الفهرس 
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Abstract
The effect of chemical sibstitution on improving the physical properties of superconducting materials is of great importance.many types of substitutions have been performed in different high-temperature superconductors and the results depend on the probable site occupancy.the substitution must be between ions of equivalent sizes.the substitution by the rare-earth elements in high-temperature superconductors has special interest in the fields of basic and applied physics.