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Revealing an unusual transparent phase of superhard iron tetraboride under high pressure
Kotmool, Komsilp,Kaewmaraya, Thanayut,Chakraborty, Sudip,Anversa, Jonas,Bovornratanaraks, Thiti,Luo, Wei,Gou, Huiyang,Piquini, Paulo Cesar,Kang, Tae Won,Mao, Ho-kwang,Ahuja, Rajeev National Academy of Sciences 2014 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF Vol.111 No.48
<P><B>Significance</B></P><P>Solids have been mainly studied at ambient conditions (i.e., at room temperature and zero pressure). However, it was realized early that there is also a fundamental relation between volume and structure and that this dependence could be most fruitfully studied by means of high-pressure experimental techniques. From a theoretical point of view this is an ideal type of experiment, because only the volume is changed, which is a very clean variation of the external conditions. In the present study we show a hard superconducting material, iron tetraboride, transforms into a novel transparent phase under pressure. Further, this phase is the first system in this class, to our knowledge, and opens a new route to search for and design new transparent materials.</P><P>First principles–based electronic structure calculations of superhard iron tetraboride (FeB<SUB>4</SUB>) under high pressure have been undertaken in this study. Starting with a “conventional” superconducting phase of this material under high pressure leads to an unexpected phase transition toward a semiconducting one. This transition occurred at 53.7 GPa, and this pressure acts as a demarcation between two distinct crystal symmetries, metallic orthorhombic and semiconducting tetragonal phases, with <I>Pnnm</I> and <I>I</I>4<SUB>1</SUB>/<I>acd</I> space groups, respectively. In this work, the electron–phonon coupling-derived superconducting T<SUB><I>c</I></SUB> has been determined up to 60 GPa and along with optical band gap variation with increasing pressure up to 300 GPa. The dynamic stability has been confirmed by phonon dispersion calculations throughout this study.</P>