dependence of the susceptibility will not follow a Curie-Weiss law exactly, and the different values of the Weiss constant which have been reported are not surprising considering the different temperature intervals which have been examined. [ C ~ ( b p y ) ~ ( [tC1O4I2. u ) ] Long-range magnetic interactions are not desirable characteristics for magnetic susceptibility standards, rather magnetically dilute compounds are preferred, Previous magnetic susceptibility studies on the polycrystalline compound [C~(bpy)~(tu)] (c104)2ruled out the possibility of intermolecular exchange interactions. Also, the EPR data indicated that the value of the exchange constant was -0.002 ~ m - ' . This ~ compound was further investigated at low temperatures and high fields, and the magnetization data are shown in Figure 5 along with the solid line calculated from eq 1 with J = 1/2 and g = 2.1175. This value of g is the average of the principle g values obtained from the single crystal EPR d a h 4 Thus, there is no parameter fitting involved and the agreement is excellent, especially for the 4.2 K data. The 1.95 K data were collected by monitoring the pressure in the sample zone and are subject to a somewhat greater error because of minor temperature fluctuations; nevertheless, the data convincingly follow the predicted behavior and this material is certainly well behaved. It is a good candidate for use in determining temperatures at high fields, via the measurement of magnetic susceptibility. [(CH3),NHCH,CH,NH(CH3)~]C~C14. In the course of our studies of the complexes formed between bifunctional bases and copper halides in acidic media, we prepared the complex tetramethylethylenediammonium tetrachlorocuprate. The observation that it appears to be a perfect Curie paramagnet attracted our attention to this material as a possible calibrant for magnetic measurements. Over the temperature range of 1.7-52 K, yeffis constant at 1.86 p ~ .The data may be fitted to a Curie-Weiss law with 8 = -0.07 K and C = 0.433. The g value obtained from the fit is 2.149. For comparison, the EPR spectrum, which is slightly asymmetric on the high-field side with a peakto-peak width of 125 G, is centered a t g = 2.168. Figure 6 shows the variation of the reciprocal susceptibility and effective magnetic moment for this compound with temperature, and the solid line represents the best Curie-Weiss law fit to this data. For the purpose of these calculations molar ligand diamagnetism of 217 X lo4 cgsu The Journal of Physical Chemistry, Vo/. 81, No. 13, 1977
1306
Hatfield et al.
TABLE I: Recommended Values of Magnetic Parameters
12G
150,
for Magnetic Susceptibility Standards
A
Compound HeCo(N CS), [c u (bpy ) * t;i
-
(ClO,), [(CH,),NHCH,CH,NH(CH,), ] CuC1,
g
J
C
0,K
2.22 2.1175
312 l/2
2.35 0.420
-3..86 0
2.149
1/2
0.433
-0.07
it is clear from these data that the complex is suitable for temperature calibration at high fields using magnetic measurements. Y
0
12
35
2L
"
50
49
-:")
Figure 6. Magnetic data for [(CH3)2NHCH2CH2NH(CH3)z]C~C14; (0) magnetic moments, ( 0 )inverse molar susceptibility; H = 10 000 G.
60
I
i36t
v
t/
0 1' 0
1
6
,
1
12
I
18
,
, 24
I
I 30
He/T ( k G a u s s i O K )
Figure 7. Magnetization data for 4.2 (0)and 1.79 K (0).
[(CH3)2NHCHzCH2NH(CH3)2]CuC14 at
and temperature independent paramagnetism ( N a )of 60 X cgsu were assumed. Several features of this complex suggest it as a good candidate for a susceptibility standard. First, it is easily prepared in nearly quantitative yield and pure form from readily available starting materials, and it exists as a tractable powder. The complex appears to be quite stable, although it may be very slightly hygroscopic, and as a precaution it should be stored in a desiccator following initial drying. Most importantly, the nearly perfect Curie law behavior suggests that the complex contains effectively isolated tetrachlorocuprate ions, with all exchange interactions being negligible. Furthermore, since zero-field splitting is of no concern for monomeric copper(I1) complexes, the field dependence of the magnetization should be readily predictable from the Brillouin function, eq 1. This is supported by the magnetization data at 4.16 and 1.79 K as shown in Figure 7. The solid line gives the calculated curve assuming a g value of 2.15 (the value derived from the Curie-Weiss law fit) and J = 1/2. The data at very high fields and low temperatures are subject to some error, since temperatures could be estimated only by monitoring the helium vapor pressure. Nonetheless,
The Journal of Physical Chemistry, Vol. 81, No. 13, 1977
Conclusions The present data allow the evaluation of these three materials as magnetic susceptibility standards. For the vibrating sample magnetometer, using a superconducting solenoid, calibration is most conveniently done a t 4.2 K, and either of these standards is suitable. The magnitudes of the susceptibility values for [C~(bpy),(tu)](ClO~)~ and [ (CHJ 2NHCH2CH2NH(CH3)2] CuC1, are substantially smaller than those of H ~ C O ( N C Sand ) ~ nickel. For Faraday or Gouy systems at high temperatures where the saturation field of nickel metal may not be easily obtained, HgCo(NCS), is clearly the best choice, since the magnitudes of the susceptibilities for the copper salts at room temperature are quite small. There is an additional disadvantage with [C~(bpy)~(tu)](ClO~),; it is more difficult to prepare and must be obtained as good crystalline material to assure reproducible results. The recommended values for the magnetic parameters of the standards are collected in Table I. Nickel is not satisfactory for determining temperatures owing to the small temperature coefficient, and HgCO(NCS)~ has the disadvantage of requiring more complex expressions to evaluate the magnetic behavior at very low temperature and very high fields. Thus, [ (CH&NHCH2CH2NH(CHJ2]CuC14is the most promising material for this purpose. Rade has recently informed us that he has determined a Weiss constant of -1.8 K using improved e q ~ i p m e n t . ' ~ Acknowledgment. This research was supported by the National Science Foundation under Grant No. MPS7411495 and by the Materials Research Center of the University of North Carolina under Grant No. DMR7203024 from the National Science Foundation.
References and Notes (1) (a) University of Vermont, Burlington, Vt. 05401. (b) University of North Carolina, Chapel Hill, N.C. 27514. (2) B. N. Figgis and R. S. Nyholm, J . Chem. SOC.,4190 (1958). (3) H. St. Rade, J . Phys. Chem., 77, 424 (1973). (4) K. T. McGregor and W.E. Hatfield, J . Chem. SOC.,Dalton Trans., 2448 (1974). (5) Princeton Applied Research: Saturation moment (field) of nickel; 4.2 K, 58.19 emu/g (6 kG); 293 K, 55.01 emu/g (8 KG). (6) R. F. Drake and W.E. Hatfield, Rev. Sci. Instrum., 45, 858 (1974). (7) B. N. Figgis and R. S. Nyholm, J . Chem. Soc., 338 (1959). (8) J. S. Smart, "Effective Field Theories of Magnetism", Saunders, Philadelphia, Pa., 1966, Chapter 1. (9) J. W. Jeffery, Nature(London), 159, 610 (1974). (IO) J. W.Jeffery, Acta Crysta//ogr., Sect. A , 16, 66 (1963). (11) B. N. Figgis, Trans. faraday SOC., 56, 1553 (1960). (12) B. N. Figgis, M. Gerloch, and R. Mason, Proc. R . SOC.London, 279, 210 (1964). (13) H. St. Rade, private communication.
