4. Results and discussion
4.3 Space charge conductivity and space charge density
63
64
(Figure 33a). On the other hand, when is kept constant, [FeTi' ] increases with T but the slope does not change.
Figure 33 FeTi' (m) concentration as function of P (a) at constant T for different [ ]Fe totand (b) at different T for [ ]Fetot =1.01 10⋅ 20cm−3(0.6at% Fe).
The P-dependence in the space charge zones deserves a special consideration:
According to the brick layer model of a polycrystalline material with grain size dg, the conductivity in the space charge region is given by:[26]
1 2
g GB
GB
d σ⊥ = Z⊥
4.6
4 GB
GB
g
Y σ = d
4.7
with ZGB⊥ and YGB being the resistance of the perpendicular boundaries and the conductance of the parallel ones, normalized with respect to the macroscopic coordinates. In the Mott-Schottky case, they are given by:[26]
*
0 0
1
2 ln( / )
GB
j j j
j j
Z z eu c c c
⊥ λ
∞
= 4.8
* 0
2ln( 0/ )
i
GB i i
i i
Y z eu c
c c λ
∞
= . 4.9
[ ]Fe tot
65 For the determination of the P-dependence of the space charge conductivity, Eq.s 2.35 and 2.45 have to be used for the differentiation, yielding, for perpendicular boundaries in the p-type regime
0 0
( ) ( / )
1 1 1
4 2 2.303
Log GB Log m Log e kT
LogP LogP LogP LogP
σ⊥ φ φ
∂ ∂ ∂ ∆ ∂ ∆
= + + −
∂ ∂ ∂ ∂ . 4.10
On the other hand, for parallel boundaries, in which electrons are the dominating charge carriers, the P-dependence of the space charge conductivity is equal to:
(
0)
0 /
1 1 1
4 2 2.303
GB e kT
Log Log m Log
Log P Log P Log P Log P
σ φ ∂ ∆φ
∂ ∂ ∂ ∆
= − − − +
∂ ∂ ∂ ∂
: 4.11
In the latter equations, the term
(
∂Log∆φ0/∂Log P) (
= ∂ ∆φ0/∂Log P)
/∆φ0 is negligible compared to(
∂∆φ0/∂Log P)
/kTe−1 as ∆φ0 kT e/ .However, in order to understand the conduction properties in the space charge region, a more pertinent parameter to be considered is the surface density of the excess charge in the grain boundary core (Σ), which is directly determined by the defect concentration within the core of the grain boundaries and induces the charge carriers rearrangement in the space charge layers.
In the present section, we intend to consider the conductivity data presented in section 4.2 in order to examine (i) how varies with P and (ii) whether the P dependence of changes between the microcrystalline and nanocrystalline samples.
Thus, the analysis presented in the following concerns undoped SrTiO3 specimens with a net acceptor (Fe impurities) content of 0.01-0.02 at%. Please note also that the nanocrystalline sample has an average grain size of 50 nm (see grey diamonds in Figure 31 and Figure 34) and is in the mesoscopic situation (The space charge layer width is larger than half the grain size).
Taking into account the surface charge density in the GB core (Σ) in the Mott-Schottky approximation (Eq. 2.48) one can rewrite Eq.s 2.54, 4.2 and 4.11 as follows:
2 2
exp( / 8 )
2 / 8
bulk GB
z mRT
z mRT
σ ε
σ⊥ ε
= Σ
Σ . 4.12
In the case of flat band approximation, Eq. 4.2 becomes exp( 2/ 8 )
bulk m
z mRT
σ ε
σ = Σ . 4.13
Σ Σ
66
In addition, the P-dependence of the GB conductivity can be expressed with respect to the GB core charge density and Eq. 4.11 can be rewritten as
2
0
1 1 2
4 18.4
GB
r
Log Log m m
LogP LogP - ekT LogP m LogP m
σ
ε ε
⊥
∂ ∂ ∂Σ Σ ∂ Σ
≈ + ⋅ ⋅ − ⋅
∂ ∂ ∂ ∂
. 4.14 In the light of Eq. 4.12 and 4.13 one could directly determine the value of Σ from the conductivity value, if the acceptor content m is known.
It is important to note that Eq. 4.14 allows predicting the P dependence of the σGB⊥ in terms of Σ. Clearly, if the dopant content m is known, this is a rather easy task (under the Mott-Schottky approximation if the dopant is not redox active) that can be carried out by using Eq. 4.12 and 4.13).
However, the situation is more complex if the dopant is redox active as in the case of Fe-doped SrTiO3 (see Eq. 2.16 ).
Hence, in such a situation, it is convenient to average Eq. 4.14 over the P-range considered:
2
0
1 1 2
4 18.4
GB
r
Log Log m m
LogP LogP - ekT LogP m LogP m
σ
ε ε
⊥
∆∆ ≈ +∆∆ ⋅∆∆Σ ⋅ Σ −∆∆ ⋅Σ
. 4.15 where instead of considering the derivatives of the different quantities with respect to
Log P, we take the finite changes of each quantity upon a finite variation of P and Σ as well as m represent the average values within the P range considered.
Now, starting from (i) the iron concentration m in the samples (0.02 at%), (ii) the defect chemistry model by Denk and (iii) the characteristic value of ∆φ0 for polycrystalline SrTiO3 (which is approximately 0.70V† as determined in this study and according to previous studies[64,73]) one can predict ∆LogσGB⊥ /∆LogP.
For the case considered here, with P ranging between 1 and 10−5 bar,
6 -2
3.28 10 C cm−
Σ = ⋅ ⋅ , m=1.76 10 cm⋅ 18 -3and ∂Σ ∂/ LogP= −3.55 10⋅ −7 are obtained.
By considering all the terms on the right side of Eq. 4.15,
/ 0.23
LogσGB⊥ LogP
∆ ∆ = 4.16 results.
† The variation of ∆φ0 with P can be neglected.
67 On the other hand, in the n-type regime, the parallel boundaries are responsible of the conduction. At very low P, the variation of m can be neglected, since all Fe atoms are ionized. Therefore, considering Eq. 2.12, Eq. 4.15 can be reformulated as
0
log 1 2
log 4 18.4
GB
P remkT LogP
σ
ε ε
∂ Σ ∂Σ
= − +
∂ ∂
. 4.17
In the following, Eq. 4.15 and 4.17 are used (in the light of the conductivity data reported in section 4.2), in order to investigate the P-dependence of Σ for the microcrystalline and nanocrystalline (dg ∼50nm) samples.
Microcrystalline SrTiO3
For the microcrystalline material, the P-dependence of σbulk determined experimentally is equal to 0.218±0.012 (see Figure 34 and Table VI), which is very close to what is expected from theoretical consideration (Eq. 4.3). The deviation is due to the Fe ionization as expressed in Eq. 4.3. The actual P-dependence of the net acceptor content m, namely ionized Fe3+, can be determined from the experimental value of ∂Logσbulk /∂Log P. In this case, (∂Log m/∂Log P)= −0.06 is obtained (Table VII). This value is slightly lower than the value predicted from the model developed by Denk et al. [39], which is equal to −0.10 (Table VII), and corresponds to a Fe content of 0.003 at%. This is clearly lower than what is indicated by the chemical analysis (0.02 at%). Notably, if we take into account also the standard deviation, (∂Log m/∂Log P) varies between −0.04 and −0.08, which still does not fit with the theoretical value of −0.10. The remaining difference (which indicate that the Fe content in the grain interior is lower than 0.01-0.02 at%) can be explained with a slight Fe segregation at the GBs, which reduces the amount of m in the grain interior. Fe segregation is typical in microcrystalline SrTiO3 due to the positive GB core and was already observed for example by Chiang et al..[72]
68
Table VI Values of the dependence of conductivity and of the surface charge density (in the high P-regime) at the GBs determined experimentally
Log Log P
σ⊥
∂
∂
Log Log P
σ
∂
∂
Log P
∂ Σ
∂
Micro
Bulk 0.218±0.012
3.68 10−7
− ⋅
GB 0.196±0.007 −0.275±0.038
Nano 0.205±0.011 −0.285±0.007 −2.64 10⋅ −7
Figure 34 Detailed view of the P-dependence of the conductivity at T=544°C for oxidizing conditions given in Figure 31. The symbols are assigned as follows: (grey diamonds) nanocrystalline sample (dg ~ 50 nm); (open red squares) bulk of the microcrystalline sample; (blue triangles) GB contribution of the microcrystalline sample. For the GB conductivity of the microcrystalline material, the conductivity was calculated considering the geometry of the sample. The values of the slopes are reported in Table VI.
Table VII P-dependence of m; (a) from Eq.4.3 , (b) from Eq. 4.15.
/
Log m Log P
∂ ∂
Denk et al.[39] −0.10±0.01
Micro
Bulk −0.06(a)
GB −0.12 (b)
69 If we now consider the GB behaviour, we can determine ∆φ0 by numerically solving Eq. 2.54 for the different P values, at which the conductivity was measured. At first, it is convenient to use Eq. 4.10, since Eq. 4.15 requires the knowledge of the exact value of m, which is actually not known. By plotting versus P (Figure 35), its P dependence can be determined, yielding to ∂∆φ0/∂Log P= −0.006 V·bar─1. This is consistent (at least qualitatively) with what was previously observed by Zhang et al. in slightly Fe-doped SrTiO3.[163] By inserting the latter value in Eq. 4.10 and taking into consideration the experimental value ∂LogσGB⊥ /∂LogP=0.196 0.007± (Table VI),
/ 0.12
Log m Log P
∂ ∂ = − (Table VII) is obtained. It is worth noting that this value is higher than the one calculated for the bulk, which corroborates the previous hypothesis of a slight Fe segregation at the grain boundaries, (under the assumption that Eq. 2.16 holds also in the boundary regions).
Figure 35 P-dependence of of the microcrystalline calculated according to Eq. 2.54.
From the experimental data, the surface charge density Σ at the GBs can be evaluated according to Eq. 2.48, considering a Fe content of 0.02 at%. The data are plotted in Figure 36. From these, one can recognize that ∂Σ ∂/ Log P
( )
<0 whichindicates that the excess positive charge within the GB core diminishes with increasing P. If we now assign the excess positive charge to VO••,[68] then their surface density in the case of a core consisting of a single atomic layer varies from 6.99 10⋅ 12 to 1.27 10⋅ 13
φ0
∆
φ0
∆
70
cm─2 when P increases from 1 to 10─5 bar. This is consistent with oxygen ions leaving the grain boundary core when the environment becomes more reducing.
Nanocrystalline SrTiO3
In this case, a flat concentration profile approximation can be assumed and thus Σcan be determined according to Eq. 4.13 for each P value considered. From this, it is possible to estimate Σ by taking into account the Fe ionization reaction, which in this case is assumed to be equal to the one obtained for the GBs of the microcrystalline material (∂Log m/∂Log P= −0.12). The resulting data are plotted in Figure 36, in which also the P dependence of Σ of the microcrystalline sample is shown for comparison.
It is evident that in both samples Σ decreases with increasing P. Notably, despite the similar tendency, two main differences can be recognized in the nanocrystalline sample compared with the microcrystalline one: (i) the charge density is lower, suggesting a lower excess positive charge (e.g. VO•• concentration) in the GB core and (ii) the P dependence is less steep. Nonetheless it is instructive to consider the variation of Σ relative to the charge density in pure oxygen. Remarkably, although the starting values of Σ are clearly different (1.74·10−6 vs. 2.23·10−6 C·cm─2), the change of Σ upon a reduction of 5 orders of magnitude of P is the same, namely an increase of 43%.
Figure 36 P dependence of the surface charge density Σ experimentally determined for both the microcrystalline (blue triangles) and nanocrystalline samples (grey diamonds). The values of the slopes are reported in Table VI.
71 Moreover, it is useful to consider the variation of Σ relative to the charge density ∂Σ Σ ⋅∂/ ( Log P). Note that
2 2
( O ) ( O )
Log s
Log p Log p
∂Σ ∂ Σ
= =
Σ ⋅ ∂ ∂ . 4.18
which – if we assign this change of Σ to the variation of the VO•• concentration in the GB core – represent a power law that is rather typical for bulk defect chemistry.
Remarkably, although the starting values of Σ are different, 1.74·10−6 vs.
2.23·10−6 C·cm−2 for the nanocrystalline and the microcrystalline sample respectively, /
Log Log P
∂ Σ ∂ is a constant with a value which is the same (about ─0.05, see Figure 37) for both samples.
Figure 37 Logarithmic plot of Σversus P. (Grey diamonds) nanocrystalline sample; (blue triangles) microcrystalline sample.
If one now takes into consideration the n-type conduction regime, logσGB/ logP
∂ ∂ was experimentally determined to be equal to −0.28 for both the nanocrystalline (as well as the microcrystalline) materials. The deviation from −0.25 (see 4.17) is an evidence that (∂Σ ∂/ LogP) 0< , since we can exclude Σ to be negative as in the n-type regime the overall conductivity of the sample is determined by the parallel grain boundaries.‡ This means that Σ decreases while increasing P.
‡ If Σwere negative, the GBs would be blocking.
72
From the experimental data, the term 2 (Σ ⋅ ∂Σ ∂/ LogP)= −7.0 10⋅ −14 C2·cm─4·bar─1 can be calculated. If we now take the same value of ∂Σ ∂/ LogP of the p-type regime,
7 2
1.62 10− C cm−
Σ = − ⋅ ⋅ results, which is one order of magnitude lower than under oxidizing conditions. This is obviously not possible as Σ ≥ 3 10⋅ −6C cm⋅ −2. Therefore, the oxygen partial pressure dependence of Σ in the n-type case is negative but less steep than in the p-type situation. This might suggest that with decreasing P the extraction of oxygen ions from the GB core become progressively less favourable.
Section conclusions:
In this part, a systematic investigation of the variation of the grain boundary core charge density Σ as a function of the oxygen partial pressure has been carried out.
Such an analysis is based on the general space charge model, which enables to correlate the results of the conductivity measurements (impedance spectroscopy) with Σ. The results indicate that, irrespective of the average grain size, Σ decreases with increasing P, which corroborate the hypothesis that the excess positive charge of the GB core results from the presence of oxygen vacancies, which are filled when P increases.
In addition, it is found that compared to the microcrystalline sample, in the nanocrystalline SrTiO3, (i) the charge density is lower but (ii) ∂LogΣ ∂/ Log p( O )2 is the same for both systems and equal to ─0.05 C·cm─2·bar─1. This suggests that the stoichiometry of the GB core is different between micro and the nanocrystalline material. Remarkably, it has been found that the percentage change of the oxygen vacancies concentration in the GB core is the same independently of the grain size.[166]
73