Coulomb splitting of atomic layers in crystal lattices of

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Physica C 245 (1995) 181-185

Coulomb splitting of atomic layers in crystal lattices of layered cuprates and nickelates S.Sh. Shilstein *, A.S. Ivanov, V.A. Somenkov Russian Research Center, " Kurchatov Institute", 123182 Moscow, Russian Federation

Received 5 October 1994

Abstract

The crystal structure data are collected and analyzed on the splitting of anion-cation layers with non-symmetric electric charge environment. It is shown that the effect is caused by Coulomb interaction with the other charged layers, mainly the nearest neighboring ones. The feasibility is demonstrated of the control of interlayer charge transfer by magnitude of the Coulomb splitting. The correlation between the Coulomb splitting of (BaO) layers, neighboring to (CuO2), and superconducting transition temperature Tc is found in yttrium-barium cuprates.

1. Introduction

It was earlier established in diffraction studies of rare-earth nickelates [1] that a particular feature exists in lattice structures of layered ionic crystals built up from electrically charged atomic layers. If the neighboring layers surrounding a given anion-cation layer have different integral charges, then the cations shift from their own layer towards the negatively charged neighboring layer while the anions move closer to the positive one (Fig. 1). As a result, the cation-anion layer appears to be split into the two sublayers of oppositely charged ions and the sign of the splitting, i.e. direction of charge displacements, unambiguously corresponds to the Coulomb charge interaction.

Presented at the 15th European Crystallographic Meeting 28 August - 2 September 1994, Dresden, Germany. * Corresponding author.

In this paper, based on a model of Coulomb interaction, the relation between the ion charges and the value of the "Coulomb" splitting is derived and compared to the real splittings of metal-oxygen (MeO) layers in crystal lattices of cuprates and nickelates. It is shown that the model of Coulomb interaction with only the nearest layers correctly predicts the sign of the splitting and its dependence on ion charges for a number of compounds with different layer stacking. It is illustrated within the series of K2NiF4-type structure compounds that the exact calculation of Coulomb crystal energy, taking into account all the ion charges in an infinite crystal, gives the proper magnitude of the splittings. It is noted that the highest-Tc compounds (Bi-, T1- and Hg-based cuprates) reveal the maximal Coulomb splitting of (MeO) layers next to the conducting (CuO 2) planes, and within one particular system of Y-123 and Y-124 a correlation is demonstrated between the magnitude of the anion-cation layer splitting and Tc values. The possible role in the mechanism of high-To super-

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S.Sh. Shilstein et al. /Physica C 245 (1995) 181-185

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conductivity of the (MeO) layers located near the (CuO 2) planes is discussed.

2. Analysis of the observed data on anion-cation layer splitting in crystals with different structures The Coulomb interaction of the ions in the layer, built up from cations (with charge qc > 0) and anions (qa < 0), with the surrounding charged layers, carrying integral charges Q1 > 0 and Q2 < 0 as counted for one planar quadratic cell of size " a " , leads to a splitting force (IFlc I + [ F l a I, Fig. 1). This force is balanced by a contracting force of interaction between the two split sublayers of opposite charges which is proportional to the magnitude A of splitting. The equilibrium condition for small values of A gives the relation [2] A = 4~raC-I[ P( Q~ - Q2)],

Tl-based cuprates) the average position parameters of ions are used which were refined in some independent and most reliable structure determinations. The analysis has been performed for the series of compounds: L a 2 _ x S r x C u O 4 , R 2 N i O 4 , R 3 N i 2 0 7 and R4Ni3010 (R = La, Pr, Nd); T12Ba2Ca,Cu,+ 1O2n+6, Bi2Sr2Ca,Cu,+102,+6, HgBa2CuO4+ 8, HgBa 2CaCu 206 + 8, and La 2- x(CaSr)l +xCu 206 + a (the bibliography is given in Ref. [2]). The analysis shows that the splitting of (MeO) layers could be experimentally detected only for a nearest environment of asymmetric charge and the obtained dependence of the splitting magnitude on the parameter for different stacking of layers (Fig. 2) indeed appears to be linear in a wide range of ~ values in accordance with Eq. (1). Nevertheless, the values calculated with Eq. (1) are several times higher than the experimental ones. This is seemingly due to approximations chosen to derive relation (1), namely, restriction of the interactions up to the nearest-neighboring layers and charges Q1 and Q2 uniformly spread over the whole plane. Indeed, the calculations of Coulomb energy of K2NiFa-type crystal structures including all the electrostatic charges in the infinite lattice give magnitudes of splittings very close to the observed ones in

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where C is a constant defined by only the particular structure of the splitting layer (C = 16.48 for the most important case of a NaCl-type layer), while the factor p = ( q ~ - qc)/[2(q~qc)] represents the average inverse charge of the ions in the split layer and characterizes its "charge softness". The relation (1) shows a linear dependence of the splitting on the combined charge parameter ~ = P(Q1 - Q2). The experimental data on anion-cation layer splittings were analyzed for the Cu- and Ni-based oxides for which the charges of all the ions might be unambiguously defined. The most precise structural data obtained by neutron (mainly for powder sampies) and X-ray (single crystals) diffraction were selected. To determine the value of (MeO) layer splitting for one particular compound (say, (BaO) in

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