STRUCTURAL INHOMOGENEITIES IN OXYGEN-DEFICIENT ERBA2CU3O6+X ASSOCIATED WITH THE TETRAGONAL-TO-ORTHORHOMBIC TRANSITION - EVIDENCE OF 1ST-ORDER BEHAVIOR

Neutron powder diffraction and ac susceptibility data on a series of ErBa2Cu3O6 + x samples prepared to achieve thermal equilibrium at low temperature provide evidence of structural inhomogeneities near the tetragonal-to-orthorhombic (T --> O) transition. In the composition range 0.25 less-than-or-equal-to x less-than-or-equal-to 0.45, the data can best be described as a mixture of orthorhombic and tetragonal phases. This observation, together with the presence of discontinuities in the order parameters, leads to the conclusion that the T --> O transition is first order in the temperature range where these samples were prepared. This interpretation provides a simple explanation for the differences between oxygen-deficient R Ba2Cu3O6 + x (R = Y, rare earths) samples prepared by different techniques.