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b-Zn 4 Sb 3 is a promising thermoelectric material due to the abundance of zinc and antimony and reports of high efficiency in bulk samples. This work establishes the high temperature properties of b-Zn 4 Sb 3 across the phase stability window. By controlling the stoichiometry, the Hall carrier concentration can be tuned from 6-9 Â 10 19 cm À3 without requiring extrinsic dopants. The trend in Seebeck coefficient on carrier concentration is rationalized with a single, parabolic band model. Extremely low lattice thermal conductivity (0.4-0.6 W m À1 K À1 ) coupled with a moderate effective mass (1.2 m e ) and mobility leads to a large figure of merit (zT of 0.8 by 550 K). The single parabolic band model is used to obtain the carrier concentration dependence of the figure of merit and an optimum carrier concentration near 5 Â 10 19 cm À3 is predicted.

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SUMMARY Atom Location by Channelling Enhanced Microanalysis (ALCHEMI) is a quantitative technique for identifying the crystallographic sites, distribution and types of substitutional impurities in many crystals. The method involves no adjustable parameters, can be applied to areas as small as a few hundred Angstroms and to impurity concentrations down to about 0·1 atomic per cent. It is capable of distinguishing neighbours in the periodic table. The method uses the incident electron beam orientation dependence of characteristic X‐ray emission and uses an energy dispersive X‐ray microanalyser fitted to a transmission electron microscope. The method does not require the specimen thickness or precise orientation to be known, and makes few assumptions about the form of the dynamical electron wavefunction, which need not be calculated or predicted. The classical problems of cation ordering in spinels, feldspars and olivine have now been studied by this method.

The valence of an atom occupying a particular crystal lattice site can be determined from the chemical shift in a transmission electron energy-loss spectrum obtained with the intensity of the incident electron wave maximized at that site by dynamical electron diffraction. The new technique is demonstrated by determining the location of Fe 2+ and Fe^ ions in a mixed-valence spinel.PACS numbers: 79.20. Kz, 71.70.Ms Bragg reflection of a plane-wave incident on a crystal creates a standing-wave intensity pattern inside the crystal unit cell. 1 " 3 Depending on the orientation of the incident wave with respect to the crystal, the intensity maxima of this pattern will be located at the atomic sites or between them, or, in the case of more complex crystals, at one or another crystal site. If the incident radiation also produces characteristic ionization processes which are sufficiently localized, 4 signals associated with the ionization will depend on the distribution of the standing-wave field inside the crystal unit cell. Several techniques based on this principle but employing different incident and emitted radiations have been developed. X rays have been used as both the primary and the emitted radiation (fluorescence) to identify the site of impurity atoms in a crystal matrix 5 and to determine the distance of adsorbed atoms from a crystal surface. 6 Electrons have been used as the primary radiation together with x rays as the emitted radiation to verify the existence of the modulated primary wave field 7 and to determine the location of minority atoms, 8 and together with energy analysis of the transmitted electrons to study the localization of the inner-shell ionization process for various light atoms. 4 In this paper we show that by detecting the chemical shift in the energy-loss spectrum of the transmitted electrons as a function of incident beam direction, the valence of an atom occupying a particular crystal site may be determined.A chromite spinel, Cr^Feo^Al^Mgo^C^, was studied. The location of the cations had been determined previously. 8 Three quarters of the Fe atoms occupy tetrahedral sites and one quarter octahedral sites. Previous evidence indicated that the Fe 2+ :Fe 3+ ratio is also 3:1, but the distribution of the Fe 2+ and Fe 3+ ions among octahedral and tetrahedral sites was not known. An ion-thinned specimen was examined in a Philips 400T transmission electron microscope equipped with a Gatan 607 magnetic sector electron energyloss spectrometer. 9 Areas about 1 /^m in diameter in the thinnest regions of the specimen (thickness -500 A) were illuminated with 100-keV electrons. The incident beam was nearly parallel to the crystal (100) planes [(h00) systematic orientation] and the beam divergence was up to 10 mrad (0.3 A" 1 ). The electrons selected for energy analysis were scattered by about 17 mrad and had an angular spread of also up to 10 mrad.Along the (100) direction in a spinel, the octahedral-site atoms (all oxygen atoms and-f of cations) occupy one set of alternate (800) planes, and the ...