A SHEAR TRANSFORMATION IN THE A-B PLANE OF YBA2CU3O7-DELTA FROM THE SUPERCONDUCTING ONSET

The discontinuities or jumps in the crystallographic thermal expansion coefficients across the superconducting to normal phase boundary have been reported to contain opposite signs and nearly equal magnitudes between the a and b crystal axes. A shear transformation is shown to account for almost all of the thermal expansion jumps in a coordinate system that is rotated by pi/4 about the c axis. The heat capacity, the shear ther-mal expansion coefficient and the elastic compliance matrix jumps in this rotated coordinate are related using second order phase transformation thermodynamics for a solid. The lattice motion for the shear transformation involves a single coupled displacement. A shear stress that gives a pure shear strain displacement will change T(c) in this crystal. The thermal expansion jumps when restricted to contain no volume terms have been emphasized in this paper. This shear thermal expansion is found to interact with the lattice through the orthorhombic shear compliance Of (S11 + S22 - 2s12).