Ganguly, P. ; Shah, N.
(1993)
*Use of an AX _{3} close-packing description of layered perovskites in understanding the role of various A ions in cuprate superconductors*
Physica C: Superconductivity and its Applications, 208
(3-4).
pp. 307-322.
ISSN 0921-4534

Full text not available from this repository.

Official URL: http://linkinghub.elsevier.com/retrieve/pii/092145...

Related URL: http://dx.doi.org/10.1016/0921-4534(93)90202-2

## Abstract

The layered perovskite oxide systems have been modelled on the basis of two types of AX_{3} close-packing found, for example, in the Ll_{2} and DO_{22} structure types of Cu_{3}Au and TiAl_{3}, respectively. The former is known to give the ABX_{3} perovskite structure. It is proposed that the latter gives the AX rock-salt structure upon the loss of XX rows for which the ionic radius ratio rA/rX > 0.73. The lower limit of the tolerance factor is also justified in such a packing scheme based on this geometrical constraint of rA/rX > 0.73 or a minimum r_{A}sim;1.02 Å. An admixture of the two kinds of packing yields the layered perovskite structure. The AX^{#}_{3} packing (X^{#} includes the anion vacancy) may be modelled in terms of pseudo-spins in the axial next-nearest-neighbor-Ising (ANNNI) model. The pseudo-spins correspond to the two relative orientations of the AX^{#} rows instead of the lengths of the domains as in the usual description of long period intermetallic superlattices using antiphase boundary (APB) structures. Just as the intermetallic compounds, the layered cuprate perovskites in general have the m, 1^{n} Fisher-Selke notation where m is the number of spins in "ferromagnetic" ...AX#.B.X^{#}X^{#}.AX^{#}... (perovskite) domains and n is the number of spins in the "antiferromagnetic" ...AX^{#}.X^{#}X^{#}.X^{#}A...(or rock-salt) domains. Three sites for the A ions may then be distinguished. The 3, 1 notation has the sequence ...A_{P}X^{#}.X^{#}X^{#}.B.A_{F}X^{#}.X^{#}X^{#}.B.A_{P}X^{#}.X^{#}X^{#}.X^{#}A_{R}.X^{#}X^{#}.ARX^{#}.X^{#} X^{#}..., where A_{P} are large cations such as La^{3+}, Ba^{2+}, Sr^{2+} in nine-fold coordination; A_{F} are cations with eight-fold coordinations such as Ca^{2+} and Y^{3+}. TlBa_{2}CaCu_{2}O_{7} is an example of such a notation. The A_{R} ions with six-fold coordination are ions such as Tl, Bi, Pb etc. It is proposed that the constraints of the close-packing model require the average size of the A_{R} ions to have an average radius of 1.02 Å. This is due to an admixture of various valence states in the A_{R} ions such as Bi, Tl, Pb to provide epitaxial stabilization. The possible role of an admixture of the valence skipping states of the A_{R} ions in enhancing the superconducting transition temperatures is discussed.

Item Type: | Article |
---|---|

Source: | Copyright of this article belongs to Elsevier Science. |

ID Code: | 13843 |

Deposited On: | 12 Nov 2010 14:37 |

Last Modified: | 31 May 2011 09:44 |

Repository Staff Only: item control page