Polymorphic transition and nucleation pathway of barium dititanate (BaTi2O5) during crystallization from undercooled liquid

Xuan Ge,Jianguo Li,Liang Yang,Wenquan Lu,Mingqin Xu,Sheng Cao,Mingxu Xia,Qiaodan Hu

doi:10.1038/s41598-019-43357-6

Xuan Ge, Jianguo Li + Show 6 more

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https://doi.org/10.1038/s41598-019-43357-6

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Abstract

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 BaTi2O5 (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.

Highlights

Most recently, a novel functional ceramic barium dititanate, BaTi2O5 (BT2), has drawn more and more attention due to its multifunctional properties and brilliant application prospect[1,2,3]
Kirby and Wechsler claimed that an liquid intermediate is required for nucleation of the BT2 phase[14] without an in-depth explanation for the reason; West et al proposed that the formation of BT2 is an example of the Ostward’s rule of successive reactions[15], but they failed to demonstrate why the crystal embryo is stable near the liquidus temperature (~1663 K) that severiously deviated from BT2 thermodynamically stable temperature range (1493 K~1503 K)
To clarify the nucleation and growth processes of BT2 from melt, the following critical questions should be answered: (i) whether BT2 crystals can form as the primary phase in melt beyond the thermodynamically stable temperature range; (ii) why BT2 is thermodynamically metastable but it can be obtained as the main phase rather than the thermodynamically stable phases such as BaTiO3 and Ba6Ti17O40 after melt quenching or rapid solidification

Summary

Introduction

A novel functional ceramic barium dititanate, BaTi2O5 (BT2), has drawn more and more attention due to its multifunctional properties and brilliant application prospect[1,2,3]. The formation and evolution of nuclei and pre-nucleation clusters have been observed directedly through colloidal particales[21] or HRTEM17,18,23, the indirect methods, such as heat events studies[26], high-speed video camera[24], microstructure studies[27,28] and computational simulations[29,30] are still the most widely used approaches for studying the nucleation path from extremely high temperature melt (especially undercooling melt) By these ex situ methods, it is diffcult to determine the crystalline phases and to identify the formation sequence simultaneously. The thermodynamically metastable phases with a high growth rate may preferentially precipitate under specific undercooling conditions, for instance, the direct growth of the peritectic-phase NdBa2Cu3O7-δ24 and FeTi28 were observed in a supercooled melt below the peritectic temperature (TP) These classical interpretations are not sufficient to explain the crystallization process of oxide ceramics. This work is expected to resolve the dispute about BT2 forming from cooling melt and to provide a convenient approach to prepare the ferroelectric γ-BT2 and the metastable phase (β-BT2)

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