PART OF THE "THIRD COMPONENT" (Ln', M) IN CRYSTAL STRUCTURE FORMATION OF Ln­(Ln',M)­X (X=S, Se, Te) SYSTEM COMPOUNDS

Matveenko I.A., Kuz'micheva G.M.

M.V. Lomonosov State Academy of Fine Chemical Technology, Moscow 117571, Russia
E-mail: kuz7micheva@glasnet.ru

 

Keywords: ternary chalcogenides, structure types, coordination polihedra, stability regions

The binary to ternary compound transition - adding of a third component which can be I­VIII group element as well as lanthanid ion with quite different characteristics may be performed either within the initial (corresponding to binary compound) structure or with formation of new structure types. In the latter case it can be assumed that changing of Ln-ion coordination encirclement is due to cation influence to each other, that partially imitates outer influences (such as pressure and temperature).

Possibility or impossibility of third component adding within an initial structure motive depends on three main factors: geometry (ionic radii ratio), energy (electronegativity ratio), and electronic configuration factor of lattice sharing cations. According to this considerations the "similarity" evaluation criterion was developed:

,

where c means electronegativity, r - cation radius, and A - constant that accounts for electronic configuration pecularities of a cation. Figure shows that growth of F criterion correspondes to reducing of Ln and M ions "similarity". According to the developed criterion and using available structure data we calculated "stability regions" for different structure types. All compounds were divided into subgroups:

  1. Depending on anion (S, Se and Te subgroups).
  2. Depending on Ln atom - dividing of La-Lu seria into 4 parts allows to take into account f­elements electronic structure pecularities.
  3. Depending on stoichiometric cation:anion ratio.

Analysis of these "stability regions" has helped to allocate different types of derived structures: polytypes, polysomes, superstructures and deficient phases and also to reveal following rules:

  1. Coordination number of I (Ln') ion decreases with F criterion growth from 8 to 4-5.
  2. As the Ln and M (Ln') cations "similarity" runs down, coordination polyhedron of Ln changes dramatically so that at maximal F values Ln ion gets quite nonpeculiar coordinati
  3. General number of specific structure types decreases with motion from sulfides to tellurides.

Developed stability fields are to be used for structure prediction of new phases and evaluation of structure refinement correctness.