(Structure stability and compressibility of iron-based superconductor Nd(O0.88F0.12)FeAs under high pressure)
J.G. Zhao, L.H. Wang, D.W. Dong, Z.G. Liu, H.Z. Liu, G.F. Chen, D. Wu, J.L. Luo, N.L .Wang, Y. Yu,
C. Q. Jin, Q.Z. Guo
The high-pressure angle-dispersive X-ray diffraction experiments on iron-based superconductor Nd(O0.88F 0.12)FeAs were performed up to 32.7 GPa at room temperature. An isostructural phase transition starts at about 10 GPa. When pressure is higher than 13.5 GPa, Nd(O0.88F0.12)FeAs completely transforms to a high-pressure phase, which remains the same tetragonal structure with a larger a-axis and smaller c-axis than those of the low-pressure phase. The ambient conditions isothermal bulk moduli B0 are derived as 102(2) and 245(9) GPa for the low-pressure phase and high-pressure phase, respectively. The structure analysis based on the Rietveld refinement methods shows the difference of pressure dependence of the Fe-As and Nd-(O, F) bonding distances, as well as As-Fe-As and Nd-(O, F)-Nd angles between the low-pressure phase and high-pressure phase.
La(O1-xFx)FeAs具有Tc为26K的超导转变温度。而当用离子半径较小的Nd、Sm等镧系元素替代La，可以大幅提高该铁基体系超导体的超导转变温度到55K左右，显示了由元素化学替代引起的物质结构内压的变化对Tc的巨大影响。Ln(O1-xFx)FeAs (Ln=La, Ce, Pr, Nd…)晶体沿C轴方向具有LnO层和FeAs层交替层叠的层状结构，在外部压力的作用下易于被压缩。因此利用高压的手段来研究该体系铁基超导体的超导性质及结构变化具有很重要的意义。我们利用高压原位x光的测量，发现Nd(O1-xFx)FeAs在9.9～13.5GPa范围内发生了等结构的相变。