The phase relation between actinides and alkaline — earth metals changes with the increase in the atomic number of the latter. Regarding the Be-related
systems, a NaZn13-type intermetallic compound (D23-structure) is observed in the Th-Be, U-Be, and Pu-Be phase diagrams.1,4 These intermetallic compounds melt congruently near the Be terminal and the decomposition temperatures are estimated to be 2203, 2273, and 2223 K for ThBe13, UBe13, and PuBe13, respectively. Since these data have at least ±50 K error, the decomposition temperatures are reasonably comparable. Figure 4 indicates the U-Be phase diagram as a typical example quoted from Okamoto,4 which was mainly constructed from the observations of Buzzard.14 The eutectic point appears near the U terminal at 1363 K, and a narrow liquid miscibility gap appears near the Be termi­nal in the U concentration region between ^0.7 and ^2.2 at.%. Although the latter was questioned by Hansen and Anderko,15 there is no other available experimental data for this system. According to Wilhelm et a/.,16 this miscibility gap is thermody­namically unlikely and its presence is possible only if there is a strong clustering in the liquid phase, and thus thermodynamic functions for the U-Be system are estimated by introducing a simple thermodynamic model. As for the solid phase, a few percent of solid solubility of Be in g-U was observed.14 Also, a spinodal composition was given in the central region.14 There are two different sources for the Pu-Be system.11,17 The significant differences between them are the congruent melting temperature of the PuBe13 and the shape of the liquidus. The latter phase diagram given in Konobeevsky17 was then modified based on the several unpublished results obtained at the Los Alamos National Laboratory, as shown by Ellinger et a/.18 The modified Pu-Be phase diagram by Ellinger et a/.18 was recommended by Okamoto,19 who showed phase relations that are mostly similar to those in the U-Be system, although the eutectic temperature lowered to ^903 K. There is also a small percent solubility of Be in e-Pu (bcc structure). The Th-Be phase diagram was mainly constructed by Okamoto19 from the observations by Badaeva and Kuznetsova.20 When neglecting the ThBe13 com­pound, the Th-Be phase diagram looks like that of a typical eutectic type, and the eutectic point was reported to be at 1503 K and at 65 at.% Th. However, according to Okamoto,19 due to the cursory nature of the work of Badaeva and Kuznetsova,20 the eutectic point is still only a rough estimation. Table 1 sum­marizes the thermodynamic functions for the Th-Be, U-Be, and Pu-Be systems, which were estimated by Okamoto19 with respect to the liquid phases. The NaZn13-type intermetallic compound was also

Table 1 Thermodynamic functions for actinide-Be systems

GO(Be, liq) = 0 GO(Th, liq) = 0 GO(U, liq) = 0 GO(Pu, liq) = 0

GO(Be, bcc) = -12 600 + 8.067T GO(Be, HCP) = -14 700 + 9.428T GO(Th, bcc) = -13807 + 6.808T GO(Th, fcc) = -17 406 + 9.012T GO(U, bcc) = -9142 + 6.497T GO(Be13Th) = -14630 + 4.902T GO(Be13U) = -17100 + 6.470T GO(Be13Pu) = -24 980 + 9.400T Gex(Th-Be, liq) = xTh(1 — xTh) (13520 -830xTh)

Gex(U-Be, liq) = xU(1 — xU) (36100 — 5090xU — 580xU) Gex(Pu-Be, liq) = xPu(1 — xPu) (10160)

Source: Okamoto, H. In Phase Diagrams of Binary Actinide Alloys-, Kassner, M. E., Peterson, D. E., Eds.; Monograph Series on Alloy Phase Diagrams No. 11; ASM International: Materials Park, OH, 1995; pp 22-24, 146-151, 164-168, 207-208, 218-219, 246-247, 297-300, 411-412, 423.

Note: values are in J mol-1. Tis in K. x is mole fraction.

observed in Pa-Be, Np-Be, Am-Be, and Cm-Be sys­tems.21- The decomposition temperatures are pre­dicted to be at least higher than 1673 K for the Np-Be system and 1773 K for the Am-Be and Cm-Be systems.

Figure 5 shows the Np-Be phase diagram preliminar­ily estimated in the present study, assuming the inter­action parameter for the liquid phase and the Gibbs energy of formation for NpBe13 are the same as those for the Pu-Be system. It is speculated that the phase relations in the Np-Be system will have a reasonably
similar shape with the Pu-Be system, with the excep­tion of the Np terminal. By measuring the thermal arrests for several compositions, especially near the Be terminal, the speculated phase diagram will be modified efficiently.

Regarding the Th-Mg system, there are some conflicting issues among the available data.24-26 The tentatively assessed phase diagram was given in Nayeb-Hashemi and Clark.27 However, the phase relations related to the gas phase were not given in the phase diagram, although the boiling point of Mg is 1380 K, which is lower than the transition temperature between a-Th and p-Th. Two interme­tallic compounds, that is, Th6Mg23 and ThMg2, exist near the Mg terminal in the low-temperature region. At the least, these decomposition temperatures are much lower than that for the ThBe13, suggesting that the stability of the Th-Mg compounds is far lower than that of the Th-Be compounds. Thermodynamic functions for the ThMg2 were determined by Novotny and Smith28 between 692 and 812 K by means of vapor pressure measurement. The derived equation for the Gibbs energy of the formation is

DfG°(Mg2Th) = — 59.871 ± 12.979 + (63.639 ± 18.000) x 10-3T (kJmol-1)

Table 2 summarizes the thermodynamic values at 750 K. As for the enthalpy and entropy of formation, the higher values in the table were recommended by Nayeb-Hashemi and Clark.27 A similar phase rela­tion near the Mg terminal was reported for the Pu-Mg system,30 although the phase relation at high tempera­ture was different. There is a miscibility gap for the liquid phase in the high-temperature region ofthe Pu- Mg system. On the other hand, there are no interme­tallic compounds in the U-Mg system, and the limited solubility even for the liquid phase was shown.31 These facts on the phase relation between actinides and Mg suggest that the miscibility between actinides and Mg becomes far poorer than that between actinides
and Be. By assuming the systematic variation in the actinide-Mg systems, partial phase diagrams for Pa — Mg, Np-Mg, and Am-Mg systems were proposed by Gulyaev and Dvorshkaya32 and there is limited solubility for the solid and liquid phases.

In the cases of the Ca-, Sr-, Ba-, and probably Ra — related systems, the miscibility between actinides and these heavy alkaline-earth metals is expected to be very poor even for the liquid phase, although the available information is very limited. Thorium metal was prepared by calciothermic reduction at around 1223 K, and the solubility of Ca in Th was found to be very low (<0.12 at.%). No binary compounds between U and Ba were observed in the determination of the U-Ba-C ternary phase dia — gram.34 The U-Ca, U-Sr, Pu-Ca, Pu-Sr, Pu-Ba, and Am-Ba systems were predicted to be immisci­ble, and the mutual solubility was extremely low even in the liquid phase.11,35-38 According to semi­empirical modeling,39,40 the limited mutual solubility and the absence of any intermetallic compounds were also predicted for the Th-Ba system, although there is no available experimental data. These suggest that, for the actinides-Ca, — Sr, — Ba, and possibly, -Ra sys­tems, the allotropic transformation temperatures of both the actinides and alkaline-earth metals will only appear in the phase diagrams. Figure 6 shows the Pu-Ca phase diagram as a typical example, which was calculated by taking very large positive values (^50 kJ mol-1) for the interaction parameters of each phase. Other systems are considered to have a similar tendency.