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Temperature-Dependent Mixed Valency in the Hexagonal Perovskite Cs3NaFe2Cl9.
David Liu1, Alexander Milder1, Jeremiah Stevens1
1Department of Chemistry and Biochemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, Ohio 43210, United States.
Journal of the American Chemical Society
|February 20, 2025
Summary
Two novel hexagonal perovskites, Cs₃NaFe₂Cl₉ and Cs₃NaMnFeCl₉, exhibit unique magnetic and electronic properties due to their bioctahedral structures. Their distinct magnetic couplings and charge transfer excitations were detailed.
Area of Science:
- Solid State Chemistry
- Materials Science
- Magnetism
Background:
- Hexagonal perovskites are a class of materials with diverse structural and electronic properties.
- Understanding the interplay between structure, electronic transitions, and magnetic coupling is crucial for designing new functional materials.
- The 6H perovskite structure, characterized by bioctahedral clusters, offers unique possibilities for transition metal ion arrangements.
Purpose of the Study:
- To synthesize and characterize two new hexagonal perovskites: Cs₃NaFe₂Cl₉ and Cs₃NaMnFeCl₉.
- To investigate their structural, optical, and magnetic properties.
- To elucidate the nature of electronic and magnetic interactions within these novel compounds.
Main Methods:
- Synthesis of hexagonal perovskite compounds.
- Structural analysis using X-ray diffraction (implied by structure determination).
- Optical characterization via diffuse reflectance spectroscopy.
- Magnetic susceptibility measurements.
- Mössbauer spectroscopy for detailed electronic state analysis.
Main Results:
- Both compounds crystallize in the 6H hexagonal perovskite structure (P6₃/mmc) featuring Fe₂Cl₉⁴⁻ and FeMnCl₉⁴⁻ bioctahedra.
- Diffuse reflectance spectroscopy identified metal-to-metal and intervalence charge transfer excitations.
- Cs₃NaFe₂Cl₉ exhibits ferromagnetic coupling (θ_CW = 16.7 K) due to rapid electron exchange within Fe dimers.
- Cs₃NaMnFeCl₉ shows antiferromagnetic coupling between Fe³⁺ and Mn²⁺ (θ_CW = -41.4 K).
- Mössbauer spectroscopy revealed dynamic electron exchange (Fe²⁺.⁵⁺) at 100 K, slowing to distinct Fe²⁺ and Fe³⁺ signals upon cooling.
Conclusions:
- The synthesized hexagonal perovskites possess unique bioctahedral arrangements influencing their electronic and magnetic behaviors.
- The observed magnetic couplings (ferromagnetic and antiferromagnetic) are directly linked to the specific transition metal ions and their electronic states.
- Dynamic electron exchange phenomena were clearly demonstrated, providing insights into charge transport mechanisms in these materials.
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