Y and La Doping in CaMnO3 Compounds: Effects of Dopant Identity and Amount on Charge Transport Kinetics

Enhancing electronic transport properties of thermoelectric oxides is of great technological importance. Oxides are promising candidates for waste heat harvesting at elevated temperatures as well as for electricity generation in low‐power applications. To this purpose, fundamental understanding of their electrical and thermal conduction mechanisms is essential. Herein, the conduction mechanism of CaMnO3 materials is focused on and how dopant identity and amount alter the kinetic properties of charge transport is investigated. Ca1−xRxMnO3 compounds with R = Y and La are synthesized, where 0 ≤ x ≤ 0.13, and the electrical conductivity and Seebeck coefficient for temperatures ranging from 300 to 1050 K, indicating that Y‐doped compounds are usually more conductive than their La‐doped counterparts, are measured. Analysis of both in terms of the small polaron hopping model reveals that Y doping reduces conduction activation energies, resulting in higher electrical conductivity and charge carrier mobility. Remarkably high values of thermoelectric power factor for the Ca0.97La0.03MnO3 compound, for example, 300 μWm−1 K−2 at 1050 K are observed; furthermore, these values are preserved for a wide temperature range, rendering this compound a good candidate for heat‐to‐electrical power generation at elevated temperatures.


Introduction
3][4][5] TE oxides are usually low cost, environmental friendly, nontoxic, and chemically stable; they show promising characteristics for waste heat harvesting at elevated temperatures [6][7][8][9] and for applications where low power is required (few μW to mW), for example, small wearable electronics and body sensors. [10,11]Therefore, enhancing their electronic properties, which depend on the ionic nature of the compounds, is of great technological importance.For this purpose, fundamental understanding of their electrical and thermal conduction mechanisms is essential.
[17][18] This value determines how many CaMnO 3 perovskite subunits are separated by an extra CaO rock-salt crystallographic plane. [19]The presence of this plane perpendicular to the c-axis is strictly linked to the peculiar TE transport behavior of the series; the boundary between CaMnO 3 and CaO acts as a scattering center for both electrons and phonons, forming an energy barrier that suppresses charge and heat transport.Combining this fascinating feature along with the charge carrier hopping nature in manganites motivates basic research on the series' electrical conduction mechanism.
[22] Furthermore, our previous studies of Ca 2Àx R x MnO 4 (m = 1) compounds with R = Y or La showed that electrical conduction obeys the small polaron hopping model. [19,23]e discovered several interesting features, among them are as follows.1) Y doping yields higher electrical conductivity compared to La doping for all samples due to reduced conduction activation energy, although their charge carrier concentrations are similar.
2) The conduction activation energy and polaron hopping energy attain minimum values for %x = 0.10, regardless of the dopant identity.Recently, we have shown that Y doping reduces the conduction activation energy compared to La doping also in Ca 3 Mn 2 O 7 (m = 2) materials, [24] corroborating our results for Ca 2 MnO 4 . [19,23]aMnO 3 is the m = ∞ derivative of the series, and in contrast to the m = 1 and 2 compounds, it consists of the perovskite structure without any extra CaO plane, but only those which are part of the CaO-MnO 2 layering.[27][28][29] In this study, we optimize the TE power factor (PF ≡ S 2 σ) of Ca 1Àx R x MnO 3 compounds by additions of Y and La in different amounts and obtain remarkable values that are stable for wide temperature range, rendering this compound a good candidate for TE power generation at elevated temperatures.We question whether the lower conduction activation energies (E σ ) of Y-doped samples, observed for Ca 2 MnO 4 and Ca 3 Mn 2 O 7 , originate solely from reduction of the energy barrier for charge transport exerted by the extra CaO planes or from the different electronic nature of the dopants as well.Since charge transport in manganites occurs by thermally activated hopping through Mn þ3 -O-Mn þ4 conduction chains, [23,30,31] lower degree of charge localization should improve charge carrier mobility by reducing the activation energy for electrical conduction.Also, we look to clarify whether the minimum observed for E σ is typical for Ca 2 MnO 4 only.
Accordingly, we synthesize and study Ca 1Àx R x MnO 3 compounds with R = Y or La, where 0 ≤ x ≤ 0.13; the x-range is chosen to yield charge carrier concentrations equivalent to those studied by us previously for Ca 2 MnO 4 , [19] considering the different unit cell volumes.It was also reported that dopant concentrations within this range usually yield optimized TE performance for dopants substituting for either Ca sites [21,22,[32][33][34][35] or Mn sites. [36,37]We hypothesize that the absence of the extra CaO planes would significantly reduce the conduction activation energy compared to Ca 2 MnO 4 materials, rendering electrical conductivity metallic instead of semiconducting.More importantly, we argue that the dopant identity determines charge localization in the CaMnO 3 lattice, despite the absence of the extra CaO energy barrier.Therefore, the differences in charge transport kinetics between Y and La doping observed for Ca 2 MnO 4 and Ca 3 Mn 2 O 7 should apply for CaMnO 3 as well, yet the extent of this effect is to be determined.

Materials Synthesis
We applied a standard solid-state reaction (SSR) routine to synthesize Ca 1Àx R x MnO 3 compounds with x = 0, 0.006, 0.02, 0.03, 0.06, 0.10, and 0.13, where R = Y or La.The pure oxide powders, CaCO 3 (Reag.Ph Eur, Merck), MnO 2 (≥99%, Sigma-Aldrich), and Y 2 O 3 (99.99%,Strem Chemicals) or La 2 O 3 (99.99%,Sigma-Aldrich), were weighed in proper stoichiometric ratio, manually milled by mortar and pestle, and underwent four 24 h SSR steps with increasing temperatures applying 100 K h À1 heating rate: 1273, 1373, 1473, and 1573 K. Prior to each step, the powders were thoroughly milled to increase the surface area for reaction.Subsequently, disc-shaped specimens were prepared for each compound by uniaxial pressing of the powder at 700 MPa and sintering of the green body at 1573 K in air for 24 h. [16,18,19,25]

Materials Characterization and Electrical Measurements
X-ray diffraction (XRD) was used to analyze crystal structure and confirm single-phase purity of the samples.Crystal structure analysis of the specimens was conducted using a Rigaku Miniflex II X-ray diffractometer for the 2θ angular range of 10°through 90°with 0.01°resolution, applying a 0.4°min À1 scanning speed.Microstructure characterization was conducted for the surface of the as-sintered samples using a Zeiss Ultra Plus scanning electron microscope (SEM) applying 5 kV acceleration voltage; secondary electron (SE) signals were recorded for imaging.Analysis of the chemical composition was done employing energy-dispersive X-ray spectroscopy (EDS) using an Oxford Instruments X-Max 80 mm 2 SDD detector, included within the SEM.Crystal lattice imaging and chemical identification of atoms were carried out using a double-corrected high-resolution scanning/transmission electron microscope (HR-S/TEM) Titan Themis G 2 60-300 (FEI/Thermo Fisher, USA) operated at 200 keV and equipped with a Dual-X detector (Bruker Corporation, USA) for EDS mapping.The chemical maps were postprocessed (by background correction and Radial Wiener filter), quantified, and analyzed using the Velox software (Thermo Fisher).The bulk density (ρ) was determined at room temperature using an Archimedes-based method [38] and was assumed to be temperature independent.Electrical conductivity (σ) and Seebeck coefficient values (S) were measured at temperatures ranging from 300 up to 1050 K in air using a Nemesis SBA-458 apparatus (Netzsch GmbH, Selb, Germany), providing instrumental accuracy of AE5 and AE7%, respectively.Hall mobility and charge carrier concentration were measured at room temperature using a 8400 Series HMS apparatus (Lake Shore Cryotronics Inc., USA), [39] applying the van der Pauw method on the disc-shaped specimens. [39]The material's thermal conductivity (κ) was expressed by: κ(T ) = α(T )⋅C p (T )⋅ρ; where α(T ) is the thermal diffusivity and C p (T ) is the constant-pressure specific heat.The thermal diffusivity values were directly measured for pellet-shaped specimens at 300 K using the MicroFlash LFA-457 laser flash analyzer (LFA; Netzsch GmbH, Selb, Germany) in air, applying short (0.5 ms) thermal pulses of 18.5 J/pulse generated by a Nd-YAG laser system, yielding an instrumental accuracy of 2%.The heat capacity was evaluated using the LFA in a comparative method with a reference sample made of pure Al 2 O 3 having similar geometry.The resulting accuracy of the thermal conductivity values was up to 10%.

Microstructure Characterization
Figure 1 shows the XRD patterns acquired from powders of the Ca 1Àx R x MnO 3 samples (R = Y or La; x = 0.006, 0.02, 0.03, 0.06, 0.10, and 0.13), matching the orthorhombic crystal structure of the Pnma space group, JCPDS # 040 078 030, in agreement with previous studies. [22,35]No secondary phase peaks are observed, verifying successfully doped single-phase compounds.
[42][43][44] The images show that the dopant's chemical identity does not affect grain size and morphology; however, increasing dopant concentration slightly reduces grain size, suggesting that both Y and La somewhat inhibit grain growth in CaMnO 3 compounds. [15,24]igure 3 shows the relative bulk densities of La-and Y-doped samples, indicating variation from 72 to 82% and 81 to 89%, respectively, suggesting that Y doping slightly improves compaction.For both series of compounds, the relative density range is %10%; nevertheless, no clear dependence on dopant concentration is observed.
A high-angle-annular dark-field (HAADF) lattice image of the Ca 0.87 Y 0.13 MnO 3 specimen acquired by scanning transmission electron microscopy (STEM) across the [001] zone axis is shown in Figure 4.The insets show atomically resolved elemental mapping obtained by EDS, revealing Ca, Y, Mn, and O atomic columns colored by red, purple, green, and blue, respectively.Layering of CaO and MnO 2 planes along the [100] direction is clearly observed, as well as substitution of the Y dopants for the Ca sites.

Thermoelectric Transport Properties
The temperature-dependent electrical conductivity and Seebeck coefficient values of the Ca 1Àx R x MnO 3 samples (R = Y or La; 0.00 ≤ x ≤ 0.13) measured between 300 and 1050 K are shown in Figure 5a,b, respectively.Most of the samples exhibit a negative temperature dependence, that is, dσ/dT < 0, with negative S-values for the entire temperature range, indicating degenerate semiconducting behavior in which electrons are the major charge carriers.The undoped sample (x = 0) shows semiconducting behavior for the entire temperature range, and the x = 0.006 and 0.02 materials exhibit an increase in conductivity at elevated temperatures, where the transition temperature increases with increasing dopant concentration.This is caused by the increasing activation of the extrinsic carriers.Expectedly, the absolute values of S decrease with increasing dopant amount, implying that the higher the x-values, the larger the charge carrier concentration is.
Similar to the behavior observed for Y-and La-doped Ca 2 MnO 4 and Ca 3 Mn 2 O 7 samples, [19,23,24] most of the Y-doped CaMnO 3 samples exhibit higher electrical conductivity values than their La-doped counterparts, for most of the temperature range.For example, the electrical conductivity of the x = 0.13 Y sample at 300 K is about 2.2 times larger than its La-doped counterpart, although their Seebeck coefficient values are practically equal.In fact, the data collected indicate similar S values for most of the sample couples (same x-values), especially for x = 0.03, 0.10, and 0.13, suggesting similar charge carrier concentrations (since S ∝ 1/n, [45] where n is the charge carrier concentration).These results suggest that Y doping yields higher charge carrier mobility than does La doping, corroborating our hypothesis.
Interestingly, the Y-and La-doped Ca 2 MnO 4 (m = 1) and Ca 3 Mn 2 O 7 (m = 2)-based compounds with the doping levels of 0.01 ≤ x ≤ 0.20 and 0.016 ≤ x ≤ 0.33, respectively, exhibit a positive temperature dependence mostly, that is, dσ/dT > 0, with negative S-values for the entire temperature range, indicating nondegenerate semiconducting behavior, where electrons are the major charge carriers. [19,23,24]Values for the cases of undoped compounds are not reported.An exception is observed for the case of Ca 2.77 La 0.33 Mn 2 O 7 , where a degenerate trend of dσ/dT < 0 is observed, which is associated with the considerably larger La doping level (x = 0.33). [24]The general trend of dσ/dT < 0 shown in the present study is opposite to the trends reported for the abovementioned m = 1 and 2 derivatives.We  associate this contradiction to the difference between the equivalent doping levels in the base CaMnO 3 structure (m = ∞) and the structures with m 6 ¼ ∞, in which the presence of the extra CaO planes increases the effective doping level. [18]We, furthermore, note that the conductivity values reported in this study range from %8 and 200 S cm À1 at 1000 K, Figure 5.For the sake of comparison, the corresponding values that were reported for Y-and La-doped Ca 2 MnO 4 (m = 1) and Ca 3 Mn 2 O 7 (m = 2)-based compounds with the doping levels of 0.01 ≤ x ≤ 0.20 and 0.016 ≤ x ≤ 0.33 are 0.2-20 and 0.5-50 S cm À1 at 1000 K, respectively. [19,23,24]The systematic behavior of σ (m = 1) < σ (m = 2) < σ (m = ∞) can be evidently associated with the density of the CaO planes that scatter charge carriers. [17]e note that an apparent irregularity in the conductivity values measured for the La-doped series is identified; the x = 0.06 sample is less conductive than the x = 0.03 one, and the x = 0.13 sample is less conductive than the x = 0.10 one.These findings suggest that mobility and charge carrier concentration balance each other in some cases.
To address the applicability of these compounds to TE power generation, we evaluate the PF that reflects the capability of the material to generate electrical power from waste heat.The PF values appear in Figure 6.It is shown that the compounds exhibiting the highest PF values are Ca 0.97 La 0.03 MnO 3 and Ca 0.97 Y 0.03 MnO 3 compounds, where the former exhibits the best performance.It is noteworthy that these values are preserved for a wide temperature range with moderate decrease from %400 μWm À1 K À2 at 300 K down to 300 μWm À1 K À2 at 1050 K, rendering this compound a good candidate for heat-to-electrical power generation at elevated temperatures.We highlight that the x = 0.03 materials exhibit peculiarity as the La-doped compound exhibits better PF values than its Y-doped counterpart, as opposed to the other Y-/La-doped couples with the same x values.][35][36] Second, our PF values exhibit  stable values throughout a wide temperature range, which is very much attractive for engineering applications.
To better elucidate the effects of Y and La doping on the materials' properties, we measure the thermal conductivity of the samples at room temperature.The results are plotted in Figure 7.It is evidently shown that Y-doped compounds exhibit larger thermal conductivities than their La-doped counterparts for all compositions.This can be associated to the significantly larger mass and ionic radii differences of the La dopants with respect to their neighboring CaMnO 3 -matrix, compared to those of the Y dopants.This encourages stronger phonon scattering in the case of La-doped compounds, thereby reducing thermal conductivities. [46]he dependence of κ values on dopant concentration (x) is, however, more complicated since different factors play opposite roles; among them are the effects of point defect concentration on phonon scattering, the strength and ionicity of interatomic bonds, the effects of dopants on local atomic vibrational frequencies, and more. [15]Interestingly, the κ values reported in this study range between %1 and 2.8 Wm À1 K À1 , whereas the corresponding values reported for Y-and La-doped Ca 3 Mn 2 O 7 (m = 2)-based compounds with the doping levels of 0.016 ≤ x ≤ 0.33 range between %0.6 and 1.5 Wm À1 K À1 . [24]his difference is associated with the CaO planes residing in the Ca 3 Mn 2 O 7 (m = 2) structure, which contribute to phonon scattering and reduce thermal conductivity. [15]he values of PF and thermal conductivities enable us to evaluate the upper limit of zT, keeping in mind that the κ-values are given at room temperature; however, practically we are interested in power generation at elevated temperatures.Since κ(T) usually decreases with temperature; the κ-values plotted in Figure 7 serve as the upper limits.As mentioned above, the Ca 0.97 La 0.03 MnO 3 compound exhibits a maximum value of PF %300 μWm À1 K À2 at 1050 K, while possessing a room-temperature κ-value of %1 Wm À1 K À1 .][35][36] We highlight that, anyway, since such oxides are directed mostly toward waste heat recovery, considerations of power generation (quantified by PF) are usually more important than efficiency (quantified by zT ).

Electron Transport Mechanisms
9][50][51] Here, σ 0 is a constant depending on optical phonon frequency, hopping distance, and charge carrier concentration, k B is the Boltzmann constant, e is the elementary electron charge, E σ is the conduction activation energy, E S is the charge carrier generation energy, and α 0 is a constant matching the Heikes expression for thermopower at the high-temperature limit. [50,52]lues of ln(σT ) and Seebeck coefficients are plotted versus reciprocal temperature, see Figure 5b and S1, Supporting Information.Good linear fits are obtained for ln(σT ) and S versus T À1 for temperatures ranging from 300 to 1050 and 400 to 1050 K (R 2 values close to unity), respectively, depending on dopant amount.We note that the slope of ln(σT ) versus T À1 drastically increases for T > 550-750 K, similar to the behavior shown for Ca 2 MnO 4 and Ca 3 Mn 2 O 7 compounds, [19,23,24] suggesting transition into a different conduction mechanism with considerably higher activation energy.In contrast to the sharp slope changes observed for Ca 2 MnO 4 samples at 750 K, here we identify smoother change starting at 550 K.The entire temperature range can, therefore, be separated into three slopes for certain samples, for example, for both x = 0.006 samples.Nevertheless, we focus on the first part of the temperature range which is represented by the first slope in the ln(σT ) plot.
Using Heikes formula, we extract the S-values at the hightemperature limit from linear regression analysis, as follows. [52]Heikes ðT !∞Þ Here, c is the fraction of Mn þ3 ions of the total number of Mn sites, and g is a factor combining spin and orbital degeneracy.The degeneracy factor for the mixed-valent Mn þ3 /Mn þ4 system considering strong Hund's coupling and high spin state reads g = 4/10. [53,54]To estimate charge carrier density in cm À3 from c, we use the unit cell volume normalized to one formula unit, Z = 1, as a constant-size occupation site of the charge carrier.The variations of this volume associated with doping and heating are neglected.
The values of charge carrier concentration evaluated for the samples are shown in Figure 8, confirming that most of the   sample couples attain similar numbers.Certain deviations from nominal values are observed, especially for high dopant concentration; these may stem from an inaccurate value of the degeneracy factor due to spin and/or orbital interactions affecting the spin arrangements, variation of unit cell volume, or interactions between charge carriers at high dopant concentrations.However, the more important result is the proximity of the values between the different samples of the same x-values, indicating that differences in electrical conductivity originate from charge carrier mobility (μ), as σ = neμ.It is noteworthy that the charge carrier concentrations obtained here are similar to those obtained by us previously for Ca 2 MnO 4 [19] ; this proximity validates the stoichiometric calculations done to achieve the desirable carrier concentrations and suggests accurate synthesis procedures.
Further validation for similar dopant amount in each couple of samples is given by the composition analysis performed using EDS, see Figure S2, Supporting Information.These results adhere to the S values and charge carrier concentration similarities observed in Figure 5b and 8.We note that for the x = 0.02, 0.03, and 0.06 La-doped samples, a small contamination of Al (0.2-0.3 at%) is detected; however, we do not observe any systematic effect on the electronic transport properties, particularly on the Seebeck coefficient, which is highly susceptible to variations in charge carrier concentration.
Overall, the above results suggest that the differences observed in electrical conductivity for each couple of samples originate from charge carrier mobility rather than carrier concentration; they clearly imply that Y doping favors higher mobility in CaMnO 3 compounds compared to La doping.A similar conclusion was drawn by us for Ca 2 MnO 4 , as well. [19]he charge carrier generation energies are shown in Figure 9a.It is shown that for each pair (same x-values), the E S values are reasonably similar; this trend is expected, as all samples possess similar charge carrier concentrations, suggesting a similar gap between the Fermi level and the polaronic band. [50]Also, E S monotonically decreases with increasing dopant amount, confirming that as carrier concentration increases, the energy required to generate the carrier decreases, so that the gap is reduced.Similar trends were reported by us for Ca 2 MnO 4 . [23]igure 9b shows the conduction activation energies.A comparison between the E σ values of the similarly doped CaMnO 3 and Ca 2 MnO 4 materials indicates that the extra CaO planes present in Ca 2 MnO 4 increase the E σ values by about an order of magnitude, that is, from 2-22 meV in CaMnO 3 (this work) to 86-127 meV for Ca 2 MnO 4 , [19,23] rendering electrical conductivity from degenerate to thermally activated semiconducting behavior.This finding provides the kinetic explanation regarding the role of the extra CaO planes in CaO(CaMnO 3 ) m materials with m = 1, 2, or 3, which act as energy barriers for charge transport. [19]Therefore, the lower the barrier is, the higher the carrier mobility is.
Remarkably, all the activation energies for conduction of the Y-doped samples are lower than their La-doped counterparts, like the results obtained for both Ca 2 MnO 4 and Ca 3 Mn 2 O 7 compounds. [19,24]As opposed to Ca 2 MnO 4 and Ca 3 Mn 2 O 7 , however, the CaMnO 3 structure lacks extra CaO planes in the crystal lattice; thus, our finding implies that Y inherently enhances charge transport within the perovskite lattice.We stress that this effect is additional to lowering the energy barrier for charge transport exerted by the extra CaO planes (in Ca 2 MnO 4 and Ca 3 Mn 2 O 7 ). [19,24]ur results confirm strictly that dopants not only contribute charge carriers, but also determine charge mobility.
Interestingly, the undoped sample exhibits an E σ value that is at least one order of magnitude higher than all the doped derivatives, that is, 227 compared to 2-22 meV, respectively.The reason for that can be explained by considering the polaron hopping process through the Mn þ3 -O-Mn þ4 conduction chain.In pure CaMnO 3 , where basically no Mn þ3 sites are present, polaron hopping through the Mn þ3 -O-Mn þ4 conduction chain is highly improbable, since no Mn þ3 sites are present to absorb the hopping charge carrier.As the Mn þ3 concentration increases, the hopping process is facilitated, the activation energy for the process decreases, and thus the hopping probability increases.That is, the low probability of hopping in pure CaMnO 3 is reflected by its large E σ values.We note that undoped CaMnO 3 samples may still contain a small amount of Mn þ3 due to oxygen vacancies or point defects, generating sporadic Mn þ3 states in the Mn þ4 sublattice.
Following this reasoning, we note that E σ attains a minimum value with respect to dopant concentration for both Y-and La-doped samples; the minimum is observed at x % 0.03.This is the same behavior as observed for Y-and La-doped Ca 2 MnO 4 . [19,23]o explain it, we evaluate the average distance between polarons in the samples, assuming that each substitution of one dopant atom for Ca þ2 generates one Mn þ3 point defect in the Mn þ4 sublattice; then, based on a simple point defect volume, we estimate which characterizes the average distance between two adjacent dopants and is equivalent to the average distance between Mn þ3 sites, that is, polarons.The average distance between polarons for CaMnO 3 , L m = ∞ , is shown in Figure 10a.In Figure 10b, it is further shown that E σ attains the minimum value for L % 12 Å regardless of the dopant identity, similar to the results in Ca 2 MnO 4 . [23]It is clear that the minimum corresponds to the case where L values approach the unit cell length, %10 Å.These results suggest that an optimal polaron-polaron distance exists in terms of hopping; at that distance, each polaron is surrounded by potential Mn þ4 hopping sites, and thus the hopping probability through the Mn þ3 -O-Mn þ4 conducting network is maximized so that activation energy is minimized.Further increase of the polaron concentration initiates repulsive polaron-polaron interactions, increasing the hopping energy and reducing the hopping probability.Additional validation for the effects of dopants on charge carrier mobility is provided by Hall effect measurements; we measure the room-temperature charge carrier concentration and Hall mobility of the Ca 1Àx R x MnO 3 samples.Figure 11a clearly shows that the mobility of the Y-doped samples exhibits greater values compared to their La-doped counterparts for most of the compositions studied.Also, further confirmation for the proximity of charge carrier concentration per couple of samples having the same x-values is shown in Figure 11b; these results correspond with the evaluation made based on the Heikes formula and the EDS composition analysis data (Figure 8 and S2, Supporting Information, respectively), as well as with the behavior reported for the Ca 2 MnO 4 -based system. [19]emarkably, the ratio between the mobility values of the Y-doped and La-doped samples exhibits a maximum at %x = 0.06, and the ratio between the E σ values of the La-and Y-doped samples yields a maximum, as well, at about the same x-value, as shown in Figure 12.We highlight that most mobility ratios exceed 1, confirming the advantage of Y doping in terms of charge carrier mobility.Interestingly, the maximum in the mobility ratio is obtained for x values similar to those yielding the minimum E σ values.These results agree with the analysis based on electrical conductivity and Seebeck coefficient measurements, confirming that Y doping enhances carrier mobility compared to La doping due to reduced E σ .

Conclusions
This work provides insights into the role of the dopant identity and amount in charge transport kinetics of Ca 1Àx R x MnO 3 materials (R = Y or La; 0 ≤ x ≤ 0.13), which is the m = ∞ derivative of the Ruddlesden-Popper CaO(CaMnO 3 ) m series.Similar to results reported for doped Ca 2 MnO 4 and Ca 3 Mn 2 O 7 compounds, [19,23,24] we show that Y doping increases electrical conductivity more than La doping, thanks to lower conduction activation energy.Comparison between the E σ values of the similarly doped CaMnO Interestingly, we also show that as for Ca 2 MnO 4 , the conduction activation energy attains a minimum value at specific dopant amount, regardless of the dopant identity.That dopant amount corresponds to polaron-polaron distances which are about unit cell size, similar to the results obtained for Ca 2 MnO 4 .Remarkably, the ratio between the Hall mobility of Y-and La-doped samples and the ratio between E σ values of the Y-and La-doped samples attain maximum at similar x-values.These observations validate the existence of an optimal charge carrier/polaron concentration in terms of activation energy for conduction and electrical mobility.This work sheds light on the effects of dopant identity and amount on kinetics of charge transport in CaO(CaMnO 3 ) m compounds.For further research, we suggest selecting the x-value corresponding to the lowest E σ (x = 0.03) and study codoped materials, namely, substitute varying dopant amounts for the Mn site (e.g., Ca 0.97 Y 0.03 Mn 1Àx Nb x O 3 ).It would be interesting to see whether and how the Mn-O-Mn conduction chains will be affected from Mn substitution and what will be the outcome on activation energies.Finally, we observe remarkably

Figure 2 .
Figure 2. SEM micrographs acquired by secondary-electron signals from the surface of as-sintered Ca 1Àx R x MnO 3 samples.a) R = La and x = 0.02, b) R = Y and x = 0.02, c) R = La and x = 0.06, d) R = Y and x = 0.06, e) R = La and x = 0.10, f ) R = Y and x = 0.10, g) R = La and x = 0.13, and h) R = Y and x = 0.13.Dopant identity and amount does not significantly affect the microstructure and morphology.Slight decrease of grain size with increasing dopant amount is observed.

Figure 3 .
Figure 3. Room-temperature values of bulk and relative density measured for the Ca 1Àx R x MnO 3 samples (R = Y or La marked with green or blue, respectively; 0.00 ≤ x ≤ 0.13).The green and blue lines connecting between the datapoints serve as guides for the eye.

Figure 4 .
Figure 4. Crystal lattice image of Ca 0.87 Y 0.13 MnO 3 specimen obtained by HAADF-STEM across the [001] zone axis, showing the layering of CaO and MnO 2 planes.Atomically resolved elemental mapping obtained by EDS is shown in insets, revealing Ca, La, Mn, and O atomic columns colored by red, purple, green, and blue, respectively.The image clearly indicates Y substitution for Ca sites.

Figure 5 .
Figure 5. a) Temperature-dependent electrical conductivity and b) Seebeck coefficient values of the undoped and Ca 1Àx R x MnO 3 samples with R = Y or La and 0.006 ≤ x ≤ 0.13, measured between 300 and 1050 K.For most compositions and most of the temperature range, enhanced electrical conductivity is observed for the Y-doped samples compared to their La-doped counterparts, while Seebeck coefficient values are similar.The quality of the linear fitting between the S-curves and expression (2) is denoted by R 2 -values close to unity.The lines connecting between the datapoints serve as guides for the eye.

Figure 7 .
Figure 7.The room-temperature values of thermal conductivity measured for the undoped (orange) and Ca 1Àx R x MnO 3 samples with R = Y (green) or La (blue) and 0.006 ≤ x ≤ 0.13.

Figure 8 .
Figure 8. Values of charge carrier concentration evaluated for the Ca 1Àx R x MnO 3 samples (R = Y or La; 0.006 ≤ x ≤ 0.13) based on analysis of the Seebeck coefficient data using the Heikes formula.The lines connecting between the datapoints serve as guides for the eye.

Figure 6 .
Figure 6.The temperature-dependent thermoelectric PF evaluated for the undoped and Ca 1Àx R x MnO 3 samples with R = Y or La and 0.006 ≤ x ≤ 0.13 in the range of 300 through 1050 K.The lines connecting between the datapoints serve as guides for the eye.

Figure 9 .
Figure 9. Values of a) charge carrier generation energy, E S , and b) conduction activation energy, E σ , evaluated for the Ca 1Àx R x MnO 3 samples (R = Y or La marked with green or blue, respectively; 0 ≤ x ≤ 0.13) by analysis of the temperature-dependent electrical conductivity and Seebeck coefficient data using the small polaron hopping model.The green and blue lines connecting between the datapoints serve as guides for the eye.

Figure 10 .
Figure 10.a) Variation of the average distance between polarons (Mn þ3 ) with respect to dopant amount (x) in Ca 2 MnO 4 and CaMnO 3 , calculated assuming that each substitution of Y þ3 or La þ3 for Ca þ2 generates Mn þ3 defect at the Mn þ4 sublattice.The dashed green line represents values close to lattice parameters or unit cell length, that is, 10 Å. b) Conduction activation energy (E σ ) values derived for the Ca 1Àx R x MnO 3 samples (R = Y or La marked with green or blue, respectively; 0.006 ≤ x ≤ 0.13) plotted versus L. The green and blue lines connecting between the datapoints serve as guides for the eye.
3 and Ca 2 MnO 4 materials indicates that the extra CaO planes drastically impede charge transport and increase E σ from 2-22 meV in CaMnO 3 to 86-127 meV in Ca 2 MnO 4 , rendering electrical conductivity from metallic to semiconducting behavior.Our results suggest that besides increasing charge carrier concentration, Y inherently enhances charge carrier mobility compared to La doping, regardless of the presence of the extra CaO planes (in Ca 2 MnO 4 and Ca 3 Mn 2 O 7 ).The effect of dopant's chemical identity on electrical mobility is further corroborated by Hall measurements, showing higher mobility for Y-doped samples compared to their La-doped counterparts.By evaluating charge carrier concentrations in our samples, we find similar values for samples doped equally with Y or La, thereby corroborating the effects of dopant on carrier mobility.

Figure 11 .
Figure 11.Values of a) Hall mobility and b) charge carrier concentration for the Ca 1Àx R x MnO 4 samples (R = Y or La marked with green or blue, respectively; 0 ≤ x ≤ 0.13) measured using Hall effect.The lines connecting between the datapoints serve as guides for the eye.

Figure 12 .
Figure 12.The ratios between the Hall mobility values measured for the Y-and La-doped samples (wine) and between the conduction activation energies of La-and Y-doped samples (violet).A maximum is observed for both ratios near the same composition of x % 0.06.The lines connecting between the datapoints serve as guides for the eye.