New superconductors open up the periodic table

For two decades, the search for superconductors that worked at high temperatures was restricted to copper. Now a new family of high-temperature superconductors based on iron has been discovered - and materials scientists are wondering what other combinations of elements they might try. So far the quest is guided by luck as much as by judgement, so researchers are hoping the new materials will provide clues to the recipe for high-temperature superconductivity.

Superconductors can carry electric current with no resistance - a property which could transform electric power generation if it were possible to maintain at room temperature. Since 1986, the field has been dominated by ceramic copper-oxide materials (cuprates), where copper-oxygen planes carry electrons between surrounding layers of various elements. Though an operating temperature of 138K (at ambient pressure) was established in 1995, progress since then has been frustratingly slow. Most other types of superconductors only work at temperatures close to absolute zero.

Materials developed this year by Japanese and Chinese researchers, however, consisting of iron-arsenic compounds sandwiched between rare earth oxides doped to provide extra charge carriers, are superconductors up to a promising 55K, though they have yet to work at the boiling point of liquid nitrogen. 'It's superconductivity in places you've never thought of,' says David Larbalestier, a materials scientist working at Florida State University's national high magnetic field laboratory.

The first of the new compounds, LaOFeAs doped with fluorine, was reported in February 2008 by Hideo Hosono and colleagues at the Tokyo Institute of Technology [1]. It was a superconductor at 26K, and higher if put under pressure. Subsequently, Xianhui Chen's team at the University of Science and Technology of China, in Hefei, replaced the rare earth lanthanum with samarium (SmO1-x FxFeAs) and got the critical temperature up to 43K [2]. Zhong-Xian Zhao's group at Beijing's National Laboratory for Superconductivity, China, have since shown that similar compounds with other rare earths are superconductors above 50K; the samarium material under pressure holds the current 55K record [3].

The crystal structures of materials made so far are very similar, but a good deal of tinkering with the [rare earth][oxygen][dopant][transition metal][group 5 element] formula is expected. 'You can try replacing just about every element in the formula,' says Pengcheng Dai, a physicist at the University of Tennessee, Knoxville. The rare earths have varied through lanthanum, samarium, praseodymium, neodymium, cerium and gadolinium. The arsenic has been replaced with phosphorus, the iron with nickel, and even the dopant has been changed from fluorine, providing extra electrons, to strontium or oxygen vacancies, providing holes - though these experiments have not reached such high critical superconducting temperatures (Tc).


Source: http://www.rsc.org/chemistryworld/News/2008/May/30050802.asp