ANISOTROPIC THERMAL-EXPANSION AND GRUNEISEN COEFFICIENTS OF THE BISMUTH CUPRATE 2-2-1-2

The structural evolution of the superconductor Bi2Sr2CaCu2O8 has been analyzed by X-ray diffraction measurements of the cell parameters a an d c between 5 and 220 K. A tetragonal modeling already applied to YBa2 Cu3O7 is used in order to characterize the evolution of the anisotropi c thermo-elastic properties of this bismuth cuprate by means of the De bye model and Gruneisen coefficients. Similarly to the case of YBa2Cu3 O7, a strong peak gamma(a) approximate to 2.8 at 45 K for the Gruneise n coefficient in the a, b plane is evidenced (gamma(c)= 0.85). The mea n value gamma(a)=1.2 is determined at 220 K. The evolution of gamma(a) (T) indicates that microstructural effects might induce an anomalous e xpansion below T-c.