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Understanding the role of flux, pressure and temperature on polymorphism in ThB 2 O 5 .
- Source :
-
Dalton transactions (Cambridge, England : 2003) [Dalton Trans] 2022 Sep 13; Vol. 51 (35), pp. 13376-13385. Date of Electronic Publication: 2022 Sep 13. - Publication Year :
- 2022
-
Abstract
- A novel polymorph of ThB <subscript>2</subscript> O <subscript>5</subscript> , denoted as β-ThB <subscript>2</subscript> O <subscript>5</subscript> , was synthesised under high-temperature high-pressure (HT/HP) conditions. Via single crystal X-ray diffraction measurements, β-ThB <subscript>2</subscript> O <subscript>5</subscript> was found to form a three-dimensional (3D) framework structure where thorium atoms are ten-fold oxygen coordinated forming tetra-capped trigonal prisms. The only other known polymorph of ThB <subscript>2</subscript> O <subscript>5</subscript> , denoted α, synthesised herein using a known borax, B <subscript>2</subscript> O <subscript>3</subscript> -Na <subscript>2</subscript> B <subscript>4</subscript> O <subscript>7</subscript> , high temperature solid method, was found to transform to the β polymorph when exposed to conditions of 4 GPa and ∼900 °C. Compared to the α polymorph, β-ThB <subscript>2</subscript> O <subscript>5</subscript> has smaller molar volume by approximately 12%. Exposing a mixture of the α and β polymorphs to HT/HP conditions ex situ further demonstrated the preferred higher-pressure phase being β, with no α phase material being observed via Rietveld refinements against laboratory X-ray powder diffraction (PXRD) measurements. In situ heating PXRD measurements on α-ThB <subscript>2</subscript> O <subscript>5</subscript> from RT to 1030 °C indicated that α-ThB <subscript>2</subscript> O <subscript>5</subscript> transforms to the β variant at approximately 900 °C via a 1st order mechanism. β-ThB <subscript>2</subscript> O <subscript>5</subscript> was found to exist only over a narrow temperature range, decomposing above 1050 °C. Ab initio calculations using density functional theory (DFT) with the Hubbard U parameter indicated, consistent with experimental observations, that β is both the preferred phase at higher temperatures and high pressures. Interestingly, it was found by switching from B <subscript>2</subscript> O <subscript>3</subscript> -Na <subscript>2</subscript> B <subscript>4</subscript> O <subscript>7</subscript> to H <subscript>3</subscript> BO <subscript>3</subscript> -Li <subscript>2</subscript> CO <subscript>3</subscript> flux using consistent high temperature solid state conditions for the synthesis of the α variant, β-ThB <subscript>2</subscript> O <subscript>5</subscript> could be generated. Comparison of their single crystal measurements showed this was identical to that obtained from HT/HP conditions.
Details
- Language :
- English
- ISSN :
- 1477-9234
- Volume :
- 51
- Issue :
- 35
- Database :
- MEDLINE
- Journal :
- Dalton transactions (Cambridge, England : 2003)
- Publication Type :
- Academic Journal
- Accession number :
- 35984644
- Full Text :
- https://doi.org/10.1039/d2dt01049f