Lonesome <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>Ag</mml:mi></mml:math> Atoms Drive Ultralow Thermal Conductivity in Argyrodite <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" displ

Because of their rapid ionic diffusion, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mi>Ag</a:mi></a:math>-based argyrodites have great potential as solid-state electrolytes. The fundamental thermal transport in these materials is important for the overall all-solid-state battery design. Using an accurate framework and a unified heat-transport theory, we compute both the particle-like (<d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><d:msub><d:mi>κ</d:mi><d:mi mathvariant="normal">p</d:mi></d:msub></d:math>) and the glass-like (<h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><h:msub><h:mi>κ</h:mi><h:mi mathvariant="normal">c</h:mi></h:msub></h:math>) components of the lattice thermal conductivity (<l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><l:msub><l:mi>κ</l:mi><l:mi mathvariant="normal">L</l:mi></l:msub></l:math>) of <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><p:msub><p:mi>Ag</p:mi><p:mn>8</p:mn></p:msub><p:msub><p:mrow><p:mi>Sn</p:mi><p:mi>Se</p:mi></p:mrow><p:mn>6</p:mn></p:msub></p:math> in the orthorhombic phase. Despite being a crystal, <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><s:msub><s:mi>Ag</s:mi><s:mn>8</s:mn></s:msub><s:msub><s:mrow><s:mi>Sn</s:mi><s:mi>Se</s:mi></s:mrow><s:mn>6</s:mn></s:msub></s:math> exhibits an exceptionally low room-temperature <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><v:msub><v:mi>κ</v:mi><v:mi mathvariant="normal">L</v:mi></v:msub><v:mo>=</v:mo><v:msub><v:mi>κ</v:mi><v:mi mathvariant="normal">p</v:mi></v:msub><v:mo>+</v:mo><v:msub><v:mi>κ</v:mi><v:mi mathvariant="normal">c</v:mi></v:msub></v:math> of 0.274 W/mK, which is nearly temperature independent over the 100–350 K range. This unusual behavior is linked to the lonesome <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><bb:mi>Ag</bb:mi></bb:math> atoms, which, in this temperature range, exhibit intense vibrations while remaining weakly bonded to the <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><eb:mi>Sn</eb:mi></eb:math>-<hb:math xmlns:hb="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><hb:mi>Se</hb:mi></hb:math> framework. While their bonding anharmonicity significantly reduces <kb:math xmlns:kb="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><kb:msub><kb:mi>κ</kb:mi><kb:mi mathvariant="normal">p</kb:mi></kb:msub></kb:math>, the <ob:math xmlns:ob="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><ob:mi>Ag</ob:mi></ob:math>-dominated extended vibrations make <rb:math xmlns:rb="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><rb:msub><rb:mi>κ</rb:mi><rb:mi mathvariant="normal">c</rb:mi></rb:msub></rb:math> the primary heat-transfer mode. By unveiling the role of <vb:math xmlns:vb="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><vb:mi>Ag</vb:mi></vb:math> in the heat transport of <yb:math xmlns:yb="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><yb:msub><yb:mi>Ag</yb:mi><yb:mn>8</yb:mn></yb:msub><yb:msub><yb:mrow><yb:mi>Sn</yb:mi><yb:mi>Se</yb:mi></yb:mrow><yb:mn>6</yb:mn></yb:msub></yb:math>, we support the notion that the bonding and structural characteristics responsible for fast ionic conduction can also help identify extreme materials with glass-like thermal transport.