DETECTION OF INTERLAYER CHARGE TRANSFER IN CRYSTALS FROM ATOMIC POSITIONAL PARAMETERS

S.Sh.Shilstein

Russian Research Center Kurchatov Institute, Moscow, 123182, Moscow, Russia

Keywords: crystal structure, layer splitting, layer charge, Coulomb interaction.

In layered crystals the layers which have neighbour layers with different charge are splitted to anion layer shifted to neighbour positive layer and cation layer shifted to neighbour negative layer. This Coulomb splitting for MeO type layer reach up to about 0.09 nm and depends on charge difference of neighbour layers. This relation is general for different structure types [1]. Based on this relation the detection of interlayer charge transfer is possible. It is especially important for HTSC oxides where the BaO or SrO layers are placed between conducting CuO₂ layers and layer which is reservoire of charge. From neutron diffraction data about change of Ba and O positional parameters the interlayer charge transfer was determined by content change in HTSC.

In YBa₂Cu₃O_7-y by transition from y=1 to y=0 the positive charge transfer from chain copper to copper in plaines reach about 0.3 electron charge (i.e. 15% of full change of the chain copper layer charge). In HgBa₂CuO_4+d and HgBa₂CuO₄Fd transfer from mercury layer to copper one reach by superstoichiometric anions addition about 0.2-0.4, i.e. from one half to full change of reservoire charge. For YBa₂Cu₄O₈ and HgBa₂Ca_n-1Cu_nO_2n+2+d oxydes change of positional parameters by high pressure indicated on positive charge transfer to conducting CuO₂ layers. Transferred charge by pressure 5 Gpa is about 0.15 and 0.3 correspondingly. The limiting transfer (up to 1 or 2) can be reached by pressure not much more than 50 Gpa. Thus use of Coulomb splitting model provide the control of interlayer charge transfer based on precise diffraction data. In contrary the describing based on valence calculation from the bond lengths give for complex cryatals not so cogent results. For instance at high pressure the Cu-O bond in mercury cuprates will be shorter but change of Hg-O bond length is practically not detectable. In YBa₂Cu₃O_7-y at change of y the change of calculated valence for Ba and O atoms are not less than for Cu atoms. On other hand it is known that in contrary to Cu atoms the Ba and O atoms have usely constant valence. In addition the BaO splitting value is more sensitive to external influence than bond lengths. Therefore it is more easy to detect the splitting change as result of weak structure change as at high pressure or small content change.

1. S.Sh.Shilstein, A.S.Ivanov, V.A.Somenkov, Physica C 245 (1995) 181-185.