Sr–Si diagram at Si contents of 55–100 at% and crystal structure of SrSi2-x

Abstract

Cubic SrSi2 has attracted attention as a thermoelectric material or Weyl semimetal candidate material. However, its physical properties have not been examined in detail. High-quality samples are required for this purpose. A previous study on the Sr–Si phase diagram reported that cubic SrSi2 is a low-temperature phase and tetragonal SrSi2 is a high-temperature phase based on little experimental evidence, highlighting the difficulty in synthesizing high-quality cubic SrSi2 samples. Therefore, a new investigation of the Sr–Si phase diagram is necessary. This study experimentally investigated the Sr–Si diagram at Si contents of 55–100 at% using arc-melted samples by performing inductively-coupled plasma optical emission spectroscopy, electron-probe microanalysis, powder X-ray diffraction analysis, and differential thermal analysis. Two intermetallic compounds (SrSi2-x and SrSi2) and two eutectic reactions existed in this Si content range. The eutectic points were approximately 57 at% and 1053.2 °C for Liquid⇔SrSi + SrSi2-x and approximately 75 at% and 1050.0 °C for Liquid ⇔SrSi2 + Si. SrSi2 had no high-temperature phase and melted congruently at 1121 °C. SrSi2-x existed at Si contents of 62.4–65.0 at%, and it is formed by the peritectic reaction Liquid + SrSi2⇔ SrSi2-x. SrSi2-x crystallized into a monoclinic variant of the √5 × √5 × 1 α-ThSi2-type superstructure with ordered Si defects at the body-center position of the cell (space group: I121, No. 5). SrSi2-x is the first example of the α-ThSi2-type superstructure.