Low Temperature X-ray Diffraction Study of the Spin-1/2 Two-leg Ladder System, Sr_0.4Ca_13.6Cu₂₄O_41+��

The spin-1/2 two-leg ladder system superconductor, Sr_0.4Ca_13.6Cu₂₄O_41+��, 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<sub>0.4</sub>Ca<sub>13.6</sub>Cu<sub>24</sub>O<sub>41+��</sub> is orthorhombic and incommensurate with modulate along c-axis (��<sub>1</sub>= (0,0,��),��=1.4263 (2)). The incommensurability (��) 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%o from 300K to 10 K. Then, the shrinkage of a-axis and c-axis are 0.05% and 0.09%, respectively. Linear coefficient of thermal expansion (��<sub>mean</sub>,�~10<sup>-6</sup>/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.