Oxides and Oxide Superconductors: Elastic and Related Properties
M Lei, H M. Ledbetter
Using both measurements and modeling, we studied the elastic and related properties of some oxides and oxide superconductors. The polycrystal elastic constants were measured using a megahertz-frequency pulse-echo method between 295 and 4 K and corrected to the void-free state by using a model for a composite material containing spherical particles. The elastic moduli of the high-Tc superconductor Y1Ba2Cu3Oτ (YBCO) were compared with that oxides, especially the perovskites BaTiO3 and SrTiO3, which are crystal-structure building blocks for the YBCO superconductor. The bulk moduli were also calculated using a Born ionic model with two energy terms: electrostatic (Madelung) and ion-core-repulsion. The calculated bulk modulus of YBCO, 98 Gpa, agrees well with measurement, 101 GPA. Based on monocrystal measurements combined with analysis-theory, elastic stiffnesses Cij for orthorhombic YBCO were estimated. The bulk modulus obtained from the estimated C^ij^ by the Voigt-Reuss-Hill averaging method agrees with the monocrystal measurement. From the measured polycrystal elastic constants, the Debye characteristic temperatures, θD, were calculated. For YBCO, θD = 437 K. The electron - Phonon parameters, γ were estimated from T^c^ and θ D using Kresin's model, which is valid for all values of γ. For YVCO, γ = 2.24. Calculating the Madelung energy allowed us to study two further features: the valence of copper and the electron hole distribution. The results show that the hole prefers the CuO2 plane at the oxygen sites. All the results are consistent with the assumption that all copper ions have valences near +2. Using a relationship between Tc6 and δ VdA^, the difference in Madelung site potentials for a hole at the apical and planar oxygens, the pressure derivative and stress and stain derivatives of Tc were calculated. The results show that Tc increases with decreasing a-axis, increasing b-axis, and decreasing c-axis.
and Ledbetter, H.
Oxides and Oxide Superconductors: Elastic and Related Properties, NIST Interagency/Internal Report (NISTIR), National Institute of Standards and Technology, Gaithersburg, MD
(Accessed June 6, 2023)