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PAPER
Electron-poor antimonides: complex framework structures with narrow band
gaps and low thermal conductivity
Ulrich Ha¨ussermann*a and Arkady S. Mikhaylushkinb
Received 31st July 2009, Accepted 22nd September 2009
First published as an Advance Article on the web 16th October 2009
DOI: 10.1039/b915724g
Binary zinc and cadmium antimonides and their ternary relatives with indium display complex crystal
structures, but reveal at the same time narrow band gaps in their electronic structure at or close to the
Fermi level. It is argued that these systems represent “electron-poor framework semiconductors”
(EPFS) with average valence electron concentrations between three and four. EPFS materials
constituted of metal and semimetal atoms form a common, weakly polar framework containing
multi-center bonded structural entities. The localized multi-center bonding feature is thought to be the
key to structurally complex semiconductors. In this respect electron-poor antimonides become related
to modifications of elemental boron. Electron-poor antimonides show promising thermoelectric
properties, especially through a remarkably low thermal conductivity. At the same time the thermal
stability of these compounds is rather limited because of temperature polymorphism and/or
comparatively low melting or decomposition temperatures (usually below 600 K).
Zn interstitials. The actual composition of Zn4Sb3 was revised
Introduction
to Zn13Sb10, or rather Zn13-d Sb10 (d = 0.2–0.5). Subsequent work
Binary zinc– and cadmium–antimony phases have been early
investigated for their narrow band gap semiconductor properties.1
These investigations, however, struggled with the complexity
showed that the randomly distributed Zn defects and interstitials
in b-Zn4Sb3 order into distinct islands consisting of clustered
defects and interstitials in low-temperature modifications with
of crystal structures and phase relations occurring in the
highly complex structures.6–8 The structural complexity of Zn4Sb3
Zn–Sb and Cd–Sb systems, and results for structural and physical
is rivaled by that of the Zn3Sb2 phases which, however, are
properties were often contradictory. Whereas the antimony richest
metastable at room temperature with respect to decomposition
phases ZnSb and CdSb are compositionally and structurally
into Zn4Sb3 and Zn.9
well defined,2 phases with larger Zn/Cd contents often display
Recently also Cd4Sb3 could be structurally characterized10 and
temperature dependent polymorphism, are metastable, and/or
ternary derivatives of zinc and cadmium antimonides with indium
have homogeneity ranges.
could be identified.11,12 The emerging family of antimonide systems
The discovery of outstanding thermoelectric properties for the
shows common characteristics: (a) complex and/or disordered
room-temperature phase of Zn4Sb3 (b-Zn4Sb3) by Caillat et al.
structures, (b) temperature polymorphism, (c) comparatively
in 1997 fueled renewed research activity.3 The room-temperature
low melting or decomposition temperatures, and (d) interesting
thermoelectric properties for b-Zn4Sb3 are S = 113 mV K-1, r =
thermoelectric properties. A true peculiarity is their low thermal
2 mX cm, k = 0.9 W mK-1 for the thermopower, electrical
conductivity feature, which is similar to clathrates, but applies to a
resistivity and thermal conductivity, respectively. It was found
wider variety of structures and thus cannot have a common origin.
that values for the thermoelectric figure of merit ZT (ZT =
While structural disorder or complex, large unit-cell crystal
structures may account for low thermal conductivity, it is not
immediately evident why zinc antimonides and related systems
should reveal narrow gap semiconductor properties leading to size-
able power factors S2/r. In this work we attempt to rationalize this
family of compounds as electron-poor framework semiconductors
(EPFS). “Framework semiconductor” refers to a material with a
(S2/rk)T, where T is the absolute temperature) exceed 1 above
500 K, and increase to almost 1.4 at 670 K. The key to the high
thermoelectric performance of b-Zn4Sb3 lies in the exceptional
thermal conductivity, which is as low as for glass-like materials.
Although ZnSb has similar values for the power factor S2/r, the
figure of merit is much lower compared to b-Zn4Sb3, due to a
three times higher thermal conductivity.
The origin of the remarkably low thermal conductivity of
structure where all constituting atoms participate in a common
framework. This expresses a weakly polar character, as opposed
b-Zn4Sb3 remained a mystery until recently when Snyder et al.
to e.g. salt-like Zintl phases (CaSi2 = [Ca2+][Si-]2). “Electron-
reported an unusual and intricate form of disorder in this
poor” refers to a sp-bonded framework with electron counts lower
compound.4,5 The disorder is expressed as a combination of Zn
than four per atom, as opposed to exactly four in tetrahedral
defects in a regular framework with composition Zn6Sb5 and
frameworks. We first introduce the concept of EPFS, and then
review and discuss the structural properties of characterized
aDepartment of Chemistry and Biochemistry, Arizona State University,
electron-poor antimonides along with their electronic densities
Tempe, AZ, 85287-1604, USA
bDepartment of Physics, Chemistry and Biology, Linko¨ping University, SE
58133, Linko¨ping, Sweden
of states (DOS). Where unknown, DOS curves were calculated
for this work. Finally, we review the thermoelectric properties of
1036 | Dalton Trans., 2010, 39, 1036–1045
This journal is
The Royal Society of Chemistry 2010
©
Haeussermann, Ulrich
