A Chemical, High-Temperature Way to Ag_1.9Te via Quasi-Topotactic Reaction of Stuetzite-type Ag_1.54Te: Structural and Thermoelectric Properties

Semiconducting silver tellurides gained reasonable interest in the past years due to its thermoelectric, magneto-caloric, and nonlinear optic properties. Nanostructuring has been frequently used to address quantum-confinement effects of minerals and synthetic compounds in the Ag–Te system. Here, we report on the structural, thermal, and thermoelectric properties of stuetzite-like Ag_1.54Te (or Ag_4.63Te₃) and Ag_1.9Te. By a quasi-topotactic reaction upon tellurium evaporation Ag_1.54Te can be transferred to Ag_1.9Te after heat treatment. Crystal structures, thermal and thermoelectric properties of stuetzite-like Ag_1.54Te (or Ag_4.63Te₃) and Ag_1.9Te were determined by ex situ and in situ experiments. This method represents an elegant chemical way to Ag_1.9Te, which was so far only accessible electrochemically via electrochemical removal of silver from the mineral hessite (Ag₂Te). The mixed conductors show reasonable high total electric conductivities, very low thermal conductivities, and large Seebeck coefficients, which result in a significant high thermoelectric figure of 0.57 at 680 K

Polymorphism in Zintl Phases ACd₄Pn₃: Modulated Structures of NaCd₄Pn₃ with Pn = P, As

NaCd₄P₃ and NaCd₄As₃ were synthesized via short-way transport using the corresponding elements and CdI₂ as mineralizer. At room temperature, the two β-polymorphs adopt the RbCd₄As₃ structure type which has been recently reported for alkali metal (A)–d¹⁰ transition metal (T)–pnictides (Pn). The title compounds crystallize rhombohedrally in space group <i>R</i>3̅<i>m</i> at room temperature and show reversible phase transitions to incommensurately modulated α-polymorphs at lower temperatures. The low-temperature phases are monoclinic and can be described in space group <i>Cm</i>(α0γ)<i>s</i> with <i>q</i> vectors of <i>q</i> = (−0.04,0,0.34) for α-NaCd₄P₃ and <i>q</i>₁ = (−0.02,0,0.34) for α-NaCd₄As₃. Thermal properties, Raman spectroscopy, and electronic structures have been determined. Both compounds are Zintl phases with band gaps of 1.05 eV for β-NaCd₄P₃ and ∼0.4 eV for β-NaCd₄As₃