Understanding the role of flux, pressure and temperature on polymorphism in ThB₂O₅

Abstract

A novel polymorph of ThB₂O₅, denoted as β-ThB₂O₅, was synthesised under high-temperature high-pressure (HT/HP) conditions. Via single crystal X-ray diffraction measurements, β-ThB₂O₅ 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 ThB₂O₅, denoted α, synthesised herein using a known borax, B₂O₃–Na₂B₄O₇, 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-ThB₂O₅ 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-ThB₂O₅ from RT to 1030 °C indicated that α-ThB₂O₅ transforms to the β variant at approximately 900 °C via a 1st order mechanism. β-ThB₂O₅ 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 B₂O₃–Na₂B₄O₇ to H₃BO₃–Li₂CO₃ flux using consistent high temperature solid state conditions for the synthesis of the α variant, β-ThB₂O₅ could be generated. Comparison of their single crystal measurements showed this was identical to that obtained from HT/HP conditions.

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1. Introduction

Traditional interest in thorium chemistry has been somewhat overshadowed by its f-block sister uranium,^2,3 however contemporary studies have unravelled significant applications and properties in the context of advanced functional material development, reinvigorating focus in the most naturally abundant actinide element.^4–6 Examples of such successes can be seen in nuclear waste separation technology,^7a radiation detection^7b and also MOF development.^7c In these, the chemical advantages derived from the typically tetravalent Th cation, as opposed to its higher actinide counterparts (U, Np, Pu), relate to its ability to adopt a relatively more spherical chemical environment whilst retaining somewhat f-block character.⁸ This allows for the construction of unique higher dimensional structures than those typically found for instance in uranyl compounds. Accordingly, together with property measurements and applications, inorganic chemists have shifted their attention towards deriving facile synthesis routes to unlock novel thorium phases and study their structural properties.

Since the exceptional Tc ion-exchangeable thorium borate, NDTB-1 (ThB₅O₆(OH)₆ BO(OH)₂·2.5H₂O) was first reported in 2010, interest in thorium borate synthesis has grown.⁹ However, surprisingly only a few thorium borates without additional countercations have been reported.^1,9,10 Prior to NDTB-1, Cousson and Gasperin in 1991 reported the first example of a thorium borate, ThB₂O₅ (denoted hereon as α-phase), which was obtained through high-temperature flux synthesis.¹ It consists of a 3D thorium borate framework structure, in which thorium–thorium stacking sequences exist with a diamond packing motif forming a 3D thorium-network. B₂O₅ dimers edge share with thorium polyhedra and fill the voids of the Th-cation framework along the [−101] direction. NDTB-1 was originally synthesised using a low-temperature boric acid flux method in which this thorium borate is constructed from crown-like B₁₀O₂₄ groups and twelve-fold coordinated thorium ions. These groups form a porous super-tetrahedral 3D thorium-borate framework. The super-tetrahedra cationic framework in NDTB-1 contributes to its superb anion exchange properties. Hinteregger et al. prepared the third member of the thorium borates system, ThB₄O₈, which was synthesised under extreme conditions (5.5 GPa/1100 °C) through a high temperature/high pressure (HT/HP) method.¹⁰ Its structure is based on 2D polyborate layers composed of corner-sharing BO₄ tetrahedra where the ten-fold coordinated thorium cations are located in the interlayer space. Currently, there are no further reports of other thorium borates than the three described.

With the motivation of understanding the structural chemistry and behavior of thorium borates under variable conditions a part of our general focus on exploring chemistry of actinide compounds,^2–9 we have examined the Th–B–O system towards expanding knowledge of it and ascertaining fundamental chemical insight. Herein, we report a new polymorph in the ThB₂O₅ family, β-ThB₂O₅, from HT/HP conditions and examine the temperature and pressure relationship it has to the only other reported polymorph,¹ α, using experimental and theoretical methods. Interestingly although β-ThB₂O₅ is found to be the preferred higher pressure and temperature phase, relative to α-ThB₂O₅, it can also be accessed through careful choice of flux under ambient pressure solid state high temperature conditions. The synthetic conditions, structural chemistry and topology, phase relationship and phase decomposition process, thermodynamics stability, theoretical analysis as well as Raman and IR spectra of both polymorphs are discussed.

2. Experiment section

2.1 Materials and methods

Thorium nitrate Th(NO₃)₄·5H₂O (International Bioanalytical Industries, Inc.), Lithium carbonate Li₂CO₃ (Alfa-Aesar, 99.9%), Boric acid H₃BO₃ (Alfa-Aesar, 99.9%), Sodium Tetraborate Na₂B₄O₇ (Alfa-Aesar, 99.5%), Boron Oxide B₂O₃ (Alfa-Aesar, 99.5%).

2.2 Synthesis

2.2.1 α-ThB₂O₅ – high temperature solid state flux growth¹. A mixture of ThO₂, Na₂B₄O₇ and B₂O₃ at a molar ratio of 1/2/10, was heated in the air at ∼1200 °C for 15 hours, and then cooled down to room temperature slowly (5 °C h⁻¹). After washing with boiling water, homogeneous product, composed of prism colourless transparent crystals of monoclinic α-ThB₂O₅ were prepared.¹ A nearly pure polycrystalline sample of α-ThB₂O₅ was prepared by reacting a mixture of Th(NO₃)₄·5H₂O (0.302 g, 0.53 mmol) and H₃BO₃ (0.073 g, 1.18 mmol) with a molar ratio of 1 : 2.2 at ∼780 °C for 36 hours. The samples were then washed with boiled water and ethanol several times to eliminate impurities.

2.2.2 β-ThB₂O₅ – high pressure/high temperature. Single crystals of monoclinic β-ThB₂O₅ were obtained using a HT/HP method. For the HT/HP method a piston cylinder module of a Voggenreiter LP 1000-540/50 device was used. The starting chemicals of Th(NO₃)₄·5H₂O (0.115 g, 0.19 mmol) and B₂O₃ (0.0795 g, 1.15 mmol) with a ratio of Th : B = 1 : 6 were used. All the reagents were thoroughly ground in an agate mortar and then transferred to a small platinum crucible of 4 mm diameter and 7 mm height which was sealed. A pressure of 4.0 GPa was applied within 0.5 hours before heating (cold piston in). The reaction mixtures were kept at 4.0 GPa for the whole experimental run. The temperature was increased up to 1000 °C in 0.5 hours. The temperature was held at 1000 °C for 6 hours then slowly cooled down to 700 °C with a rate of 5 °C h⁻¹. The following next step cooled the assembly down to 450 °C at a rate of 10 °C h⁻¹ followed by quenching to room temperature and subsequent pressure release. For extracting the resulting products, the platinum crucible was broken. Colorless pallet shaped crystals were isolated.

2.2.3 β-ThB₂O₅ – high temperature solid state flux growth. For high temperature flux synthesis of β-ThB₂O₅, Th(NO₃)₄·5H₂O (0.105 g, 0.18 mmol), H₃BO₃ (0.0618 g, 1.00 mmol) and Li₂CO₃ (0.0182 g, 0.25 mmol) were mixed in a mortar and grinded thoroughly, then transferred into a platinum crucible. The mixture was held in a box furnace at 450 °C for 1 hour to decompose the thorium nitrate and boric acid. Then the temperature was increased to 970 °C for 5 hours until the homogeneous melt was formed. After this, the melt was cooled down to 700 °C at a rate of 5 °C h⁻¹ and then cooled down to room temperature with a cooling rate of 10 °C h⁻¹. The resulting product was washed with hot water to remove the excess of boric acid. Large colorless block shaped crystals of β-ThB₂O₅ were obtained.

A nearly pure polycrystalline sample of β-ThB₂O₅ was prepared quantitatively by reacting a mixture of Th(NO₃)₄·5H₂O(0.306 g, 0.54 mmol) and H₃BO₃ (0.090 g, 1.45 mmol) with a molar ratio of 3 : 8 and exposed to conditions of 4 GPa/900 °C for 36 hours. The samples were then washed with boiling water several times to eliminate impurities. Energy dispersive X-ray spectroscopy (EDS) elemental analysis on several single crystals gave an average molar ratio of Th : O = 1 : 5.35 for β-ThB₂O₅, which is in good agreement with those obtained from single crystal X-ray diffraction studies (Fig. S1†).

2.3. Ex situ and in situ experimentation on α and β phases

2.3.1 High-temperature/high-pressure treatment of α- and β-ThB₂O₅ mixture. To understand how the pressure influence the polymorphism we exposed both polymorphs to high-pressure conditions using the piston cylinder module of a Voggenreiter LP 1000-540/50. The starting material contained a mixture of β-ThB₂O₅ and α-ThB₂O₅ (0.2055 g) with ratio about 15% to 85%, respectively. The mixture was thoroughly ground, pressed and then sealed in a platinum crucible. The pressure was raised to 4.0 GPa within 0.5 hours and then the mixture was kept under 4.0 GPa for the whole experimental run. The temperature was increased up to 900 °C in 0.5 hours, after the pressure was stabilised at 4.0 GPa. The temperature was held at 900 °C for 6 hours and then cooled down to 500 °C at a rate of 10 °C h⁻¹ followed by quenching to room temperature. The pressure was released after sample cooling in half an hour. For extracting the resulting products, the platinum crucible was carefully opened. The colourless powder was isolated and characterised with PXRD diffraction (Fig. S2†).

2.3.2 Thermal analysis. The thermal behavior of the dried powder of β-ThB₂O₅ up to 1200 °C was studied by differential scanning calorimetry (DSC) analysis coupled with thermogravimetry (TG) in air at a heating rate of 10 °C min⁻¹ using a Netzsch STA 449C Jupiter apparatus. The sample (20.5 mg) was loaded in a platinum crucible, which was covered by a platinum cover. During the measurements a constant air flow of 20–30 mL min⁻¹ was applied.

2.4 Analysis crystallographic study and X-ray powder diffraction

2.4.1 Single crystal diffraction. Diffraction data for β-ThB₂O₅ were collected on an Agilent Technologies SuperNova diffractometer with Mo-Kα radiation (λ = 0.71073 Å) at room temperature. All data sets were corrected for Lorentz and polarisation factors as well as for absorption by the multi-scan method.¹¹ Structure of the compound was solved by direct method and refined with a full-matrix least-squares fitting on F² by SHELX.¹¹ No higher symmetry elements were found after checking using the PLATON with the ADDSY Malgorithms.¹¹ Crystallographic data and structural refinement information for this β-ThB₂O₅ is summarised in Table 1. The crystal chemical calculations for both ThB₂O₅ phases were performed with TOPOS 4.0 Professional software.¹¹ Further information for selected bond distances and angles are listed in Table S1.†

Table 1 Crystal data and structure refinement details for β-ThB₂O₅ and Crystal Data for α-ThB₂O₅¹

a R 1 = Σ||Fo| − |Fc||/Σ|Fo| b wR2 = {Σw[(Fo)^2 − (Fc)^2]^2/Σw [(Fo)^2]^2}^1/2
Compound	β-ThB2O5	α-ThB2O5^1
FW	333.66	333.66
Space group	P21/m	C2/c
a (Å)	4.2535(2)	11.545(3)
b (Å)	6.8733(3)	6.937(2)
c (Å)	6.3242(3)	10.263(3)
α (°)	90	90
β (°)	106.316(5)	101.5(3)
γ (°)	90	90
V (Å^3)	177.445(14)	805.44
Z	2	8
λ (Å)	0.71073
F(000)	280
D c (g cm^−3)	6.245	5.500
GOF on F^2	0.851
R 1	0.0261
wR2^b	0.0609

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2.4.2 In situ high temperature X-ray diffraction. High-temperature X-ray powder diffraction data were collected on a Panalytical MPD powder diffractometer (Cu-radiation, Ni-filter, X'Celerator detector, Anton Paar HTK1200N heating chamber) with Bragg–Brentano geometry. Experiments were performed in the range from 27 °C to 1027 °C for pure α-ThB₂O₅ (see Fig. 5) and 27 °C to 1147 °C for the mixture of α-/β-ThB₂O₅ (see Fig. S5†) with a step of 20 °C. Each diffraction pattern was recorded with a step size of 0.0167°. The pure phase α-ThB₂O₅ data was analysed via sequential refinements with the Le Bail method, results are giving in Fig. S3 and S4.†

2.4.3 PXRD analysis. Powder X-ray diffraction data (PXRD) was measured on a Bruker-AXS D4 Endeavor diffractometer, 40 kV/40 mA, in Bragg–Brentano geometry. The diffractometer is equipped with a copper X-ray tube and a primary nickel filter producing CuKα radiation. A linear silicon strip LynxEye detector (Bruker-AXS) was used. Data¹² were recorded in the range of 2 θ = 10–80° with 5 s/step and a step width of 0.02°. The aperture of the fixed divergence slit was set to 0.2 mm and the receiving slit with 8.0 mm, respectively. The discriminator of the detector was set to an interval of [0.16–0.25 V]. Collected data for the α-ThB₂O₅/β-ThB₂O₅ mix pre- and post-HT/HP treatment were analysed using the Rietveld refinement method using the program GSAS-II¹³ Starting models were used derived from SC-XRD structural solutions where the peak shapes were modelled using a pseudo-Voigt function and the background was estimated using a 6 term shifted Chebyschev function. The scale factor, detector, zero point, lattice parameters, atomic coordinates and atomic displacement parameters for cations and phase fractions were refined together with the peak profile parameters.

2.5 Scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDS) analysis

Scanning electron microscopy image and energy-dispersive X-ray spectroscopy (SEM/EDS) was collected on a FEI Quanta 200F Environment Scanning Electron Microscope with a low-vacuum mode at 0.6 mbar. SEM/EDS results were given as ESI (Fig. S1†).

2.6 Raman and IR spectroscopy

Unpolarised Raman spectra were measured with a Horiba LabRAM HR spectrometer using a Peltier cooled multichannel CCD detector. An objective lens with a 50× magnification was linked to the spectrometer, allowing the analysis of samples as small as 2 μm in diameter. The samples were in the form of single crystals. The incident radiation was produced by a He–Ne laser line at a power of 17 mW (λ = 632.8 nm). The focal length of the spectrometer was 800 mm, and an 1800 gr mm⁻¹ grating was used. The spectral resolution was approximately 1 cm⁻¹ with a slit of 100 μm. The spectrum was recorded in the range of 100–4000 cm⁻¹. Infrared (IR) spectrum was measured using polycrystalline samples mixed with KBr powder, with FT-IR spectrometer Bruker Equinox 55 at room temperature.

2.7 Bond-valence analysis

As a common method in coordination chemistry to evaluate the oxidation states of atoms, BVS of all atoms in the thorium borate phase were calculated and agree well with corresponding formal values of oxidation states. The bond-valence parameters for Th(IV)–O and B(III)–O were used according to Brese and O'Keeffe.¹⁴

2.8 Ab initio calculations

The computational studies of the ThB₂O₅ system were performed with density functional theory (DFT) method using Quantum-ESPRESSO¹⁵ simulation package. Because we are interested in good computations of structures of considered materials we applied PBEsol exchange/correlation functional,¹⁶ which was successfully applied in the previous studies of an-bearing systems.^8,17 The core electrons of the constituent atoms were modeled with ultrasoft pseudopotentials¹⁸ and the 6s²6p⁶6d²7s² electrons of thorium atom were computed explicitly. Based on previous experience with computing Th-bearing systems, we selected 50Ryd as plane-wave energy cutoff.¹⁹

The α- and β-ThB₂O₅ structures were modeled with 16 and 64 atoms contained supercells and the calculations were performed on the 6 × 4 × 4 and 2 × 4 × 3 Methfessel–Paxton k-points grids.²⁰ The calculations of phonons and vibrational entropy contributions to Gibbs free energies were performed with the density functional perturbation theory using PHONON and DYNMAT packages of the Quantum-ESPRESSO code.

3.1 Results and discussion

3.1 Syntheses

β-ThB₂O₅ was successfully synthesised via a HT/HP method with conditions of 4 GPa and temperature of 1000 °C. It was further found that β-ThB₂O₅ could be successfully synthesised through a high temperature flux method with H₃BO₃–Li₂CO₃. It was found that by switching the fluxes from H₃BO₃–Li₂CO₃ to B₂O₃–Na₂B₄O₇ the known α polymorph¹ could be obtained. It is pertinent that we used different initial reactants (Th(NO₃)₄·5H₂O/Li₂CO₃/H₃BO₃) for preparation of β-ThB₂O₅ compared to (ThO₂/Na₂B₄O₇/B₂O₃) the synthesis of α-ThB₂O₅. It was found that by heating a sample α-ThB₂O₅ between 800 to 1200 °C, β-ThB₂O₅ could be obtained indicating that α transforms to β with increasing temperature. If a mixture of α-ThB₂O₅ and β-ThB₂O₅ or a single sample of α-ThB₂O₅ was exposed to HT/HP conditions of 4 GPa, ∼900 °C, only the β polymorph with some additional minor ThO₂ was obtained. This indicates that a pressure induced phase transition occurs from α to β-ThB₂O₅ with increasing pressure which is logical because β-phase has significantly higher density. A difference in volume of ∼12 ų, at P = 4 GPa, corresponds to ΔVP = 29 kJ mol⁻¹, which is larger than the difference in the free energy calculated between α and β phases (Table 2) from ab initio calculations. Hinteregger et al. reported the synthesis of ThB₄O₈, under similar HT/HP conditions to that of the present investigation (5.5 GPa, 1000 °C).¹⁰ The density of ThB₄O₈ (5.970 g cm⁻³) is larger than that of α-ThB₂O₅ (5.50 g cm⁻³) and smaller compared to that of β-ThB₂O₅ (6.245 g cm⁻³).

Table 2 The difference in Gibbs free energy between α-ThB₂O₅ and β-ThB₂O₅ phases. The energies are reported in kJ mol⁻¹

	ΔU (0 K)	ΔU + ZPE (0 K)	ΔG (300 K)	ΔG (1000 K)	ΔG (1200 K)	ΔG (1500 K)
ΔG = G(β) − G(α)	34.4	31.8	28.1	12.6	8.1	1.5

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3.2 Structural description of β-ThB₂O₅ and comparison to α-ThB₂O₅

Structural refinements against single crystal X-ray diffraction data reveal that novel β-ThB₂O₅ crystallises in the monoclinic space group P2₁/m. It consists of one crystallographically independent Th, two B and four O atoms. The 3D framework of β-ThB₂O₅ is based on 1D B chains and 2D Th layers (see Fig. 1). In the structure of β-ThB₂O₅, B(1) atoms are existing as B(1)O₄ tetrahedra and B(2) atoms are forming B(2)O₃ planar triangles. B(1)O₄ tetrahedra and B(2)O₃ triangles are connected with each other through corner sharing, forming a 1D zigzag B–O chain along the a-axis (see Fig. 1b). The oxo-borate fragments are polymerising into 2D layers in the structure of the phases from the family of closely related compounds AB₂O₅ (A = Zr, Hf).^21,22 B atoms occur only as BO₄ tetrahedra within the structures of AB₂O₅ (A = Zr, Hf).

Fig. 1 Polyhedral representation of β-ThB₂O₅ structure. The 3D thorium borate structure along the a-axis (a), a 1D boron–oxygen chain along the a-axis (b), a 2D thorium layer on ab-plane (c). Thorium polyhedra, BO₃ triangles and BO₄ tetrahedra are shown in yellow and green, oxygen atoms are shown in red, respectively.

In β-ThB₂O₅, two BO₄ tetrahedra are corner sharing with two ThO₁₀ polyhedra, forming four-membered ring channels along the a-axis. B–O bond lengths in B(1)O₄ tetrahedra are found to range from 1.455(7) Å to 1.472(11) Å. The O–B(1)–O bond angles are in the range of 105.8(7)°–113.1(5)°. The B–O bond distances in B(2)O₃ are in the range from 1.356(11) Å to 1.390(11) Å. The O–B(2)–O bond angles are in the range of 114.2(7)°–127.4(8)°, which are consistent with the borates reported previously (see Table S2†).²³

The Th atoms in β-ThB₂O₅ are coordinated by eight shorter and two longer Th–O bonds giving a ten-fold oxygen coordination, resulting in ThO₈₊₂ tetra-caped trigonal prisms (Fig. 2 and Fig. S6). The shorter eight Th–O bonds range in length from 2.302(5) Å to 2.583(6) Å, whereas the additional longer two Th-O bonds are 3.067(6) Å. These ThO₈₊₂ polyhedra are edge and face sharing with each other forming 2D sheets parallel to the ab-plain (see Fig. 1c). The zigzag B–O chains further link the 2D Th-based sheets, through corner (BO₄), edge (BO₃), and face (BO₄) sharing, and form the 3D Th borate framework (see Fig. S7†). In the 3D thorium borate framework, a four-membered ring channel can be observed along the a-axis. The size of this channel is ∼4.5 Å × 3.5 Å (distances are between two opposite O–O atoms) (see Fig. S8†). BVS calculations indicate that Th cations are tetravalent with a value of 3.97. The BVS values suggest that the B cation is trivalent with B(1) and B(2) BVS of ∼3.07 and ∼2.97, respectively.¹⁴

Fig. 2 (a) The oxygen coordination environment of thorium cation with ball and stick mode, (b) the four caped trigonal prismatic shape of thorium polyhedra for β-ThB₂O₅. Th and O atoms are shown as yellow and red, respectively.

Contrasting the structures of α-ThB₂O₅ and β-ThB₂O₅, both possess 3D framework structures, which are composed of two independent B and one Th atoms per unit cell. However, B atoms are only presented as BO₃ triangles in the α phase.

In the structure of α-ThB₂O₅, two BO₃ triangles are corner sharing linked forming isolated B₂O₅ dimers. Each BO₃ triangle is corner sharing with two and edge sharing with one Th polyhedra, forming BTh₃ fragments with linear geometry (see Fig. S9†). In the structure of β-ThB₂O₅, as described above, B(1)O₄ tetrahedra and B(2)O₃ triangles are corner sharing and form 1D zigzag chains. Within the 1D B–O chains, the two shortest B–B distances are 2.489(3) Å and 2.594(1) Å, respectively. For comparison, the shortest B(1)–B(2) distance in the structure of α-ThB₂O₅ is 2.445(1) Å. Th cations are eight-fold coordinated in the structure of α-ThB₂O₅ and exist as distorted one-caped pentagonal bipyramids. These ThO₈ distorted one-caped pentagonal bipyramids are edge sharing with each other, forming a 3D Th–O framework. Each Th cation is connected to four other Th cations in a diamond packing mode in the structure of α-ThB₂O₅. The four shortest Th–Th distances are ranging from 4.071(1) Å to 4.131(3) Å in the structure of α-ThB₂O₅. There are seven neighboring Th cations with Th-Th distances ranging from 3.965(5) Å to 4.411(2) Å in the structure of β-ThB₂O₅ (see Fig. 3a and b). We suspect that changing of packing modes between α- and β-ThB₂O₅ is necessary for the large molar volume change (∼12%), where there is a similar density increase α to β phase. This is comparable with to the ThMo₂O₈ polymorphs, where a large volume change (∼20%) for the HT/HP–ThMo₂O₈ modification is described compared with its other ambient pressure polymorphs.¹⁹

Fig. 3 The adjacent Th–Th bond distances in two polymorphs for α-ThB₂O₅ (a) and β-ThB₂O₅ (b).

3.2.1 Coordination environment and geometry of B and Th centres. Tetravalent Th cations have been observed to possess complex oxygen coordination environments where CNs range from six to fifteen are reported, depending on the type of ligands.^24,25 The connection between Th polyhedra and oxo-borate groups produces numerous thorium borate architectures. In the structure of β-ThB₂O₅, B(1)O₄ tetrahedra are found to be face sharing with one, corner sharing with three and edge sharing with one ThO₈₊₂ polyhedra, denoted as ThTh₃BTh, existing in form of the square pyramid geometry (see Fig. 4a). B(2)O₃ triangles are edge sharing with one and corner sharing with two Th polyhedra, exhibiting a planar triangle geometry as ThBTh₂ (see Fig. 4b). ThO₁₀ polyhedra are corner, edge and face sharing with five BO₄ tetrahedra and three BO₃ triangles, forming a Th-centred hexagonal bipyramidal (ThB₈) geometry (see Fig. 4c). This Th coordination environment is apparently the first reported for inorganic Th bearing compounds. To better observe the Th–Th coordination environments and spatial interactions more clearly, Voronoi–Dirichlet polyhedra (VDP) of Th atoms in Th sub-lattices have been described for both polymorphs of ThB₂O₅ (see Fig. S10†). The Th VDP of β-ThB₂O₅ show a truncated cuboctahedron with the combinatorial–topological type (CTT) of 4⁵·5²·6⁵. This face distribution symbol means that there are three different numbers (5,2,5) of different (4,5,6) faces. Whereas the CTT of the Th atoms for α-ThB₂O₅ is considerably different, shown as 3⁴·4⁴·5⁸·10⁴ in Fig. S10.† The large differences between CTTs for the Th atoms in the VDP indicate that, Th atoms have a totally different spatial distribution in the two polymorphic modifications.

Fig. 4 For β-ThB₂O₅; the coordination environment of a B(1)O₄ tetrahedron (a), coordination environment of a B(2)O₃ triangle (b), coordination environment of ThO₈₊₂ polyhedra (c) and the corresponding schematic topological representation of square pyramid geometry (d), schematic topological representation of a trigonal geometry (e), schematic topological representation of a hexagonal bipyramidal shape of Th coordination (f).

3.2.2 Structural topology analysis. To examine the cationic topology of the ThB₂O₅ polymorphs, we omitted the anions whilst retaining the connectivity between the cations from the 3D thorium borate framework of β-ThB₂O₅. As shown in Fig. S11,† B1 and B2 are linked along the a-axis forming a zigzag B chain⋯B1B2B1B2B1⋯(see Fig. S11a†). Th cations are connected with six other neighboring ones forming a 2D Th layer parallel to the ab-plain. Each Th is a [3⁶] node within the 2D Th-sheet topology (Fig. S11b†). The zigzag B-chains are bridged over those parallel Th-sheets along the c-axis and constructed the 3D ThB₂ framework (see Fig. S11c†). The simplified ThB₂ network can be depicted as a new 3-nodal net topological type with a point symbol of {3⁴, 4¹⁰, 5¹⁰, 6⁴}{3⁴, 4¹⁰, 5⁶, 6}{3⁴, 4⁴, 5²}.

3.3 Temperature and pressure relationship between α- and β-ThB₂O₅

To understand the temperature and pressure dependence of ThB₂O₅ polymorphs, both were examined using in situ powder X-ray diffraction, and under ex situ HT/HP conditions (post PXRD examination).

3.3.1 In situ HT-PXRD measurement analysis. In situ PXRD measurements were made against a pure sample of α-ThB₂O₅ and mixed sample of α- and β-ThB₂O₅ (∼ ratio α/β = 3/1), a mixture sample was chosen to observe which phase would be preferred at higher temperature and to compare the relative change to lattice volume with heating (ESI Fig. S2†). For pure α-ThB₂O₅ PXRD measurements were collected incrementally as the sample was headed towards 1030 °C from RT (Fig. 5) and the data analysed using sequential refinements with the Le Bail method. It can be observed that between 630 and 840 °C reflections corresponding to α-ThB₂O₅ begin sharply changing intensity and positions. From refinement analysis, this region corresponds to non-isotropic thermal expansion where considerable contraction of the a parameter occurs whereas the b, c and β parameters showed increased expansion (see ESI Fig. S3†).

Fig. 5 Temperature dependence of a portion of the PXRD profiles for α-ThB₂O₅ heated from 0 to 1030 °C illustrating the transition from the α to β phase between 807 °C and 927 °C.

Interestingly, continual heating above 840 towards 927 °C further reflections can be observed to occur as can be seen in Fig. 5 but also particularly highlighted in Fig. 6. That these reflections occur independently with respect to the main reflections of β-ThB₂O₅ indicates that the sample is undergoing a discontinuous 1^st order phase transformation. This corresponds to the formation of β-ThB₂O₅. The phase transformation motif is further supported when the refined lattice volumes are compared (Fig. S4†). β-ThB₂O₅ is found to occur only over a narrow temperature range as further heating above 1020 °C results in its decomposition. This is supported by TGA measurements performed on α-ThB₂O₅ to 1200 °C (see section below).

Fig. 6 Portion of PXRD data between 13 and 24° for ThB₂O₅ heated from 807 to 1007 °C. The occurrence of additional reflections above 887 °C (most noticeable at 14.9, 19.5 and 22° as shown by the arrows) indicates a 1^st order phase transformation is occurring resulting in β-ThB₂O₅.

3.3.2 Thermogravimetric analysis. TGA measurements were performed in the temperature range of 200–1200 °C (see Fig. S12†). From TGA measurements of a single-phase sample of α-ThB₂O₅ indicated there is no obvious weight loss up to 1200 °C, based on the TG curve (Fig. S12†). TGA-DSC measurements of a single-phase sample of α-ThB₂O₅ indicated a thermal event at ∼630 °C. This is consistent with the onset of the phase transformation observed in the in situ PXRD measurements. The DSC curve shows a strong endothermic peak at ∼1053 °C, which corresponds to the decomposition of the β-phase, which is supported by the in situ PXRD results described earlier. The TG-DSC curves, in situ PXRD pattern and HT/HP-ThB₂O₅ PXRD pattern are shown in Fig. S12† and Fig. 5, Fig. S2.†

3.3.3 Ex situ HT/HP PXRD measurement analysis. To examine the pressure influence on α- and β-phase stability, a mixture of these was treated under HT/HP conditions at 900 °C and 4 GPa and the resulting product analysed with PXRD. Collected PXRD data for the pre- and post-HT/HP α-ThB₂O₅/β-ThB₂O₅ mixtures were analysed using the Rietveld method in the program GSAS-II. The refinement profiles are given in Fig. 7.

Fig. 7 Rietveld refinements profiles for the α-ThB₂O₅/β-ThB₂O₅ mix pre- (top) and post- (bottom) HT/HP treatment. For top; β-ThB₂O₅ was refined in a monoclinic structure (SG = P2₁/m), where a = 4.2400(16) Å, b = 6.8504(7) Å, c = 6.302(3) Å, and β = 106.300(7)°, α-ThB₂O₅ was refined in a monoclinic structure (SG = C2/c), where a = 11.548(7) Å, b = 6.9360(20) Å, c = 10.272(6) Å, and β = 101.56(2)°, ThO₂ was refined in a cubic structure (SG = Fm3̄m), where a = 5.597(1) Å and where wR = 15.58%. For bottom; β-ThB2O5 was refined in a monoclinic structure (SG = P2₁/m), where a = 4.2417(8) Å, b = 6.8549(5) Å, c = 6.3047(19) Å, and β = 106.305(4)°, ThO₂ was refined in a cubic structure (SG = Fm3̄m), where a = 5.5304(8) Å, and where wR = 17.60%.

For the initial (pre-HT/HP) α-ThB₂O₅/β-ThB₂O₅ mixture monoclinic models in space groups C2/c and P2₁/m could be refined against the PXRD data consistent with α-ThB₂O₅ and β-ThB₂O₅ respectively. An additional ThO₂ impurity phase was also identified, speculated to arise from the initial as synthesised ThB₂O₅. Phase analysis indicated the sample to be 85.6(15), 12.9(6) and 1.48(2) wt% for α-ThB₂O₅, β-ThB₂O₅ and ThO₂ respectively largely consistent with initial experimental input amounts. For the post-HT/HP α-ThB₂O₅/β-ThB₂O₅ mixture, monoclinic and cubic models in space group P2₁/m and Fm3̄m respectively consistent with β-ThB₂O₅ and ThO₂ could be successfully refined against the collected PXRD data. Phase analysis indicates the sample to be 97(1) and 2.3(3) wt% for β-ThB₂O₅ and ThO₂ respectively. From these ex situ measurements and analysis, it indicates that the application of HT/HP conditions to the α-ThB₂O₅/β-ThB₂O₅ mix results in the conversion of α-ThB₂O₅ to β-ThB₂O₅, such that β-ThB₂O₅ is the preferred phase under HT/HP conditions. The occurrence of a slightly increased relative amount of ThO₂ is speculated to be related to partial sample decomposition occurring.

3.4 Vibrational spectroscopy

Raman spectrum of β-ThB₂O₅ was measured in the region of 100–1500 cm⁻¹. As shown in Fig. S13,† the modes in low-frequency region from 187–389 cm⁻¹ are raised from the lattice vibrations. The Raman band with a very strong peak around 526 cm⁻¹ can be attributed to the bending character ν₄ mode of BO₄ tetrahedra. Raman bands with weak peaks around 800 cm⁻¹ are due to the symmetric stretching ν₁ mode of BO₄ units. The Raman band at around 966 cm⁻¹ were assigned to the B–O–B symmetrical stretching ν₃ mode in BO₃ triangles. The peaks around 1362 cm⁻¹ can be attributed to the doubly degenerate asymmetrical stretching ν₄ mode of the B–O bonds in the trigonal BO₃ units. For α-ThB₂O₅, Raman band with a strong peak at 472 cm⁻¹ are attributed to the doubly degenerate in-plane O–B–O bending ν₁ mode from the planar trigonal BO₃ groups. The weak peaks at 700 cm⁻¹ and 803 cm⁻¹ can be assigned to the B–O bending ν₂ mode in the BO₃ triangles. The symmetrical stretching ν₃ modes of BO₃ groups have caused the peaks at Raman bands of 918 cm⁻¹ and 1000 cm⁻¹. The Raman band with a weak peak at 1416 cm⁻¹ is attributed to the doubly degenerate asymmetrical stretching ν₄ mode of BO₃ units.^26,27

IR spectra of both phases were measured in the range of 4000–400 cm⁻¹ (see Fig. 8 black curve). The IR spectra of both phases show very high transmittance in the range of 4000–1600 cm⁻¹ (2.50–6.25 μm). The absorption bands between 1027 cm⁻¹ and 1438 cm⁻¹ in both curves can be assigned to the antisymmetric stretching vibrations of the BO₃ groups in the two compounds. The peaks associated with the BO₄ symmetric stretch vibrations appeared in the range of 928–735 cm⁻¹ for β-ThB₂O₅. The absorption peaks at 529–652 cm⁻¹ can be assigned to bending vibrations of BO₃ and BO₄ groups for α-ThB₂O₅ and BO₃ groups for β-ThB₂O₅. These assignments are consistent with the borates reported previously.²²

Fig. 8 IR spectra experimental (black) and calculated (red) curves of α-ThB₂O₅ and β-ThB₂O₅.

3.5 DFT investigations of α- and β-polymorphs

To understand the thermodynamic stability and spectroscopic properties of studied polymorphic modifications we performed a DFT study of both structures. The computed lattice parameters for the α-ThB₂O₅ and β-ThB₂O₅ phases are reported in Table S3.† The lattice parameters of β-ThB₂O₅ phase are reproduced with surprisingly good accuracy with relative error much smaller than 1% and the volume being off from the experimental value by just 0.6%. On the other hand, the lattice parameters of α-ThB₂O₅ are off the experimental values by up to 1.5% with the volume being overestimated by 2.5%, although this is still reasonably good performance of DFT. Despite these offsets, we notice that both structures are well reproduced, and the DFT-based geometry relaxation did not result in significant deformation of the experimental structures, which were taken as starting configurations. In Table 2 we present the values of relative internal energies (enthalpies) and free energies computed via adding vibrational entropy and zero point energy contributions for temperatures of 300K (ambient conditions), 1000 K, 1200 K and 1500 K. The Gibbs free energies are computed as:

G = U + ZPE − ST

where U is the internal energy, ZPE is the zero-point energy contribution, S is the vibrational entropy and T is the temperature. We report the differences in Gibbs free energies of the two phases ΔG = G(β-ThB₂O₅) – G(α-ThB₂O₅) at selected temperatures (see Fig. 9).

Fig. 9 The computed difference in Gibbs free energy between α-ThB₂O₅ and β-ThB₂O₅ phases. ΔG = G(β-ThB₂O₅) − G(α-ThB₂O₅).

The calculations show that α-ThB₂O₅ phase is the more stable phase at low temperature with the internal energy difference at 0 K of 34.4 kJ mol⁻¹. This is consistent with experimental observation which have been discussed in thermal and in situ PXRD analyses section. Increasing the temperature, the free energy difference decreases reaching zero at temperature just above 1500 K.

4. Conclusions

Only the second known polymorph of ThB₂O₅, β-ThB₂O₅, was synthesised from HT/HP conditions and characterised using X-ray single crystal, powder diffraction and spectroscopic methods. The structure of β-ThB₂O₅ is starkly different in comparison to the known α-phase¹ and to the wider family of AB₂O₅ compounds (A = Zr, Hf).^21,22 The β-modification features a dense 3D thorium borate framework, which is composed of [3⁶] topological 2D thorium layers on the ab-plane and 1D boron–oxygen zigzag chains along the a-axis. The structure further possesses tetra-capped trigonal prisms of ThO₁₀ polyhedra that are apparently the first observed in inorganic thorium compounds. The previously known α variant with regard to the β, was found to be the lower temperature and lower pressure preferred phase as shown by HT/HP synthesis experiments, in situ heating PXRD measurements and supported by ab initio calculations. From in situ heating PXRD measurements the α polymorph was observed to transform to the β form at approximately 900 °C via a 1^st order discontinuous phase transformation. β-ThB₂O₅ was found to exist only over a narrow temperature range, decomposing above 1050 °C. Interestingly it was found that by switching fluxes from borax, used to synthesis the α variant under high temperature conditions, to carbonates, the β variant could be synthesised, which was identical to that obtained from HT/HP synthesis. This study demonstrates how subtle changes to synthesis conditions, including temperature, pressure but also and pertinently choice of flux, can dramatically influence different results in structure formation of ThB₂O₅ polymorphs and potentially maybe applicable in synthesis design of other inorganic compounds.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Authors are grateful to the Natural Science Foundation of the Education Department of Anhui Province (KJ2019A0831), Hefei University Talent Funding (18-19RC14), Natural Science Foundation of Anhui Province (2008085QB56), HFUU Bilingual Course Construction Project (2021Yyykc02) and Teaching and Research Project of Hefei 2019hfjyxm09. E. V. A. is grateful for DFG for the funding within the AL1527/3-1 project.

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Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic files in CIF format, SEM and EDS measurement, selected bond distances and angles, TG-DSC curves, and vis-IR spectroscopy. CCDC 2106445. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2dt01049f

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