Abstract
Spinels represent an important class of technologically relevant materials, used in diverse applications ranging from dielectrics, sensors and energy materials. While solid solutions combining two “single spinels” have been explored in a number of past studies, no ordered “double” spinels have been reported. Based on our first principles computations, here we predict the existence of such a double spinel compound MgAlGaO4, formed by an equimolar mixing of MgAl2O4 normal and MgGa2O4 inverse spinels. After studying the details of its atomic and electronic structure, we use a cluster expansion based effective Hamiltonian approach with Monte Carlo simulations to study the thermodynamic behavior and cation distribution as a function of temperature. Our simulations provide strong evidence for short-ranged cation order in the double spinel structure, even at significantly elevated temperatures. Finally, an attempt was made to synthesize the predicted double spinel compound. Energy Dispersive X-ray Spectrometry and X-ray diffraction Rietveld refinements were performed to characterize the single-phase chemical composition and local configurational environments, which showed a favorable agreement with the theoretical predictions. These findings suggest that a much larger number of compounds can potentially be realized within this chemical space, opening new avenues for the design of spinel-structured materials with tailored functionality.
Highlights
Spinels represent an important class of technologically relevant materials, used in diverse applications ranging from dielectrics, sensors and energy materials
While the MgAl2O4–MgIn2O4 and MgGa2O4–MgIn2O4 binary systems do not like to mix at low temperatures, there exists a specific equimolar ordered configuration for the MgAl2O4–MgGa2O4 ( [Ga]t[MgAl]oO4) system that is distinctly favored by a negative enthalpy of mixing
Our DFTcomputed ΔE = (EInverse − ENormal) values of 0.192, −0.077, and −0.092 eV per formula unit for MgAl2O4, MgGa2O4 and MgIn2O4, respectively, are again in good agreement with those reported in previous calculations[62,63]
Summary
Spinels represent an important class of technologically relevant materials, used in diverse applications ranging from dielectrics, sensors and energy materials. The fact that most of the main group and transition metals can be synthesized in a stable spinel structure makes it a relatively large family of compounds with manifold compositions, cation ordering, electron configurations, and valence states[9] Owing to this large synthetic flexibility and a wide range of atomic and electronic configurations accessible within this chemical space, spinels exhibit interesting magnetic[5,10,11], electronic[12,13], optical[14], and catalytic[15,16] properties useful for a diverse range of applications, including data storage[17], high frequency electronic devices[18], dielectrics[19], transparent conducting oxides[20,21,22,23], lasers[24,25], sensing[2], energy storage[26,27], superconductivity[28], and biotechnology[29]. While MgIn2O4 exhibits a high degree of inversion at room temperature, reported structures for MgGa2O4 scatter over a wide range of intermediate degrees of inversion[30,31]
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