Ca₃Mn₂O₇‐layered perovskites: Effects of La‐ and Y‐doping on phase stability, microstructure, and thermoelectric transport

Abstract

We investigate phase stability, microstructure, and thermoelectric transport of polycrystalline bulk Ca3−xRxMn2O7 samples prepared by standard solid‐state reaction, where R = Y or La and 0 ≤ x ≤ 0.33. Ab‐initio calculations predict that Y‐doping at Ca‐sites should reduce the potential energy barrier for electron transport, as opposed to La‐doping. We find that Y‐doping prompts transformation from Ca3Mn2O7 to Ca2MnO4, whereas La‐doping is accompanied by no phase transformation. La‐doping significantly hinders grain growth, for example, the average grain size decreases from 4.44 ± 0.24 to 1.20 ± 0.03 μm for x = 0 (undoped) and x = 0.33 upon La‐doping, respectively. Electrical conductivity and Seebeck coefficients are measured for the temperature range of 300–1300 K, and analyzed in terms of the small polaron hopping model. We find that Y‐doping reduces the activation energy for conduction compared to La‐doping, for example, 43 and 63 meV, respectively. This suggests that Y reduces the energy barrier for polaron transport, in accordance with computational predictions. This trend is further supported by calculations of selected electronic, structural, and vibrational properties, highlighting the intriguing correlation between electronic transport governed by small polarons and elastic properties, thereby shedding light on charge transport and thermoelectric properties of such layered perovskites.

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

“…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] CaMnO 3 is the m = ∞ derivative of the series, and in contrast to the m = 1 and 2 compounds, it consists of the perovskite DOI: 10.1002/aesr.202300191…”

“…[36,[40][41][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] Figure 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.…”

Resistive Avalanches in La1–xSrxCoO3−δ (x = 0, 0.3) Thin Films and Their Reversible Evolution by Tuning Lattice Oxygen Vacancies (δ)

“…The thermopower S can be written as S = ± k normalb e ( E S k normalb T + B ) where E S is the charge carrier generation energy and B is a constant. The small polaron activation energy E a is a combination of mainly the polaron binding energies E p and E s (can be obtained from the temperature-dependent thermopower measurement) and related by eq , E a = E normalP 2 + E s …”

“…However, according to the in situ measurement, the activation energy decreased. Azulay et al reported the anomaly in the activation energies with respect to the doping of La in Ca 3 Mn 2 O 7 . Similarly, the presence of different oxygen-deficient crystallites or the differential incorporation of O – ions in different crystallites in in situ O 2 annealing may avert the films from exhibiting a systematic nature in the activation energy .…”

Thermoelectric properties of hot‐pressed Ruddlesden‐Popper phases CaO(CaMnO3)m

“…This double effect is discussed in a study of the m = 2 derivative (Ca 3 Mn 2 O 7 ) doped with La or Y. 42 To investigate whether the low density is the cause here, we evaluated the intrinsic thermal conductivity associated with the fully-densified bulk materials applying the Maxwell-Eucken relation for pore correction: κ = κ o (1 -P)/(1 + βP), where κ o is the intrinsic thermal conductivity with zero porosity, P is the porosity volume fraction, and β is a phenomenological parameter that equals 2 for the case of spherical pores. This relation neglects both conductive and radiative processes within the pores and applies for approximately random and homogeneous distribution of spheroidal pores.…”