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 <i>c</i>-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 <i>c</i>-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 <i>c</i>-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 <i>ZT</i> value of 0.033 at high temperatures.Read less <