The structural effects of alkaline- and rare-earth element incorporation into thorium molybdates

Abstract

Four novel thorium molybdates containing alkaline-earth or rare-earth metals were isolated from high-temperature solid-state synthesis. The incorporation of divalent and trivalent cations into the thorium molybdate system results in complex structural topologies. A₂MgTh₃(MoO₄)₈ (A = K, Rb) which crystallizes in the space group C2/c is the first instance among the thorium molybdate family that incorporates alkaline-earth metals. Its crystal structure is based on complex channels composed of ThO₈ square antiprisms and MoO₄ tetrahedra arranged in a corner-sharing manner. K₂SrTh₂(MoO₄)₆, as the first thorium polymolybdate compound, is constructed from ThO₈ square antiprisms and Mo₄O₁₆ tetramers. The Mo₄O₁₆ tetramers, lying in the (001) plane, are built from four edge-sharing MoO₆ octahedra. Nd₂Th₃(MoO₄)₉ is the first thorium molybdate containing rare-earth cations. The resemblance of Nd₂Th₃(MoO₄)₉ with hexagonal-ThMo₂O₈ reveals its potential as a host for different trivalent transuranium elements. Raman spectra analysis shows that the different Mo polyhedral geometries (MoO₄ tetrahedra and MoO₆ octahedra) have significant effects on the vibrational bands of these compounds. The thermal behavior and stability of the newly obtained materials have been studied.

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Introduction

The chemical behaviour of synthetic and natural actinide molybdates has received considerable attention for a few decades due to their important roles in nuclear safety management.¹ Molybdenum, in most cases, is one of the significant fission products produced in the spent nuclear fuel.^2–4 The chemical reactions between actinide elements and molybdenum may produce complex compounds and these compounds could affect the nuclear fuel behaviour.^5,6 In addition, actinide molybdates are considered as important materials for the long-term evolution of a geological repository for the spent nuclear fuel.^7–12 From a more fundamental perspective, actinide molybdates bear complex structures and are capable of forming diverse topological configurations. In the past few decades, extended studies on actinide molybdate compounds have been mostly focused on high-valent uranium(VI) phases consisting of approximately linear uranyl cations UO₂²⁺.^13–16 The uranyl UO₂²⁺ exhibits various coordination geometries, because of its ability to link four to six equatorial ligands and form tetragonal, pentagonal and hexagonal bipyramidal configurations. Based on this, a vast number of uranium molybdates incorporating alkali,^14,17–19 alkaline-earth,^4,20 transition^21–24 and even lanthanide metals²⁵ have already been isolated, which have exhibited exceedingly rich structural flexibility.

In comparison, molybdate compounds containing lower-valent actinides such as Th(IV), Np(IV) and Pu(IV) are barely reported. These elements also exhibit a versatile structural chemistry. For example, the coordination number of Th(IV) can be varied from six to fifteen.^26–28 Unlike most of the early actinides possessing various oxidation states, thorium is practically stable in tetravalent state Th(IV). This makes thorium much easier to handle, and thus it is frequently used as a surrogate for studying the chemical behaviour of more radiotoxic transuranic elements in valence state (IV) such as Pu(IV) and Np(IV).²⁹

Most of the studies on thorium molybdates were carried out more than two decades ago.^30–33 Recently, thorium has been considered in new generations of nuclear fuel cycles, which is spurring more attention on thorium compounds.^34,35 The first reported thorium molybdate compound is orthorhombic ThMo₂O₈.³⁶ It is based on corner-sharing ThO₈ antiprisms and MoO₄ tetrahedra. Using micro thermal analysis and powder X-ray diffraction accordingly, the initial work on quaternary thorium molybdate compounds containing alkali metals has been performed by Tabuteau and Pages.³⁷ Thereafter, the crystal structures and some physicochemical properties of a series of thorium molybdate compounds, namely, K₂Th(MoO₄)₃, K₄Th(MoO₄)₄ as well as K₈Th(MoO₄)₆ have been described by Huyghe et al.^38–40 Recently, Dahale et al. presented the syntheses and thermal stabilities of Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄.⁴¹ Subsequently, the study of thorium molybdates containing alkaline cations was completed by recently reported Rb–Th–Mo (ref. 42) and Cs–Th–Mo (ref. 43) families. In addition to these synthetic compounds, Orlandi et al. discovered the first two natural thorium molybdate hydrates, Th(MoO₄)₂·3H₂O (ichnusaite)⁴⁴ and Th(MoO₄)₂·H₂O (nuragheite).⁴⁵ The structures of these two minerals are based on ThO₉ and MoO₄ polyhedra, which are connected to each other by corner-sharing. However, besides the above mentioned pure thorium molybdates or alkaline thorium molybdates, no other phases are reported from this system.

In order to develop a better understanding of the structural chemistry of actinides with the purpose of modelling the complicated processes in nuclear waste disposal sites, we have undertaken a series of phase synthesis and characterization studies of thorium compounds containing hexavalent cations (S⁶⁺, Se⁶⁺, Te⁶⁺, Cr⁶⁺, W⁶⁺, Mo⁶⁺).^{13,43,46–54} We found that, compared to uranium molybdates, which favorably form 2D sheets,^50,51 the thorium counterparts are structurally more versatile. This can be exemplified by one family of thorium molybdates, Rb–Th–Mo, where all compounds share common polyhedral units of MoO₄ tetrahedra and ThO₈ antiprisms but adopt different structural dimensionalities, from simple clusters in Rb₈Th(MoO₄)₆, to a complex three-dimensional framework in Rb₄Th₅(MoO₄)₁₂. Continuing this study, here we perform an investigation of the thorium molybdate family that contains alkaline-earth metals and rare-earth metals. In this report, we discussed the influence of alkali- and rare earth-elements on the structures and stability of thorium molybdates.

Experimental section

Syntheses

The four titled thorium molybdate compounds were prepared by a solid-state reaction method using A.R. grade chemicals Th(NO₃)₄·5H₂O (Merck) and MoO₃ (Alfa-Aesar) without further purification. Th used in this study is an α-emitting radioisotope and thus standard precautions for handling radioactive materials should be strictly obeyed at all times.

Single crystal preparation of K₂MgTh₃(MoO₄)₈

Th(NO₃)₄·5H₂O (A.R. grade), KNO₃ (A.R. grade), Mg(NO₃)₂ (A.R. grade) and MoO₃ (A.R. grade) with a molar ratio of 1 : 1 : 2 : 2 were loaded into a Pt crucible after being fully ground. The mass weights of the reactants were 100.0 mg for Th(NO₃)₄·5H₂O, 17.7 mg for KNO₃, 52.0 mg for Mg(NO₃)₂ and 50.5 mg for MoO₃. The Pt crucible was heated up to 1000 °C within 4 h in a furnace (CARBOLITE CWF 1300) and held constant at 1000 °C for 2 h in order to melt homogeneously. Afterwards, the mixture was cooled to 400 °C at a rate of 6.5 °C h⁻¹, followed by quenching. The resulting products were washed with hot water and bulk colourless crystals of good quality were obtained. Due to the undistinguishable optical behaviour of the synthesized crystals and glassy pieces, the yield couldn't be calculated.

Single crystal preparation of Rb₂MgTh₃(MoO₄)₈

Rb₂MgTh₃(MoO₄)₈ was prepared by heating a mixture of Th(NO₃)₄·5H₂O (100.0 mg), RbNO₃ (25.9 mg), Mg(NO₃)₂ (52.0 mg) and MoO₃ (50.5 mg) with a molar ratio of 1 : 1 : 2 : 2 in a Pt boat. The remaining experimental conditions were identical to K₂MgTh₃(MoO₄)₈. After being washed with hot water, lots of colourless crystals with prismatic shape were isolated. The yield couldn't be obtained because of the undistinguishable shapes between the crystals and glassy pieces.

Single crystal preparation of K₂SrTh₂(MoO₄)₆

The reactants Th(NO₃)₄·5H₂O (100.0 mg), KNO₃ (53.2 mg), Sr(NO₃)₂ (148.5 mg) and MoO₃ (50.5 mg) with a molar ratio of 1 : 3 : 4 : 2 were loaded into a Pt crucible. The mixture was ground and heated in a furnace at 950 °C for 3 h. The heating rate was 500 °C h⁻¹ and the cooling rate was 5 °C h⁻¹. The resultant mixture in the crucible consisted of well-formed colourless prisms of K₂SrTh₂(MoO₄)₆ as well as crystals of K₂MoO₄ and other undefined amorphous products.

Single crystal preparation of Nd₂Th₃(MoO₄)₉

Th(NO₃)₄·5H₂O (100.0 mg), Nd₂O₃ (23.6 mg) and MoO₃ (20.2 mg) with a molar ratio of 5 : 2 : 4 were loaded into a Pt crucible after being fully ground in an agate mortar. The crucible was then heated in a furnace for 6 h at 1100 °C, and slowly cooled to 400 °C at a rate of 5 °C h⁻¹ followed by quenching. The resulting products consisted of pale violet Nd₂Th₃(MoO₄)₉ crystals and a glassy mass that was subsequently isolated. The yield couldn't be determined because of the similar shapes between the crystals and glass pieces.

Preparation of the pure phases

All the obtained thorium molybdates were also prepared in the form of pure polycrystalline powder via standard solid-state methods. Using the same reagents and devices as for the single crystal syntheses, the pure thorium molybdate powders were prepared for each stoichiometric ratio. The procedure was as follows: after being thoroughly ground, the reaction mixtures were initially heated in air up to 400 °C and kept at this temperature for 30 h. After that, room-temperature X-ray powder diffraction analysis was required to analyze the phase content and purity. The same grinding and heating steps were repeated several times, increasing the temperature stepwise (50 °C per step), until pure phases were obtained. Finally, the pure phases of K₂MgTh₃(MoO₄)₈, Rb₂MgTh₃(MoO₄)₈, K₂SrTh₂(MoO₄)₆ and Nd₂Th₃(MoO₄)₉ could be prepared at temperatures starting at 550 °C, 600 °C, 650 °C and 600 °C, respectively.

Crystallographic studies

The crystals of thorium molybdates selected for data collection were mounted on glass fibres and then optically aligned on an Agilent SuperNova (Dual Source) diffractometer. All data were collected using monochromatic Mo-Kα radiation with an incident wavelength of 0.71073 Å, running at 50 kV and 0.8 mA providing a beam size of approximately 30 μm in diameter. More than a hemisphere of data was collected for each crystal, and the three-dimensional data were reduced and outliers identified using the software CrysAlis^Pro. Data were corrected for Lorentz, polarization, absorption and background effects. The SHELXL-97 program was used for the determination and refinement of the structures.⁵⁵ The data and crystallographic information are given in Table 1. The structures were solved by direct methods and refined to R₁ = 0.020 for K₂MgTh₃(MoO₄)₈, R₁ = 0.034 for Rb₂MgTh₃(MoO₄)₈, R₁ = 0.053 for K₂SrTh₂(MoO₄)₆ and R₁ = 0.042 for Nd₂Th₃(MoO₄)₉.

Table 1 Crystallographic data of K₂MgTh₃(MoO₄)₈, Rb₂MgTh₃(MoO₄)₈, K₂SrTh₂(MoO₄)₆, and Nd₂Th₃(MoO₄)₉

Compound	K2MgTh3(MoO4)8	Rb2MgTh3(MoO4)8	K2SrTh2(MoO4)6	Nd2Th3(MoO4)9
Mass	2078.15	2170.89	1589.54	2424.03
Space group	C2/c	C2/c	P1̄	P63/m
a (Å)	18.568(7)	18.6792(4)	8.0197(10)	17.556(3)
b (Å)	18.0282(19)	18.0812(5)	8.4470(17)	17.556(3)
c (Å)	9.4188(14)	9.4616(3)	16.030(2)	6.2713(10)
α (deg)	90	90	101.550(15)	90
β (deg)	90.35(3)	90.230(2)	102.893(12)	90
γ (deg)	90	90	97.388(13)	120
V (Å^3)	3152.9(13)	3195.56(15)	1019.9(3)	1673.9(7)
Z	4	4	2	1
λ (Å)	0.71073	0.71073	0.71073	0.71073
F(000)	3648.0	984.0	1400.0	1952.0
μ (cm^−1)	17.546	20.078	21.194	19.716
ρ calcd (g cm^−3)	4.378	4.512	5.176	4.820
R(F) for Fo^2 > 2σ(Fo^2)	0.0200	0.0342	0.0526	0.0414
wR2 (Fo^2)	0.0434	0.0611	0.1300	0.1435

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Powder X-ray diffraction

X-ray powder diffraction patterns were collected on a Bruker-AXS D4 Endeavor diffractometer in the Bragg–Brentano geometry, equipped with a copper tube and a primary nickel filter providing Cu K[small alpha, Greek, macron] radiation (λ = 1.54187 Å). A one-dimensional silicon strip LynxEye detector (Bruker-AXS) was used for data collection. A voltage of 40 kV and an electric current of 40 mA (1.6 kW) were set for the experiment. Data were recorded in the range of 2θ = 10–80° (total counting time = 10 s per step with the step width of 0.02°). The aperture of the fixed divergence slit was set to 0.2 mm and that of the receiving slit to 8.0 mm. The discriminator of the detector was set to an interval of 0.16 to 0.25 V. The X-ray powder diffraction patterns for K₂MgTh₃(MoO₄)₈, Rb₂MgTh₃(MoO₄)₈, K₂SrTh₂(MoO₄)₆ and Nd₂Th₃(MoO₄)₉ are provided in the ESI† (Fig. SI1).

Raman studies

Unpolarized Raman spectra were recorded with a Horiba LabRAM HR spectrometer using a Peltier cooled multi-channel 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. All the samples were in the form of polycrystalline powders. The incident radiation was produced by a He–Ne laser at a power of 17 mW (λ = 632.81 nm). The focal length of the spectrometer was 800 mm and an 1800 grooves per mm grating was used. The spectral resolution was around 1 cm⁻¹ with a slit of 100 μm. The spectra were recorded in the range of 100–1050 cm⁻¹. No photoluminescence (PL) was observed.

Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS)

Scanning electron microscopy images and energy-dispersive X-ray spectroscopy (SEM/EDS) data were collected on a FEI Quanta 200 F Environmental Scanning Electron Microscope. The instrument is equipped with an Apollo Silicon Drift Detector (SSD) from EDAX. The measurements were carried out in a low-vacuum mode at 60 Pa (30 kV, spot size 4, working distance 10 mm). The EDS results shown in the ESI† (Fig. S2 and Table S1) are in good agreement with the expected chemical compositions for all four thorium compounds.

Thermal analysis

The thermal behaviour of the dried powders was studied from room temperature up to 1250 °C by differential scanning calorimetry analysis (DSC) coupled with thermogravimetry (TG) in air, at a heating rate of 10 °C min⁻¹, using a Netzsch STA 449C Jupiter apparatus. 20 mg of each sample were loaded in a platinum crucible which was closed with a platinum cover. During the measurements a constant air flow of 20–30 ml min⁻¹ was applied.

Results and discussion

Structure descriptions

Crystal structure of A₂MgTh₃(MoO₄)₈ (A = K, Rb) series. A₂MgTh₃(MoO₄)₈ (A = K, Rb) features a 3D open-framework network, as shown in Fig. 1(a). It contains two Th and four Mo atoms in an asymmetrical unit. As is observed in most thorium molybdate compounds, the Th and Mo atoms in A₂MgTh₃(MoO₄)₈ (A = K, Rb) are eight- and four-fold coordinated, forming a distorted square antiprismatic and tetrahedral geometry, respectively. For both phases the Th–O distances range from 2.346(4) to 2.513(4) Å. This range is typically observed in other inorganic thorium molybdates reported previously.^56,57 In K₂MgTh₃(MoO₄)₈, the Mo–O(1) bond distance (1.790(4) Å) is the longest one, which is further connected to Th(1), and the shortest bond distance is Mo–O(16) (1.687(4) Å), where O(16) is a terminal oxygen atom. For Rb₂MgTh₃(MoO₄)₈, the Mo–O bond distances range from 1.699(7) to 1.788(6) Å, with an average value of 1.757(6) Å, which is comparable to that found in the potassium-based structure (1.759(8) Å). The Th–O–Mo angles in both structures are in the range from 129.6(2)° to 176.9(2)°, demonstrating quite flexible linkages between the ThO₈ and MoO₄ polyhedra.

Fig. 1 (a) Crystal structure of A₂MgTh₃(MoO₄)₈ (A = K, Rb) projected along the c axis. The channels with elliptical cross-section are highlighted with blue color. (b) The local connection of ThO₈ and MoO₄ polyhedra in the crystal structure of K₂MgTh₃(MoO₄)₈. The yellow and green polyhedra represent Th and Mo, respectively. Mg cations are shown as black nodes. The alkali metals (K or Rb) are shown in red.

In the structure of A₂MgTh₃(MoO₄)₈ (A = K, Rb), all ThO₈ square antiprisms are eight-fold coordinated by MoO₄ tetrahedra. The local coordination environment of ThO₈ in K₂MgTh₃(MoO₄)₈ that is chosen as an example is exhibited in Fig. 1(b). All O atoms of the Mo(1)O₄ tetrahedra are bridging oxygen atoms to ThO₈ polyhedra, whereas Mo(2)O₄ and Mo(4)O₄ each share three O atoms with ThO₈ polyhedra, and Mo(3)O₄ only shares two O atoms with adjacent ThO₈ polyhedra. As one can easily imagine, these non-shared terminal O atoms in Mo(2)O₄, Mo(3)O₄ and Mo(4)O₄ directly lead to an open-framework structure with channels running along the [001] direction. The resulting channels, having an elliptical cross-section, are filled with Mg²⁺ cations. As can be seen from Fig. 1(a), additional small voids within the framework that are created by ThO₈ square antiprisms corner sharing with MoO₄ polyhedra are accompanied by K⁺ cations.

The most notable feature of A₂MgTh₃(MoO₄)₈ (A = K, Rb) is the unusual complex channels, highlighted in Fig. 1(a). Fig. 2(a and b) show the local connections of ThO₈ and MoO₄ polyhedra inside the cylindroid channels. Using the black and white nodal representation,⁵⁸ the idealized unfolding version of the topological graph for such a Th–Mo channel is shown in Fig. 2(c). It is assembled with four-connected black nodes (Th) and two-connected white nodes (Mo). It is obvious that its topology is composed of so-called basic building tapes (highlighted in Fig. 2(c)).⁵⁹ The adjacent building tapes are joined by Mo(3) cations. In order to obtain the channels observed in the structure of A₃MgTh₃(MoO₄)₈, one needs to fold and glue the corresponding building tapes by the following procedure: as a first step, label the equivalent points on the sides of the tapes by the letters a, b, c, d, e, f and g. Then fold the tape by joining the associated opposite sides (a–a′, b–b′, c–c′, d–d′, e–e′, f–f′ and g–g′), as demonstrated in Fig. 2(c and d). It is notable that the method of basic building tapes not only allows us to recognize the internal Th–Mo polyhedral connections along the channels, but is also helpful to understand the symmetry inside the channels.⁶⁰ The basic tapes are related by a two-fold screw axis perpendicular to the channel direction. Due to the inversion centre, as required by the space group C2/c, the cylindroid channels demonstrate an achiral cylindroid topology within this structure. Such a method was also adopted by Krivovichev et al. to present the linkage of U and Mo polyhedra inside the channels of a rare open-framework structure of (NH₄)₄[(UO₂)₅(MoO₄)₇](H₂O)₅.⁶¹ In contrast to A₂MgTh₃(MoO₄)₈, the equivalent points on the sides of the tapes for (NH₄)₄[(UO₂)₅(MoO₄)₇](H₂O)₅ are not oppositely orientated to each other, thus their folding procedure results in a chiral U–Mo channel topology.

Fig. 2 Representation of the cylindroid channels in the structure of A₂MgTh₃(MoO₄)₈ (A = K, Rb). (a and b) Construction of the channels connected by Th and Mo atoms. (c) Black and white nodal presentation of strips of Th and Mo polyhedra. The basic building strip is highlighted. (d) The idealized cylindroid channel is formed by folding the building strips around the c axis.

Structure of K₂SrTh₂(MoO₄)₆

K₂SrTh₂(MoO₄)₆ crystallizes in the space group P1̄, consisting of ThO₈ square antiprisms and Mo₄O₁₆ tetramers. The crystal structure of K₂SrTh₂(MoO₄)₆ is exhibited in Fig. 3. There are six crystallographically unique Mo sites and two distinct Th sites. Each Mo cation is coordinated by six O atoms forming a MoO₆ octahedral configuration. Th(1) and Th(2), with a square antiprismatic coordination geometry, reveal varying Th–O bond distances ranging from 2.34(1) Å to 2.56(2) Å, with the mean value being 2.44 Å, which is slightly larger than that observed in Cs₂Th(MoO₄)₃ (average Th–O = 2.41 Å),⁴³ but agrees well with values reported for many molybdates containing eight-fold coordinated Th.^38,39,62 In K₂SrTh₂(MoO₄)₆, two Th(1)O₈ polyhedra, related by an inversion centre, share a common O(8)–O(8) edge to construct a Th(1)₂O₁₄ dimeric unit, whereas the Th(2)O₈ polyhedra are found to be isolated from neighbouring ThO₈ polyhedra.

Fig. 3 View of the crystal structure of K₂SrTh₂(MoO₄)₆. (a) Projection along the a axis. (b) The Th₂O₁₄ thorium dimers. (c) The Mo₄O₁₆ tetramers lying in the ab plane. (d) Projection along the b axis shows the rectangular voids occupied by Sr²⁺ cations. The Th and W polyhedra are shown in yellow and green, respectively. The K and Sr atoms are presented as red and black nodes, respectively.

The most interesting structural feature of K₂SrTh₂(MoO₄)₆ is the Mo₄O₁₆ tetramers lying in the ab plane. They are built by four MoO₆ octahedra connecting with each other in an edge-sharing manner. Each Mo₄O₁₆ tetramer contains two kinds of bridging oxygen atoms, that is, μ₂-oxo and μ₃-oxo groups. The Mo–O bond distances within the MoO₆ octahedra vary from 1.72(2) Å to 2.44(2) Å. The average Mo–O bond distance is 1.96 Å, with some lengthening observed for Mo-μ₃O (around 2.15 Å). The O–Mo–O bond angles for O atoms in cis positions vary from 76.4(2)° to 106.4(3)°, and are far from an ideal right angle. To our best knowledge, such a Mo₄O₁₆ tetramer has not been reported in any actinide family before. However, it can be typically found in polyoxomatellate compounds,^63,64 where Mo can be aggregated in corner-, edge- and even face-sharing environments to form large and complex 3D frameworks. It also has been reported as a fundamental structural unit in some simple structures, such as Ag₂Mo₂O₇.⁶⁵

Each Mo₄O₁₆ tetrameter shares 14 oxygen atoms by attachment to three Th(1)₂O₁₄ dimers on the one side and two Th(2)O₈ square antiprisms on the other side. As a result, two types of channels can be found in the crystal structure of K₂SrTh₂(MoO₄)₆. The first one, with interior dimensions of approximately 11.5 × 1.9 Å, is formed propagating in the [100] direction. These channels are located within the Th–O–Mo anionic network and are occupied by the charge-compensating K⁺ and Sr²⁺ cations. Additional small channels with a rectangular shape are occupied by Sr²⁺ cations and can be seen clearly if one views along the [010] direction. These two kinds of channels intersect with each other, resulting in an open-framework structure.

Structure of Nd₂Th₃(MoO₄)₉

Nd₂Th₃(MoO₄)₉ crystallizes in the space group P6₃/m with a 3D framework structure, as shown in Fig. 4. The structure of Nd₂Th₃(MoO₄)₉ consists of one ThO₉ site, two independent NdO₆ sites, and three independent MoO₄ sites. Each Th atom, coordinating to nine O atoms, connects to nine MoO₄ tetrahedra forming a distorted and capped square antiprism coordination. The Th–O bond distances range from 2.42(1) to 2.51(1) Å, with an average bond distance equal to 2.44 Å.

Fig. 4 View of the crystal structure of Nd₂Th₃(MoO₄)₉ along the c-axis in (a) and along [110] in (b), respectively. The local coordination environment of Th and Nd polyhedra are shown in (c) and (d), respectively. Blue symbolizes NdO₆ octahedra; yellow and green represent ThO₉ and MoO₄ polyhedra, respectively.

This bond distance is slightly shorter than those found in other synthetic thorium molybdates having nine-fold coordinated Th polyhedra, such as 2.46 Å in hexagonal ThMo₂O₈,⁶⁶ but is still consistent with the ionic radii reported by Shannon.⁶⁷ The Nd atoms are six-coordinated in a highly distorted octahedral geometry. The neighbouring Nd polyhedra are linked into the chains by sharing of two octahedral faces (Fig. 4(a) and (b)). As given in Fig. 4(d), each NdO₆ octahedron is polarized by a three-fold rotation axis running through its parallel triangular faces. As a result, the Nd³⁺ cations move towards the octahedral face. This distortion is obviously reflected by a cis O–Nd–O configuration diverging from the ideal angle of 90°, exhibiting small (around 83°) and large (around 97°) angles. Each MoO₄ tetrahedron is four-fold connected to both types of polyhedra, i.e. ThO₉ and NdO₆. The variation of Mo–O bond distances is appreciable, ranging from 1.70(1) to 1.81(1) Å, with the average distance ~1.75 Å. This is in good agreement with previous results of thorium molybdates in MoO₄ tetrahedra.^39,62

The structures of a series of compounds Ln₂M₃(MoO₄)₉ (Ln = La–Lu, Y, Sc; M = Zr, Hf)^68,69 have a similar stoichiometric composition to Nd₂Th₃(MoO₄)₉. Both can be derived from the octahedral–tetrahedral framework of the mineral kosnarite (NaZr₂(PO₄)₃), a structure type that has been proposed as a nuclear waste form due to its capacity of accommodating a large variety of radionuclides present in high-level wastes.⁷⁰ Ln₂M₃(MoO₄)₉ is also based upon a 3D framework composed of three kinds of corner-sharing polyhedra: MoO₄ tetrahedra, MO₆ octahedra and nine-fold coordinated LnO₉. One of the interesting features of Ln₂M₃(MoO₄)₉ compounds is that they have been demonstrated as an acceptor for immobilizing tri- and tetravalent actinides, which elucidates the chemical behaviour of actinides in processes of treatment and disposal of radioactive waste.⁷¹ As a new member of this family, Nd₂Th₃(MoO₄)₉ may also become a potential waste form, but further investigation involving Am³⁺ and Cm³⁺ is necessary in order to confirm this hypothesis.

It is also noteworthy that there is a close structural relation between Nd₂Th₃(MoO₄)₉ and hexagonal ThMo₂O₈. Both compounds show similar unit-cell parameters. The a and b parameters of Nd₂Th₃(MoO₄)₉ are shorter than those of hexagonal ThMo₂O₈, and this shortening can be explained by the inclusion of the smaller Nd³⁺ cation. Fig. 5 shows the polyhedral representations of both compounds. The similarity can best be demonstrated by projecting Nd₂Th₃(MoO₄)₉ and hexagonal-ThMo₂O₈ along each crystallographic c-axis. However, further structural analysis reveals that all six-coordinated sites (NdO₆ octahedra) in Nd₂Th₃(MoO₄)₉ are connected via face-sharing, while the corresponding sites (ThO₆ octahedra) observed in hexagonal ThMo₂O₈ are completely separated (Fig. 5(c) and (d), respectively). For this reason, the associated Nd–Nd bond distance in Nd₂Th₃(MoO₄)₉ is remarkably short, i.e. only around 3.1 Å. The fact that Nd₂Th₃(MoO₄)₉ and hexagonal-ThMo₂O₈ are based on a similar structural framework while containing different cations indicates that Th⁴⁺ cations can be partially substituted by trivalent ions while keeping the same structural skeleton. This means that such a framework has the ability to host diverse cations of various radii as well as valences without a considerable change of geometric parameters. Therefore, Nd₂Th₃(MoO₄)₉ can be used as a host phase for rare-earth elements and minor actinides (Am³⁺, Cm³⁺…).

Fig. 5 Comparison of the crystal structures of Nd₂Th₃(MoO₄)₉ and hexagonal ThMo₂O₈. (a and b) The polyhedral representations in Nd₂Th₃(MoO₄)₉ and hexagonal ThMo₂O₈, respectively. (c) Face-sharing NdO₆–NdO₆ chain in Nd₂Th₃(MoO₄)₉. (d) Isolated ThO₆ octahedra in hexagonal ThMo₂O₈.

Raman analysis

The Raman spectra of all thorium molybdates shown in Fig. 6 can be coarsely divided into two parts: a low frequency part between 100 cm⁻¹ and 250 cm⁻¹ in which the bands are associated with lattice phonon modes, and a high-frequency part from 250 cm⁻¹ to 1000 cm⁻¹, which is assigned to the internal motions of oxo-anion groups (MoO₄²⁻ tetrahedron or MoO₆²⁻ octahedron).^72–76 As discussed in the structure analysis part, except for K₂SrTh₂(MoO₄)₆ which contains Mo₄O₁₆ tetramers constructed from edge-sharing of four MoO₆ octahedra, all remaining units are based on separate MoO₄²⁻ groups. For the MoO₄²⁻ group, the vibrational data are well defined by a large number of available references.^73,77–79 However, very little research has been undertaken so far on actinide molybdate materials. The free MoO₄²⁻ tetrahedron has a T_d symmetry and four normal modes which are assigned as A₁(ν₁), E(ν₂), F₁(rot), and F₂(trans, ν₃, ν₄), where A₁, E and F₂ are Raman permitted whereas F₂ is also IR active. ν₁, ν₂, ν₃ and ν₄ are denoted as non-degenerate symmetrical stretching mode, double degenerate symmetrical bending mode, and triple degenerate anti-symmetrical stretching and anti-symmetrical bending vibrations, respectively. The stretching vibrations ν₁ and ν₃ in ideal MoO₄²⁻ are located in the region 700–1000 cm⁻¹, while the bending vibrations (ν₂ and ν₄) are situated in the region 300–500 cm⁻¹.⁷⁶ When it comes to a specific structure, however, due to crystal field effects, the MoO₄²⁻ groups are distorted, and therefore the number of possible modes is increased. Because of having a large number of atoms in an asymmetrical unit (e.g. 35 crystallographically different atoms observed in K₂SrTh₂(MoO₄)₆), the vibrational properties are remarkably complex for our reported compounds, thus here we only assign selected frequencies based on a comparison to literature data. The Raman-active shifts are listed in Table S2† and the complete Raman fitting results for each thorium molybdate can be found in the ESI† (Fig. S3).

Fig. 6 Raman spectra of K₂MgTh₃(MoO₄)₈, Rb₂MgTh₃(MoO₄)₈, K₂SrTh₂(MoO₄)₆ and Nd₂Th₃(MoO₄)₉.

Like most of the reported molybdate compounds containing MoO₄ tetrahedral units,^77,80 strong vibrational bands for K₂MgTh₃(MoO₄)₈, Rb₂MgTh₃(MoO₄)₈ and Nd₂Th₃(MoO₄)₉ are observed related to the symmetric stretching modes (v₁), while the asymmetric stretches (v₃) are relatively weak. The Raman spectra of the isostructural compounds K₂MgTh₃(MoO₄)₈ and Rb₂MgTh₃(MoO₄)₈ are virtually identical. Both have a total of 276 normal vibrational modes (68A_g + 68A_u + 70B_g + 70B_u) predicted by factor group analysis.⁸¹ Only minor vibrational differences occur between these two compounds. In particular, the highest peak located at 953(1) cm⁻¹ in K₂MgTh₃(MoO₄)₈ can be assigned to the v₁ of MoO₄, while it is shifted to a lower frequency (951(1) cm⁻¹) in Rb₂MgTh₃(MoO₄)₈. Such a distinction mainly originated from different Mo–O bond distances, which has been elucidated in detail by Hardcastle and Wachs.⁸² The most significant feature of Nd₂Th₃(MoO₄)₉ is the strong bands in the region from 750 cm⁻¹ to 1000 cm⁻¹. The high and sharp peak located at 965(1) cm⁻¹ can be assigned to the symmetrical stretching mode v₁ of the MoO₄²⁻ tetrahedron. As for K₂SrTh₂(MoO₄)₆, because of the large number of atoms in an asymmetrical unit and Mo–O–Mo connections, the Raman profile shows complex broad and overlapping bands, originating from varying Mo–O bond distances and Mo–O–Mo bond angles.⁸³ The stretching modes of K₂SrTh₂(MoO₄)₆ occur at wavenumbers from 700 to 1000 cm⁻¹, and the bending modes from 250 to 450 cm⁻¹, which is comparable to other molybdates containing MoO₆ octahedra.^83–85

Thermal analysis

Fig. 7 shows DSC and TG measurements of as-synthesized pure thorium molybdate phases. Black lines represent the DSC curves and blue lines the percentage mass loss (TG). The DSC curves exhibit different profiles for thorium molybdates containing different types of cations. The thermal behaviour of K₂MgTh₃(MoO₄)₈ is quite similar to that of alkaline thorium molybdates, such as Rb₄Th₅(MoO₄)₁₂ (ref. 42) and Na₄Th(MoO₄)₄.⁴¹ K₂MgTh₃(MoO₄)₈ reveals two endothermic peaks, at around 921(5) °C and 1116(5) °C. The first peak indicates sample melting, and the broad and weak one, appearing immediately after the start of mass loss, shows the incongruent melting behaviour. A similar thermal evolution has also been observed for the isostructural counterpart Rb₂MgTh₃(MoO₄)₈. For K₂SrTh₂(MoO₄)₆ the DSC curve shows a strong endothermic peak at 985(5) °C indicating decomposition. For Nd₂Th₃(MoO₄)₉ decomposition seems to start at 667(5) °C, detectable by powder X-ray diffraction while heating the sample above this temperature. The main decomposition products can be indexed as hexagonal ThMo₂O₈ (see ESI,† Fig. S4). The associated mass loss due to MoO₃ evaporation in this range can serve as additional proof.

Fig. 7 DSC and TG behaviour of K₂MgTh₃(MoO₄)₈, Rb₂MgTh₃(MoO₄)₈, K₂SrTh₂(MoO₄)₆ and Nd₂Th₃(MoO₄)₉.

Conclusion

In conclusion, the isolation of a series of alkaline-earth or rare-earth thorium molybdates allows the elucidation of the influence of divalent and trivalent cations on the structural and physiochemical properties of thorium molybdates. Generally, the structural and thermal stability disparities between the thorium molybdates are strongly attributed to the radii and charge of the counter cations. In comparison with thorium molybdate (ThMo₂O₈), the thorium molybdate compounds with alkaline-earth or rare-earth metals are more complex. Specifically, A₂MgTh₃(MoO₄)₈ (A = K, Rb) are based on unusual complex cylindroid channels composed of ThO₈ square antiprisms and MoO₄ tetrahedra. Up to now, all known thorium molybdates are exclusively built by MoO₄ tetrahedra. K₂SrTh₂(MoO₄)₆ is the first phase being constructed from MoO₆ octahedral units. Nd₂Th₃(MoO₄)₉, comparable to octahedral–tetrahedral based natural kosnarite, has a structure similar to that of hexagonal ThMo₂O₈ but with NdO₆ instead of ThO₆ octahedra. The lanthanide to actinide substitution observed within the Nd₂Th₃(MoO₄)₉ framework may be an indication of the potential of this structure as a matrix for the immobilization of actinides.

The DSC measurements demonstrate that all the studied compounds in this work exhibit high thermal stability. Unlike the simple thorium molybdate ThMo₂O₈, which exhibits tremendous polymorphic diversity with three different polymorphs stabilized at ambient pressure, no phase transition was observed for alkaline-earth or rare-earth thorium molybdates up to the associated melting temperature. Nd₂Th₃(MoO₄)₉ starts to decompose upon melting at around 667(5) °C; this is the lowest melting point observed for all known alkaline-earth or alkali metal thorium molybdates.

Associated content

X-ray crystallographic files for K₂MgTh₃(MoO₄)₈, K₂SrTh₂(MoO₄)₆, Nd₂Th₃(MoO₄)₉, and Rb₂MgTh₃(MoO₄)₈ in CIF format (CSD depository number 430398-430401).

Acknowledgements

The authors are grateful to Dr. Martina Klinkenberg (IEK-6, Forschungszentrum Jülich) for her kind help in electron microscopy and EDX experiments. We are also grateful to the Helmholtz Association for funding within the VH-NG-815 grant.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: PXRD patterns, BSE image and EDS measuring points and Raman fitting results. CCDC 1430749–1430752. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ce02040a

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