Phase transitions and electrical conductivity of selenium disulfide at high pressure

Sulfur and selenium demonstrate one of the most complex behavior under high pressure among all elements of the periodic table. Despite being known to form interchalcogens, the properties of these compounds have not been widely explored in the dense state. Through a series of diamond anvil experiments combined with x-ray diffraction, optical spectroscopy, and electrical resistance measurements, we explore the properties of selenium disulfide <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mo>(</a:mo> <a:msub> <a:mi>SeS</a:mi> <a:mn>2</a:mn> </a:msub> <a:mo>)</a:mo> </a:math> up to pressures of 150 GPa. At ambient pressure, <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:msub> <b:mi>SeS</b:mi> <b:mn>2</b:mn> </b:msub> <b:mtext>−</b:mtext> <b:mi mathvariant="normal">I</b:mi> </b:mrow> </b:math> represents a substitutional solid solution of S and Se atoms, forming an eight-membered molecular ring arrangement analogous to <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mi>γ</d:mi> </d:math> -S. The band gap of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:msub> <e:mi>SeS</e:mi> <e:mn>2</e:mn> </e:msub> <e:mtext>−</e:mtext> <e:mi mathvariant="normal">I</e:mi> </e:mrow> </e:math> rapidly closes upon compression, and above 27 GPa, there is a transformation to a metallic tetragonal phase <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:mo>(</g:mo> <g:msub> <g:mi>SeS</g:mi> <g:mn>2</g:mn> </g:msub> <g:mtext>−</g:mtext> <g:mi>III</g:mi> <g:mo>)</g:mo> </g:mrow> </g:math> , in which atoms form square helical chains. Upon further compression, we observe a phase sequence from incommensurate modulated <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:mrow> <h:msub> <h:mi>SeS</h:mi> <h:mn>2</h:mn> </h:msub> <h:mtext>−</h:mtext> <h:mi>IV</h:mi> </h:mrow> </h:math> above 59 GPa, transforming to rhombohedral <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mrow> <i:msub> <i:mi>SeS</i:mi> <i:mn>2</i:mn> </i:msub> <i:mtext>−</i:mtext> <i:mi mathvariant="normal">V</i:mi> </i:mrow> </i:math> by 116 GPa.