High-Pressure Synthesis of High-Entropy Spinel-Type Semiconducting Manganates and Perovskite-Type Ferroelectric Niobates

We have studied high-entropy (HE) chalcogenides[1] and oxides prepared at high pressure. The high-pressure method has several advantages in stabilizing HE compounds; for example, enclose the volatile elements and quick-quench from high temperature. Here, we report our recent study on HE spinel-type manganates as a thermistor and HE perovskite-type niobates, possibly as ferroelectric relaxors.<br/><br/>We focused on NiMn₂O₄ as a pristine that is a typical NTC (Negative Temperature Coefficient) thermistor compound. NiMn₂O₄and partially (minor component) substituted ones show proper resistivity for monitoring temperatures between 300–500 K in various electric devices. The efficiency of a thermistor is evaluated with the B constant (corresponding to sensitivity) in general. Most commercial ones show B = 3,000–4,000 K. Our objective of this HE-spinel project is to expand the temperature range up to 700 K while keeping B = 3,000–4,000 K by controlling temperature dependence of resistivity with the HE techniques.<br/><br/>Two types of crystal systems appear in AMn₂O₄ (A=Mg, Co, Ni, Cu, and Zn) spinel: cubic in A=Ni and Co and orthorhombic in A=Mg, Cu, and Zn. It is interesting to see which structural system is stable in HE spinels and how to change a temperature dependence of resistivity depending on the composition and structure. We mixed up equal molar selected 2–5 elements among Mg, Co, Ni, Cu, Zn, plus Ti in assuming A site. TiMn₂O₄ could not be stabilized in spinel since tetravalent Ti in ilmenite-type MnTiO₃ and rutile-type TiO₂ are stable in the ambient condition. However, we expected Ti would be included in the spinel structure with the HE stabilized effect if we were starting from TiO with divalent in a closed golden cell of high-pressure synthesis assembly. We prepared a single phase of tetragonal spinel (Mg, Co, Ni, Zn, Ti)Mn₂O₄ with sintering at high pressure (4 GPa) and quenched from ca. 1250 K to 350 K in several seconds. In contrast, samples sintered in the air were a mixture of tetragonal and cubic phases or two sets of tetragonal phases, whether quench (liquid nitrogen) or slow cooling. The temperature dependence of resistivity showed semiconducting behavior with B= 5,000–5,500 K at 300-700 K. In addition, resistivity's absolute value is higher than conventional ones, an advantage in sensitivity at higher temperatures. Ti may be located in the B site, and part of Mn moved to the A site. The Cu version (Mg, Co, Ni, Zn, Cu)Mn₂O₄ showed lower B=3,000 K at 300-700 K and lower resistivity. The properties look sensitive to cooling processes and oxygen deficiency. We are also preparing A and B double sites' HE spinel.<br/><br/>Another HE project is to obtain a ferroelectric relaxor in perovskite-type niobates. ANbO₃(Li, Na, K, and Ag) are well-known ferro(antiferro)electric compounds. Structure types and or distortion types of these compounds differ depending on the A element; however, they allow partial substitution of each other. For example, in KNbO₃, Li and Ag could be substituted 5-10 % of K, and NaNbO₃ makes a complete solid solution with KNbO₃. In this study, we adapted KNbO₃ as a pristine and prepared HE niobates with equal molar selected 2–5 elements among Li, Na, K, Ag, and others in A site at ambient and high pressure. The high-pressure method effectively suppresses alkaline metals' vaporization (Li, Na, and K). We found LiNbO₃ is more stable in a LiNbO₃-type structure at high pressure. Therefore, equal molar HE compounds may be challenging in this system. Once it stabilized in a single phase, even if Li concentration is lower than other metals, it expected a ferroelectric relaxor in the temperature dependence of dielectric constant.<br/><br/>We also present our progress in XAS and XPS studies in HE pyrite (Fe, Co, Ni, Cu)X₂ (X = S and Se). It investigates a local configuration and interaction difference by comparing A element in a single metal AX₂ (A=Fe, Co, Ni, and Cu).<br/><br/>[1] AZ. Laila et al., J. Phys. Soc. Jpn. 91 084802.