Defect study in intrinsic CoSb 3
6.3 Results and discussion
6.3.1 Phase width of CoSb 3
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.
variance due to sintering conditions. 4) Investigated compositions are chosen to have a stoichiometry Co:Sb= 4 : (12+x) close to 1:3 (x = -0.06~0.17) in order to minimize quantities of secondary phases. With all these synthesis conditions determined, we hope to synthesize CoSb3 in both Co-rich and Sb-rich conditions with the least amount of secondary phase, which shall allow us to study the defect type in CoSb3 without influences from these impurities.
Binary CoSb3 is often considered to be a line compound up to 874℃. However, we challenge this by trying to measure the temperature dependent phase width of CoSb3. 21 Samples with various compositions are synthesized in a similar manner except with different annealing temperatures (See Chapter 2, Section 2.2 for more detail). The synthesized compositions as well as their annealing temperatures are shown in Table. 6.1, where × denotes the indicated composition is synthesized and annealed at the corresponding temperature.
Table 6.1 Compositions synthesized with corresponding annealing temperatures.
Annealing T(℃)
Co4Sb11.94 Co4Sb11.97 Co4Sb12 Co4Sb12.03 Co4Sb12.13 Co4Sb12.15 Co4Sb12.17
850 × × ×
800 × × × × ×
700 × × × × ×
650 × ×
600 × × ×
500 × × ×
The phase purity of annealed ingots was checked with SEM and XRD. The stoichiometry of the CoSb3 phase was confirmed by EPMA. SEM photos and XRD patterns for samples annealed at 500℃, 700℃, and 850℃ (on ingots or hot pressed samples) were chosen to show in Figures 6.3- 6.8. The secondary phases (mostly determined from SEM unless otherwise specified from XRD) and EPMA compositions (on annealed ingots) are summarized and shown in Tables 6.2-6.4. After hot pressing, the phases were again checked by XRD; however, unfortunately, the stoichiometry of CoSb3 phase on hot pressed samples was not double-checked. As we shall discuss in Section 6.3.2 later, this might cause problems in analyzing the correct defect type. For all the samples, the EPMA compositions for CoSb3 phase confirm a stoichiometry of Co:Sb close to 1:3 after annealing and before hot pressing. The phase width can be determined from secondary phase identification for each annealing temperature.
Table 6.2 Phase purity and composition analysis of CoSb3 samples annealed at 500℃. Nominal
Composition
EPMA composition
Secondary phase
Co4Sb11.97 12.05±0.06 CoSb2
Co4Sb12 12.05±0.08 Sb
Co4Sb12.17 12.06±0.06 Sb
Figure 6.3 XRD patterns of CoSb3 samples annealed at 500℃ (after hot pressing).
Figure 6.4 SEM photos of a) Co-rich sample Co4Sb11.97; b) on stoichiometry sample Co4Sb12; c) Sb- rich sample Co4Sb12.17 after annealing at 500℃.
As we can see from Table 6.2, the phase region of CoSb3 at 500℃ should be between the compositions Co4Sb11.97 and Co4Sb12. The Sb secondary phase for nominal composition Co4Sb12 (as can be seen in Figure 6.4b) disappears after hot pressing in Figure 6.3 (no Sb peak on the green curve), which is consistent with previous findings34.
Table 6.3 Phase purity and composition analysis of CoSb3 samples annealed at 700℃. Nominal
Composition
EPMA composition
Secondary phase
(a) (b) (c)
Co4Sb11.94 12.04±0.07 CoSb2 (XRD)
Co4Sb11.97 12.03±0.04 None
Co4Sb12 12.05±0.04 None
Co4Sb12.03 12.06±0.09 Sb
Co4Sb12.15 12.00±0.09 Sb
Figure 6.5 XRD patterns of CoSb3 samples annealed at 700℃ (ingots or samples after hot pressing).
Figure 6.6 SEM photos of a) Co-rich sample Co4Sb11.94; b) Co-rich sample Co4Sb11.97; c) on stoichiometry sample Co4Sb12; d) Sb-rich sample Co4Sb12.03; e) Sb-rich sample Co4Sb12.15 after annealing at 700℃.
As we can see from Table 6.3, the phase region of CoSb3 at 700℃ should be between the compositions Co4Sb11.94 and Co4Sb12.03. The CoSb2 phase for nominal composition Co4Sb11.94 is invisible in SEM but is detected by XRD. The Sb secondary phase for nominal composition Co4Sb12.03 (as can be seen in SEM) disappears after hot pressing in XRD.
(a) (b) (c)
(d) (e)
Table 6.4 Phase purity and composition analysis of CoSb3 samples annealed at 850℃. Nominal
Composition
EPMA composition
Secondary phase
Co4Sb11.94 12.05±0.04 CoSb2 (XRD)
Co4Sb11.97 11.95±0.08 CoSb2 (XRD)
Co4Sb12.03 12.04±0.08 Sb
Figure 6.7 XRD patterns of CoSb3 samples annealed at 850℃ (ingots after annealing).
Figure 6.8 SEM photos of a) Co-rich sample Co4Sb11.94; b) Co-rich sample Co4Sb11.97; c) Sb-rich sample Co4Sb12.03 after annealing at 850℃.
As we can see from Table 6.4, the phase region of CoSb3 at 850℃ should be between the compositions Co4Sb11.97 and Co4Sb12.03.
If we combine all the information collected at each temperature, a rough estimate of the temperature dependent phase width of CoSb3 can be depicted in Figure 6.9. As we can see, the phase width (where green circles are in) is very narrow throughout the temperature range investigated, with (∆𝑥𝑥)𝑚𝑚𝑚𝑚𝑚𝑚 < 0.1 for Co4Sb12+x. Due to the accuracy of EPMA measurements, it is hard to distinguish
(a) (b) (c)
experimentally if the skutteurdite phase is Sb rich or Co rich. Appearance of secondary phases can serve as a guide, but this is true only on annealed samples because of possible modification on the stoichiometry due to high temperature sintering. This work is useful in guiding researchers to synthesize off-stoichiometry CoSb3 with least secondary phases.
Figure 6.9 Temperature dependent phase width of CoSb3. Pink downward triangle symbols represent samples in a two-phase region of CoSb2 and CoSb3. Green circle symbols represent samples in a single-phase region of pure CoSb3. Purple upward triangle symbols represent samples in a two-phase region of CoSb3 and Sb. The orange solid line represents the peritectic temperature for CoSb3 874℃.