Increased transition temperature in superconducting Na2CsC60 by intercalation of ammonia
Increased transition temperature in superconducting Na2CsC60 by intercalation of ammonia
复制标题
通过插入氨提高超导 Na2CsC60 的转变温度
作者：
O. Zhou;R. Fleming;D. Murphy;M. Rosseinsky;A. P. Ramirez;R. B. Dover;R. Haddon
合成了多种碱金属富勒化物(AxC60)，其化学计量比在1=x=11之间变化。大多数x=3的化合物都是具有异常高的转变温度(T_C)的超导体，其转变温度随着面心立方晶胞尺寸的增大而增大。晶格膨胀时T_c的增加归因于导带宽度的减小，因此人们最终预计T_c会饱和或下降，随后会发生金属-绝缘体转变。我们试图通过插入能够溶解A+离子的中性分子来扩展A3C60结构来探索这一机制。本文报道了氨水与Na2CsC60(T_c=10.5K，晶格常数α=14.132Å)反应生成的化合物(NH_3)_4Na_2CsC_(60)，其晶胞参数a=14.473Å，T_c增加到29.6K，结构中八面体中心含有Na(NH_3)_4~+。
A WIDE variety of alkali-metal fullerides (AxC60) have been prepared, with stoichiometries varying between 1=x=11 (refs 1,2). Most of the compounds with x = 3 are superconductors with unusually high transition temperatures (Tc) which increase with the size of the face-centred cubic unit cell3. The increase in Tc on lattice expansion is attributed to a decrease in the conduction band width, and therefore one ultimately expects a saturation or downturn of Tc followed by a metal–insulator transition. We have sought to explore this regime by expanding the A3C60 structure through intercalation of neutral molecules capable of solvating the A+ ions. Here we report that the reaction of ammonia with Na2CsC60 (Tc = 10.5K; lattice constant α = 14.132 Å) produces the compound (NH3)4Na2CsC60, which has an expanded unit cell (a = 14.473 Å) and an increased Tc of 29.6 K. The structure contains the Na(NH3)+4 cation on the octahedral site.