New class of high-entropy pseudobrookite titanate with excellent thermal stability, low thermal expansion coefficient, and low thermal conductivity

As a type of titanate, the pseudobrookite (MTi₂O₅/M₂TiO₅) exhibits a low thermal expansion coefficient and thermal conductivity, as well as excellent dielectric and solar spectrum absorption properties. However, the pseudobrookite is unstable and prone to decomposing below 1200 ℃, which limits the practical application of the pseudobrookite. In this paper, the high-entropy pseudobrookite ceramic is synthesized for the first time. The pure high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ with the pseudobrookite structure and the biphasic high-entropy ceramic composed of the high-entropy pseudobrookite (Cr,Mn,Fe,Al,Ga)₂TiO₅ and the high-entropy spinel (Cr,Mn,Fe,Al,Ga,Ti)₃O₄ are successfully prepared by the in-situ solid-phase reaction method. The comparison between the theoretical crystal structure of the pseudobrookite and the aberration-corrected scanning transmission electron microscopy (AC-STEM) images of high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ shows that the metal ions (M and Ti ions) are disorderly distributed at the A site and the B site in high-entropy (Mg,Co,Ni,Zn)Ti₂O₅, leading to an unprecedentedly high configurational entropy of high-entropy (Mg,Co,Ni,Zn)Ti₂O₅. The bulk high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ ceramics exhibit a low thermal expansion coefficient of 6.35×10⁻⁶ K⁻¹ in the temperature range of 25–1400 ℃ and thermal conductivity of 1.840 W·m⁻¹·K⁻¹ at room temperature, as well as the excellent thermal stability at 200, 600, and 1400 ℃. Owing to these outstanding properties, high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ is expected to be the promising candidate for high-temperature thermal insulation. This work has further extended the family of different crystal structures of high-entropy ceramics reported to date.