Unraveling Non-polymorphic Phase Evolution in Mg-Ag-Sb for Designing Thermally Recoverable Thermoelectrics

P-type MgAgSb with excellent room-temperature thermoelectric performance holds great promise for cooling and power generation. However, its practical application has been significantly limited by phase transitions, which are long considered a polymorphic transformation where the compositions remain identical but the crystal structures differ. Here we report the non-polymorphic nature of the complex phase transitions in Mg-Ag-Sb during thermal cycling, yielding four compositionally distinct ternary compounds that crystallize in three different structures. During heating, the tetragonal <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>α</a:mi> </a:math> -phase MgAgSb decomposes into the tetragonal <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>β</c:mi> </c:math> -phase <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mrow> <e:msub> <e:mrow> <e:mi>Mg</e:mi> </e:mrow> <e:mn>5</e:mn> </e:msub> <e:msub> <e:mrow> <e:mi>Ag</e:mi> </e:mrow> <e:mn>4.32</e:mn> </e:msub> <e:msub> <e:mrow> <e:mi>Sb</e:mi> </e:mrow> <e:mn>4.5</e:mn> </e:msub> </e:mrow> </e:math> , and subsequently into the cubic <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mrow> <g:msub> <g:mi>γ</g:mi> <g:mi mathvariant="normal">H</g:mi> </g:msub> </g:mrow> </g:math> -phase <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:mrow> <j:msub> <j:mrow> <j:mi>Mg</j:mi> </j:mrow> <j:mn>5</j:mn> </j:msub> <j:msub> <j:mrow> <j:mi>Ag</j:mi> </j:mrow> <j:mn>2.11</j:mn> </j:msub> <j:msub> <j:mrow> <j:mi>Sb</j:mi> </j:mrow> <j:mn>4</j:mn> </j:msub> </j:mrow> </j:math> , with the progressive precipitation of <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mrow> <l:msub> <l:mrow> <l:mi>Ag</l:mi> </l:mrow> <l:mn>3</l:mn> </l:msub> <l:mi>Sb</l:mi> </l:mrow> </l:math> and Sb phases. Upon cooling, the <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:mrow> <n:msub> <n:mi>γ</n:mi> <n:mi mathvariant="normal">H</n:mi> </n:msub> </n:mrow> </n:math> -phase <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"> <q:mrow> <q:msub> <q:mrow> <q:mi>Mg</q:mi> </q:mrow> <q:mn>5</q:mn> </q:msub> <q:msub> <q:mrow> <q:mi>Ag</q:mi> </q:mrow> <q:mn>2.11</q:mn> </q:msub> <q:msub> <q:mrow> <q:mi>Sb</q:mi> </q:mrow> <q:mn>4</q:mn> </q:msub> </q:mrow> </q:math> converts to the cubic <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"> <s:mrow> <s:msub> <s:mi>γ</s:mi> <s:mi mathvariant="normal">L</s:mi> </s:msub> </s:mrow> </s:math> -phase <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"> <v:mrow> <v:msub> <v:mi>Mg</v:mi> <v:mn>5</v:mn> </v:msub> <v:msub> <v:mi>Ag</v:mi> <v:mn>1.18</v:mn> </v:msub> <v:msub> <v:mi>Sb</v:mi> <v:mn>3.63</v:mn> </v:msub> </v:mrow> </v:math> , accompanied by the <x:math xmlns:x="http://www.w3.org/1998/Math/MathML" display="inline"> <x:mi>α</x:mi> </x:math> phase with substantial <z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline"> <z:mrow> <z:msub> <z:mrow> <z:mi>Ag</z:mi> </z:mrow> <z:mn>3</z:mn> </z:msub> <z:mi>Sb</z:mi> </z:mrow> </z:math> and Sb phases. The electron localization function and bonding analysis indicate that the Mg-Ag-Sb compound is characterized by ionic interactions, wherein the Ag-Sb bond is notably weaker compared to the Mg-Sb bond. Molecular dynamics simulations reveal that significant Mg and Ag migration within the Sb sublattices underlies the phase evolution. Surprisingly, the rapid atomic diffusion also enables the restoration of the <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"> <bb:mi>α</bb:mi> </bb:math> -phase MgAgSb upon annealing at low temperatures. Successful recovery of the <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"> <db:mi>α</db:mi> </db:math> phase can be realized even after 500 thermal cycles between 423–673 K, demonstrating unprecedented thermal recoverability.