Comparisons with Pnictides

The chemistry of phosphorus generally resembles that of arsenic much more closely than that of nitrogen. Whereas nitrogen is only trivalent and forms covalent linkages to not more than four other atoms, arsenic, antimony and bismuth have d orbitals and, like phosphorus, can exhibit tri or penta valency and form five and six-coordinated compounds. 

Nitrogen often forms multiple bonds whereas the remaining pnictide elements have less tendency to do so. Phosphorus, in particular, has much greater tendency than nitrogen to catenate and form continuous chains of single bonds, that is, -P-P-P-P-. All pnictide elements except nitrogen (N) occur in more highly polymerised form. 

There are formal similarities, but structural differences, between various simple N and P compounds. The latter usually exist in more highly polymerised form, and the corresponding As compounds are usually isostructural  

Some series of simple pnictide compounds show a systematic change in properties  

Element Symbol Atomic Number Atomic Wt Stable Isotopes % % 

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The Eu and Yb pnictides in comparison with the alkaline-earth pnictides. Phases marked with contain partly or exclusively (the NaCl-type phases)... 

Parks et al. (1984) and Wieliczka et al. (1984b) noticed that the two-peak structure present in cerium systems also appeared in Pr and Nd systems, but the structures there were further removed from Ef. This seems to rule out a Kondo explanation for the upper peak since in that model such a feature is inherently tied to p It is, therefore, of some interest to see whether our approach followed for the Ce compounds would explain the Pr and Nd pnictides in the same manner as it did the Ce pnictides. Comparison will be made with photoemission data on PrSb and NdSb (unfortunately, no resonance data exist). 

Comparison of the electronic specific heat of the Ln pnictides with those of the metallic (superconducting) La chalcogenides and those of the degenerate narrow-band (superconducting Tc 0.4 K) semiconductors GeTe (rhomboh.) and SnTe (NaCl type) as well as with that of degenerate p-type germanium, Ag and NiS. (tie = number of conduction elections per formula unit.)... 

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.

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