Reversible Property Switching, Thermoelectric Performance, and d¹⁰−d¹⁰ Interactions in Ag₅Te₂Cl

Abstract

Thermoelectric and phase analytical measurements were performed to investigate the physical properties of trimorphic Ag5Te2Cl. The material is a mixed electron/silver ion conductor featuring drastic property changes during a silver order/disorder phase transition at 334 K. The transition is characterized by a jump in the total electric conductivity by 2 orders of magnitude directly affecting the electric and thermoelectric properties. Silver ions are arranged in well-defined strands along the crystallographic c-axis characterized by a set of not fully occupied sites. Heat capacity measurements show a large effect, whereas the thermopower and thermal diffusivity drop significantly at the temperature of transition. Right after the transition, an attractive d10−d10 interaction within the disordered silver substructure occurs affecting the c-lattice parameter upon heating. Due to this interaction a modulation of the electronic structure and the thermoelectric properties can be observed which have been investigated in detail. While the thermopower stays low with increasing temperature the thermal diffusivity relaxes fast to values before the transition. At 355 K, the thermopower starts rising again, which is consistent with a small effect in the heat capacity and a reduction of the c-lattice parameter upon heating. Further heating leads to a reduction of the d10−d10 interactions and a drastic increase in the thermopower. The observed phenomenon must be regarded as a new example of a compound following the recently discovered concept of low-dimensional partially covalent-bonded structure units that can positively influence thermoelectric properties in bulk materials. Ag5Te2Cl is the first example where mobile d10 ions interact to create low-dimensional partially covalent-bonded subunits in a solid, which then leads to a switching of thermoelectric and electronic properties. The system shows very low thermal conductivities between 0.19 W m−1 K−1 and 0.60 W m−1 K−1 in the temperature range 298 to 500 K, reaching a maximal ZT value of 0.033 at high temperatures.

The Influence of d10-d10 Interactions in Ag5Te1.8Se0.2Cl and Ag5Te1.6Se0.4Cl on Structural and Thermoelectric Properties

“…In summary, the results of the thermopower measurements and the thermal diffusivities are comparable to the properties of Ag 5 Te 2 Cl [11].…”

“…4). This phenomenon was also demonstrated for the ternary counterpart Ag 5 Te 2 Cl [11]. A negative thermal expansion coefficient was located in supercooled liquid tellurium, which could be related to the accruement of a Peierls-distorted arrangement of Te chains [20].…”

“…Trimorphic Ag 5 versible order/disorder transitions (β -α) at 303 (1) and 273(1) K, respectively [11]. In Fig.…”

“…The appearance of attractive interactions in In 4 Se 3−δ (Peierls-distorted In chain) [4], Ag 10 Te 4 Br 3 (Peierls-distorted Te chain) [5] and Ag 5 Te 2 Cl (d 10 -d 10 interaction) [11] has a significant influence on the physical properties. A modulation of the electronic structure can be achieved based on these low-dimensional interactions which do significantly influence the thermopower and the thermal diffusivity of these materials [12,18].…”

“…Significant variations of the thermoelectric properties like thermopower, thermal diffusivity and the thermoelectric figure of merit can be assigned to such oriented, low-dimensional interactions around the silver order/disorder phase transition [11,12].…”