Influence of Doping on the Transport Properties of Y1-xLnxMnO3+ (Ln: Pr, Nd)

It has been documented that the total electrical conductivity of the hexagonal rare-earth manganites Y0.95Pr0.05MnO3+δ and Y0.95Nd0.05MnO3+δ, as well as the undoped YMnO3+δ, is largely dependent on the oxygen excess δ, which increases considerably at temperatures below ca. 300 ◦C in air or O2. Improvement for samples maintaining the same P63cm crystal structure can exceed 3 orders of magnitude below 200 ◦C and is related to the amount of the intercalated oxygen. At the same time, doping with Nd3+ or Pr3+ affects the ability of the materials to incorporate O2, and therefore indirectly influences the conductivity as well. At high temperatures (700–1000 ◦C) and in different atmospheres of Ar, air, and O2, all materials are nearly oxygen-stoichiometric, showing very similar total conduction with the activation energy values of 0.8–0.9 eV. At low temperatures in Ar (δ ≈ 0), the mean ionic radius of Y1−xLnx appears to influence the electrical conductivity, with the highest values observed for the parent YMnO3. For Y0.95Pr0.05MnO3+δ oxide, showing the largest oxygen content changes, the recorded dependence of the Seebeck coefficient on the temperature in different atmospheres exhibits complex behavior, reflecting oxygen content variations, and change of the dominant charge carriers at elevated temperatures in Ar (from electronic holes to electrons). Supplementary cathodic polarization resistance studies of the Y0.95Pr0.05MnO3+δ electrode document different behavior at higher and lower temperatures in air, corresponding to the total conduction


Introduction
Multiferroic materials, showing the coexistence of ferroelectricity and magnetism, have become of special interest for physicists, not only due to fascinating and complex magnetoelectric properties but also considering their possible application for data storage, capacitors, transducers, actuators, etc. [1,2]. Among the compounds, hexagonal manganites LnMnO 3 (Ln: Y, In, Sc, and Ho-Lu) appear to be especially interesting [3][4][5][6], with, e.g., ferroelectric order (below ∼ 900 K) and antiferromagnetic order (below ∼ 80 K) documented for YMnO 3 [7]. The emergence of ferroelectricity in YMnO 3 is associated with a transition between the centrosymmetric (aristotype) P6 3 /mmc and non-centrosymmetric P6 3 cm space group, associated with tilting of the oxygen bi-pyramids surrounding Mn ions, as well as bending the Y layers [8]. This can also be considered as the asymmetric movement of Y 3+ cations from their positions in the aristotype structure [9,10]. The distorted, lower temperature crystal structure shows electric polarization along the c-axis. At low temperatures, the magnetic properties reflect the presence of Mn 3+ cations, which are ordered within the a-b planes (forming triangular sublattice), also with the possible effect of canting of the Mn-related spins [7]. The five-fold coordination of Mn 3+ results in the splitting of the 3d 4 electronic orbitals into three sets: empty a', and filled up e', and e". This X-ray diffractometry (XRD) was performed on both powders after annealing at 1000 • C in Ar, as well as on the sintered pellets.
Crystal structure and phase composition of the materials considered were examined using the Empyrean (PANalytical, The Netherlands) diffractometer with CuKα radiation and PIXcel3D detector. HighScore Plus (PANalytical, The Netherlands) software with PDF-4+ (ICDD, USA) database were used for phase identification. Rietveld refinements were done using GSAS software with EXPGUI graphical interface [25,26].
The oxygen content (δ) of the samples was investigated with the TG method using Q5000 IR thermobalance (TA Instruments, USA). Weight changes were recorded in the 25 mL min −1 flow of Ar, synthetic air and O 2 (in such order). Two cycles of heating and cooling (2 • C min −1 rate) were done in the temperature range from 30 • C to 500 • C. Discussed data concern cooling step. Measurements were carried out for the sintered samples of the same densities as those used for conductivity and thermoelectric measurements.
Electrical conductivity measurements were carried out using the direct current 4-wire technique. For each material, 2 cycles of heating and cooling (2 • C min −1 ) were performed in the range from ca. 50 • C to 1000 • C. Discussed in the paper data show a cooling step in the 2nd cycle. Similar to TG analyses, tests were done in the flow of Ar, air, and O 2 (20 mL min −1 ). Samples of rectangular shape (approx. 7 × 5 × 1 mm) were mounted inside the Probostat holder (NorECs, Oslo, Norway), and data were collected with Keithley 2000 (Tektronix, Beaverton, OR, USA) multimeter, and measurement was controlled by Omega2 (NorECs, Oslo, Norway) software. Pt electrodes were used in the studies-these were prepared by doctor-blading Pt paste on the opposite faces of the samples and firing at 980 • C for 15 min. Since the relative density of all samples ranged between 79-83%, a porosity-related correction factor was applied in the calculations, according to Bruggeman's model [27,28]. For each sample, resistance was measured across its longest dimension.
Thermoelectric power of Pr005 material was measured using the same sample as for the conductivity studies. Testes were also done in the flow of Ar, air and O 2 (20 mL min −1 ), in the temperature range from 50 • C to 1000 • C. Data were collected during the 2nd cycle upon cooling (2 • C min −1 ). Temperature gradient (approx. 10 • C across 7.56 mm long sample) was maintained by appropriately moving the Probostat holder off the center of the furnace. Data were collected with Keithley 2000 m and Omega2 software. Voltage was measured using attached Pt electrodes. To evaluate thermoelectric power, the recorded thermoelectric voltage was divided by the temperature gradient, with no additional corrections done.
Phenom XL Desktop (Thermo Fisher Scientific, USA) scanning electron microscope (SEM) equipped with a silicon drift detector was used to investigate the morphology of the pellets after electrical measurements. Samples were reduced (δ = 0) in argon at 1000 • C prior to the experiment. Energy-dispersive X-ray spectroscopy (EDS) was done to prepare elemental maps of samples' surfaces. The applied voltage for EDS analysis was 15 kV. The results were analyzed with the use of ProSuite (Thermo Scientific, USA) software.
To determine the possibility of application of the Pr005 for manufacturing air electrodes for solid oxide fuel cells, tests regarding thermochemical stability of the material in contact with two commercial electrolyte materials were done. LSGM and GDC powders were used in the measurements. Pr005 powder was mixed in 1:1 mass proportion with LSGM and respectively with GDC. The mixtures were then annealed at 1000 • C for 10 h and at 800 • C for 100 h in atmospheric air. XRD tests were done to check for possible interactions between the powders.
Electrode polarization resistance for Y 0.95 Pr 0.05 MnO 3+δ was determined by studying Pr005|LSGM|Pr005 symmetrical cell. Dense LSGM electrolyte pellet was sintered at 1450 • C for 8 h. Pr005 electrodes (ca. 6 mm in diameter) were screen-printed on both sides of the LSGM pellet (10.3 mm in diameter, 1.5 mm thick). Electrode paste was prepared by mixing Pr005 (fine powder was received by grinding the sintered pellet thoroughly in mortar) with an organic binder and starch (used as a pore former) in the wt. ratio of 1.5:2:0.05. Two layers were printed on each side of the electrolyte, and the cell was fired at Crystals 2021, 11, 510 4 of 13 1100 • C for 4 h. Polarization resistance was measured by impedance spectroscopy using Solartron 1252A frequency response analyzer, with the cell mounted in the Probostat holder. The data were recorded on cooling from 900 • C, in the 0.1 Hz to 300 kHz frequency range, with 10 mV amplitude.

Results and Discussion
3.1. Crystal Structure and Phase Composition after Sintering X-ray diffraction data confirmed that each of the sintered pellets comprised singlephase material crystallized in hexagonal P6 3 cm (PDF-4+ reference pattern: 01-082-3832) symmetry ( Figure 1). As expected, for larger values of the mean ionic radius of Y 1−x Ln x in doped materials, the unit cell volume enlarged (Table 1). This growth, however, wwas related only to the increase of the a parameter, while the c parameter was found to be very similar for all compounds. This is in accordance with the results of previous studies [24] and can be linked with layered-type of the arrangement of Y 1−x Ln x -and Mn-related structural units along the c-axis. It should be noted that annealing at the high temperature of 1400 • C in air, needed to obtain relatively dense sinters, resulted in the shrinkage of the unit cell, as compared to YMnO 3 or Pr005 samples synthesized at 900 • C and 950 • C, respectively in Ar [24]. This might be explained as due to the partial oxidation of the pelletized samples, ongoing while cooling in air. The effect is discussed in more detail below. mixing Pr005 (fine powder was received by grinding the sintered pellet thoroughly in mortar) with an organic binder and starch (used as a pore former) in the wt. ratio of 1.5:2:0.05. Two layers were printed on each side of the electrolyte, and the cell was fired at 1100 °C for 4 h. Polarization resistance was measured by impedance spectroscopy using Solartron 1252A frequency response analyzer, with the cell mounted in the Probostat holder. The data were recorded on cooling from 900 °C, in the 0.1 Hz to 300 kHz frequency range, with 10 mV amplitude.

Oxygen Content-Thermogravimetric Analyses
It has already been established that the ability to absorb oxygen in LnMnO 3+δ is mainly dependent on two factors: (1) mean ionic radii of rare-earth elements occupying Ln sublattice; and (2) specific surface area of the particular sample [14,23,24,29]. The first discovered relationship indicates that if a proper doping level of larger lanthanides is introduced into Y 1−x Ln x MnO 3+δ , resulting in the increased mean radius being close to the critical value, more oxygen can be incorporated into the materials, and the process occurs with a higher rate, as well as in atmospheres with less oxygen [24]. The second factor results from the nature of the oxidation process itself, as the oxygen molecules need to be adsorbed, dissociated, and reduced first, and then oxygen anions can enter the oxide's structure. Since a number of the active surface sites is proportional to the specific surface, as well as the (bulk) diffusion length depends on the particle (or sintered pellet) size, it is obvious that the oxidation rates for fine powders are much higher, compared to coarse material or pellets [11][12][13][14][15]23,24]. Importantly, while doping may influence both thermodynamics and kinetics of the reactivity with oxygen, the material's optimized fine morphology can only help to achieve faster oxidation (as well as oxygen release) rates.
As presented in TG data gathered in Figure 2a-c, slow cooling in different atmospheres of YMnO 3+δ , Nd005, and Pr005 pellets enables to obtain partially oxidized materials in air and O 2 , while cooling in Ar gas results in practically oxygen-stoichiometric samples. As for the reference point in the studies, it was assumed, similarly to previous papers, that all materials are stoichiometric at 500 • C [12,13,15,30]. It should be emphasized that the oxidation did not proceed to the values of δ as high as the ones reported for the fine powders (Table 2). This is due to the limited surface area of sintered pellets (see SEM images in Figure S1), hindering the rate of oxygen incorporation. This is crucial for the discussed below electrical conductivity studies: in all of the cases, the materials remained in the initial P6 3 cm symmetry, with no transition to the oxygen-loaded Hex1 (R3c symmetry) or other phases, as well as with no indication of the formation of two-phase mixtures. In other words, limiting the oxidation degree up to only δ ≈ 0.05 (Pr005 sample oxidized in O 2 ) enables to maintain the original crystal structure in the studied materials, and therefore allows to eliminate possible influence of the phase transitions. It is worth mentioning that, for all of the compounds studied, the expected relationships were obtained; doping with Nd 3+ and Pr 3+ allowed for larger oxygen excess in the materials, as well as δ was found to be the largest for samples cooled in pure oxygen, intermediate after cooling in air, and close to zero if the process was conducted in argon. Table 2. Comparison of the oxygen excess δ at room temperature evaluated for the sintered pellets, with reference data for the fine powders [24]. Values given after cooling in air and in pure O2.  It is worth mentioning that, for all of the compounds studied, the expected relationships were obtained; doping with Nd 3+ and Pr 3+ allowed for larger oxygen excess in the materials, as well as δ was found to be the largest for samples cooled in pure oxygen, intermediate after cooling in air, and close to zero if the process was conducted in argon.

Electrical Conductivity Measurements
As presented in Figure 3a-c, the total electrical conductivity of all of the studied materials in the range of 700-1000 • C was found to be very similar and practically not influenced by the gas atmosphere, confirming near oxygen-stoichiometric composition of the compounds. The exemplary (and corrected by the porosity-related factor) values are: 6.30·10 −2 S cm −1 for YMnO 3+δ , 6.35·10 −2 S cm −1 for Nd005, and 6.98·10 −2 S cm −1 for Pr005 at 800 • C in Ar. Similarly, the respective activation energy values of the conductivity are in the range of 0.8-0.9 eV (Table 3). A phase transformation from high-temperature P63/mmc symmetry to P63cm (Hex0) was reported to occur at ca. 950-1000 °C in YMnO3+δ [10]. Nevertheless, there was no visible impact of this transformation on the conductivity behavior of any material examined here. Interestingly, Gibbs et al. observed secondary isosymmetric transition in the Hex0 structure at about 650 °C. It was suggested that this transition involves polar displacement of the Mn-O equatorial planes and is related to hybridization of the Y-O bond along the c direction. In our experiments, for YMnO3+δ in Ar, the log(δ)-1000/T relationship appears to deviate from the linear dependence at temperatures near to the transition mentioned above (i.e., at approx. 600 °C) [7,10], resulting in the lowered Ea, and overall highest conductivity values among the studied samples at low temperatures in this atmosphere. Since all compounds are oxygen-stoichiometric in Ar (Figure 2a-c), lower conductivity observed for Nd-and Pr-doped oxides seems to relate to the larger unit cell parameter a (Table 1), which affects the effectiveness of the orbital overlapping in Mn-related layers. Different degrees of structural distortion (in comparison to the aristotype P63/mmc symmetry) may also affect the conduction, but a full description of the properties would require additional data from high-temperature structural studies.

Atmosphere
Temperature Range [°C]  A phase transformation from high-temperature P6 3 /mmc symmetry to P6 3 cm (Hex0) was reported to occur at ca. 950-1000 • C in YMnO 3+δ [10]. Nevertheless, there was no visible impact of this transformation on the conductivity behavior of any material examined here. Interestingly, Gibbs et al. observed secondary isosymmetric transition in the Hex0 Crystals 2021, 11, 510 7 of 13 structure at about 650 • C. It was suggested that this transition involves polar displacement of the Mn-O equatorial planes and is related to hybridization of the Y-O bond along the c direction. In our experiments, for YMnO 3+δ in Ar, the log(δ)-1000/T relationship appears to deviate from the linear dependence at temperatures near to the transition mentioned above (i.e., at approx. 600 • C) [7,10], resulting in the lowered E a , and overall highest conductivity values among the studied samples at low temperatures in this atmosphere. Since all compounds are oxygen-stoichiometric in Ar (Figure 2a-c), lower conductivity observed for Nd-and Pr-doped oxides seems to relate to the larger unit cell parameter a (Table 1), which affects the effectiveness of the orbital overlapping in Mn-related layers. Different degrees of structural distortion (in comparison to the aristotype P6 3 /mmc symmetry) may also affect the conduction, but a full description of the properties would require additional data from high-temperature structural studies.
Since electrical conductivity measurements were performed for the same samples as those used in TG experiments, and because the cooling rate was identical, it can be assumed that conduction data presented in Figure 3b reflect the actual changes induced by the oxygen incorporation in the materials in air (Figure 2a-c). An intermediate range at ca. 300-400 • C is clearly visible, with deviation from linearity occurring from ca. 500 • C, which suggests that even a small amount of the intercalated oxygen has a profound influence on the total electrical conduction. A similar plateau was observed in [31] for YMnO 3+δ , however, shifted towards higher temperatures (400-600 • C), which might be due to different experimental conditions: (1) oxygen transfer between atmosphere and specimen was hindered by the sample's surface area (lower porosity, less than 10% compared to 18% in this work); and (2) the heating/cooling rate was set to 10 • C min −1 , being 5 times faster than in our experiments. It should also be emphasized that all materials remain homogenous after tests (Figures S1 and S2).
Previous studies indicated that in hexagonal rare-earth manganites, the absorbed oxygen is introduced (mainly) into the manganese-related layers [13,32]. To keep the electroneutrality, Mn cations are oxidized concurrently, yielding mixed +3/+4 states located in the respective layers of the (largely undisturbed at the initial stages of the oxygen incorporation) triangular sublattice. This should result in the significantly enhanced electrical conduction of the material, as indeed was observed. It should be noted, however, that since in the presented results, the oxidation did not proceed to the equilibrium values, the recorded data should be interpreted as corresponding to the respective oxygen excess values, as evaluated in the TG experiments (Figure 2a-c). Interestingly, at lower temperatures total electrical conductivity of Nd005 and Pr005 is higher than that of YMnO 3+δ , which can be interpreted as due to the larger oxygen excess in the doped materials.
Proof that higher oxygen excess positively affects the conduction comes from data presented in Figure 3c, in which the highest conductivity was observed for the Pr005 sample in O 2 , corresponding to the higher degree of the oxidation (Figure 2c). The exemplary values at 250 • C were 7.24·10 −6 S cm −1 , 1.13·10 −3 S cm −1 , and 3.51·10 −3 S cm −1 for Ar, air, and oxygen atmospheres respectively. In the figure discussed, two more points are marked, corresponding to the values registered in the 20 h relaxation process shown in Figure 4. After switching the gas from Ar to air, the conductivity rises rapidly and then increases more slowly. Since the prolonged oxygen incorporation should proceed to even higher δ values (up to ca. 0.24-0.29 for Hex1 phase [24]), it is not surprising that the conduction of Pr005 exceeds in this case that measured in O 2 during the cooling process.
values at 250 °C were 7.24·10 −6 S cm −1 , 1.13·10 −3 S cm −1 , and 3.51·10 −3 S cm −1 for Ar, air, and oxygen atmospheres respectively. In the figure discussed, two more points are marked, corresponding to the values registered in the 20 h relaxation process shown in Figure 4. After switching the gas from Ar to air, the conductivity rises rapidly and then increases more slowly. Since the prolonged oxygen incorporation should proceed to even higher δ values (up to ca. 0.24-0.29 for Hex1 phase [24]), it is not surprising that the conduction of Pr005 exceeds in this case that measured in O2 during the cooling process.  As can be seen in Figure 5, for the experiments performed in air and O 2 , thermoelectric power was found to be positive for Y 0.95 Pr 0.05 MnO 3+δ in the entire temperature range. With a decrease of temperature, the Seebeck coefficient rises up to high values of 1500 µV K −1 , as can be expected for semiconducting samples. In the range of the ongoing oxidation, the values stabilize, and at lower temperature decrease, with changes more strongly visible for data measured in oxygen. Since Pr005 oxidizes to higher δ in O 2 , the respective decrease of Seebeck coefficient seems to relate to the excess of oxygen, and therefore, the increase of concentration of Mn 4+ upon oxidation. As commonly understood, positive thermoelectric power suggests the dominance of the electronic holes as main charge carriers [33], which in this case may be related to Mn 4+ . The behavior observed here for Pr005 was similar to that previously reported for YMnO 3+δ in air-the sign of the Seebeck coefficient was also positive and exhibited its maximum at 600 • C [31].

Thermoelectric power of Y0.95Pr0.05MnO3+δ
Surprisingly, a negative Seebeck coefficient was recorded at high temperatures in Ar, with a sign change to positive at ca. 510 • C ( Figure 5). This unexpected behavior cannot be easily explained, as the oxygen content in Pr005 should be close to stoichiometric in such conditions, similarly as in air or O 2 at high temperatures. One possible explanation is that a limited reduction occurs in argon, which may cause an appearance of Mn 2+ cations acting as the effectively negative charge carriers, which may also be facilitated by a presence of Pr cations possibly showing a mixed +3/+4 state. Nevertheless, full elucidation of the behavior in this range requires additional experiments. Below 510 • C, the thermoelectric power of the material rises sharply to values reaching ca. 2000 µV K −1 , while further below, it could not be measured due to a lack of reliable thermoelectric voltage readout. A similar problem occurring for the other two atmospheres arises in the range in which the compound shows high resistance.
ble for data measured in oxygen. Since Pr005 oxidizes to higher δ in O2, the respective decrease of Seebeck coefficient seems to relate to the excess of oxygen, and therefore, the increase of concentration of Mn 4+ upon oxidation. As commonly understood, positive thermoelectric power suggests the dominance of the electronic holes as main charge carriers [33], which in this case may be related to Mn 4+ . The behavior observed here for Pr005 was similar to that previously reported for YMnO3+δ in air-the sign of the Seebeck coefficient was also positive and exhibited its maximum at 600 °C [31]. Surprisingly, a negative Seebeck coefficient was recorded at high temperatures in Ar, with a sign change to positive at ca. 510 °C ( Figure 5). This unexpected behavior cannot be easily explained, as the oxygen content in Pr005 should be close to stoichiometric in such conditions, similarly as in air or O2 at high temperatures. One possible explanation is that a limited reduction occurs in argon, which may cause an appearance of Mn 2+ cations acting as the effectively negative charge carriers, which may also be facilitated by a presence of Pr cations possibly showing a mixed +3/+4 state. Nevertheless, full elucidation of the behavior in this range requires additional experiments. Below 510 °C, the thermoelectric power of the material rises sharply to values reaching ca. 2000 µV K −1 , while further below, it could not be measured due to a lack of reliable thermoelectric voltage readout. A similar problem occurring for the other two atmospheres arises in the range in which the compound shows high resistance.

Stability of Y0.95Pr0.05MnO3+δ vs. Solid Electrolytes
Despite moderate specific conductivity of Pr005 (as well as other studied materials) at high temperatures, which indicates rather limited possibility of application for manufacturing of air electrodes for solid oxide fuel cells, chemical stability data of the material in relation to oxygen-conducting solid electrolytes is of interest, as studies of cathodic polarization resistance of the cells bring valuable, additional information about reactivity  Despite moderate specific conductivity of Pr005 (as well as other studied materials) at high temperatures, which indicates rather limited possibility of application for manufacturing of air electrodes for solid oxide fuel cells, chemical stability data of the material in relation to oxygen-conducting solid electrolytes is of interest, as studies of cathodic polarization resistance of the cells bring valuable, additional information about reactivity towards oxygen [34,35]. As presented in Figure 6a towards oxygen [34,35]. As presented in Figure 6a,b, Pr005 is fully stable in relation to La0.8Sr0.2Ga0.8Mg0.2O3−d (LSGM) and Ce0.9Gd0.1O1.95 (GDC), with no additional phases appearing after annealings in air (at 800 °C for 100 h and 1000 °C for 10 h), as well as with a very limited shift of the recorded peaks. Consequently, in the following experiment, LSGM was used as a solid electrolyte for the preparation of a symmetrical cell.
(a) (b) Figure 7a-c show exemplary impedance spectroscopy data (Nyquist plots) recorded for the symmetrical Pr005|LSGM|Pr005 cell in air. As can be seen, with lowering of temperature character of the recorded curves changes from a shifted and depressed semicircle to more complex behavior with one additional arch appearing, which at the lowest temperatures dominates in the recorded data. With the usage of the respective equivalent cir-  7a-c show exemplary impedance spectroscopy data (Nyquist plots) recorded for the symmetrical Pr005|LSGM|Pr005 cell in air. As can be seen, with lowering of temperature character of the recorded curves changes from a shifted and depressed semicircle to more complex behavior with one additional arch appearing, which at the lowest temperatures dominates in the recorded data. With the usage of the respective equivalent circuits, the R1, R2, and R2a resistance components could be evaluated. The temperature dependence of the values is shown in Figure 8. Generally, R1 can be interpreted as an ohmic-related component (of the solid electrolyte) [36], but data in the literature are usually not shown below 500 • C. In the recorded high-temperature characteristics, indeed, the R1 component seems to reflect conductivity of the LSGM pellet (after recalculation considering pellet's dimensions), with the activation energy (Table 4) also being in agreement with the literature [37,38]. At lower temperatures (below 350 • C), R1 acts strange, being temperature independent. The high-frequency intercept is not directly observed (Figure 7c), so this may be combined artifacts from the measurement of some highest frequencies and also from the R-CPE fitting. It appears that R1 and R2a together correspond to the solid electrolyte and possibly electrode/electrolyte interfacial resistance, and therefore, only the R2 component can be interpreted as related directly to the Pr005 electrode.      The temperature changes of R2 seemed to follow the electrical conductivity data for this material in air (Figure 3c). However, the activation energy values were different (higher) in the respective temperature ranges. This is even more clearly visible in Figure S3. Notably, the recorded electrical conductivity values at temperatures, in which Pr005 oxidizes in air, were much higher compared to the values extrapolated from the high-temperature conduction behavior. Similar characteristics were visible for R2, which together allow us to conclude that at this temperature range, the material shows improved activity towards the oxygen reduction reaction and allows for the effective incorporation of oxygen into the structure (Figure 2c). This is also in accordance with the emerging ionic conduction, which has to take place if oxygen anions are to diffuse into the crystal structure.

Conclusions
The influence of doping with larger Pr 3+ and Nd 3+ of the parent YMnO 3+δ was studied, showing the major role of the oxygen content in the materials, determining total electrical conductivity. Modified Y 0.95 Pr 0.05 MnO 3+δ and Y 0.95 Nd 0.05 MnO 3+δ , as well and the reference YMnO 3+δ , could be partially oxidized, and in the limited range of changes of δ, all materials remained in the P6 3 cm crystal structure. Despite rather small oxygen content variations (up to ca. 0.05), the improvement of conduction for Pr005 could exceed 3 orders of magnitude in relation to the oxygen-stoichiometric compound. Also, the mean ionic radius of Y 1−x Ln x visibly influenced the electrical conductivity, but only for the stoichiometric materials. Interestingly, the recorded dependences of the Seebeck coefficient on the temperature in different atmospheres for Y 0.95 Pr 0.05 MnO 3+δ oxide were found to be complex but generally reflecting the oxygen content variations. With supplementary data concerning cathodic polarization resistance of the Pr005 electrode, it could be confirmed that the material shows enhanced reactivity towards oxygen at lower temperatures in air, corresponding to the range of effective oxidation.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/cryst11050510/s1, Figure S1: SEM images of the a) YMnO3, b) Nd005 and c) Pr005 pellets reduced at 1000 • C in argon, Figure S2: EDS elemental maps of the a) YMnO3, b) Nd005 and c) Pr005 pellets reduced in Ar after electrical conductivity measurements, Figure S3: Comparison of conductivity and oxygen content dependencies on temperature, depicted in linear temperature scale, Table S1: Fitted values of the equivalent circuit elements of the Pr005|LSGM|Pr005 cell. Thickness of LSGM electrolyte: 1.46 mm. Effective surface area of single Pr005 electrode: 27.3 mm 2 .

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.