[Paper Review] Thermoelectric properties of LaRh$_{1-x}$Ni$_x$O$_3$

Soichiro Shibasaki, Yoshiyuki Takahashi+1 more|ArXiv.org|Dec 11, 2007

Advanced Thermoelectric Materials and Devices1 references3 citations

TL;DR

This study demonstrates that Ni-doped LaRh1-xNixO3 exhibits a large thermopower of 185 μV/K at 800 K due to the high-temperature stability of the low-spin state of Rh3+/Rh4+ ions, enabling a ZT value of 0.044 at x=0.3—three times higher than Ni-doped LaCoO3—highlighting spin-state control as a key strategy for high-temperature oxide thermoelectrics.

ABSTRACT

We report measurements and analyses of resistivity, thermopower, and thermal conductivity of polycrystalline samples of perovskite LaRh$_{1-x}$Ni$_x$O$_3$. The thermopower is found to be large at 800 K (185 $μ$V/K for $x=$0.3), which is ascribed to the high-temperature stability of the low-spin state of Rh$^{3+}$/Rh$^{4+}$ ions. This clearly contrasts with the thermopower of the isostructural oxide LaCoO$_3$, which rapidly decreases above 500 K owing to the spin-state transition. The spin state of the transition-metal ions is one of the most important parameters in oxide thermoelectrics.

Motivation & Objective

• To investigate the high-temperature thermoelectric properties of perovskite-type LaRh1-xNixO3 as a potential alternative to Co-based oxides.
• To determine whether the spin-state stability of Rh3+/Rh4+ ions enables sustained high thermopower above 500 K, unlike in LaCoO3.
• To compare the thermoelectric performance of LaRh1-xNixO3 with that of isostructural LaCo1-xNixO3 to evaluate the role of transition metal ion spin state.
• To assess the potential of Rh-based oxides for high-temperature thermoelectric applications by measuring resistivity, thermopower, and thermal conductivity.
• To explore Ni doping as a strategy to enhance the power factor and ZT in Rh-based perovskites while maintaining structural and electronic stability.

Proposed method

• Polycrystalline LaRh1-xNixO3 samples were synthesized via solid-state reaction at 1273 K for 24 h, followed by sintering at 1373 K for 48 h in air.
• X-ray diffraction (XRD) with θ–2θ geometry and Rietveld refinement were used to confirm phase purity and lattice parameter changes with Ni doping.
• Electrical transport properties (resistivity and thermopower) were measured using a four-probe technique from 4.2 K to 800 K in vacuum.
• Thermal conductivity was measured using a steady-state technique from 8 K to 300 K, with electronic contribution estimated via the Wiedemann-Franz law.
• Magnetic susceptibility measurements from 5 K to 400 K were used to determine the valence state of Ni ions, confirming Ni2+ in a high-spin state.
• ZT was calculated using the formula ZT = S²T / (ρκ), with κ estimated from lattice contribution and Wiedemann-Franz law, and compared to LaCo1-xNixO3.

Experimental results

Research questions

• RQ1 Does Ni doping in LaRh1-xNixO3 stabilize high-temperature thermopower comparable to layered Rh oxides?
• RQ2 How does the spin-state behavior of Rh3+/Rh4+ ions in LaRh1-xNixO3 influence its thermoelectric performance above 500 K compared to LaCoO3?
• RQ3 What is the effect of Ni substitution on resistivity, thermopower, and power factor in LaRh1-xNixO3 at elevated temperatures?
• RQ4 Can the ZT of Ni-doped LaRh1-xNixO3 exceed that of Ni-doped LaCoO3 at high temperatures, and if so, why?
• RQ5 To what extent does the lattice thermal conductivity limit ZT in LaRh1-xNixO3, and can further optimization improve performance?

Key findings

• The thermopower of LaRh0.7Ni0.3O3 reaches 185 μV/K at 800 K, significantly higher than the 15 μV/K measured for LaCo0.8Ni0.2O3 at the same temperature.
• The resistivity of LaRh0.7Ni0.3O3 is 25 times higher than that of LaCo0.8Ni0.2O3 at 800 K, but its thermopower is 12 times larger, resulting in a power factor 6 times higher.
• The ZT value of LaRh0.7Ni0.3O3 is calculated to be 0.044 at 800 K, which is approximately three times higher than the ZT of 0.015 for LaCo0.95Ni0.05O3 at the same temperature.
• The high thermopower in LaRh1-xNixO3 is attributed to the stability of the low-spin state of Rh3+/Rh4+ ions, which prevents the spin-state transition that degrades thermopower in LaCoO3 above 500 K.
• Magnetic susceptibility measurements confirm that Ni ions are divalent (Ni2+) in a high-spin state, and the lattice parameters remain nearly unchanged with doping due to similar ionic radii.
• The thermal conductivity of LaRh1-xNixO3 is dominated by lattice contributions, with electronic contribution estimated at less than 1 mW/cmK at 300 K, and is comparable to that of LaCo1-xNixO3.