It is well known that the grain size of high-entropy ceramics is quite small owing to the sluggish diffusion effect. However, abnormal grain growth often occurs in high-entropy pseudobrookite ceramics, ultimately resulting in the formation of many abnormally grown grains with a grain size as large as 50 μm. To study this phenomenon, the grain growth behavior of high-entropy pseudobrookite ceramics was systematically investigated in this paper. The results demonstrate that the starting material powders first react with each other to form a high-entropy intermediate phase and calcined TiO₂ powders (TiO₂-1100 °C), and then as the sintering temperature increases, the formed high-entropy intermediate phase further reacts with TiO₂-1100 °C to form high-entropy pseudobrookite ceramics. Thus, in this system, in addition to the sluggish diffusion effect, the grain sizes of the high-entropy intermediate phase and TiO₂-1100 °C also affect the morphology of high-entropy pseudobrookite. Compared to nanosized TiO₂, micron-sized TiO₂ has a lower sintering activity. Therefore, the high-entropy intermediate phases (Mg,Co,Ni,Zn)TiO₃ and TiO₂-1100 °C prepared with micron-sized starting materials exhibit lower grain sizes, finally resulting in the formation of high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ with small grain sizes. Moreover, nano-indentation and thermal conductivity tests were carried out on high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ with different morphologies. The results show that the hardness of high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ increases from 6.05 to 9.95 GPa as the grain size increases, whereas the thermal conductivity decreases from 2.091±0.006 to 1.583±0.006 W·m⁻¹·K⁻¹. All these results indicate that high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ with a small grain size is a potential material for thermal protection.

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.