The influence of Mn doping on the properties of Ge sub(4)Sb sub(2)Te sub(7)

Ge sub(3)MnSb sub(2)Te sub(7) and Ge sub(3.5)Mn sub(0.5)Sb sub(2)Te sub(7) are doped variants of Ge sub(4)Sb sub(2)Te sub(7), a material from the homologous series (GeTe) sub(n)Sb sub(2)Te sub(3). 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 similar to 300 degree C, a phase transition leads to a trigonal layered phase, predominantly corresponding to the 33R-Ge sub(3)As sub(2)Sb sub(6) type. However, traces of a rocksalt-type phase remain. A rocksalt-type high-temperature phase forms between 500 degree C and similar to 590 degree C. EPR spectra reveal the oxidation state +II for Mn [g = 2.001(5)], which is also supported by an effective magnetic moment mu sub(eff) = 5.713 mu sub(B). The Mn super(2+) 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 Theta sub(P) = -43 K indicates antiferromagnetic fluctuations that set in at similar to 50 K. Mn doping reduces the electrical conductivity, while the Seebeck coefficient increases significantly up to 190 mu V/K. The thermoelectric properties are affected by the phase transitions. In a first heating cycle, ZT reaches a maximum of similar to 0.5 below 300 degree C and again at 500 degree 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 Ge sub(4)Sb sub(2)Te sub(7), the ZT value of Mn-doped samples is higher for the quenched phase and at high temperatures.