Cite this: DOI: 10.1039/c3ra40457a Mg 3 Sb 2 -based Zintl compound: a non-toxic, inexpensive and abundant thermoelectric material for power generation3 Received 25th January 2013, Accepted 20th March 2013 DOI: 10.1039/c3ra40457a www.rsc.org/advances A. Bhardwaj, A. Rajput, A. K. Shukla, J. J. Pulikkotil, A. K. Srivastava, A. Dhar, Govind Gupta, S. Auluck, D. K. Misra* and R. C. Budhani The deployment of thermoelectric materials for deriving an enhanced figure of merit (ZT) for power generation in inexpensive, non-toxic and relatively abundant bulk homogeneous solid relies on the extent of achieving the ‘‘phonon-glass electron crystal’’ (PGEC) characteristics. Here, a proof of principal has been established experimentally in the present work for a Zintl compound of Mg 3 Sb 2 and its derivative of isoelectronically Bi doped Bi; Mg 3 Sb 22x Bi x (0 ¡ x ¡ 0.4) alloys in Mg 3 Sb 2 . Single phase p-type Mg 3 Sb 2 compounds, with Mg and Sb powders as starting materials, have been prepared directly by spark plasma sintering (SPS) in a one step process. The structural refinements of this hexagonal Zintl compound by X-ray diffraction analysis (XRD) and high resolution transmission electron microscopy (HRTEM) investigation reveal that they are single phase devoid of any oxides or Sb precipitates. Transport measurements indicate low thermoelectric figure of merit (ZT = 0.26 at 750 K) for Mg 3 Sb 2 . However, an optimum doping of 0.2 at% with iso-electronic Bi ions at the Sb site enhances the ZT to 0.6 at 750 K, which is comparable with the present day industrial materials such as Bi based tellurides and selenides which are toxic. We note that the system becomes metal with carrier density exceeding 15 6 10 20 /cm 3 for x ¢0.25. The substantial increase in ZT in Mg 3 Sb 22x Bi x (0 ¡ x ¡ 0.4) owes to a partial decoupling of the electronic and phonon subsystem, as expected for a Zintl phase compound. While the reduction in thermal conductivity in Mg 3 Sb 22x Bi x (0 ¡ x ¡ 0.4) accounts to mass fluctuations and grain boundary scattering, the enhancement in the electronic power-factor is attributed to the presence of heavy and light bands in its valence band structure. The latter has been confirmed by means of both X-ray photo electron spectroscopy studies and first-principles density functional based calculations. These measurements established that a high figure of merit can be achieved in this class of materials with appropriate doping. Further, relative abundance of the material ingredients combined with its one step synthesis leads to a cost effective production and less toxicity makes the material an environmentally benign system for thermoelectric power generation. I. Introduction The design of thermoelectric materials for deriving an enhanced figure of merit in a bulk homogeneous solid relies on the extent of achieving the ‘‘phonon-glass electron crystal’’ (PGEC) 1 characteristics. A complete manifestation of PGEC in a thermoelectric material is less practical as it suggests a decoupling of the electronic and phonon degrees of freedom of the crystal. In the recent past the Zintl phase compounds, which have a unique chemical bonding, 2–10 have been found to partly satisfy the PGEC criteria. These compounds are composed of highly electropositive cations (typically, related to group 1 and 2) that donate their electrons to electronegative anions; which, in turn, are used to form bonds in order to satisfy valency. The extent of charge transfer by the cations to the anion network establishes the ‘‘electron crystal’’ features for improving electronic transport, while the electropositive ions which act as phonon scattering centers, stands to its ‘‘phonon glass’’ behavior leading to a reduction in the thermal conductivity. As a result, a high thermoelectric figure of merit (ZT) has been attained in a variety of Zintl compounds such as skutterudites, 11–16 calthrates, 17–25 and others. 2,26–48 However, the composition of such materials generally has expensive rare-earth elements and toxic chalcogenides as constituents, together with the complex synthesis routes that make them less useful for large scale waste heat recovery applications. Other than the Zintl phase compound, remarkable progress on state-of-art materials such as bismuth telluride based, 49–52 III-nitride based e.g. AlInN, InGaN 53–57 and silicon germanium CSIR-Network of Institutes for Solar Energy, Materials Physics & Engineering Division, CSIR-National Physical Laboratory, Dr K. S. Krishnan Road, New Delhi, 110012, India. E-mail: misradk@nplindia.org; dakkmisra@gmail.com 3 Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3ra40457a RSC Advances PAPER This journal is ß The Royal Society of Chemistry 2013 RSC Adv. Downloaded by University of Delaware on 29/04/2013 14:54:50. Published on 26 April 2013 on http://pubs.rsc.org | doi:10.1039/C3RA40457A View Article Online View Journal