Ge Xuan, Hu Qiaodan, Lu Wenquan, Cao Sheng, Yang Liang, Xu Mingqin, Xia Mingxu, Li Jianguo
Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, 200240, Shanghai, P.R. China.
Monash Centre for Additive Manufacturing (MCAM), Monash University, Clayton, VIC, 3800, Australia.
Sci Rep. 2019 May 10;9(1):7207. doi: 10.1038/s41598-019-43357-6.
The nucleation pathway plays an important role in vitrification, preparation of glass-ceramic composites and synthesis of metastable materials. In this paper, we studied the nucleation pathway of a novel ferroelectric BaTiO (BT2) during crystallization from undercooled liquid by aerodynamic levitation (ADL) containerless processing and structural analysis. An interesting polymorphic transition of BT2 regulated by the undercooling was observed during the crystallization process: the ferroelectric monoclinic phase (γ-BT2) was fabricated at low undercoolings and the paraelectric orthorhombic metastable phase (β-BT2) was obtained from hypercooled liquid. This polymorphic transition phenomenon corresponds to a non-classical nucleation pathway: metastable β-BT2 preferentially nucleates from undercooled melt and γ-BT2 is generated from β phase by solid-state phase transition. The two-step nucleation pathway stems from the structural heredity between the undercooled liquid and crystals. A stronger structural homology exists between the undercooled melt and β-BT2 than γ-BT2 based on diffraction data and atomic configurations analysis. This structural homology coupled with nucleation barrier calculation was used to elucidate the non-classical nucleation pathway of BT2 crystallization: the similarity of the structural unit (Ti-O polyhedra) between the undercooled liquid and the metastable β-BT2 reduces the nucleation barrier and contributes to the preferential precipitation of β-like clusters. This work reveals the formation route of BT2 from cooling melt, which not only benefits the synthesis and application of this novel functional material but also provides a guideline of the crystallization process of titanates from melt at atomic level.
成核途径在玻璃化、玻璃陶瓷复合材料制备和亚稳材料合成中起着重要作用。本文通过气动悬浮（ADL）无容器处理和结构分析，研究了新型铁电体BaTiO（BT2）从过冷液体结晶过程中的成核途径。在结晶过程中观察到了由过冷度调节的BT2有趣的多晶型转变：在低过冷度下制备出铁电单斜相（γ-BT2），从深度过冷液体中获得顺电正交亚稳相（β-BT2）。这种多晶型转变现象对应于一种非经典成核途径：亚稳β-BT2优先从过冷熔体中形核，γ-BT2由β相通过固态相变产生。两步成核途径源于过冷液体与晶体之间的结构遗传性。基于衍射数据和原子构型分析，过冷熔体与β-BT2之间的结构同源性比γ-BT2更强。这种结构同源性与成核势垒计算相结合，用于阐明BT2结晶的非经典成核途径：过冷液体与亚稳β-BT2之间结构单元（Ti-O多面体）的相似性降低了成核势垒，有助于β类团簇的优先析出。这项工作揭示了BT2从冷却熔体中的形成途径，这不仅有利于这种新型功能材料的合成和应用，而且在原子水平上为钛酸盐从熔体中的结晶过程提供了指导。
