Evolution of precipitate morphology during heat treatment and its implications for the superconductivity in KxFe1.6+ySe2 single crystals
Evolution of precipitate morphology during heat treatment and its implications for the superconductivity in KxFe1.6+ySe2 single crystals
复制标题
DOI：
10.1103/physrevb.86.144507
复制
发表时间：
2012-10-08
影响因子：
3.7
通讯作者：
Lograsso, T. A.
中科院分区：
文献类型：
--
作者：
Liu, Y.;Xing, Q.;Lograsso, T. A.
我们研究了自熔剂法生长的KxFe1.6+ySe2单晶中析出相形貌与超导性的关系。扫描电子显微镜 (SEM) 测量表明，在铁空位有序-无序转变温度 T-s 以上淬火的样品中，超导相形成网络，而在炉冷样品中，超导相聚集成微米大小的矩形条并排列为断开的链。伴随着形态的这种变化，超导屏蔽部分大大减少。通过在 T-s 以上进行后退火，然后在室温水中淬火，网络恢复后，炉冷样品的超导屏蔽率接近 80%。通过对显示大屏蔽分数的退火样品进行二次热处理，实现了从网络链到棒链的可逆变化，即加热到 T-s 以上，然后在 T-s 上缓慢冷却。 KxFe1.6+ySe2 单晶中观察到的大屏蔽分数实际上是由超导相均匀且连续的分布造成的。通过温度相关的X射线衍射测量发现，KxFe1.6+ySe2单晶中通过T-s冷却，超导相析出，同时铁空位有序相一起形成。它是T-s以上的固溶体，其中铁原子在铁空位无序状态下随机占据Fe1和Fe2位点；相分离是由冷却时铁空位有序-无序转变驱动的。然而，起始混合物中添加的铁和高温淬火都不能将混溶间隙扩大到 KFe2Se2 一侧。
We study the relationship between precipitate morphology and superconductivity in KxFe1.6+ySe2 single crystals grown by self-flux method. Scanning electron microscopy (SEM) measurements revealed that the superconducting phase forms a network in the samples quenched above iron vacancy order-disorder transition temperature T-s, whereas it aggregates into micrometer-sized rectangular bars and aligns as disconnected chains in the furnace-cooled samples. Accompanying this change in morphology the superconducting shielding fraction is strongly reduced. By post-annealing above T-s followed by quenching in room temperature water, the network recovers with a superconducting shielding fraction approaching 80% for the furnace-cooled samples. A reversible change from network to bar chains was realized by a secondary heat treatment in annealed samples showing a large shielding fraction, that is, heating above T-s followed by slow cooling across T-s. The large shielding fraction observed in KxFe1.6+ySe2 single crystals actually results from an uniform and contiguous distribution of superconducting phase. Through the measurements of temperature dependent x-ray diffraction, it is found that the superconducting phase precipitates while the iron vacancy ordered phase forms together by cooling across T-s in KxFe1.6+ySe2 single crystals. It is a solid solution above T-s, where iron atoms randomly occupy both Fe1 and Fe2 sites in the iron vacancy disordering status; and phase separation is driven by the iron vacancy order-disorder transition upon cooling. However, neither additional iron in the starting mixtures nor as-quenching at high temperatures can extend the miscibility gap to the KFe2Se2 side.