April, 1962 A 30-cm. length of tungsten wire, 0.005 cm. in diameter, served as the torsion filament. Oscillations were damped out magnetically. The effusion cell was heated by radiation from a surrounding hollow tantalum cylinder which was in turn heated by high frequency induction. By shielding the cell and heating indirectly, a coupling effect2 betmen the cell and the high frequency field was avoided, leading to entirely satisfactory operation. A number of 0.2-cm. diameter holes drilled in the susceptor side. and a larger hole in the removable lid allowed for evacuation of the cell region and escape of vapor molecules. Thermal radiation shielding of the susceptor was provided by several layers of tantalum foil joined in such a way as to present a high resistance to eddy currents. A fused silica tube enclosed the susceptor and cell arrangement and, while measurements were in progress, was evacuated to a pressure of 5 X 10-5 mm., or lower. The bottom of the tube contained a planar optical window protected by a movable shutter. Temperatures were measured with a calibrat,ed disappearing-filament optical pyrometer sighting through the opticd window and a hole in the susceptor bottom into a cylindrical black body cavity in the bottom of the effusion cell. The cavity was 1.0 cm. deep and 0.25 cm. in diameter, Temperatures were corrected for reflection losses a t window and prism surfaces. The remainder of t,he system, the method of determining the torsion constant and the measurement of angular deflection are essentially the same as described by other^.^,^ In this work, the torque angle could be observed directly to within 0.001 radian.
755
NOTICS
ture dependence of pressure, one calculates a second law AHzg8of 89.3 kcal./mole, in good agreement with the third law value. In Table 11, the results of this research are compared with third law heats of sublimation derived from the work of other investigators, in all cases using free energy functions from ref. 9. UncerTABLE I1 COMPARISON OF RESWLTS FOR Au(s) = Au(g) Investigator
Ref.
AHzsa, kcal./mole
P. Harteck L. D. Hall R. K. Edwards E. G, Rauh A. N. Nesmeyanov, el al. R. D. Freeman P. Grieveson, et al.
12 13 14 14
90.7 84.7 rt 0 . 7 87.0 87.2 i0.8 87.3 88.4 88,0-89,3 88.3 rt 0.9
15 16 17
This Research
tainties are included where they have been estimated by the various investigators. All results except those of this research were obtained by the Knudsen effusion technique, although no details concerning the work of Edwards14 or Rauh14 are as yet available. Grieveson, et aZ.,l7used both Knudsen and transpiration techniques; a trend of 1.3 kcal./mole in the derived third law heats over the range of their measurements, however, indicates a temperature dependent error. The ratio of orifice area to surface area employed in the effusion measurements of Harteck12 appears too large to yield equilibrium pressures and the AH298 value obtained from his data is high by about 0.6 kcal./ mole. Except for the unaccountably high pressures obtained by Hall,13 who used a radioactive tracer technique to monitor the effusion transport, the various sets of data are in reasonable agreement and indicate a best value of 88.0 f 1 kcal./mole for the heat of sublimation of gold a t 298OK.
Results The experiment'al vapor pressure data and derived heats of sublimation are given in Table I and are listed in t.he order of measurement'. Data were obtained with t'hree different orifice sizes; the three sets of result's are in good agreement, indicating the observed pressures to be equilibrium values. There is no reason to suspect a reaction of the metal with graphit'e, since it does not form a stable carbide.8 The extent to which the results may be affected by solution of carbon in gold, with subsequent lowering of the activity, is not known with certainty, 'but t'he effect' is believed to be inappreciable. The same gold sample was used throughout the measurement's, with no observed change in pressure with time such as might be (13) P. Hrtrteck, 2. phgsik. Ghem., 1S4, 1 (1928). expected for a gradual solution of carbon in the (13) L. D. Hall, J . A m . Chem. Sac., 73, 757 (1951). met,al in significant' amount,. Experiment'al pres(14) Private communication, quoted by R. Hultgren, et c d "Thersures were within the molecular flow range, as re- niodynainic Properties of Metals and Alloys," Minerals Research quired for application of equation 1. Laboratory, University of California, Berkeley, California, 1960. (16) A . N. Nesmeyanov, L. A. Smakhtin, D. Ya. Choporov and V. I. Free energy .functions used in the third law Zhur. Fiz. Khim., 33, 342 (1959). treatment of the vapor pressure dat,a u-ere taken Lebeder, (16) R. D. Freeman, Oklahoma State University, private communfrom Stull and Sinke.g The assumption of mono- ication. meric vapor implicit in the ca,lculatioiis appears (17) P. Grieveson, G. W. Hooper and C . B. Alcock, "Physical t'o be valid since mass spectra of the vapor obtained Chemistry of Process Metallurgy," Interscience Publishers, h e . , New under both free evaporationlo and Knudsenll con- York, N. Y., 1861, pp. 341-352. ditions indicate less than one mole per cent. polymeric species to be present. The average t.hird EFFECT OF INTENSITY ON THE law value of the heat of sublimation a t 298'K., 88.3 kcal./mole, is assigned an over-all accuracy KADIATION INDUCED DECOMPOSITION uncertainty of i.0.9 kcal./mole, based on an analysis OF INORGANIC NITRATES1 of experiment'al errors. An estimate of a possible BYEVERETTR. JOHNSON error of 15' in temperature meanuremeut' contribDepartment of Chemmtry, Broolchaaen National Laboratory, Upton utes most of the uncertaint,y. From tmhetemperaLong Island, NEW Yorlo ( 8 ) L. Brewer, L. A. Wromley, P. W. Gilies and N. L. Lofgren, Paper
in "The Chemistry and Metallurgy of :\fiscellaneous Materials: Thermodynamics," Nahional Nuclear Energy Series, Vol. 19B, MeGraw-Hill Book Co., New York, N. Y., 1950. (9) D. R. Stull and G . C. Sinke, "Thermodynamic Properties oi the Elements," Advances in Chemistry Series, No. 18, American Chemical Society, Washington, D. C., 1956. (10) J. Drowart and R. E. Honig, J . Phys. Chem., 61, 980 (1987). Chem. I'h?/s., 2 6 , 1276 (1987). 4
Received Augmt 16, 1961
The radiolysis of the solid inorganic nitrates has been shown to be complex. The yields are dependent upon lattice parameters,?S3changes which (1) Research performed in part under the auspices of the U. S. A toniic Energy Commission, Brookhaven National Laboratory, Upton, L. I., N. I,.,anclin part under contract No. BT(30-1) 1824.
SOTEX
756
0
10
Voi. 66
50 60 70 10-20. Fig. 1.-Effect of intensity on decomposition of KNOa. 30
20
40
E.v./g.
x
80
90
110
16(
14C 12c
e
1oc 8C
5 6C
4a 20
a 0
10
20
30
40
50
E.v. = Fig. 2.-Effect
60
70
80
90
100
110
120
10-20/g.
of intensity on decomposition of CsNOs.
occur in the lattice during irradiation,$-5and possibly on the stability of the primary products formed, vix., the nitrite ion. One of the factors which has not been clarified in these studies is the effect of intensity on the nitrite yield. It is the intent of this communication to clarify this aspect (2) (a) J. Cunningham and H. G. Heel, Trans. Faraday SOL, 64, 1355 (1968); (b) J. Cunningham, J . Phys. Chem.. 66, 628 (1961). (3) G. Hennig, R. Lees, and M. 9. Matheron, J . Chem. Phvs., 21, 664 (1953). (4) J. Forten and E. R. Johnson, J . Phys. and Chem. Solids, 15, 218 (1960). (5) E. R. Johnson and J. Forten, Discussions Faraday Soc., 31, 238 (9161).
of the decomposition of the inorganic nitrates by ionizing radiation. Experimental One and one-half M e V . electrons from a Van de Graaf accelerator were used m the radiation source. The total current passing through the sample was determined by a current integrator designed by the Electronics Department of the Broolihaven National Laboratory.6 Nitrite ion was determined by Shinn’s method.7 The molar extraction a t 546 mp was 53,200. To ensure uniform analyses, the entire sample wm dissolved and an aliquot taken for nitrite ion analysis.
-_____
(6) R.Shuler and A. 0. Allen, J . Chem. Phys.. 24,56 11956). (7) M.R. Shinn, I n d . Ens. Chem., Anal. E d . , 13, 33 (1941).
NOTES
April, 1962
lo'*
501 A :=
e
1019
3
I O 20 = IO2' /CORRECTED FOR TEMP. 1 INCREASE
0 =
o
A
il
The radiation cells were flat cylinders of Pyrex glass, 14 mm. i.d. and an average of 2.7 mm. inside length in the direction of the beam. This is about half the range of the electrons. The front face of the cell was about 5 to 10 mils thick. There was a single entrance to the cell from the. edge, 3 mm. i.d. and about 1 cm. long for sample filling, which was left open during the irradiation. During the irradiation, the cell was placed behind a grounded 0.25-in. ailuminum plate, 3 in. in diameter with a 15-mm. hole directly in front of the cell. Only that portion of the electron beam which penetrated the radiation cell was collected. Behind the cell, a 0.25411. thick insulated aluminum plate collected the current that passed through the sample. A 2 mil platinum wire attached to the back plate was inserted B few millimeters into the neck of the cell to prevent the accumulation of any static charge in the sample. During irradiation all samples were cooled by compressed air.
Results The dose rate for the nitrates varied from to lozoe.v./g./sec. The lower value was obtained from y-ray results reported in a previous communication.4 Dosimetry for the electron beam experiments was determined by filling the individual radiation cells with ferrous sulfate dosimetric solution and making a direct determination of the dose. The standard deviation in determining dose was AS%. This deviation was due primarily to the fact that exposures f o r the dosimetry experiments were of the order of 0-2 see., and the mechanism for cutting off the beam involved an uncertain amount of human error. The value used for dosimetry calculations was 15.45 molecules of Fe++ oxidized per 100 e.v.6 The results are summarized in Fig. 1-4. KNOB.-The decomposition of potassium nitrate by ionizing radiation, as with most of the nitrates, yields nitrite ion and 0xygen.~8,3~kS,9 NOa-
-+
NO,-
+ '/~OZ
I n the case of KNOB a distinct break in the nitrite yield us. dose curve is f0und.2~~4The G-value before the break (initial slope) was found (8) C. J. Hochanadel and T. W. Davis, -7. Chem. Phys., S7, 333 (1957). (9) A. 0. Albnand J. A. Ghormley, {bid., in, 205 (1947).
10 20 30 40 E.v. X 10-2o/g. Fig. 4.-Effect of intensity on decomposition of Pb(NO&. Dose rates e.v./g./sec.: = 1018; 0 = lozo.
0
to be 1.48 i 0.21 and after the break 0.96 i 0.06. These may be compared with the values of 1.3g4, 1.4562b f 0.011, and l.5@ found for the initial slope, and 0.934 and 0.902 f 0.029 for that portion of the curve after the break using Co60 y-r ay s, During the radiolysis of both KNOa and NaKOs it was found that anomalously high yields of nitrite were obtained during the high intensity experiments (IOzo e.v./g./sec). This could be shown to be due to poor current collection efficiency, which gave a low value of the absorbed dose and consequently a high G-value. The poor collection current was maGifested by a sloiv but steady
NOTES
758
decrease in the collected electron beam current during an exposure. It was found that cutting off the beam for a few minutes when the current had decreased 1-2% remedied the situation. Apparently both KN03 and S a x 0 3 stored a large amount of charge; by cutting off the beam for a few minutes, this static charge could bleed off through the platinum wire in contact with the sample. At low intensities the rate at which the static charge bled off was about equal to or less than the rate a t which charge could build up. This effect, however, was not observed in either Csr\;03or Pb(KO&. CsNO,.-The resuIts on intensity effects are shown graphically in Fig. 2. The G-value for the initial slope was found to be 1.53 f 0.08 for the intensity range 1018-1020e.v./g./sec. This may be compared with the values of 1.44,41.68 f 0.05,8 and 1.72 f 0.lZbobtained by y-radiolysis. In the high dose region there is no basis for comparison with ?-rays from previous work. NaN03.-The results obtained for the decomposition of NaN03 are shown in Fig. 3. The G-value using only the data obtained at a dose rate of IOL8and lox9e.v./g.-l sec.-l was 0.22 f 0.02. This may be compared with the values of 0.16,4 0.25 f 0.02,8 and 0.200 f 0.0042b obtained by y-rays. The results obtained at the highest dose rate (lozoe.v./g.-l sec.-l) do appear to show an intensity effect. However, although these samples mere cooled during irradiation, some unavoidable warming amounting to 8-12' mas observed. Cunningham found about a 50% increase in the G-value of ;?JaSOawhen the temperature was increased from 25 to 60'. The results at the high dose rate accordingly were corrected assuming an average temperature rise of 10'. Pb(N03)2.-The decomposition of lead nitrate (Fig. 4) was studied only in the initial decomposition range. The G-value found for the initial slope was 0.53 f 0.08. This may be compared with the values of 0.48,4 0.44 f 0.048 and 0.432 found by y-ray radiolysis.
VOl. 66
mental error, that there is any appreciable intensity effect. Certainly there is no evidence for reaction (d) above. The agreement between y-ray results and electron beam experiments is as good in most cases as that shown among the different laboratories for the various nitrate decompositions by y-rays. It was not feasible to extend this work to higher dose rates because of the difficulty of dissipating the large amount of heat generated.
THE STRUCTURE OF LANTHASUX FAMILY SILICIDES1 BY A. G. THARP~ Department of Mineral Technology, Unzversity of Caltjornia. Berkeley, Calzfornaa Received August 24, 1961
Perri, Binder, and Post3p4 have described the structures of several rare earth silicides with MSiz ideal lattice types. Their work agreed with, and expanded upon, that previously reported by Brauer and Haag.6 The structures of LaSiz, CeSiz, PrSis, and EuSiz were shown to be tetragonal while structures of NdSiz, SmSiz, GdSiz, DySiz, and YSiz are orthorhombic. Perri, et al., postulated that the tetragonal lattice is characteristic of disilicides of those rare earths with relatively large atomic radii. The orthorhombic structure is a slight distortion of the LaSi2 (tetragonal) type structure. At slightly elevated temperatures, where the effective diameters of the atoms are increased by thermal vibration, the orthorhombic phases transform to the tetragonal structure. Radii of the lanthanum family atoms generally decrease with increased atomic number except that preference for certain electronic configurations causes the radii of some of the atoms to lie off the smooth curve of diameter vs. atomic number. The present work was undertaken to compare the structures of disilicides of the higher atomic number lanthanum family elements, erbium, thulium, ytterbium, and lutetium. If the postulate of Perri, et al., is valid for these elements, YbSiz,. if Discussion such a phase could be prepared, should crystallize The primary step in the decomposition of the in the tetragonal system as do EuSL and disilicides inorganic nitrates has been postulated to be2bss-11 of lower atomic number rare earths, while erbium, thulium, and lutetium disilicides should have the Nos- --e- NOz0 (a) orthorhombic or some other structure. The oxygen fragment produced in (a) reacting
+
as1a,g38
NOzNO$0
+
+ 0 +KOs +XOz-
+
0 2
(b) (c)
and possibly 0+0-402
(d)
It was shown previously5 that a kinetic scheme using reactions (a), (b), and (c) above gave excellent agreement with experiments for the radiolysis of NaN03. It is quite possible that a similar reaction scheme ill show good agreement for some of the other nitrates, particularly Csx'03 and AgNQ3.
Considering the range of intensities used in these studies it does not appear, within experi(10) P. Dolgan and T. IT, Davis. J Chem. Phys , 27, 333 (1957). (11) L. K. NnrayLnswainv, Trans Faraday Soc., 31, 1411 (1935).
Experimental Preparation of Samples .-Disilicides of thulium and ytterbium vrrere prepared by direct syntheses from the elements. Disilicides of erbium and lutetium were synthesized by reducing the sesquioxides of the rare earths with an excess of silicon. The rare earth oxides, rare earth metals, and the silicon were 99.9+% pure. The reactions between thulium and silicon and between ytterbium and silicon were carried out in tungsten crucibles under a slight positive pressure of helium or argon or in vacuo. Vacuum preparation wa2 somewhat undesirable since considerable quantities of thulium or ytterbium would vaporize from the reaction chamber. The argon or helium was dried by passing it through a 30411. column of phos(1) This work was supported by the Office of Naval Research. ( 2 ) Department of Chemistry, Long Beach State College, Long Beach, California. (3) J. A. Perri, I. Binder, and B. Post, J . Phys. Chem., 63, 616 (1959).
B. Post, ibid., 63, 2073 (1959). ( 5 ) G. Brauer and €1. Heap, 2.anorg. allgem. Chem., 267, 198 (1952)
(4) J. A. Perri, I. Binder, and
