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Equilibrium and structural properties of the PuH system
Journal
of the Less-Common
EQUILIBRIUM SYSTEM*
JOHN
133
(1987)
AND STRUCTURAL
M. HASCHKE,
Rockwell
Metals,
International,
ANGELO P.O. Box
Golden,
155
- 166
PROPERTIES
E. HODGES, 464,
155
OF THE Pu-H
III and ROBERT CO 80402-0464
L. LUCAS (U.S.A.)
Summary
The findings of recent work on Pu-H are described and employed in resolving several inconsistencies in the reported properties of plutonium hydride. Results of pressure measurements establish the nature of a large hysteresis effect and suggest revisions in the phase diagram. The existence of five hydride phases between the dihydride and trihydride compositions is indicated. Recently reported properties and neutron diffraction data are employed in formulating conceptual models for bonding and hydrogen accommodation in the fluorite-related hydride solid solution. Evidence for the existence of vacancy clusters in the cubic hydride is presented, and the influence of cation and anion ordering on various hydride properties is discussed.
1. Introduction
The reaction of plutonium metal with hydrogen is facile at low temperatures and finds application in plutonium recovery and in powder metallurgy [l, 21. Although the hydride is known to react pyrophorically with air at room temperature, and strict safety procedures must be employed in hydride operations, some investigators have reported that the hydride is stable in air up to 150 “C [3]. Our interest in plutonium hydride has been promoted by the desire to resolve this and other discrepancies in the literature and to characterize the hydride more fully. The basic features of Pu-H have been known for more than thirty years. The existence of the dihydride and of a continuous solid solution, PuH,, in the range 2 < x < 3 was first indicated by tensimetric measurements made by Johns during the Manhattan Project [4]. The thermodynamic values for the hydride were more thoroughly defined in subsequent studies by Mulford and Sturdy [ 5, 61. In addition to a limited region of hydrogen *Paper presented at a Symposium on Solid-State held at the 192nd National Meeting of the American September 7 - 12,1986. 0022-5088/87/$3.50
0 Elsevier
Actinide Chemical
Chemistry and Physics, Society, Anaheim, CA,
Sequoia/Printed
in The Netherlands
156
solubility in the metal, these authors reported the existence of an f.c.c. (calcium difluoride-type) dihydride and a fluorite-related solid solution over the range 2.0
2. Results and discussion 2.1. Phase equilibria and thermodynamic properties In our initial tensimetric study of Pu-H, microbalance methods were used to determine the composition dependence of the hydride phase as a function of temperature at constant hydrogen pressure [lo, 111. Equilibration was rapid and reversible even at room temperature. When In P(H,) is plotted as a function of T-’ for constant x values in the range 1.9 < x G 2.9, a set of linear relationships is found [lo]. The lines have a common intercept and their slopes decrease systematically with increasing composition. Such behavior is characteristic of Cox charts for completely miscible organic liquids [ 121. As suggested by the results of Johns [4], these relationships indicate that plutonium forms a continuous cubic solid solution hydride over a range bounded by the substoichiometric dihydride and the trihydride compositions. Other of our tensimetric results are consistent with the findings of Mulford and Sturdy [ 5,6], and show a large hysteresis effect in Pu-H. These P-T-x data were obtained by progressively dehydriding PuH,.,, in a Sievert’s (P-V-T) apparatus. The stoichiometric hydride was first prepared by reacting the metal with excess hydrogen at high temperature and pressure. In Fig. 1, the observed x dependence of the partial molar free energy of hydro-
157
gen at 75 “C is compared with that derived from the microbalance data. The hysteresis is most severe near x = 2.9, where the pressures differ by a factor of 104. Since X-ray diffraction data show that only the cubic hydride is present at 3t < 2.7, coincidence of the data sets is anticipated in this region. A ten-fold difference in the pressures is observed. Although experimental error cannot be excluded, the lower pressure observed with the Sievert’s apparatus is attributed primarily to the effects of extensive annealing of the hydride during those measurements. The shape of the lower curve in Fig. 1 also implies the existence of a two-phase region in the range 2.7
2.6
2.8
3.0
1
.5
XioPuHx
2.0 Xin
2.5
3.0
PuH,
Fig. 1. Partial molar free energy results for hydriding (upper curve from microbalance data) and for dehydriding (lower curve from P-V-T data) for PuH, at 75 “C. Fig. 2. Revised
phase diagram
for Pu-H
derived
from
dehydriding
isotherms.
158
phase above x = 2.95 [ 161, phase V may have a related structure. Diffraction data are not available for PuHs., because of safety constraints in handling plutonium and because the hydride decomposes unless the equilibrium hydrogen pressure of the phase is maintained or exceeded at all times. The preparation of Debye-Scherrer samples requires that capillaries be loaded and sealed in a glove-box atmosphere containing 5% to 10% Hz. The in situ preparation of phase V in the sample holder of a diffractometer is precluded by the need for high temperatures and high hydrogen overpressures during reaction of the elements. Although the existence of phase VI is evidenced only by the appearance of G isotherms with similar slopes in the upper region of the (III + VI) gap, the f.c.c. and h.c.p. metal lattices are coherent, and a solid solution could form by intergrowth of fluorite and tysoniterelated domains. One must remember that the phase relationships indicated in Fig. 2 are not fully defined because the phase fields must be separated by two-phase regions not evident in our pressure data. The enthalpies of formation derived for the plutonium hydrides from the tensimetric data [ 11, 131 are presented in Table 1. The results show the consequences of hysteresis. The enthalpies from the two data sources are identical at x values across the cubic region, but differ markedly above x = 2.7. The hexagonal hydride is clearly more stable than the cubic form in this region. The tensimetric results, which are confirmed by calorimetric measurements on several cubic compositions [17], have resolved the discrepancy in the thermochemical properties of the trihydride, and the availability of heat capacity data [18] has permitted the internal consistency of the results to be checked [ 131. Whereas one set of our tensimetric results agree with the results of Johns, the other is consistent with the findings of Mulford and Sturdy. The key to understanding the origin of these differences and the source of the hysteresis effect is found in the preparative procedures. In the microbalance studies, the cubic dihydride was first prepared by slow reaction of the elements at temperatures below 100 “C. Tensimetric measurements were then made by adding and removing hydrogen at low temperatures. Although TABLE 1 Integral enthalpies of formation for plutonium hydride derived from tensimetric data obtained during hydriding (microbalance measurements) and dehydriding (P-V-T measurements) x in PuH,
1.9 2.2 2.5 2.8 3.0
AW, (kcal mol-l) Hydriding
Dehydriding
-37.6 -41.0 -44.4 -47.8 -50.1
-37.6 -41.0 -44.5 -55.7 -57.2
159
measurements were made at temperatures up to 500 “C, they never exceeded 200 “C when x was greater than 2.7. In contrast, samples for the Sievert’s apparatus were prepared by reacting the elements at 400 “C and maintaining a hydrogen overpressure of about 3 bar as the product cooled. At low temperatures, the metal atoms are unable to transform from the f.c.c. to the h.c.p. arrangement when x exceeds 2.7. Consequently, hydrogen is accommodated by the cubic lattice up to the trihydride composition as sufficient overpressure is applied. At high temperature, kinetic energy is available and the metal atoms transform to the more stable h.c.p. arrangement when x exceeds 2.7. The differing “equilibrium” results obtained by earlier workers and by us can be traced to the experimental procedures. As with Pa-H, in which the hysteresis correlates with structural transitions of the hydride phases [19], the hysteresis in Pu-H apparently has structural origins. The perplexing observation in both systems is that the transformations are hindered in the hydriding direction, but are facile for dehydriding. 2.2. Other physicochemical properties An evaluation of other hydride properties such as lattice parametercomposition data, decomposition kinetics, electrical conductivity and spectroscopic information reveals additional inconsistencies in our knowledge of plutonium hydride and provide a basis for advancing our understanding of Pu-H. The composition dependence of the lattice parameter of the cubic hydride is shown in Fig. 3. Our recently measured a0 values [20] agree with earlier results for 2.0 < x < 2.7 [ 91. The steady decrease in a0 with increasing x is similar to that found for the cubic Ln(II,III)F, systems [15], but its magnitude is only one-third of that for the fluorides. The existence of the two-phase region below the dihydride is evidenced by a constant ao, and a second two-phase region at x > 2.7 is implied by the constant parameters observed by Muromura et al. [ 91. This result is, however, inconsistent with the phase diagram because only cubic hydride was observed at x = 3.00. A plausible explanation for the anomalous diffraction results at high compositions is found in microbalance data for decomposition of the cubic
. 00
5 34 -
0
1.8
2.0
22
2.4 2.6 x in PuH,
2.3
3.0
Fig. 3. Composition dependence of lattice parameters for cubic et al. [ 9 ] (filled symbols) and from this study (open symbols).
PuH,
from
Muromura
160
hydride [21]. When a 1 g sample of cubic hydride (.lt = 2.9) is subjected to dynamic vacuum at 25 “C, its composition decreases rapidly with time. After a few minutes, the composition asymptotically approaches a constant value near x = 2.7. This facile loss of hydrogen is consistent with the activation energies observed for decomposition of the cubic hydride. E, decreases linearly with 3c from a constant value of 27.3 f 1.4 kcal mol-’ in the (I + II) diaphasic region to approximately zero at x = 2.75 [21]. In contrast to this result, microbalance data for the hexagonal hydride show that its rate of decomposition is undetectably slow at 25 “C, but becomes measurable only at temperatures above 150 “C. An E, of 11.7 kcal mol-’ was obtained for the 160 to 240 “C temperature range with a sample having x = 2.8. Since the preparative procedures employed by Muromura et al. [9] would have produced cubic hydride and since the hydrogen pressure was not maintained during preparation of the X-ray specimens, all samples with x > 2.7 probably decomposed to that stoichiometry prior to analysis. A discrepancy in the reported pyrophoricity of plutonium hydride was noted in the introduction. Since the rate at which the hydride loses hydrogen depends on its structure (i.e. whether it is cubic or hexagonal) and since its structure is determined by the conditions of preparation, the pyrophoric tendency of a given hydride sample is determined by its preparative history. Indeed, the non-pyrophoric hydride described by Brown et al. [3] was obtained by a rapid reaction at 270 “C, and this hypothesis has been repeatedly verified in our laboratory. When the hydride is prepared slowly so that self-heating does not produce temperatures in excess of 100 “C!, the product is a fine powder which often ignites spontaneously on exposure to air. We have observed that the powder is not altered or oxidized during the initial phase of the reaction, but that a bluish flame bums above the sample as if it were a gas burner. The spontaneous ignition and burning characteristics of cubic hydride samples with high stoichiometries suggest that pyrophoricity is also related to the rate of hydrogen evolution. When the hydride is prepared by rapid reaction of the elements at high temperature (greater than 300 “C) and pressure (greater than 3 bar), the product is a coarse material which only ignites in air when heated above 270 “C. One must be intrigued by the apparent importance of the x = 2.7 composition in Pu-H. This stoichiometry marks the upper boundary of thermodynamic stability of the fluorite-related hydride. The lattice parameter data and the decomposition results also indicate that this point is unique. The boundary lies well beyond the maximum composition of x = 2.4 observed for the lanthanide fluoride systems, and an adequate explanation for this behavior is not suggested by the equilibrium or the kinetic results. 2.3. Electronic and spectroscopic properties Electronic properties and photoelectron spectra of the cubic hydride have recently been measured [20]. The electrical conductivity of five compositions in the range 1.93 < x < 2.65 were determined over the temperature range 1 to 300 K. At x = 1.93, the observed conductivity (1.4 X lo6 !Z’
161
m-‘) is slightly less than that of a-phase plutonium metal (5.0 X lo6 sip1 m-l) and is comparable with that of lead and bismuth. This behavior is different from the lanthanide hydrides, which have noticeably higher conductivities than their respective metals. Ward has noted that the lower value for PuH, suggests 5f involvement in bonding [22]. The measured conductivities also decrease steadily with increasing x from metallic behavior at 1.93 towards semiconductor behavior at 2.65. Although the absolute values are uncertain because of microcracks in the samples at x > 2, our qualitative visual observations are consistent with this trend. Whereas the dihydride exhibits a bright silver-grey lustre, samples with 3t near 2.7 are charcoal black. X-ray photoelectron spectra obtained for a sample with x = 2.1 show evidence for a 5f-6d hybridization band 1.8 eV below the Fermi level and indicate that plutonium is trivalent in the hydride. The observed 4f binding energy for the hydride (426.6 eV) is in excellent agreement with the 426.4 eV value reported for the oxide of trivalent plutonium [ 231. 2.4. Structural properties Neutron powder diffraction measurements have been performed to investigate the possibility that the hydrogens accommodated on the octahedral sites of the fluorite-related structure are ordered [24]. Studies of the PuD~.~~ composition at several temperatures in the 20 to 290 K range show Fm3m symmetry (a,, = 5.344 A) and antiferromagnetic ordering below 60 K. The ordered magnetic moment of 0.8 pa per plutonium atom is consistent with the presence of trivalent metal, but the data show no evidence for ordering of hydrogen on the octahedral sites at low temperature. The calculated intensities obtained by placement of plutonium and deuterium on fluorite positions (plutonium on 4a and deuterium on 8c) and by random placement of the additional deuterium atoms on the octahedral (4b) sites did not agree with the observed intensities. Agreement was achieved by allowing for vacancies on the tetrahedral sites and refining on the occupancy factor. The results show that 4% to 5% of the 8c sites are vacant at x = 2.25 over the temperature range of the measurements. Additional neutron diffraction results have recently been reported for other deuteride compositions [25]. The findings of the earlier study are confirmed, but differences are seen in the fraction of tetrahedral vacancies. The results for x = 2.65 show that approximately 10% of the 8c sites are vacant and 80% of the octahedral sites are occupied. Refinement of the structure also shows that deuterium in the octahedral interstices are displaced by approximately 0.4 A from the ideal 4b positions. 2.5. A conceptual
model for cubic PuH,
Although the development of a comprehensive model for bonding in the hydride is not possible, new insight into the question can be gained by evaluating recent results. The close similarities between Pu-H and the fluoride systems of the lanthanides suggests that plutonium hydride is ionic in
162
nature. A similar conclusion was reached by Gibb and Schumacher for all the lanthanide and actinide hydrides after an extensive correlation of structural data [26]. For a coordination number (CN) of 4, these authors derived an ionic radius of 1.29 A for H- . This value is less than that for F- , but comparison of lattice parameters for isostructural hydrides and fluorides of the lanthanides and actinides shows that the hydride parameters are consistently 10% less than those for the respective fluorides. Our recalculation of the H- radius from these data and Shannon’s cationic radii [27] gives a value of 1.20 + 0.05 A. Since plutonium is trivalent in the hydride and the hydrogens appear to be hydridic, a convenient formulation for the dihydride is Pu3’(H-),(e-). The observed electrical conductivity is consistent with one electron per formula unit in the conduction band. The trend in the electronic properties of PuH, is particularly important in defining the nature of the hydrogen species entering the lattice in the solid solution range beyond the dihydride. The observed decrease in electrical conductivity with increasing x suggests that electrons are progressively removed from the conduction band and bound as H- ions in the lattice as x increases. The cubic solid solution hydride can therefore be formulated as Pu~‘(H-),(~-)~_.. The observed decrease in a0 with increasing x, as well as the relative magnitude of that decrease, are also consistent with this ionic model. The changes observed in a0 across the composition ranges of the cubic lanthanide fluoride phases are a factor of three greater than for PuH, because both the trivalent cation to divalent cation ratio and the anion to cation ratio increase with increasing x. In the hydride, plutonium is trivalent at all compositions, and the modest decrease in a0 results only from the increasing anion to cation ratio. The process by which hydride ions are accommodated by the cubic phase in the solid solution region should also be considered. The neutron diffraction results show that a simple model of random occupancy of octahedral interstices in the fluorite lattice is inaccurate. Since hydrogen shows a preference for tetrahedral sites in metal lattices, the existence of tetrahedral vacancies in PuH, suggests that they are an integral part of the accommodation process. The octahedral to tetrahedral distance in the dihydride is 2.30 A, but the hydride diameter is apparently 2.4 to 2.6 A. Each octahedral hydrogen simultaneously interacts with H- ions on eight adjacent sites. Although Gibb and Schumacher have noted that the H- radii in UH3 must penetrate [26], the plutonium hydride structure is able to relieve the H--H- repulsion by forming tetrahedral vacancies. The existence of vacancy clusters in cubic PuH, has been suggested [24], but a more extensive examination of the model is instructive. The tetrahedral (8~) sites are energetically more favorable for H- occupancy than octahedral (4b) sites, but the energetics of a concerted process favor vacancy formation. When an H- enters a 4b site, the anion on one of the adjacent 8c sites can move to one of the three remaining 4b sites surrounding the vacated tetrahedron. Since both anions on the 4b sites can move towards the faces of the vacant tetrahedron, their repulsive interactions are reduced
Fluorite cell IlOO] projeclion
Fig. 4. The (100)
Expanded
cluster
projection
of fluorite
and the expanded
Pu1,jH26 vacancy
cluster.
and a more stable configuration is attainable. The energetics are further favored by the presence of two unoccupied 4b sites adjacent to the vacancy. Since additional energy is not expended by filling these sites, the spontaneous formation of a vacancy cluster is anticipated. The basic cluster is composed of four metal atoms surrounding a vacant tetrahedral site and four hydrogens occupying octahedrally related positions above the faces of the tetrahedron. Consideration of an expanded cluster permits definition of the cluster limit and correlation of the vacancy concentration with hydride composition. A projection of the fluorite structure on the (100) plane is shown at the top left-hand side of Fig. 4. Filled and open circles indicate plutonium at 0 and 1 and at * along the projection axis respectively. Hydrogens at a and + are shown by crosses. If one assumes that each hydrogen on an octahedral site interacts with one tetrahedral vacancy (i.e. that vacancies on the other seven SC sites around the octahedron are precluded), the expanded cluster shown at the top right-hand side of Fig. 4 emerges. The vacancy, which is indicated by the square, may be at a or z and is completely surrounded by an array of occupied tetrahedra having the dimensions of the unit cell projected at the top left of Fig. 4. A three-dimensional perspective of the expanded Pu,,Hz6 cluster is shown at the bottom of Fig. 4. Since the model predicts a limit of one cluster per unit cell, a maximum of $ or 12.5% of the tetrahedral sites can be vacant in the fluorite-related hydride. If as proposed above, all four octahedra around each vacancy are occupied, the occupancy of octahedral sites and x can be calculated as a function of the vacancy concentration. The results in Table 2 show these relationships and reveal the most interesting prediction of the model. At the maximum vacancy concentration, the octahedral sites are fully occupied and x is 2.75. The vacancy cluster model is simple, but permits prediction and interpretation of several structural and physicochemical properties of cubic PuH,. The critical assessment of the model is made by comparing the theoretical and observed levels of tetrahedral vacancies for the experimental compositions. At the x = 2.25 and 2.65 stoichiometries, the calculated vacancy concentrations are 4.2% and 10.8% respectively, and the corresponding
164 TABLE 2 Calculated octahedral occupancies tetrahedral vacancies Tetrahedral vacancy (%) 0 5.0
10.0 12.5
and hydride compositions
Octahedral occupancy (%I 0
40 80 100
for selected percentages of
x in PuH,
2.00 2.30 2.60 2.75
measured concen~tions are 4% to 5% and 10%. The rem~kable accuracy of the model is further evidenced by the observed displacement of hydrogen from the ideal octahedral positions. Perhaps the most intriguing result is found in our ability to interpret the equilibrium and decomposition behavior and the pyrophoricity of the cubic hydride. As shown in Table 2, the octahedral sites are fully occupied at x = 2.75. As the model predicts, high composition cubic hydride can be prepared by increasing the hydrogen overpressure and occupying the tetrahedral vacancies. As expected, these stoichiometries are thermodynamically and kinetically unstable, and their tendency to rapidly decompose greatly enhances their pyrophoricity. Inconsistencies between the observed behavior and the predictions of the vacancy cluster model are seen in the failure to detect long-range order at low temperatures. According to the model, a superstructure should result from ordering of the tetrahedral vacancies on a specific set of sites formed by $ of the 8c positions. The existence of ordered LnsH, is reported for several lanthanide hydride systems by Greis et al. [ 281, but differences in the formation and equilibration kinetics of the hydrides suggest that hydrogen is significantly more mobile in cubic PuH, than in the lanthanide analogs. It is not surprising that order is absent at room temperature where hydrogen is mobile, or at low vacancy concentrations where the clusters are able to act somewhat independently. Evidence for long-range order is, however, expected for the x = 2.65 composition at 20 K. Although selected neutron diffraction samples were cooled slowly to room temperature from 400 “C, a retrospective re-evaluation of the procedures suggests that slow cooling rates should have been used during attainment of subambient temperatures. The disorder present at room temperature may have been frozen in during rapid cooling on the spectrometer. Indeed, nuclear magnetic resonance (NMR) data for PuH, show that hydrogen is mobile above -20 “C and that the hydride lattice is rigid below -80 “C [29]. Whereas the disordered LnH, phase was obtained by cooling at a rate of 25 K h-i, the ordered LnsH, phases were obtained by cooling at 4 K h-’ [ 281. Further consideration of the situation suggests that long-range order may not be observable in cubic plutonium hydride. Examination of a model for packing of expanded clusters shows that the tetrahedral vacancies should order at a and i on a symmetry-related subset of 8c positions. An imper-
165
fection in this extended array is introduced if a given tetrahedral site of the subset remains occupied. The consequences are two-fold. The stoichiometry of the hydride is slightly reduced, and the vacancy array is shifted to a different subset of 8c. Repetition of this process permutes the vacancy array into ordered domains over all the 8c positions. If the domains are sufficiently small, the diffraction pattern from each crystallite would indicate random vacancies. The partial molar free energy isotherms and the diffraction results suggest that a change in anion ordering occurs near x = 2.25. Although this composition corresponds to one-third completion of cluster formation and ordering of clusters is suggested, characterization of the process may be difficult.
3. Conclusions Our study of Pu-H shows that the system is extremely complex and that the physicochemical properties of the hydride are strongly influenced by structural features. The pronounced hysteresis in the hydrogen pressures and the apparent discrepancies in the equilibrium behavior of the hydride are consequences of cation ordering. Finer features of the phase diagram and the chemical reactivity of the cubic hydride are influenced by anion ordering. Hydride pyrophoricity is apparently determined by ordering of both cations and anions. If the preparative conditions are such that the metals pack in the h.c.p. array, the hydride is not pyrophoric. If the f.c.c. hydride is obtained, it is pyrophoric, especially at high compositions where anion ordering promotes rapid evolution of hydrogen. Progress has also been made in the development of models for bonding and for accommodation of hydrogen in the fluorite-related solid solution. Various properties of PuH, suggest that it is saline in nature. The development of a vacancy cluster model provides a framework for clarifying relationships between structural and physical properties of the hydride. Indeed the analogy between the cubic hydride and the lanthanide fluorides is supported by the existence of anion clusters formed by displacing fluorides from tetrahedral sites and inserting regions of close-packed anions [30]. Although defect clusters may exist in the cubic lanthanide hydrides, the cluster mechanism is probably different from that found in either the Ln-F or the Pu-H systems. The upper boundaries of the cubic LnH, phases are found to vary with preparative temperature, unusual behavior is not evident for x = 2.7 and the Ln3H, stoichiometry is unique [28]. Our attempts to detect long-range ordering of clusters in PuH, have been unsuccessful, and the true nature of the cubic solid solution in Pu-H remains uncertain. Instead of a continuously varying composition, the hydride may be composed of a series of closely spaced finite compositions with ordered structures. Unfortunately, the resolution of such uncertainties will not be an easy task, and much work remains.
166
References 1 S. Fried and H. L. Baumbach, U.S. Potent, 2915 (1963) 362. 2 G. L. Stifter and M. H. Curtis, The Preparation of Plutonium Powder by a Hydriding Process: Initial Studies, U.S. Atomic Energy Commission Rep., HW-64289, March 1960. 3 F. Brown, H. M. Ockenden and G. A. Welch,J. Chem. Sot., 1955 (1955)3932. 4 I. B. Johns, Plutonium Hydride and Deuteride, U.S. Atomic Energy Commission Rep., ~DDC-717, February 1947. 5 R. N. R. Mulford and G. E. Sturdy,J. Am. Chem. Sot., 77 (1955) 3449, 6 R. N. R. Mulford and G. E. Sturdy, J. Am. Chem. Sot., 78 (1956) 3897. 7 F. L. Oetting, Chem. Rev., 67 (1967) 261. 8 M. I. Ivanov and N. S. Podol’skaya, 2. Fiz. Khim., 45 (1971) 2963. 9 T. Muromura, T. Yahata, K. Ouchi and M. Iseki, J. Znorg. Nucl. Chem., 34 (1972) 171. 10 J. M. Hasehke, A. E. Hodges III, C. M. Smith and F. L. Oetting, J. Less-Common Met., 73 (1980) 41. 11 J. M. Haschke, Thermodynamic Properties of the Cubic Plutonium Hydride Solid Solution, U.S. DOE Rep., RFP-3099, December 1981. 12 R. R. Dreisbach, PVT Relationships of Organic Compounds, Handbook Publishers, Sandusky, OH, 1952. 13 H. E. Flotow, J. M. Haschke and S. Yamauchi, in F. L. Oetting (ed.), The Chemical Thermodynamics of Actinide Elements and Compounds, Part 9, The Actinide Hydrides, International Atomic Energy Agency, Vienna, 1984, pp. 49 - 65. 14 J. M. Haschke, in K. A. Gschneider and L. Eyring (eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 4, North-Holland, Amsterdam, 1979,.pp. 89 151. 15 0. Greis and J. M. Haschke, in K. A. Gschneidner and L. Eyring feds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 5, North-Holland, Amsterdam, 1982, pp. 387 - 460. 16 0. Greis, 2. Anorg. Allg. Chem., 441 (1978)39. 1’7 C. M. Smith, A. E. Hodges III, J. M. Haschke and F. L. Oetting, J. Chem. Thermodynamics, 14 (1982) 115. 18 F. L. Oetting, A. E. Hodges III, J. M. Haschke and H. E. Flotow, J. Chem. Thermodynamics, 16 (1984) 1089. 19 J. M. Haschke, J. W. Ward and W. Bartscher, J. Less-Common Met., 107 (1985) 159. 20 J. 0. Willis, J. W. Ward, J. L. Smith, S. T. Kosiewicz, J. M. Haschke and A. E. Hodges III, Physico B, 130 (1985) 527. 21 J. M. Haschke and J. L. Stakebake, in G. J. McCarthy, J. J. Rhyne and H. B. Silber (eds.), The Rare Earths in Modern Science and Technology, Vol. 2, Plenum, New York, 1980, pp. 577 - 582. 22 J. W. Ward, in A. J. Freeman and C. Keller (eds.), Handbook on the Physics and Chemistry of Actinides, Elsevier, New York, 1985, pp. 1 - 74. 23 D. T. Larson and J. M. Haschke, Znorg. Chem., 20 (1981) 1945. 24 W. Bartacher, A. Boeuf, R. Caciuffo, J. M. Fournier, J. M. Haschke, L. Manes, J. Rebizant, F. Rustiehelli and J. W. Ward, Solid State Commun., 52 (1984) 619. 25 W. Bartscher, A. Boeuf, R. Caciuffo, J. M. Fournier, J. M. Haschke, L. Manes, J. Rebizant, F. Rustichelli and J. W. Ward, Physica B, I30 (1985) 530. 26 T. R. P. Gibb and D. P. Schumacher, J. Phys. Chem., 60 (1960) 1407. 27 R. D. Shannon, Acta Crystallogr. A, 32 (1976) 751. 28 0. Greis, P. Knappe and H. Mueller, J. Solid State Chem., 39 (1981) 49. 29 G. Cinidar, D. Zamir and Z. Hadari, Phys. Rev. B, 14 (1976) 912. 30 D. J. M. Bevan, J. Straehle and 0. Greis,J. Solid State Chem., 44 (1982) 75.
of the Less-Common
EQUILIBRIUM SYSTEM*
JOHN
133
(1987)
AND STRUCTURAL
M. HASCHKE,
Rockwell
Metals,
International,
ANGELO P.O. Box
Golden,
155
- 166
PROPERTIES
E. HODGES, 464,
155
OF THE Pu-H
III and ROBERT CO 80402-0464
L. LUCAS (U.S.A.)
Summary
The findings of recent work on Pu-H are described and employed in resolving several inconsistencies in the reported properties of plutonium hydride. Results of pressure measurements establish the nature of a large hysteresis effect and suggest revisions in the phase diagram. The existence of five hydride phases between the dihydride and trihydride compositions is indicated. Recently reported properties and neutron diffraction data are employed in formulating conceptual models for bonding and hydrogen accommodation in the fluorite-related hydride solid solution. Evidence for the existence of vacancy clusters in the cubic hydride is presented, and the influence of cation and anion ordering on various hydride properties is discussed.
1. Introduction
The reaction of plutonium metal with hydrogen is facile at low temperatures and finds application in plutonium recovery and in powder metallurgy [l, 21. Although the hydride is known to react pyrophorically with air at room temperature, and strict safety procedures must be employed in hydride operations, some investigators have reported that the hydride is stable in air up to 150 “C [3]. Our interest in plutonium hydride has been promoted by the desire to resolve this and other discrepancies in the literature and to characterize the hydride more fully. The basic features of Pu-H have been known for more than thirty years. The existence of the dihydride and of a continuous solid solution, PuH,, in the range 2 < x < 3 was first indicated by tensimetric measurements made by Johns during the Manhattan Project [4]. The thermodynamic values for the hydride were more thoroughly defined in subsequent studies by Mulford and Sturdy [ 5, 61. In addition to a limited region of hydrogen *Paper presented at a Symposium on Solid-State held at the 192nd National Meeting of the American September 7 - 12,1986. 0022-5088/87/$3.50
0 Elsevier
Actinide Chemical
Chemistry and Physics, Society, Anaheim, CA,
Sequoia/Printed
in The Netherlands
156
solubility in the metal, these authors reported the existence of an f.c.c. (calcium difluoride-type) dihydride and a fluorite-related solid solution over the range 2.0
2. Results and discussion 2.1. Phase equilibria and thermodynamic properties In our initial tensimetric study of Pu-H, microbalance methods were used to determine the composition dependence of the hydride phase as a function of temperature at constant hydrogen pressure [lo, 111. Equilibration was rapid and reversible even at room temperature. When In P(H,) is plotted as a function of T-’ for constant x values in the range 1.9 < x G 2.9, a set of linear relationships is found [lo]. The lines have a common intercept and their slopes decrease systematically with increasing composition. Such behavior is characteristic of Cox charts for completely miscible organic liquids [ 121. As suggested by the results of Johns [4], these relationships indicate that plutonium forms a continuous cubic solid solution hydride over a range bounded by the substoichiometric dihydride and the trihydride compositions. Other of our tensimetric results are consistent with the findings of Mulford and Sturdy [ 5,6], and show a large hysteresis effect in Pu-H. These P-T-x data were obtained by progressively dehydriding PuH,.,, in a Sievert’s (P-V-T) apparatus. The stoichiometric hydride was first prepared by reacting the metal with excess hydrogen at high temperature and pressure. In Fig. 1, the observed x dependence of the partial molar free energy of hydro-
157
gen at 75 “C is compared with that derived from the microbalance data. The hysteresis is most severe near x = 2.9, where the pressures differ by a factor of 104. Since X-ray diffraction data show that only the cubic hydride is present at 3t < 2.7, coincidence of the data sets is anticipated in this region. A ten-fold difference in the pressures is observed. Although experimental error cannot be excluded, the lower pressure observed with the Sievert’s apparatus is attributed primarily to the effects of extensive annealing of the hydride during those measurements. The shape of the lower curve in Fig. 1 also implies the existence of a two-phase region in the range 2.7
2.6
2.8
3.0
1
.5
XioPuHx
2.0 Xin
2.5
3.0
PuH,
Fig. 1. Partial molar free energy results for hydriding (upper curve from microbalance data) and for dehydriding (lower curve from P-V-T data) for PuH, at 75 “C. Fig. 2. Revised
phase diagram
for Pu-H
derived
from
dehydriding
isotherms.
158
phase above x = 2.95 [ 161, phase V may have a related structure. Diffraction data are not available for PuHs., because of safety constraints in handling plutonium and because the hydride decomposes unless the equilibrium hydrogen pressure of the phase is maintained or exceeded at all times. The preparation of Debye-Scherrer samples requires that capillaries be loaded and sealed in a glove-box atmosphere containing 5% to 10% Hz. The in situ preparation of phase V in the sample holder of a diffractometer is precluded by the need for high temperatures and high hydrogen overpressures during reaction of the elements. Although the existence of phase VI is evidenced only by the appearance of G isotherms with similar slopes in the upper region of the (III + VI) gap, the f.c.c. and h.c.p. metal lattices are coherent, and a solid solution could form by intergrowth of fluorite and tysoniterelated domains. One must remember that the phase relationships indicated in Fig. 2 are not fully defined because the phase fields must be separated by two-phase regions not evident in our pressure data. The enthalpies of formation derived for the plutonium hydrides from the tensimetric data [ 11, 131 are presented in Table 1. The results show the consequences of hysteresis. The enthalpies from the two data sources are identical at x values across the cubic region, but differ markedly above x = 2.7. The hexagonal hydride is clearly more stable than the cubic form in this region. The tensimetric results, which are confirmed by calorimetric measurements on several cubic compositions [17], have resolved the discrepancy in the thermochemical properties of the trihydride, and the availability of heat capacity data [18] has permitted the internal consistency of the results to be checked [ 131. Whereas one set of our tensimetric results agree with the results of Johns, the other is consistent with the findings of Mulford and Sturdy. The key to understanding the origin of these differences and the source of the hysteresis effect is found in the preparative procedures. In the microbalance studies, the cubic dihydride was first prepared by slow reaction of the elements at temperatures below 100 “C. Tensimetric measurements were then made by adding and removing hydrogen at low temperatures. Although TABLE 1 Integral enthalpies of formation for plutonium hydride derived from tensimetric data obtained during hydriding (microbalance measurements) and dehydriding (P-V-T measurements) x in PuH,
1.9 2.2 2.5 2.8 3.0
AW, (kcal mol-l) Hydriding
Dehydriding
-37.6 -41.0 -44.4 -47.8 -50.1
-37.6 -41.0 -44.5 -55.7 -57.2
159
measurements were made at temperatures up to 500 “C, they never exceeded 200 “C when x was greater than 2.7. In contrast, samples for the Sievert’s apparatus were prepared by reacting the elements at 400 “C and maintaining a hydrogen overpressure of about 3 bar as the product cooled. At low temperatures, the metal atoms are unable to transform from the f.c.c. to the h.c.p. arrangement when x exceeds 2.7. Consequently, hydrogen is accommodated by the cubic lattice up to the trihydride composition as sufficient overpressure is applied. At high temperature, kinetic energy is available and the metal atoms transform to the more stable h.c.p. arrangement when x exceeds 2.7. The differing “equilibrium” results obtained by earlier workers and by us can be traced to the experimental procedures. As with Pa-H, in which the hysteresis correlates with structural transitions of the hydride phases [19], the hysteresis in Pu-H apparently has structural origins. The perplexing observation in both systems is that the transformations are hindered in the hydriding direction, but are facile for dehydriding. 2.2. Other physicochemical properties An evaluation of other hydride properties such as lattice parametercomposition data, decomposition kinetics, electrical conductivity and spectroscopic information reveals additional inconsistencies in our knowledge of plutonium hydride and provide a basis for advancing our understanding of Pu-H. The composition dependence of the lattice parameter of the cubic hydride is shown in Fig. 3. Our recently measured a0 values [20] agree with earlier results for 2.0 < x < 2.7 [ 91. The steady decrease in a0 with increasing x is similar to that found for the cubic Ln(II,III)F, systems [15], but its magnitude is only one-third of that for the fluorides. The existence of the two-phase region below the dihydride is evidenced by a constant ao, and a second two-phase region at x > 2.7 is implied by the constant parameters observed by Muromura et al. [ 91. This result is, however, inconsistent with the phase diagram because only cubic hydride was observed at x = 3.00. A plausible explanation for the anomalous diffraction results at high compositions is found in microbalance data for decomposition of the cubic
. 00
5 34 -
0
1.8
2.0
22
2.4 2.6 x in PuH,
2.3
3.0
Fig. 3. Composition dependence of lattice parameters for cubic et al. [ 9 ] (filled symbols) and from this study (open symbols).
PuH,
from
Muromura
160
hydride [21]. When a 1 g sample of cubic hydride (.lt = 2.9) is subjected to dynamic vacuum at 25 “C, its composition decreases rapidly with time. After a few minutes, the composition asymptotically approaches a constant value near x = 2.7. This facile loss of hydrogen is consistent with the activation energies observed for decomposition of the cubic hydride. E, decreases linearly with 3c from a constant value of 27.3 f 1.4 kcal mol-’ in the (I + II) diaphasic region to approximately zero at x = 2.75 [21]. In contrast to this result, microbalance data for the hexagonal hydride show that its rate of decomposition is undetectably slow at 25 “C, but becomes measurable only at temperatures above 150 “C. An E, of 11.7 kcal mol-’ was obtained for the 160 to 240 “C temperature range with a sample having x = 2.8. Since the preparative procedures employed by Muromura et al. [9] would have produced cubic hydride and since the hydrogen pressure was not maintained during preparation of the X-ray specimens, all samples with x > 2.7 probably decomposed to that stoichiometry prior to analysis. A discrepancy in the reported pyrophoricity of plutonium hydride was noted in the introduction. Since the rate at which the hydride loses hydrogen depends on its structure (i.e. whether it is cubic or hexagonal) and since its structure is determined by the conditions of preparation, the pyrophoric tendency of a given hydride sample is determined by its preparative history. Indeed, the non-pyrophoric hydride described by Brown et al. [3] was obtained by a rapid reaction at 270 “C, and this hypothesis has been repeatedly verified in our laboratory. When the hydride is prepared slowly so that self-heating does not produce temperatures in excess of 100 “C!, the product is a fine powder which often ignites spontaneously on exposure to air. We have observed that the powder is not altered or oxidized during the initial phase of the reaction, but that a bluish flame bums above the sample as if it were a gas burner. The spontaneous ignition and burning characteristics of cubic hydride samples with high stoichiometries suggest that pyrophoricity is also related to the rate of hydrogen evolution. When the hydride is prepared by rapid reaction of the elements at high temperature (greater than 300 “C) and pressure (greater than 3 bar), the product is a coarse material which only ignites in air when heated above 270 “C. One must be intrigued by the apparent importance of the x = 2.7 composition in Pu-H. This stoichiometry marks the upper boundary of thermodynamic stability of the fluorite-related hydride. The lattice parameter data and the decomposition results also indicate that this point is unique. The boundary lies well beyond the maximum composition of x = 2.4 observed for the lanthanide fluoride systems, and an adequate explanation for this behavior is not suggested by the equilibrium or the kinetic results. 2.3. Electronic and spectroscopic properties Electronic properties and photoelectron spectra of the cubic hydride have recently been measured [20]. The electrical conductivity of five compositions in the range 1.93 < x < 2.65 were determined over the temperature range 1 to 300 K. At x = 1.93, the observed conductivity (1.4 X lo6 !Z’
161
m-‘) is slightly less than that of a-phase plutonium metal (5.0 X lo6 sip1 m-l) and is comparable with that of lead and bismuth. This behavior is different from the lanthanide hydrides, which have noticeably higher conductivities than their respective metals. Ward has noted that the lower value for PuH, suggests 5f involvement in bonding [22]. The measured conductivities also decrease steadily with increasing x from metallic behavior at 1.93 towards semiconductor behavior at 2.65. Although the absolute values are uncertain because of microcracks in the samples at x > 2, our qualitative visual observations are consistent with this trend. Whereas the dihydride exhibits a bright silver-grey lustre, samples with 3t near 2.7 are charcoal black. X-ray photoelectron spectra obtained for a sample with x = 2.1 show evidence for a 5f-6d hybridization band 1.8 eV below the Fermi level and indicate that plutonium is trivalent in the hydride. The observed 4f binding energy for the hydride (426.6 eV) is in excellent agreement with the 426.4 eV value reported for the oxide of trivalent plutonium [ 231. 2.4. Structural properties Neutron powder diffraction measurements have been performed to investigate the possibility that the hydrogens accommodated on the octahedral sites of the fluorite-related structure are ordered [24]. Studies of the PuD~.~~ composition at several temperatures in the 20 to 290 K range show Fm3m symmetry (a,, = 5.344 A) and antiferromagnetic ordering below 60 K. The ordered magnetic moment of 0.8 pa per plutonium atom is consistent with the presence of trivalent metal, but the data show no evidence for ordering of hydrogen on the octahedral sites at low temperature. The calculated intensities obtained by placement of plutonium and deuterium on fluorite positions (plutonium on 4a and deuterium on 8c) and by random placement of the additional deuterium atoms on the octahedral (4b) sites did not agree with the observed intensities. Agreement was achieved by allowing for vacancies on the tetrahedral sites and refining on the occupancy factor. The results show that 4% to 5% of the 8c sites are vacant at x = 2.25 over the temperature range of the measurements. Additional neutron diffraction results have recently been reported for other deuteride compositions [25]. The findings of the earlier study are confirmed, but differences are seen in the fraction of tetrahedral vacancies. The results for x = 2.65 show that approximately 10% of the 8c sites are vacant and 80% of the octahedral sites are occupied. Refinement of the structure also shows that deuterium in the octahedral interstices are displaced by approximately 0.4 A from the ideal 4b positions. 2.5. A conceptual
model for cubic PuH,
Although the development of a comprehensive model for bonding in the hydride is not possible, new insight into the question can be gained by evaluating recent results. The close similarities between Pu-H and the fluoride systems of the lanthanides suggests that plutonium hydride is ionic in
162
nature. A similar conclusion was reached by Gibb and Schumacher for all the lanthanide and actinide hydrides after an extensive correlation of structural data [26]. For a coordination number (CN) of 4, these authors derived an ionic radius of 1.29 A for H- . This value is less than that for F- , but comparison of lattice parameters for isostructural hydrides and fluorides of the lanthanides and actinides shows that the hydride parameters are consistently 10% less than those for the respective fluorides. Our recalculation of the H- radius from these data and Shannon’s cationic radii [27] gives a value of 1.20 + 0.05 A. Since plutonium is trivalent in the hydride and the hydrogens appear to be hydridic, a convenient formulation for the dihydride is Pu3’(H-),(e-). The observed electrical conductivity is consistent with one electron per formula unit in the conduction band. The trend in the electronic properties of PuH, is particularly important in defining the nature of the hydrogen species entering the lattice in the solid solution range beyond the dihydride. The observed decrease in electrical conductivity with increasing x suggests that electrons are progressively removed from the conduction band and bound as H- ions in the lattice as x increases. The cubic solid solution hydride can therefore be formulated as Pu~‘(H-),(~-)~_.. The observed decrease in a0 with increasing x, as well as the relative magnitude of that decrease, are also consistent with this ionic model. The changes observed in a0 across the composition ranges of the cubic lanthanide fluoride phases are a factor of three greater than for PuH, because both the trivalent cation to divalent cation ratio and the anion to cation ratio increase with increasing x. In the hydride, plutonium is trivalent at all compositions, and the modest decrease in a0 results only from the increasing anion to cation ratio. The process by which hydride ions are accommodated by the cubic phase in the solid solution region should also be considered. The neutron diffraction results show that a simple model of random occupancy of octahedral interstices in the fluorite lattice is inaccurate. Since hydrogen shows a preference for tetrahedral sites in metal lattices, the existence of tetrahedral vacancies in PuH, suggests that they are an integral part of the accommodation process. The octahedral to tetrahedral distance in the dihydride is 2.30 A, but the hydride diameter is apparently 2.4 to 2.6 A. Each octahedral hydrogen simultaneously interacts with H- ions on eight adjacent sites. Although Gibb and Schumacher have noted that the H- radii in UH3 must penetrate [26], the plutonium hydride structure is able to relieve the H--H- repulsion by forming tetrahedral vacancies. The existence of vacancy clusters in cubic PuH, has been suggested [24], but a more extensive examination of the model is instructive. The tetrahedral (8~) sites are energetically more favorable for H- occupancy than octahedral (4b) sites, but the energetics of a concerted process favor vacancy formation. When an H- enters a 4b site, the anion on one of the adjacent 8c sites can move to one of the three remaining 4b sites surrounding the vacated tetrahedron. Since both anions on the 4b sites can move towards the faces of the vacant tetrahedron, their repulsive interactions are reduced
Fluorite cell IlOO] projeclion
Fig. 4. The (100)
Expanded
cluster
projection
of fluorite
and the expanded
Pu1,jH26 vacancy
cluster.
and a more stable configuration is attainable. The energetics are further favored by the presence of two unoccupied 4b sites adjacent to the vacancy. Since additional energy is not expended by filling these sites, the spontaneous formation of a vacancy cluster is anticipated. The basic cluster is composed of four metal atoms surrounding a vacant tetrahedral site and four hydrogens occupying octahedrally related positions above the faces of the tetrahedron. Consideration of an expanded cluster permits definition of the cluster limit and correlation of the vacancy concentration with hydride composition. A projection of the fluorite structure on the (100) plane is shown at the top left-hand side of Fig. 4. Filled and open circles indicate plutonium at 0 and 1 and at * along the projection axis respectively. Hydrogens at a and + are shown by crosses. If one assumes that each hydrogen on an octahedral site interacts with one tetrahedral vacancy (i.e. that vacancies on the other seven SC sites around the octahedron are precluded), the expanded cluster shown at the top right-hand side of Fig. 4 emerges. The vacancy, which is indicated by the square, may be at a or z and is completely surrounded by an array of occupied tetrahedra having the dimensions of the unit cell projected at the top left of Fig. 4. A three-dimensional perspective of the expanded Pu,,Hz6 cluster is shown at the bottom of Fig. 4. Since the model predicts a limit of one cluster per unit cell, a maximum of $ or 12.5% of the tetrahedral sites can be vacant in the fluorite-related hydride. If as proposed above, all four octahedra around each vacancy are occupied, the occupancy of octahedral sites and x can be calculated as a function of the vacancy concentration. The results in Table 2 show these relationships and reveal the most interesting prediction of the model. At the maximum vacancy concentration, the octahedral sites are fully occupied and x is 2.75. The vacancy cluster model is simple, but permits prediction and interpretation of several structural and physicochemical properties of cubic PuH,. The critical assessment of the model is made by comparing the theoretical and observed levels of tetrahedral vacancies for the experimental compositions. At the x = 2.25 and 2.65 stoichiometries, the calculated vacancy concentrations are 4.2% and 10.8% respectively, and the corresponding
164 TABLE 2 Calculated octahedral occupancies tetrahedral vacancies Tetrahedral vacancy (%) 0 5.0
10.0 12.5
and hydride compositions
Octahedral occupancy (%I 0
40 80 100
for selected percentages of
x in PuH,
2.00 2.30 2.60 2.75
measured concen~tions are 4% to 5% and 10%. The rem~kable accuracy of the model is further evidenced by the observed displacement of hydrogen from the ideal octahedral positions. Perhaps the most intriguing result is found in our ability to interpret the equilibrium and decomposition behavior and the pyrophoricity of the cubic hydride. As shown in Table 2, the octahedral sites are fully occupied at x = 2.75. As the model predicts, high composition cubic hydride can be prepared by increasing the hydrogen overpressure and occupying the tetrahedral vacancies. As expected, these stoichiometries are thermodynamically and kinetically unstable, and their tendency to rapidly decompose greatly enhances their pyrophoricity. Inconsistencies between the observed behavior and the predictions of the vacancy cluster model are seen in the failure to detect long-range order at low temperatures. According to the model, a superstructure should result from ordering of the tetrahedral vacancies on a specific set of sites formed by $ of the 8c positions. The existence of ordered LnsH, is reported for several lanthanide hydride systems by Greis et al. [ 281, but differences in the formation and equilibration kinetics of the hydrides suggest that hydrogen is significantly more mobile in cubic PuH, than in the lanthanide analogs. It is not surprising that order is absent at room temperature where hydrogen is mobile, or at low vacancy concentrations where the clusters are able to act somewhat independently. Evidence for long-range order is, however, expected for the x = 2.65 composition at 20 K. Although selected neutron diffraction samples were cooled slowly to room temperature from 400 “C, a retrospective re-evaluation of the procedures suggests that slow cooling rates should have been used during attainment of subambient temperatures. The disorder present at room temperature may have been frozen in during rapid cooling on the spectrometer. Indeed, nuclear magnetic resonance (NMR) data for PuH, show that hydrogen is mobile above -20 “C and that the hydride lattice is rigid below -80 “C [29]. Whereas the disordered LnH, phase was obtained by cooling at a rate of 25 K h-i, the ordered LnsH, phases were obtained by cooling at 4 K h-’ [ 281. Further consideration of the situation suggests that long-range order may not be observable in cubic plutonium hydride. Examination of a model for packing of expanded clusters shows that the tetrahedral vacancies should order at a and i on a symmetry-related subset of 8c positions. An imper-
165
fection in this extended array is introduced if a given tetrahedral site of the subset remains occupied. The consequences are two-fold. The stoichiometry of the hydride is slightly reduced, and the vacancy array is shifted to a different subset of 8c. Repetition of this process permutes the vacancy array into ordered domains over all the 8c positions. If the domains are sufficiently small, the diffraction pattern from each crystallite would indicate random vacancies. The partial molar free energy isotherms and the diffraction results suggest that a change in anion ordering occurs near x = 2.25. Although this composition corresponds to one-third completion of cluster formation and ordering of clusters is suggested, characterization of the process may be difficult.
3. Conclusions Our study of Pu-H shows that the system is extremely complex and that the physicochemical properties of the hydride are strongly influenced by structural features. The pronounced hysteresis in the hydrogen pressures and the apparent discrepancies in the equilibrium behavior of the hydride are consequences of cation ordering. Finer features of the phase diagram and the chemical reactivity of the cubic hydride are influenced by anion ordering. Hydride pyrophoricity is apparently determined by ordering of both cations and anions. If the preparative conditions are such that the metals pack in the h.c.p. array, the hydride is not pyrophoric. If the f.c.c. hydride is obtained, it is pyrophoric, especially at high compositions where anion ordering promotes rapid evolution of hydrogen. Progress has also been made in the development of models for bonding and for accommodation of hydrogen in the fluorite-related solid solution. Various properties of PuH, suggest that it is saline in nature. The development of a vacancy cluster model provides a framework for clarifying relationships between structural and physical properties of the hydride. Indeed the analogy between the cubic hydride and the lanthanide fluorides is supported by the existence of anion clusters formed by displacing fluorides from tetrahedral sites and inserting regions of close-packed anions [30]. Although defect clusters may exist in the cubic lanthanide hydrides, the cluster mechanism is probably different from that found in either the Ln-F or the Pu-H systems. The upper boundaries of the cubic LnH, phases are found to vary with preparative temperature, unusual behavior is not evident for x = 2.7 and the Ln3H, stoichiometry is unique [28]. Our attempts to detect long-range ordering of clusters in PuH, have been unsuccessful, and the true nature of the cubic solid solution in Pu-H remains uncertain. Instead of a continuously varying composition, the hydride may be composed of a series of closely spaced finite compositions with ordered structures. Unfortunately, the resolution of such uncertainties will not be an easy task, and much work remains.
166
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