| RESEARCH HIGHLIGHTS |


[RESEARCH HIGHLIGHTS]


Iron-tellurium conducts electricity best when in a disordered amorphous phase.


Materials science: DISORDERLY CONDUCT


AN ANALYSIS OF ELECTRON BEHAVIOR EXPLAINS WHY THE PHASE-CHANGE MATERIAL IRON-TELLURIUM BEST CONDUCTS ELECTRICITY IN ITS DISORDERED AMORPHOUS PHASE


Solid materials whose atoms are arranged in a well-ordered crystalline structure are usually better conductors of electricity than randomly structured, or amorphous, solids. Recently, however, A*STAR researchers found that iron–tellurium (FeTe) breaks this rule, displaying higher conductivity, and optical reflectivity, in the amorphous phase. A recent study, published in the journal


Acta Materialia, describes their efforts to understand why FeTe’s behavior is counterin- tuitive to expectations1


. FeTe is a phase-change material, with


the ability to rapidly switch its state from crystalline to amorphous and back again when it is heated or cooled, a property which makes


4 A*STAR RESEARCH


it useful for data storage and memory appli- cations. Conventional phase-change materials such as germanium–antimony–tellurium (GST), commonly used in rewritable DVDs, display higher optical reflectivity and electrical conductivity in their crystalline state because the highly ordered structuring of atoms in the crystal results in more electron vacancies, or holes, that act as charge carriers. “FeTe behaves differently from other


phase-change materials,” explains Kewu Bai at the A*STAR Institute of High Performance Computing, who worked on the project with scientists from the National University of Singapore. “We hypothesized that these unusual characteristics may be connected


with the behavior of ‘lone-pair’ electrons. This refers to a pair of electrons from any one atom that are not involved in the bonding of materials.” The team prepared thin films of FeTe at


room temperature to produce amorphous structures, and at 220 degrees Celsius to acquire crystalline samples, and showed that the films could be flipped between the two states using a fast pulsing laser. They analyzed the molecular structure of the different films using X-ray spectroscopy, electron microscopy and first-principles calculations to investigate these unusual properties of FeTe. The researchers confirmed the existence of lone-pair electrons in both the amorphous


ISSUE 5 | OCTOBER – DECEMBER 2016


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