Theory of electron distributions and63Cu and17O nuclear quadrupole interactions in YBa2Cu3O7 and YBa2Cu3O6

Abstract

Ab initio unrestricted Hartree-Fock Cluster investigations have been carried out on the electronic structures of the YBa2Cu307 and YBa2Cu306 systems. The results of these investigations provide satisfactory explanations of available 63Cu and 170 nuclear quadrupole interaction data. The electron distributions obtained rule out the presence ofCu 3 ÷ ions and are supportive of the presence ofCu 2 +, Cu 1 ÷~ O 1-ions in the O7 system and Cu 2 ÷, Cu 1 ÷ and 02-in the 06 system with actual charges departing significantly from the formal charges, especially in the O7 system, indicating the importance ofcovalency effects. Suggestions wilt be made regarding possible sources that can bridge the remaining gap between theoretical and experimental results for the nuclear quadrupole interaction parameters.

Relativistically parametrized extended Hueckel calculation of net charges on atoms in yttrium barium copper oxide (YBa2Cu3O7)

“…In the absence of reliable information on the third ionization energy of barium, this value has been used by us to calculate the lower limit of E. Using eq 1 we find that the reallocation of two electrons from orbital 85 [0( 1) and O(4) orbitals] to orbitals 124 [Cu (2) orbital] and 137 [Ba orbital] leads to the lowest E (Tables V and VI). Only a minor adjustment of the environmental charges is required now.…”

“…All these examples show that the hole is associated with copper. Most of these indicate the hole to be on Cu(l), but the calculation on [Cu30io]"13 points out the hole to be on Cu (2). In summary, the location of the hole depends on the size, shape, and composition of the cluster under investigation.…”

“…The ground-state configuration is 32*...53*54*55*, and the hole has more or less migrated to the two 0(4) atoms. The transition from orbital 53 is essential for decreasing E. Compared to the migration from orbital 40, the migration from orbital 32 leads to a lower £¡nt, but the interaction with the environment stabilizes the usual configuration to a great extent-so much so that the possibility of finding a hole on Cu (2) is indicated. The result is presumably less reliable, since the environment ofCu(l) within the cluster is far from being adequate.…”

“…They also found a net charge of +1.49 on Cu(1). However, X-ray absorption and photoemission spectroscopies3v4 point out that in YBa2Cu307 both Cu( 1) and Cu (2) are in the +2oxidationstate. Thisclearly indicates the need for a calculation on bigger clusters which can be better representatives of the material.…”

“…It is important to know about the net charges on different atoms of the high-T, cuprates, for with this knowledge one can identify the mechanism responsible for normal-state transport in these systems . Sulaiman et a1. 2 carried out an ab-initio unrestricted HartreeFock calculation on the [CUJOI~I-I~ cluster, where the environmental effect was generated by employing the Watson sphere model for the basis set for oxygen atoms and by augmenting the Hartree-Fock potential experienced by the electrons in thecluster with the potential from the rest of the lattice while treating the lattice ions as point charges. They found that the hole resides on O(4).…”