Actinide nitrides

The lowest-energy band F6+ is the band of almost pure U6s states. The next band is the T - band of almost pure U6p states ( 1.0 Ry). The energy interval —0.6 to —0.45 Ry contains the Tg. band of mainly N2s character with some admixture of U6s states. L4 5 bands (Tg-) represent almost pure U6p states, and the L - band is due to the hybridisation of U6p and N2s states. The lowest-energy bands in Fig. 3.20 Fg-, Fg- (Lg-, L4-5-) consist of the contributions from Up, U5/, U5d and N2p states. Their composition varies greatly depending on the value of the k vector, but the main contribution is due to the N2p states. The F7- (Lg- - 7+) band with an energy between 0.6 and 0.85 Ry 

Here Nf = 14 is the total number of atomic / states per unit cell. is the number of / electrons in the N2p-band (as a result of hybridisation of 5/ and N2p states). F is the occupation number of nonhybridised /-bands, i.e., F = n /14, where Uf is the number of 5/ electrons per actinide atom. Wf,Cf,Ci, are the canonical parameters of the band structure Wf is the width of the 5/-band, Cf and Cp are the canonical energies of the band centres for U5/ and N2p-bands, respectively. The first term in (3.1) gives a parabolic dependence on nf with a minimum at = 7. This term characterises the contribution of the metallic bonding to pressure P. The second term, which is linear in F, describes the contribution from the 5/-N2p hybridisation. Superposition of these two dependences leads to a shift of the minimum of P(F) towards 7, which qualitatively explains 

Below we present a list of references on electronic structure calculations carried out for binary iiia-via metal nitrides. 

Nonempirical cluster methods Nemoshkalenko et al (1980), Sheludchenko, Kucherenko and Aleshin (1981). 

Semiempirical cluster methods Gubanov, Shveikin and Kurmaev (1977), Zhukov et al (1980a), Ivanovsky, Gubanov, Kurmaev and Shveikin (1981). 

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The results of the LMTO and RLMTO calculations are very similar to those of the RKKR calculations just described. The principle difference appears to come from the different exchange approximations used to create the potentials, rather than the band calculations themselves. The recent calculations by Podloucky and Weinberger for US were made with a similar exchange and correlation approximation to the LMTO and RLMTO calculations. In Fig. 9 a and b we show the TX bands of the actinide nitrides NpN-PuN from self-consistent LMTO and RLMTO calculations, which may be compared with the RKKR calculations of Podloucky and Weinberger. The agreement between the two sets of relativistic bands is very good. 

Fig. 13. Lattice parameters of the actinide nitrides from LMTO (labelled Pauli pramagnetic), RLMTO (labelled Dirac) and LMTO spin polarized (labelled Pauli spin polarized) calculations. The black filled circles are the experimental lattice parameters...

The trend in lattice parameter across the actinide nitride series is reproduced by an energy band theory in which it is assumed that the f-electrons are itinerant. The results with and without spin polarization do not differ greatly until AmN is reached but in this... 

Actinide nitrides are known for Th through Cm. All of the nitrides are high melting compounds with melting points of 2630 °C, 2560 °C, and 2580 °C for Th, Np, and Pu, respectively. The actinide nitrides can decompose to give N2. Thorium, uranimn, and plutonium nitrides are well known and can be used as nuclear fiiels. Fuels of this type, especially uranium and mixed uranium plutonium nitrides, can be used in lead-cooled fast reactors, which have been proposed as a possible next-generation nuclear reactor and for use in deep-sea research vehicles. 

Salt-like or ionic nitrides, in which N forms primarily ionic bonds to alkali metals, rare earth metals, and members of group IIIA. Although actinide nitrides are also included in this grouping, they may equally well be classified as metallic. Compounds in this group are readily hydrolyzed and must thus be protected from moisture. 

Molten-Tin Process for Reactor Fuels (16). Liquid tin is being evaluated as a reaction medium for the processing of thorium- and uranium-based oxide, carbide, and metal fuels. The process is based on the carbothermic reduction of UO2 > nitriding of uranium and fission product elements, and a mechanical separation of the actinide nitrides from the molten tin. Volatile fission products can be removed during the head-end steps and by distilling off a small portion of the tin. The heavier actinide nitrides are expected to sink to the bottom of the tin bath. Lighter fission product nitrides should float to the top. Other fission products may remain in solution or form compounds with... 

The volume contribution by the metallic 5f-5f and the covalent cation 5f-N2p bonds to a virial-theorem formulation of the equations of state of a series of light actinide nitrides was calculated in the self-consistent linear muffin tin orbital (LMTO), relativistic LMTO, and spin-polarized LMTO approximations [46]. The results for ThN give the same lattice spacing in all three approximations higher by ca. 3% than the experimental value, which discrepancy is attributed to the assumed frozen core ions [47]. 

Fig. 3.25 presents some dispersion curves by Brooks et al for NpN, PuN and AmN. The most pronounced changes in this series are observed for the Aj band of 5/ origin. This band becomes more and more narrow when going along this series of compounds. Finally the bands of M5/ and of higher M5/, 6d and N2p states become separated. However, the hybridisation of 5/ and N2p states remains considerable. For example, the state Tjs of NpN consists of 47% df states and 53% N2p states, while Fj of AmN consists of 46% 5/ states and 50% N2p states. The calculated and experimental values of the lattice constant versus the atomic number of the actinides are presented in Fig. 3.26. As can be seen, the experimental dependence exhibits a minimum for UN and is very different from the dependence for rare earth nitrides. The latter is monotonic and exhibits an anomaly for CeN, where Ce has an anomalous valency. While the dependence observed for rare earth nitrides can easily be explained by lanthanide compression, in the case of actinide nitrides the interpretation of such a dependence is far from trivial. The explanation proposed by Brooks et al (1984) is based on a simplified equation of state using canonical band theory. The equation takes into account only /-/ and f-p...

Fig. 3.26 Calculated and experimental lattice constant for actinide nitrides. Solid curve scalar relativistic calculations with spin polarisation. Dashed curve fully relativistic calculations. Dash-and-dot curve scalar relativistic calculations without spin polarisation. Dots experimental lattice constant.

In the context of using uranium and actinide nitride materials as nuclear fuels, Holleck et al. (1968, 1969) reported total miscibility of the rare-earth nitrides with UN, as well as Ettmayer et al. (1979) who noted that the lattice parameters of the RN-UN solid solutions (R=La, Ce, Pr, Nd, Sm, Gd, Dy, Er) generally showed negative deviations from the additivity rule. It can be noted here that in the system U2N3-LaN, a ternary nitride material La2U2Ns was observed and characterized by Waldhart and Ettmayer (1979), with the metal atoms, in the tetragonal unit cell, located in the same positions as in the CsCl-type structure. ThN is also completely miscible with LaN, CeN, PrN, NdN, SmN, GdN,... 

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