Curium metal

Properties of Curium Metal. Report UCRL-844 (AECD-2912). J. Amer. chem. Soc. 73, 493 (1951). 

Curium metal and its compounds are radioactive bone-seeking poisons that attack the skeletal system of humans and animals. Care must be used in handling them. 

Americium, californium, and einsteinium oxides have been reduced by lanthanum metal, whereas thorium has been used as the reductant metal to prepare actinium, plutonium, and curium metals from their respective oxides. Berkelimn metal could also be prepared by Th reduction of Bk02 or Bk203, but the quantity of berkelium oxide available for reduction at one time has not been large enough to produce other than thin foils by this technique. Such a form of product metal can be very difficult to handle in subsequent experimentation. The rate and yield of Am from the reduction at 1525 K of americium dioxide with lanthanum metal are given in Fig. 2. 

Curium metal is stable at ambient temperature, but oxidizes on heating to curium(lll) oxide, Cm203. 

Curium metal liberates hydrogen from dilute acids, forming the trivalent, Cu3+ ion in the solution. 

Curium metal was found to be antiferromagnetic Its Curie-WeiB behaviour above the Neel point is in good agreement with the localized picture (pett = 8 Pb as in Gd metal). In the case of Bk, again the Curie-WeiB law gives the expected effective moment value ... 

Recent Results for Curium Metal Under Pressure. 125... 

In Part II.3.a it was pointed out that the structures proposed for curitim metal and reported in Table 1 and Fig. lb represented an anomaly in the trend encoimtered when applying pressure on lanthanide and heavier actinide metals. Thus, curium metal should have represented a somewhat unexplained intermediate stage between lighter and heavier actinides. 

Then, curium metal is antiferromagnetic and its paramagnetic effective moment definitively supports the picture of a localized 5 f configuration the same is true for what is known about berkelium and californium metals ... 

Curium metal is antiferromagnetic and its resistivity has been measured as well as that of isostructural non magnetic americium metal The resistivity difference is taken as the magnetic contribution and is shown in Fig. 13. It passes through a maximum for... 

Fig. 13. Magnetic contribution to the electrical resistivity of Curium metal. (Schenkel )...

The importance of these observations lies in the fact that the 298 value is the basis point for the free-energy functions for both the solid and the gas. With a reasonably accurate estimate of the solid crystal entropy, the gaseous spectroscopic data and precise vapor pressure measurements, it is possible to calculate all the thermodynamic values for the metal, up to the highest temperatures of measurement. Also, a self-consistent heat capacity curve starting at 298 K is produced for the intensely-radioactive and scarce trans-curium metals, normal calorimetry may never be possible, and these techniques become extremely important tools. A detailed example of a typical calculation will be given under Californium below. 

Curium metal exists in two modifications, a double hexagonal close-packed (dhcp) structure (a-lanthanum type) and a high-temperature cubic close-packed (fee) structure. Using Cm, the dhcp form was found to have lattice constants a = 3.496(3)andc = 11.331(5) A, giving a calculated density of 13.5 gem and a metallic radius of 1.74 A [27,97]. In a recent pressure study, slightly less accurate lattice constants were obtained (yielding a calculated density of 13.8 g cm ) but did establish that the dhcp phase was stable at least to 6.5 GPa [162]. 

Conversion to the dhcp form was observed on cooling as well as by coldworking samples, indicating that metallic curium was indeed present. This apparently rules out possible stabilization of the fee phase by trace amounts of CmN, which has a very similar lattice constant, a — 5.041(2) A, although there is an unusually large difference in the two reported values for the fee metal. A phase attributed to CmO, fee, a = 5.09 A, was seen in early preparations of curium metal [27]. Zachariasen used a metallic radius of 1.743 A for curium [32]. 

Curium metal has an entropy of vaporization remarkably similar to that of gadolinium, although its vapor pressure is about double that of gadolinium over the measured range [33]. The metal vapor pressure of triply distilled Cm metal has been measured between 1300 and 2000 K and obeys the following relations [33] ... 

The direct reactions of curium metal with non-metals such as P, As, Sb, S, and Se have been reported, and binary compounds with N, P, As, and Sb have been prepared by reactions using curium hydride (see next section) [31,46,47]. 

Table 9.2 Crystallographic data for curium metal, alloys, and compounds. 

A phenomenological model based on crystal structure, metallic radius, melting point, and enthalpy of sublimation has been used to arrive at the electronic configuration of berkelium metal [140]. An energy difference of 0.92 eV was calculated between the 5f 7s ground state and the 5f 6d 7s first excited state. The enthalpy of sublimation of trivalent Bk metal was calculated to be 2.99 eV (288 kJmol ), reflecting the fact that berkelium metal is more volatile than curium metal. It was also concluded that the metallic valence of the face-centered cubic form of berkelium metal is less than that of the double hexagonal close-packed modification [140]. 

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