Defect study in intrinsic CoSb 3
6.2 Literature study
6.2.1 Bonding chemistry of CoSb3
Skutterudite is a type of Zintl compound in which the total number of valences from both cations and anions is zero. According to Zintl chemistry128, the formal valence of a cation and an anion can be calculated following the general valence rule168. As given in Eq. 6.1, is the number of available valence electrons of the cation element and is the number of cation-cation two- electron bonds and nonbonding lone pair electrons. Similarly, the anion formal valence is given by Eq. 6.2, where is the number of anion-anion two-electron bonds.
(Eq. 6.1)
(Eq. 6.2) Binary CoSb3 compounds are diamagnetic semiconductors, so there should be no unpaired spin in the bonding scheme. For each Sb atom with electronic configuration 5s25p3, it is bonded to its four nearest neighbors: two Sb atoms and two Co atoms. The Sb-Sb bonding has Sb-Sb-Sb bonding angle of 90°, which forms Sb4 rings shown in Figure 6.2c. The remaining three valence electrons
participate in the Co-Sb bonding. According to Eq. 6.2, Sb has -1 state (5+2-8 = -1). The distance between the Co atoms is too large for there to by any significant Co-Co bonding. Co atom with electronic configuration 4s23d7 is thus only bonded to Sb atoms in an octahedral coordination (N = 6). For each Co atom, 3 electrons occupy the d2sp3 hybrid orbitals that are the essence of the Co-Sb bonding. The Co atom is in a 3+ state (9-6 = 3) with the remaining 6 electrons paired up leading to a low-spin d6 state. The electronegativity of Sb (2.05) is close to that of Co (1.88), so these bonds have little ionic character. The bonding scheme is shown in Figure 6.1. The structure of [CoSb6] octahedral and Sb4 ring is shown in Figure 6.2.
Figure 6.1 Bonding illustration in CoSb3 from Dudkin’s bonding model169.
Figure 6.2. (a) Structure of skutterudite CoSb3 (Co: red; Sb: blue), (b) [CoSb6] octahedron, (c) Sb4
ring. © IOP Publishing. Reproduced with permission. All rights reserved.31 6.2.2 Synthesis condition and defect type
Various attempts were made by previous researchers to synthesize either polycrystalline (melt- annealing method, mechanical alloying (ball milling)31,32, high pressure assisted synthesis34, melt- melt-spinning method33 etc.) or single crystalline skutterudites (gradient freeze technique5,6, flux- assisted growth35, chemical vapor deposition36 etc.).
The influence of synthesis methods and conditions on the defect type in CoSb3 compounds is undeniable. Single crystal CoSb3 can only be grown from Sb-rich melt (90 ~ 97 at% Sb) due to the phase diagram limitation38, which is usually p-type5,6,35,36. However for polycrystalline CoSb3, inconsistent results are reported among various groups. Both p- and n-type intrinsic CoSb3 are reported with even the same synthesis method. Confusing though these data are, some results are still enlightening and worthy of attention.
Sharp et al33 found that for samples synthesized in a melt-annealing method followed by cold/hot pressing, samples that are cold pressed show p-type while the ones hot pressed with the same starting powder composition show n-type. Deficiency of Sb due to hot pressing was speculated;
however, there was no measured stoichiometry data to confirm.
Similarly, Liu et al32 reported the sintering temperature can play a large role in the defects. For samples synthesized by mechanical alloying and SPS, when the sintering temperature is at 300℃, the samples show p-type, whereas when the sintering temperature is increased to 600℃, the samples show n-type. The EDS investigation shows a change in the Co:Sb ratio from 1:3.0 to 1:2.7 with increasing SPS temperature from 300℃ to 600℃, which was accounted for the p-n change. Liu et al31 then studied the effect of Sb compensation on the point defects of CoSb3. It was found when excess Sb was increased from zero to 8 at%, the Seebeck coefficient at room temperature changes from −430 to 40μVK−1for samples synthesized with a fixed MA-SPS synthesis route. Sb-vacancy was concluded as the reason for the n-type behavior in Sb-deficient sample.
Nakagawa et al170 tried to study the effect of stoichiometry (with nominal Sb:Co = 2.6~3.4) on thermoelectric properties of CoSb3 using a melt-annealing method. Transport properties are measured on slices of annealed ingot to avoid influence of high temperature sintering process.
However no clear trend was obtained. The presence of large quantities of secondary phases (CoSb2
for Co-rich samples, Sb for Sb-rich samples) complicates the investigation. Recently, a study171 on the effect of CoSb2 and Sb on the electronic properties of CoSb3 was revealed by a multi‐band Hall effect analysis. While CoSb2 increases the charge carrier density, the influence of the highly mobile charge carriers introduced by elemental Sb is worth noting as well.
By applying a high pressure (1.5GPa-3.5GPa) and high temperature (950K) assisted synthesis starting with stoichiometric Co and Sb powders, Su et al34 reported that all samples synthesized are
p-type, more interestingly, it is found that the carrier concentration can be a factor of 10 less when the pressure increases. This stress-dependent defect concentration is attributed to anti-site defects, whose formation energy may vary with the synthesis pressure. However, there was no experimental data to confirm this hypothesis. Excess Sb was observed in the lower pressure sample, which disappears for higher-pressure samples. This can be explained by the high volatility of Sb.
From the literature review we can conclude that a few factors are important in the experimental defect study of intrinsic CoSb3:
1) The form and purity of starting material. Powder raw elements tend to introduce O2 up to 1000ppm into the system. Fe or Ni impurities, which are easy to be found in Co, can contribute to p- or n-type doping as well.
2) If we compare the most common synthesis methods, traditional melt-annealing and mechanical alloying (ball milling), the latter is more prone to introduce impurities such as oxygen or Fe (if the vial is made of steel).
3) Consolidation conditions are important. The high sintering temperature can alter the stoichiometry to be Sb-deficient in CoSb3, resulting an n-type behavior. The increasing sintering pressure leads to a decreasing p-type carrier concentration, the reason to which remains unclear.
4) When considering the influence of off-stoichiometry (either Co-rich or Sb-rich) on defects, often large amount of secondary phases is introduced as well. The influence of these secondary phases is not fully clear yet. It is thus required to synthesis of Co-rich/Sb-rich samples with little secondary phases.