Chalcogenides lattice constant

Numerous ternary systems are known for II-VI structures incorporating elements from other groups of the Periodic Table. One example is the Zn-Fe-S system Zn(II) and Fe(II) may substimte each other in chalcogenide structures as both are divalent and have similar radii. The cubic polymorphs of ZnS and FeS have almost identical lattice constant a = 5.3 A) and form solid solutions in the entire range of composition. The optical band gap of these alloys varies (rather anomalously) within the limits of the ZnS (3.6 eV) and FeS (0.95 eV) values. The properties of Zn Fei-xS are well suited for thin film heterojunction-based solar cells as well as for photoluminescent and electroluminescent devices. 

The behaviour of A in various compounds across the RE series corresponds quite well to, for example, that of the lattice constant. When A is less than 2 eV, the 4/"+1 divalent configuration is preferred. Due to the extra screening of the nuclear charge the radius of RE ions increases by about 20% this is reflected in the larger lattice constant, see Fig. 11, for the typical case of the RE chalcogenides. 

Ternary Rhenium and Technetium Chalcogenides 1599 Table 3. Lattice constants for the compounds MoftCln, Re6Se6Cl6, and Cs6Rc6S 2. ... 

FIGURE 1 Lattice constants of the elemental lanthanides (top), their chalcogenides (middle) (after Jayaraman, 1979), and pnictides (bottom). For the elemental lanthanides, it is the atomic sphere radius, 5, that is shown instead of the lattice parameter, where S is defined as V = 4/3tiS with V the unit cell volume. 

The jumps in the lattice constants in Figure 1, seen for the elemental Eu and Yb, as well as at the chalcogenides of Sm, Eu, Tm, and Yb, are due to the change in valence from trivalent to divalent. If a transition to the trivalent state were to occur, the lattice constant would also follow the monotonous behaviour of the other lanthanides, as seen in Figure 2, where the ionic radii of trivalent lanthanide ions are displayed. For the pnictides, only CeN shows an anomaly, indicating a tetravalent state, whereas all the other compounds show a smooth, decreasing behaviour as a function of the lanthanide atomic number. 

Fig. 37. Exchange interactions between the ESR probe G

Figure 12 gives the lattice constants for the sulfides, selenides and tellurides of the lanthanides (Campagna et al. 1976). These constants show a regular behaviour except for some deviations for Sm, Eu, Tm and Yb. The regular curves are for the normal trivalent chalcogenides. The anomalous lattice constants occur for the divalent lanthanides, which due to the additional 4f electron have a larger ionic radius. Thus one sees that under normal conditions SmS, SmSe and SmTe are divalent (with respect to the Sm ion) but they can be driven into the mixed valent state by external pressure (Jayaraman et al. 1970). 

Similar observations can be obtained for the thulium chalcogenides. TmTe is obviously divalent. With increasing internal pressure (decreasing lattice constant), the tendency towards trivalency increases. This means that TmS is purely trivalent and TmSe is mixed valent as is evident from the magnitude of their lattice constants. One of the first intermediate valent system studied by photoemission was the... 

Let us now move to TmSe in the center of fig. 60. Due to the reduced lattice constant the crystal-field splitting lODq of the 5d conduction band increases and - since the separation 4f -5d center of gravity remains constant (as shown also for the Sm chalcogenides in fig. 37) - the bottom of the conduction band now overlaps with the 4f... 

The Eu chalcogenides crystallize in the fee rocksalt structure. The lattice constant is increasing with increasing anion radius from 5.14 A for EuO to 6.60 A for EuTe (see table 19.1). Since the Eu chalcogenides form divalent compounds... 

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... 

Fig. 56. The measured (filled triangles and circles) and calculated (indicated by full line) lattice constants of the uranium mono-chalcogenides and monopnictides.

Fig. 40. Dependence of hyperfine field (left) and isomer shift (right) on lattice constant for Np mono-pnictides (circles) and mono-chalcogenides (squares),...

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