THENEPTUNIUM-HYDROGEN SYSTEM
1641
The Neptunium-Hydrogen System'
by R. N. R. Mulford and T. A. Wiewandt University of Cal$ornia, Loa Alamos Scientific Laboratory, Loa Alamoa, New Mexico (Receiaed November 213 1964)
Pressure-temperature-coniposition measurements and X-ray data are presented for the neptunium-hydrogen system. Two hydride phases, N P H ~ +(0~ _< x 5 0.7) and NpH3 were found. NpH?,, has a face-centered cubic structure, presumed to be of the fluorite type; SpHs is hexagonal, isostructural with PuH3. The decomposition pressure of XpH? is given by log p(atm.) = 6.257 - 6126/T(OK.) (623-898'K.). The heat of formation of NpHz from solid neptunium metal and hydrogen gas is AH = -28 kcal./mole.
Introduction The neptunium-hydrogen system has not been investigated previously except for one microscale preparation of hydride by Fried and Davidson,?who found a hydride of composition KpH3.6-3.8. As uranium and plutonium, on either side of neptunium in the periodic table, form very different hydride phases, it is of some interest to examine the behavior of neptunium with hydrogen in more detail, now that quantities of neptunium metal exist which are larger than those that were available to Fried and Davidson in 1944.
Experimental The apparatus was a standard Sievert's style glass system. Hydrogen was measured in a water-jacketed buret c'onsisting of a number of bulbs with internal reference points between the bulbs. All mercury levels were read with a cathetometer. The device for measuring hydrogen pressure over the sample was a combined RlcLeod gauge and constant volumemercury manometer. The accuracy of pressure measurement was about =kO.l% for pressures above 2 mm. and about * l % for those below 2 mm. which were read on the 1lcLeod gauge. The estimated accuracy of measurement of the quantity of hydrogen was such that it is believed that the compositions quoted are accurate to at least 0.02 in the H/Np ratio, although the internal consistency of the data suggests that an accuracy of *0.01 was probably achieved. Before any hydrogen was admitted, the tolal dead space was measured with helium with the sample in place by comparison with the known buret volumes The part of the dead space at furnace temperature was calculated from the geometries of the
sample bulb and connecting tubing. The volumes of the buret bulbs were measured by weighing the amount of mercury contained. The sample bulb was of fused silica and was connected to the rest of the apparatus by a graded seal. Sample temperature was measured with a Pt-Pt-lO% Rh thermocouple in a well fused to the bottom end of the sample bulb. Tests were run with another thermocouple in the sample position to determine whether any temperature gradient existed between the sample position and the position of the measuring thermocouple. No gradient was detected. The measuring thermocouple was calibrated against a Pt-Pt-10% Rh couple certified by the Sational Bureau of Standards. The neptuniuni sample was contained in an yttrium oxide crucible which was sealed inside the sample bulb. The crucible was degassed a t 850" in the system before adding the sample; then the sample was degassed in place a t 800" before adding hydrogen. The sample bulb was enclosed in a silver block to smooth temperature fluctuations and prevent gradients. The furnace surrounding the block was controlled by a Brown circular chart controller-recorder which operated as an on-off control with the control thermocouple in contact with the silver block. With this arrangement, a temperature fluctuation of about *0.1" could be detected with the measuring thermocouple during the controller on-off cycle. Analysis of the neptunium metal showed 230 p.p.m. by weight of carbon to be the principal impurity. Of (1) Work performed under the auspices of the U. S. Atomic Energy Commission.
(2) 5.Fried and N. Davidson, J . Am. Chem. SOC.,70, 3539 (1948).
Volume 69.Number 6
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R. N. R. MULFORD AND T . A. WIEWANDT
1642
35 metallic elements looked for, all were below the limit of spectrographic detection. Micrographic examination showed a minor amount of a residual eutectic constituent in the grain boundaries and no nonmetallic inclusions. Radiochemical assay showed that 200 p.p.m. by weight of Pu-238 was the only radioactive impurity present. No correction was made to the computed hydride compositions for impurities. The hydrogen, used directly from the cylinder, was Riatheson Co. Ultrapure grade, claimed by the manufacturer to contain less than 10 p.p.m. of total impurities. The procedure followed in obtaining data for sample 1 was to add hydrogen in successive increments until the highest H/Np ratio measured with this sample was reached. Small amounts of hydrogen were then removed from the system and P-T-C data were obtained with a successively decreasing H/Np ratio. With any fixed amount of hydrogen in the system, pressure readings obtained were approached from temperatures both above and below the equilibration temperature. As the furnace control could not be set exactly on a predetermined temperature, the isotherm points in Figures 1 and 2 represent short interpolations from a plot of the raw data. Since it was evident that the data from sample 1 gave satisfactory equilibrium pressures, sample 2 was used to check only a few points on the plateaus and to explore better the composition range close f,o NpH3.
0
0 0.1
1
1
-
1
I
I
1
1
l
I
-
--- ----__J
, 350.
"
1
l
I
1
1
,
1
I
1
1
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-
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1
Results Figures 1 and 2 show the observed P-T-C data plotted as isotherins on pressure-composition coordinates. I t is seen that solubility of hydrogen in neptunium metal is small in the temperature range studied, and that two solid phases, neptunium and NpH2, coexist. As the ratio of hydrogen to metal exceeds 2, solid solubility of hydrogen in NpHz occurs up to a limit of about KPH2.7. If more hydrogen is added to the i\TpH2.7 phase, a new phase forms of composition essentially KpH,. The isotherms are believed to represent good equilibrium over the composition range from neptunium to XPH2.7, as is shown by reproducibility on adding or removing hydrogen from the system, reproducibility with two different samples, and reproducibility when approached from higher or lower temperatures. Between xpH2.7 and NpH3, however, hysteresis was observed upon temperature cycling. It appeared from the shape of the working plots of temperature us. pressure that the hexagonal trihydride phase would convert to t2he cubic NpH2.7 phase in a manner that appeared to approach equilibrium as the temperature was increased, but that the conversion of the xpH2.7 phase to the SpH3 phase which occurred The Journal of Physical Chemistry
H:Np
RATIO
Figure 2. Pressure isotherms for the composition range NpH, to NpHa. Same code as for Figiire 1.
with decreasing temperature was sluggish. That is, with increasing temperature, the pressure rose abruptly from the low value characteristic of the hexagonal trihydride to the higher value of the cubic YpH2.7, while
THEP~EPTUXIIJM-HYDROGEN SYSTEM
1643
with decreasing temperature, the high n'pH2.7 pressure approached the lower NpH3 pressure only gradually. ?009 The hysteresis loop observed was not modified appreciably by increasing the hold time at the various temperatures. A reasonable interpretation of these data is afforded if it is assumed that the NpH, to N P H ~ . ~ 100 transformation gives pressures close to equilibrium, while the reverse transformation does not. Accordingly, Figure 3 was plotted. The abrupt change in pressure with increasing temperature was taken to represent the equilibrium pressure over a two-solidphase mixture of KpH3 arid XpH2.7. A single curve (B) to illustrate the hysteresis is shown in the small inset in Figure 3. From this plot, the plateau pressures in Figure 2 are derived. The curves plotted in IOOO/T . K-' Figure 3 are for constant total amount of hydrogen Figure 3. Log pressure vs. reciprocal temperature for the in the sample bulb which includes hydrogen present as cubic-hexagonal phase transition. gas as well as hydrogen combined in the solid, rather than for constant solid composition. The results of examination of X-ray powder patterns Table I : X-Ray Data of various compositions are given in Table I. It was found necessary to anneal the X-ray samples a t about WNP 200" for a t least 24 hr. in order to obtain sharp diffracSample ratio Phases present Lattice parameters, A. tion patterns. Although some of the compositions 1 0.5 Cubic only 5.3434 f 5 listed lie in a range which the isotherm data show to 2 1.78 5.3428 f 8 Cubic only contain neptunium metal and XpH2, lines from the 3 2.18 5.3431 f 7 Cubic only 4 2.36 Cubic only metal pattern were not visible on the films; only the 5.3463 f 7 5 2.42 Cubic only 5.3478 f 8 lines from the face-centered cubic SpHz showed. Cubic a. = 5.360 f 3 6 -2.5" Cubic + hex. Neptunium metal gives an X-ray powder pattern conHex. a. = 3.774 f 3 taining many lines, all of which are weak, and it is not co = 6.72 f 1 surprising that only the strong cubic pattern of the 7 2.80 Strong cubic, weak Cubic aa = 5.355 f I hex. hydride appeaxed. The fact that sample 8, which 8 2.98 Hex. a. = 3.771 f 1 Strong hex., very should have been single-phase XpH3, showed two weak weak cubic co = 6.713 f 2 lines from the cubic phase demonstrates that true a Sample 6 was not annealed and gave a verv poor diffraction equilibrium was not quite established with hydrogen pattern. The cubic lattice parameter is definitely larger t h a n contents above KpHz.7. those of the annealed samples but may not represent an equiThe NpHz+=phase (0 5 z _< 0.7) is face-centered librium value. cubic and isostructural with P U H ~ + ~The . structure is presumed to be of the fluorite type. The calculated density is 10.4 g . / ~ m . ~The . ?;pH, phase is hexagonal from Figure 1 that the cubic NpH, phase at the lower and isostructural with P u H and ~ ~ GdH3.4 The calcuconiposition limit becomes richer in hydrogen as the lated density is 9.64 g . / ~ m . ~Comparison . of the temperature increases. In the 600 and 625' isotherms lattice parameters of the neptunium hydrides with those the plateaus extend to N ~ H wwhile , a t 400" the plateau of the correspondi!g plutonium hydrides shows that ends a t NpHz.0. For PuH2, the cubic phase becomes F p H z (ao = 5.343 A.) is smaller than PuHz (a" = 5.359 poorer in hydrogen as temperature increases, thus at A.), arid that ?;pH3 (ao = 3.77, co 6.71 k)is smaller 600" the composition is, for plutonium hydride, P u H ~ . ~ ~ than PuH3 (ao = 3.78, co = 6.76 A.). I t is seen from and a t 500", PuH1.88. The same behavior is obTable I that the cubic hydride shows an increase in unit cell edge as the hydrogen content increases. This (3) R. N. R. Mulford and G . E. Sturdy, J . Am. Chem. S o c . , 78, 3897 behavior is opposite to that observed for the isostruc(1956). tural PuHZ5 and rare earth dihydride6 phases. (4) G . E. Sturdy and R. N. R. Mulford, ibid., 78, 1083 (1956). In addition to the anomalous increase in lattice con(5) F. H. Ellinger, unpublished data. stant with increasing hydrogen content, it is evident (6) C. E. Holley, et al., J . Phys. Chem., 59, 1??6 (1955).
r
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R. N. R. MULFORD AND T. A. WIEWANDT
1644
served in the rare earth hydrides as in plutonium hydrides. If the logarithms of the plateau pressures of Figure 1 are plotted as a function of the reciprocal of the absolute temperature, the slope of the straight line through the data gives the heat per mole of hydrogen gas for the formation of one phase from the other. Strictly, the stoichiometry of the phases concerned must be taken into account, but in the present case it appears sufficient to consider that the reaction involves pure solid neptunium and neptunium dihydride. The relation between decomposition pressure and temperature is log p(atm.)
=
6.257 - 6126/T (OK.)
This gives a heat for the assumed process Kp(s)
+ Hdg) = NpHz(s) (AH = -28.0
kcal./mole)
which may be considered to be close to the standard heat of formation of NpH2 in the temperature range investigated (3FiO to 625"). This heat lies between those for UH3 (AH298= -20.3 kcal./mole of H2)' and for PuH2 (AH873 = -37.4 kcal./mole of H2).3 These heats of formation are for different temperatures. The
The Journal of Physical Chemistry
temperature correction is unknown, but it will not affect the trend with atomic number. As the melting point of pure neptunium is about 640°, it is presumed that our measurements apply to the equilibrium involving a solid neptunium metal phase. Similarly to the above treatment, a heat of conversion of cubic NpH2.7 to hexagonal NpH3 may be obtained from the data of Figure 3. The equation for the univariant line in Figure 3 is log p(atm.)
=
6.92 - 3736/T('K.)
+
The process and the heat are: 6.67NpH2.7 Hz + 6.67NpH3(s), AH = -17.1 kcal./mole of Ha. The entropy change is A S = -31.7 cal. deg.-' mole-' of HB which means that the cubic SpH2., and the hexagonal NpH3 have very nearly the same absolute entropy, as might be expected.
Acknowledgment. Thanks are due Professor W. H. Zachariasen for part of the X-ray work, Miss Marian Gibbs for measuring X-ray films, and Dr. W. AI. Olson for setting up and calibrating the glass apparatus. (7) B. M . Abraham and H. E. Flotow, J . A m . Chem. SOC.,7 7 , 1446 (1955).
