W. 111. OLSONAND R. N. R. MULFORD
2932
The Melting Point and Decomposition Pressure of Neptunium Mononitride
by W. M. Olson and R. N. R. Mulford Unhersity of California, Los Alamos Scientific Laboratory, Los Alamos, New Mexico
(Receiaed March 11, 1966)
+
Decomposition pressures for the reaction NpN(s) = Np(1) 0.5hTz(g)in the temperature range 2210-2830" are presented. NpN melts congruently a t 2830 f 30" under a nitrogen pressure of about 10 atm. An equation, log p (atm) = 8.193 - [(29.54 X 103)/T] 7.87 X 10-18T6, was found to describe the decomposition pressuretemperature relation for NpN. A lattice parameter of a = 4.8987 f 0.0005 A was measured for the cubic nitride.
+
Introduction At ambient nitrogen pressures below the pressure where NpN melts congruently the following reaction occurs. NpN(s) = Np(1, saturated with N)
+ O.si\rTz(g)
At sufficiently high temperatures the solid will liquefy as soon as tbe decomposition pressure exceeds the ambient nitrogen pressure. Therefore the decomposition pressure-temperature relation can be determined by observing the apparent melting point as a function of nitrogen pressure. If the ambient nitrogen pressure is high enough to suppress decomposition, the congruent melting point may be observed.
Experimental Section Apparatus. The apparatus and procedure have been fully described previously.' Briefly, about 5 mg of NpN powder was placed in a 30" vee formed in a tungsten strip which was heated resistively. The sample was observed and its temperature determined by means of a calibrated Pyro micro optical pyrometer sighted through a quartz window and prism located a t the top of the stainless steel vacuum-pressure can. Temperatures were corrected for the absorption of the prism and window, and also for the emissivity of the tungsten vee. It is estimated that the accuracy of the temperatures reported is *30". Preparation of NpN. Most of the NpN was made in the following manner. Neptunium metal, which contained 230 ppm of carbon as the principal impurity, was cleaned in an inert atmosphere by removing the surface with a file. The cleaned metal was reduced to filings which were placed in a tungsten vee and heated resistively under vacuum for approximately 5 min a t The Journal of Physical chemistry
about 500" to remove absorbed gases. The filings were then cooled to room temperature, and 1 atm of spectroscopic grade nitrogen, to which had been added about 0.5% hydrogen (Linde Ultra Pure) to serve as a catalyst, was introduced into the system. The reaction between the neptunium filings and the gas was started by heating the filings to 600". After the reaction appeared to be complete, the temperature was increased to 1500" and the gas was pumped off. This procedure decomposed any neptunium hydride which had not been converted to nitride during the heating at 600" and volatilized any unreacted neptunium metal. An X-ray diffraction pattern showed only NpN to be present. The resulting NpN powder was stored under vacuum, and small portions were used for the melting point determinations. Procedure. In determining the melting point, or decomposition temperature, about 5 mg of NpN powder was placed in a tungsten vee, the apparatus was evacuated, and the sample heated to about 1000". Spectroscopically pure nitrogen was then introduced into the system until the desired pressure was attained. Then the temperature was raised until the sample started to melt. Temperature and pressure readings were taken just before and just after melting had occurred, and the true values were assumed to be the averages. The before and after temperature readings were usually about 10" apart.
Results The decomposition pressures obtained for NpN are plotted in Figure 1 as log p (atm) vs. 10,00O/T (1) W. M. Olson and R. N. R. Mulford, J. Phys. Chem., 67, 952 (1963).
2933
DECOMPOSITION PRESSURE OF NEPTUNIUM MONONITRIDE
F '
1
I
I
I
I
I
I
i k 4: I W
5rn
1.0
rn w a a
0.001
I
3.2 3.3
1
M
I
3.5
!
3.6
!
3.7
I
'3.8
3.9
40
4.1.
IOOOO/ T ~ -1 K Figure 1. Decomposition pressure of NpN.
(OK). The data points are considerably scattered, but it is apparent that they fall along a gentle curve which intersects the vertical line representing congruent melting a t a pressure of about 10 atm. The congruent melting point, as established by this vertical line on the plot, is 2830 f 30". In previous work, oxygen impurity had been found to have considerable influence on the decomposition of ThN.2 I n the present work with NpN no clear influence of oxygen was detected. Some of the points in Figure 1 were obtained with nitride made from sawdust which definitely contained oxide as a result of the sparking and burning that accompanied the sawing process. The other points were obtained from nitride prepared from bulk metal by the method previously described. There was no apparent difference between the pressure-temperature results from the two nitride sources. No oxide was detected in X-ray patterns from the nitride made from the bulk metal. Neptunium, like plutonium, forms only a mononitride. The lattice parameter of this cubic, sodium chloride type nitride was found to be a = 4.8987 f 0.0005 A which agrees well with the value reported by Za~hariasen,~ a = 4.897 f 0.002 A.
Discussion Since the liquid neptunium in equilibrium with the nitride is saturated with nitrogen, and therefore its activity is Some unknown value less than unity, the be present data to provide an accurate value for the standard heat of formation of NpN.
The solubility of nitrogen in liquid neptunium presumably varies with temperature, the solubility increasing, and, consequently, the neptunium activity decreasing as the temperature increases. This temperature dependence of the solubility is reflected in the curvature shown by the data in Figure 1. A limit for AHoZg8 may be obtained from the lowest experimental point, where the activity of the neptunium was nearest to unity, by combining the experimental value with entropy and heat capacity estimates. If, for the formation of l mole of NpN from pure neptunium and nitrogen gas, we assume A S O 2 9 8 = -20 cal/deg mole and ACpo = 1.5 cal/deg mole as an average between 298 and 25OO0K, the heat of formation at 298°K is found to be more negative than -61 kcal/mole. The entropy of formation is taken to be the same as that of UN, as calculated from the measured absolute entropy of UN.4 I n previous similar work with GN,I and ThN,2 it was found that an equation of the form log p =A ( B / T ) CT5fitted the experimental points well. This equation, which was derived empirically has the property of approaching a straight line as the temperature becomes lower. This behavior is plausible for the nitrogen pressure over a univariant mixture of liquid metal and nitride; that is, the deviation from linearity is caused by the lowering of the activity of the metal in the liquid phase. As the temperature decreases, less and less nitrogen remains in solution and the metal activity approaches unity. For the decomposition pressure of NpN, the data in Figure 1 are fitted by ( T is in O K )
+
+
l o g p (atm)
=
8.193
-
[(29.54 X 103)/T]
+
7.87
x
10-18~5
It is of some interest to compare the stability of NpN with the stabilities of the other actinide nitrides.'v2r6 For accurate comparisons, the standard free energies of formation of the nitrides are needed, but these cannot be obtained because the activities of the metals in their liquid phases are unknown. One is thus forced to compare decomposition pressures. Small differences among the decomposition pressures may reflect only the variation of nitrogen solubility in the liquid metals, but it is likely that a large difference (2) W. hl. Olson and R. N. R. Mulford, J . Phys. Chem., 69, 1223 (1965). (3) w. H. Zachariasen, 2, 388 (1949). (4) E. F. Westrum, Jr., "International Symposium on Compounds of Interest in Nuclear Reactor Technologv," University of Colorado, Boulder, Colo., Aug 3-5, 1964. ( 5 ) W. M. Olson and R. N. R. Mulford, J . Phys. Chem., 68, 1048 (1964).
Volume 70, Number 9 September 1966
2934
W. M.OLSONAND R. N. R. MULFORD
between the decomposition pressures of two nitrides is a true indication of different stabilities or bond strengths. Comparison of the known decomposition pressures of the actinide shows an increase in decomposition pressure at constant temperature as atomic number increases. At 2500°K, the lowest temperature for which measured pressures are available, comparison of the logarithms of the decomposition pressures calculated from the equations shows that, while UN, NpN, and PUS are not too dissimilar in stability, ThN is possibly significantly more stable than the other three. However, small differences in entropy or heat capacity or solubility of nitrogen in the liquid
metal phase could easily account for the differences among observed pressures. Similarly, differences in stoichiometry of the nitride phases could affect the observed pressures. Benz and Bowman6 have found a departure from stoichiometry at high temperatures for
UN * Acknowledgment. We are grateful to F. H. Ellinger for making the X-ray measurements on NpN. This work was performed under the auspices of the U. S. Atomic Energy Commission. (6) R. Benz and SI. G. Bowman, J . Am. Chem. SOC.,88, 264 (1966)
The Americium-Hydrogen System]
by W. M. Olson and R. N. R. Mulford Uninersity oj California, Los Alamos Scientific Laboratory, Los Alamos, .Vew Mexico
(Received April 18, 1966)
Pressuretemperaturecomposition measurements and X-ray data are presented for the americium-hydrogen system. Two hydride phases were found : one, face-centered cubic of composition AmH2+$; the other, hexagonal of composition AmH3. Both phases are isostructural with the corresponding plutonium and neptunium hydrides. The plateau pressures for the Am-AmHz univariant composition range are represented by log p(atm) = 7.190 - 8812/T("K). The derived heat of formation of AmHz is AHr = -40.3 kcal/ mole.
Introduction The uranium-hydrogen,2 neptunium-hydrogen, and plut~nium-hydrogen~~~ systems have been studied previously. Now, with sufficient americium metal available, it seemed appropriate to extend our knowledge of the hydrides to this seventh member of the actinide series. Such a study should be especially interesting because no very regular behavior, as is seen for the rare earth hydrides, has yet been found for the actinide hydrides.
Experimental Section The apparatus and method have been described in detail p r e v i ~ u s l y . ~ Briefly, a sample of americium The Journal
of
Physical ChemWEry
metaI WM contained in an yttria crueibIe which was placed in a silica tube attached to a standard Sievert's apparatus. The sample was out-gassed at 800'. Then, measured amounts of hydrogen were added to the sample, the pressure in the system being determined by means of a combination mercury manometer ~~
~
~~
~~
(1) Work done under the auspices of the U. S. Atomic Energy Com-
mission. (2) F. H. Spedding, et al., Nucleonics, 4, 4 (1949). (3) R. N. R. Mulford and T. A. Wiewandt, J . Phys. Chem., 69, 1641 (1965). (4) R. N. R. Mulford and G. E. Sturdy, J . Am. Chem. SOC.,77,3449 (1955). (5) R. N. R. Mulford, ibid., 78, 3897 (1956).
