AN ANISOTROPIC EHRENFEST EQUATION FOR ORTHORHOMBIC SHEAR STRAINS IN YBA2CU3O7

A [110] shear transformation has previously been shown to account for the crystallographic thermal expansion coefficient's anomalies or discontinuities on both the a and b crystal axes. The mean-field jump in the thermal expansion coefficient of the normalized orthorhombic strain is shown here to be this same shear strain transformation. The anomaly at T(c), assuming piece-wise continuous orthorhombic strains as determined from x-ray and neutron diffraction data, agrees with single-crystal thermal expansion jumps measured exactly at T(c). The shear thermal expansion coefficient measured from the shear strains of (b - a)/(b + a), at temperatures well above T(c), is constant. Finally, Ehrenfest's equations have been integrated and an exponential form relating T(c) to the applied shear stress and anomalies in the shear thermal expansion coefficient and heat capacity is presented. The Ehrenfest anomaly is shown here to include the inverse thermal expansion coefficient discontinuity with a characteristic temperature of approximately 9 X 10(-5) K. The shear stress is compared to a structural contribution to the superconducting onset across the superconducting phase boundary of 23 GPa in YBa2Cu3O7; this value is compared to the estimated in-plane shear modulus of 58-98 GPa.