Actinide pnictides

The impact on actinide solid state physics of Zachariasen s ideas has been great. It has triggered a very fruitful effort in structure determination. In the meantime, it has underlined the importance, in the theory of actinides, of knowing exactly atomic volumes as an indicator of the complexities of the metallic bond. The same picture of the variation of atomic radii appears for compounds having partial metallic character. In Fig. 5, for instance, the variation vs. Z of the lattice parameter ao of actinide pnictides is compared with the atomic radii of metals. Actinide pnictides have an NaCl type structure. Their... 

Chemical transport with iodine as transporting agent can be generally applied for the growth of actinide pnictides in single crystal form ... 

Therefore, and to avoid possible reactions with the quartz wall, it was attempted to combine synthesis and crystal growth of actinide pnictides in a modified van Arkel process Actinide metal or carbide - the latter obtained by carboreduction of the oxide - are heated in the presence of the pnictogen and of the transporting agent at the... 

Table 2. S)Tithesis temperature of actinide pnictides by the Van Arkel process ...

Self-consistent energy band calculations have now been made through the LMTO method for all of the NaCl-type actinide pnictides and chalcogenides . The equation of state is derived quite naturally from these calculations through the pressure formula extended to the case of compounds . The theoretical lattice parameter is then given by the condition of zero pressure. 

Potter, P. E., Mills, K. C., and Takahashi, Y. (1986) The Chemical Thermodynamics of Actinide Elements and Compounds, part 7, The Actinide Pnictides, STI/PUB/424/7, International Atomic Energy Agency, Vienna, in preparation. 

Notes on the crystal chemistry of selected alloys and compounds of the 15th group elements. Among the pnictides, several series of numerous isostructural compounds are found. The 1 1 compounds with Sc, Y, lanthanides and actinides, having the NaCl-type structure, generally correspond to very stable phases, strongly... 

The general shape of these curves at least for pnictides up to AnAs indicates clearly that the metallie bond is dominated by the electronic structure of the actinide atom. In the further discussion of this chapter (and in Chaps. C-F) it will be made clear that the shape is explained, contrarily to Zachariasen s hypothesis, by means of the f participation in the bond. 

The compositions of the most numerous actinide compounds with elements of the group V of the periodic table (X = N, P, As, Sb, Bi) belong to the types AnX2, An3Xt, AnX. These pnictides can be synthesized in solid-gas-reactions with actinide hydride, or with metal powder obtained by thermal dissociation of hydrides. 

In actinide binary compounds an equation of state can also be developed on the same lines. The difference in electronegativity of the actinide and the non-actinide element plays an important role, determining the degree of mixing between the actinide orbitals (5 f and 6 d) and the orbitals of the ligand. A mixture of metallic, ionic and covalent bond is then encountered. In the chapter, two classes of actinide compounds are reviewed NaCl-structure pnictides or chalcogenides, and oxides. 

Another way to approach the lattice parameter trend is to consider the actinide series of a given pnictide or chalcogenide. The f-p bond should then draw the minimum in lattice parameter towards the beginning of the series. A series of calculations for the... 

The succeeding actinides (Cm, Bk, Cf, Es, Em, Md, No, Lr) mark the point where the list of isolated compounds tends to involve binary compounds (oxides, halides and halide complexes, chalcogenides, and pnictides) rather than complexes. Those studies of complexes that have been made are usually carried out in solution and, from Em, onwards, have been tracer studies. 

The trend in the lattice constant of a given chalcogenide or pnictide across the actinide series differs from the trend for the elemental metals in that the dip in the... 

The sum of the two contributions is a skew parabola with its minimum moved towards the beginning of the series. Further, the skewness and the distance of the minimum from the centre of the series increase as the strength of f-p hybridization increases, as shown in fig, 59. A model similar to that summarized in fig. 58 may also be used for actinide transition metal compounds, with d bands replacing the p bands of the chalcogenide or pnictide. The resulting state densities are then similar to those in fig. 40, modified to include f states in the bands, as in sect. 5.3 for cerium compounds. 

Pressure-induced structural transitions in actinide carbides, pnictides, and chalcogenides AnX (An = Th, U, Np, Pu). For each compound the table lists, from top to bottom the structure of the high-pressure phase, the pressure (range) of upstroke transition (GPa), the volume decrease on transition, and references. (The, ThN no transition in pressure range studied, ThBi, ThTe B2 as ambient pressure phase no transition, + means compound exists, but no HPXRD study known). 

Fig. 21. Logarithm of upstroke-transition pressure versus cation/anion radius ratio for the B1 to B2 transition in lanthanide and actinide compounds. Trivalent ionic radii are used for all cations. Some transitions to other high-pressure structures are included for comparison and have been marked as such in the graphs, (a) Mono-pnictides with As, Sb, and Bi. The isolated data for the phosphides CeP and ThP are not included. No mononitrides have been observed to transform to the B2 type under pressure, (b) Mono-chalcogenides with S, Se, and Te. The EuO transition is outside the graph.

Fig. 22. Lattice parameters of actinide compounds of the ThjP type, (a) pnictides and (b) chalcogenides.

A large number of metaffic compounds with the NaCl-type structure are found at the AnX stoichiometry. These form when X is a chalcogenide, a pnictide or carbon. An3X4 compounds having the cubic Th3P4-type structure frequently coexist with the NaCl-type compounds in systems of the actinides with the chalcogenides and pnictides. 

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