Time Induced Changes in Phase Transition Behavior and Stability of Zn 4 Sb 3
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Time Induced Changes in Phase Transition Behavior and Stability of Zn4Sb3 Birgitte Lodberg Pedersen1, Henrik Birkedal1, Eiji Nishibori2, Makoto Sakata2, and Bo Brummerstedt Iversen1 1 Department of Chemistry and Interdisciplinary Nanoscience Center, University of Aarhus, Langelandsgade 140, 8000 Aarhus C, Aarhus, Denmark 2 Department of Applied Physics, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Nagoya, Japan
ABSTRACT The stability of high performance thermoelectric Zn4Sb3 has been studied, by using synchrotron powder diffraction to establish differences in phase transition temperatures of two samples. High resolution multi temperature diffraction data has been collected, with a time interval of 13 months, and the phase transition temperature was determined based on the results of Rietveld refinements. The refinements show a difference in transition temperature from data collected the first time till data collected the second time. Furthermore the samples showed impurity peaks after being exposed to air for 13 months, indicating that the sample decomposes over time. INTRODUCTION Zn4Sb3 is one of the stable compounds in Zn-Sb binary system. It exists in three crystalline forms; the α phase, stable below 263 K; the β phase, stable between 263 and 765 K; and the γ phase, stable from 765 K until it melts congruently at 841 K [1]. The β phase has attracted a lot of attention since it was discovered to be a high performance thermoelectric material in the intermediate temperature range (200-400ºC), where there are no good alternatives [1]. The performance is characterized by the dimensionless thermoelectric Figure of Merit, defined as α 2σ ZT = T, κ where α is the Seebeck coefficient, σ is the electrical resistivity, κ is the thermal conductivity and T is the absolute temperature. The high ZT of Zn4Sb3 is due to the low thermal conductivity, which is believed to originate from the complex structure including three interstitial Zn sites [2] as shown in figure 1.
Figure 1. The R 3 c crystal structure of Zn4Sb3. Blue = Sb, purple = Zn framework, black = interstitial Zn sites. In order for candidate thermoelectric to become commercially applicable, a number of requirements must be fulfilled. The most important requirement is stability of the compound, both thermally and as a function of time. During our present investigation, we address the time stability of two doped samples by synchrotron powder diffraction investigations of the phase transition behaviors. EXPERIMENTEL DETAILS Two samples, Mg0.04Zn3.96Sb3 and Cd0.04Zn3.96Sb3 were synthesized. The samples were prepared from 99.99% zinc shots and 99.5% antimony powder. Zinc, antimony and the dopant element were weighed in stoichiometric ratios in a quartz ampoule. After being evacuated and sealed, the ampoule was placed horizontally in a furnace, and heated with a 400 K/h ramp till 1023 K under continuous rotation. The sample was held at this temperature for 2 h before quenching in water with ice. Synchrotron powder diffraction data were measured f
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