3RD-ORDER ELASTIC-CONSTANTS OF THE HIGH-TEMPERATURE SUPERCONDUCTOR LA1.8SR0.2CUO4

The complete set of third-order elastic constants of the high-temperat ure superconductor La1.8Sr0.2CuO4 is calculated by taking into account the interactions between nine nearest-neighbour atoms in the lattice. Finite deformation theory is used to obtain the strain energy density phi of a tetragonal 2-1-4-type single crystal of the high-temperature superconductor La1.8Sr0.2CuO4. This is compared with the strain-depen dent lattice energy in continuum model approximation. The second-order derivative of the potential function is obtained from the measured C- 11. The third-order derivative is evaluated from the Mie-Gruneisen int eratomic potential. The second-order elastic constants of La1.8Sr0.2Cu O4 are calculated and compared with the available measured values. The third-order elastic constants of La1.8Sr0.2CuO4 are negative and thei r absolute magnitudes are one order higher than those of the second-or der elastic constants.