Monochalcogenides

Monochalcogenides, LnZ (Z = S, Se, Te), have been prepared for all the lanthanides except Pm, mostly by direct combination.They are almost black and, like the monoxides, have the NaCl structure. However, with the exceptions of SmZ, EuZ, YbZ, TmSe and TmTe, they have metallic conductivity and evidently consist of Ln -t- Z ions with 1 electron from each cation delocalized in a conduction band. EuZ and YbZ, by contrast, are semiconductors or insulators with genuinely divalent cations, but SmZ seem to be intermediate and may involve the equilibrium ... 

Most monochalcogenides of the Group 3 metals adopt the rock salt (NaCl) structure. Note that the crystal chemistry of divalent europium is very similar to that of the alkaline earths, particularly strontium, as the radius of Eu is almost the same as that of Sr ". For the Yb compounds, the cell dimensions are practically identical with those of the Ca compounds. 

All the 12 monochalcogenides of the IIB metals crystallize in the tetrahedral zinc blende (ZB) or wurtzite (W) structures, as shown in the table below, with the exception of HgS that exists also in a distorted rock salt (RS) form. 

Tertiary phosphine chalcogenide Diphosphine dichalcogenide Diphosphine monochalcogenide... 

Asymmetric /) .vamidocyclodiphosph(V,III)azane monochalcogenides (40), containing a phosphorus (V) and a phosphorus (III) centre, are accessible from stoichiometric oxidation of the parent feamidocyclodiphosphazane with... 

The monochalcogenides 43 can also theoretically exist in more than one tautomeric form an N-H tautomer 43a, a P-H tautomer 43b or an E-H tautomer 43c. Solution and solid-state studies on these compounds show that, depending upon the nature of the R substituents, either the N-H or the P-H tautomer predominates. Substituents which lead to increased basicity of the phosphorus lone-pair (e.g., R = NMe279, Pr80) have been shown to favour the formation of the P H tautomer, whereas in systems where the phosphorus lone pair is less basic (e.g., R = Ph81) the N-H tautomers are favoured. 

Through optical work on the monochalcogenides of Eu 9d), Yb (96) and Sm(9c,excitation energies which increase in the order -0, -S, -Se, -Te. This very unusual ordering customarily has been ascribed to an upward shift in the t g state, originating from a reduction in TO Dq with the increase in Uq. Such a description is however surely inappropriate for f -> d excitations in the present chalcogenides. 

In the case of the/ samarium monochalcogenides 9c,d), one finds then for the energy of the first / -> d optical absorption peak — which is associated with the transitions 4 / / Fq) 4/ ( Hj) tfg - the following rising sequence of values -... 

Fig. 8. Resistivity of samarium monochalcogenides versus pressure. The data from Ref. (5) has been extrapolated to p = 2 x 10 fl-cm, which is taken as appropriate to fuU crossover. The behaviour of SmS that would occur, if no first order transition occurred, is indicated here, and again in Fig. 9.

The situation close to that described above for sulfur is characteristic for the chal-cogenide elements. Atomic selenium [1,26,82-86] and tellurium [86] are part of di-, tri-, and polynuclear clusters, while diatomic molecules of these elements, when acting as ligands, have mostly a bridge function 52. Examples of di- and trinuclear monochalcogenide compounds are the complexes 53 and 54 [82-85] ... 

O. Vogt and K. Mattenberger, Magnetic measurements on rare earth and actinide monopnictides and monochalcogenides 301... 

Tetrahedral coordination is again a dominant theme. All the monochalcogenides adopt either a wurtzite or zinc blende structure, described below, in which the metal is tetrahedrally coordinated. Various aspects of close packing in relation to the structures of ZnO, ZnS, and other zinc compounds are depicted in Figure 1. 

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