Flux Growth of Single-Crystalline Hollandite-Type Potassium Ferrotitanate Microrods From KCl Flux.

Fumitaka Hayashi,Kenta Furui,Tomohito Sudare,Katsuya Teshima,Hiromasa Shiiba,Kunio Yubuta,Chiaki Terashima

doi:10.3389/fchem.2020.00714

Fumitaka Hayashi, Kenta Furui + Show 5 more

Open Access

https://doi.org/10.3389/fchem.2020.00714

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Abstract

Hollandite-type crystals have unique and interesting physical and chemical properties. Here, we report the flux growth of hollandite-type single-crystalline potassium ferrotitanate (KFTO) with faceted surface features from a KCl flux. We varied the flux growth conditions, including the kind of flux, holding temperature, and solute concentration for growing faceted crystallites. KCl was found to be the best flux to grow the single-crystalline KFTO particles, while heating at or above 900°C was needed to yield the KFTO single crystals. The crystal growth was only weakly dependent on the solute concentration. Next, we characterized the grown single crystals and discussed the manner of their growth from the KCl flux. TEM images with clear electron diffraction spots indicated that the KFTO crystals grew along the <001> direction to form microrods ~10 μm in size. DFT calculation results indicated that the surface energy of the (100) face is lower than that of the (001) face. Based on these characterization results, we proposed a possible growth mechanism of the KFTO crystals.

Highlights

Hollandite-type minerals such as silicate exist naturally in the Earth’s lower crust and upper mantle and possess significant structural stability (Miura and Iura, 1986)
We first examined the effect of different flux species on the growth of KFTO crystals, because the cation and anion in the flux can influence the dissolution of the solutes
Single-phase KFTO crystals were grown from the KCl flux

Summary

INTRODUCTION

Hollandite-type minerals such as silicate exist naturally in the Earth’s lower crust and upper mantle and possess significant structural stability (Miura and Iura, 1986). Inorganic crystals with hollandite structure are very interesting, because of their unique chemical and physical properties such as fast ion conduction and ion exchange (Michiue and Watanabe, 1999), which allow them to function as solid electrolytes and adsorbents for radioactive elements, respectively (Yoshikado et al, 1995; Aubin-Chevaldonnet et al, 2007; Xu et al, 2016; Tumurugoti et al, 2017; Hassan et al, 2018; Cao et al, 2019). To assess the physical and chemical properties of hollandite type crystals, it is important to prepare KFTO crystals with defined morphology and faceted surface features. We explored several flux growth conditions, namely the flux species, holding temperature, and solute concentration in order to yield large KFTO crystals with faceted surface features. To calculate the surface facets, slab thicknesses >20 Å were chosen for all facets with a vacuum thickness of 20 Å

RESULTS AND DISCUSSION

CONCLUSION

DATA AVAILABILITY STATEMENT