Identifying different electronic transport mechanisms in nanoporous inorganic electrides – a combined study using Hall measurements and electron paramagnetic resonance spectroscopy

Abstract

Mayenite [Ca₂₄Al₂₈O₆₄]⁴⁺(2O²⁻) is a nanoporous compound, whose properties are strongly determined by its degree of reduction. It consists of a positively charged, subnanometer sized cage structure [Ca₂₄Al₂₈O₆₄]⁴⁺ and two additional oxygen ions O²⁻, which ensure charge neutrality. The O²⁻ ions occupy 1/6th of the cages and can move almost freely between them. A transition from insulating behavior to metallic behavior occurs when replacing the O²⁻ ions in the cages by electrons, finally generating the electride [Ca₂₄Al₂₈O₆₄]⁴⁺(4e⁻). This wide tunability together with its mechanical and chemical stability make the [Ca₂₄Al₂₈O₆₄]⁴⁺(2O²⁻)_(1−x)(4e⁻)_x system highly attractive for various electronic device applications. However, a complete understanding of the charge transport mechanisms of the [Ca₂₄Al₂₈O₆₄]⁴⁺(2O²⁻)_(1−x)(4e⁻)_x system and their variation with the degree of reduction x is far from complete. Here, we present the characterization of a series of polycrystalline [Ca₂₄Al₂₈O₆₄]⁴⁺(2O²⁻)_(1−x)(4e⁻)_x with different x using Hall-measurements and electron paramagnetic resonance (EPR) spectroscopy in the full temperature range between 10 K and 300 K. The study of temperature dependence of the effective carrier concentrations, the effective mobility, and the spin relaxation processes provides detailed insight into the electronic properties of the material. Three different transport mechanisms are identified which dominate at different temperatures. The crossover-temperature between these mechanisms is observed in both types of experiments and depends on x, which determines the effective charge carrier concentration. In addition, the interplay of the three transport mechanisms explains discrepancies which occur when solely the EPR signal intensity is used as a measure of the effective carrier concentrations 4x per unit cell volume of partially reduced [Ca₂₄Al₂₈O₆₄]⁴⁺(2O²⁻)_(1−x)(4e⁻)_x.