Low temperature X-ray diffraction study of the spin-1/2 two-leg ladder system, Sr_0.4Ca_13.6Cu₂₄O_41+δ

The spin-1/2 two-leg ladder system superconductor, Sr_0.4Ca_13.6Cu₂₄O_41+δ, showing the superconducting transition at 13 K under pressure of 3∼6 GPa, has been investigated by using a low temperature powder X-ray diffractometer at 10∼300 K. A first order structural phase transition and new superlattice peaks have not been observed by low temperature X-ray measurements, while the hump in the temperature dependency of electric resistance has been observed at about 50∼150 K. The whole-powder-pattern decomposition method has been used for further structural analysis. Crystal structure of Sr_0.4Ca_13.6Cu₂₄O_41+δ is orthorhombic and incommensurate with modulate along c-axis (σ₁= (0,0, γ), γ= 1.4263 (2)). The incommensurability (γ) is constant to the temperature change. The lattice parameters decrease monotonously with a decrease of temperature. But, the lattice shrinks anisotropically with temperature. The thermal -expansion coefficient of b-axis is the largest and the shrinkage is 0.35% from 300 K to 10 K. Then, the shrinkage of a-axis and c-axis are 0.05% and 0.09%, respectively. Linear coefficient of thermal expansion (γ̄, × 10^-6/K) in the temperature region of 300∼250 K is 3.7, 18.9, 4.8, 3.8 against a, b, c₁ and C₂ axis, respectively. At about 150 K, the coefficient of thermal expansion for b-axis showed a little discontinuous change, which suggests the possibility of a second phase transition.