DETERMINATION OF SOME ANISOTROPIC ELASTIC COEFFICIENTS OF THE COMPOUND YBA2CU3O6.7F0.3

The orthorhombic compound YBa2Cu3O6.7F0.3 noted F-(123) is obtained by chemical addition of 0.3 fluorine atom into the pure (123) compound w ith 6.7 oxygen atoms. The high pressure study by neutron powder diffra ction at low temperature leads to volumic expansion coefficients at 29 5 K alpha(v) = 43 and 47 x 10(-6) K-1 under ambient and 4.9 kbar press ures respectively, a compressibility coefficient chi(v) = 5.4 X 10(-12 ) Pa-1, i.e. a bulk modulus B = chi(v)-1 = 187 GPa. These values are c lose to those still found for the pure (123) but disagree with other o nes from the literature. The value 2.9 +/- 0.6 is found for the mean G runeisen coefficient. It is abnormally high and far from the value 1.2 found for the bismuth cuprate (2212). The origin of this observed ano maly is discussed.