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Page 16 of 18 Seo et al. Energy Mater. 2025, 5, 500123 https://dx.doi.org/10.20517/energymater.2025.38

structure calculations using the TB-LMTO methods revealed that the p-type Cu doping indeed led to
increased carrier concentrations and eventually to improved electrical conductivities. The ELF analysis
further demonstrated that the Cu substitution altered the bond polarity within the anionic frameworks,
improving carrier concentration and finally electrical conductivities. TE property measurements proved
that the co-substitution of Yb and Cu significantly increased the σ compared to that of the parental
compound Ca Zn Sb despite the reduced carrier mobility. This observation demonstrated that the
4.5
9
9
increased carrier concentration had a more significant effect on electrical conductivity than the decreased
carrier mobility. The increase in σ also led to an enhancement in the κ , which caused an overall increase
elec
in the κ . However, the increase in phonon scattering by the Yb and Cu co-substitution resulted in the
tot
reduction in κ , which suppressed the overall rise in κ . As a result of these combinations, the optimized
latt
tot
TE performance was achieved in Ca YbZn Cu Sb with the maximum ZT of 0.81 computed at 843 K.
8
4.4
0.1
9
DECLARATIONS
Authors’ contributions
Designed the experiment: Seo, N.; You, T. S.
Wrote the manuscript: Seo, N.; You, T. S.
Data analysis: Seo, N.; Lee, J.
Performed the physical characterization: Seo, N.; Lee, J.; Lee, Y.; Pi, J. H.; Lee, K. H.
Performed the SXRD measurements: Choi, M. H.; Ok, K. M.
Revised the manuscript: Lee, J.; You, T. S.
Acquired funding: Lee, K. H.; You, T. S.
Supervised students and developed concepts: You, T. S.

Availability of data and materials
The original supplementary crystallographic data for this study is openly available in the Cambridge
Crystallographic Data Center (12 Union Road, Cambridge CB21EZ, UK; fax: +44-1223-336033) at www.
ccdc.cam.ac.uk/data_request/cif (accessed on 10 February 2025), or by emailing
data_request@ccdc.cam.ac.uk. The reference/accession number to be used is CCDC 2422809-2422812.
Furthermore, the raw data supporting the conclusions of this article will be made available by the authors
on request.

Financial support and sponsorship
This work was supported by the Basic Science Research Program through the National Research
Foundation of Korea (NRF), funded by the Ministry of Science and ICT (RS-2024-00337629 and RS-2022-
NR068194).

Conflicts of interest
All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate
Not applicable.

Consent for publication
Not applicable.

Copyright
© The Author(s) 2025.