The influence of Mn doping on the properties of Ge4Sb2Te7

• Metastable solid solutions Ge4−xMnxSb2Te7 (x = 0.5, 1) exhibit the NaCl structure type.
• Thermoelectric properties can be correlated with structural changes upon heating.
• Mn doping increases ZT values of Ge/Sb/Te materials at high temperatures.
• EPR and magnetic measurements reveals the valence state +II for Mn.
• Antiferromagnetic fluctuations are detected below ∼50 K.

Ge3MnSb2Te7 and Ge3.5Mn0.5Sb2Te7 are doped variants of Ge4Sb2Te7, a material from the homologous series (GeTe)nSb2Te3. Phase purity was confirmed by ICP-AES, EDX and quantitative yield according to powder X-ray diffraction. Quenched solid solutions are metastable and exhibit a rocksalt-type average structure and contain multiply twinned domains with superstructures. At ∼300 °C, a phase transition leads to a trigonal layered phase, predominantly corresponding to the 33R-Ge3As2Sb6 type. However, traces of a rocksalt-type phase remain. A rocksalt-type high-temperature phase forms between 500 °C and ∼590 °C. EPR spectra reveal the oxidation state +II for Mn [g = 2.001(5)], which is also supported by an effective magnetic moment μeff = 5.713 μB. The Mn2+ content of 7.2(5) at% as derived from the magnetic susceptibility agrees with the nominal content of 7.7 at%. A temperature-dependent change in the line width and shape of the EPR signal below 20 K indicates magnetic ordering, which is in agreement with magnetic measurements. A paramagnetic Curie temperature ΘP = −43 K indicates antiferromagnetic fluctuations that set in at ∼50 K. Mn doping reduces the electrical conductivity, while the Seebeck coefficient increases significantly up to 190 μV/K. The thermoelectric properties are affected by the phase transitions. In a first heating cycle, ZT reaches a maximum of ∼0.5 below 300 °C and again at 500 °C. After the first cycle the first maximum disappears as the metastable phase is accessible only by quenching, but the second one is reproducible. Compared to Ge4Sb2Te7, the ZT value of Mn-doped samples is higher for the quenched phase and at high temperatures.

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