In the search for a suitable crystalline wasteform, synthetic analogues of some of the naturally occurring rare-earth phosphate minerals have been identified as a potential host matrix for HLW storage.60,81,100 The rare-earth phosphate minerals are both compositionally and structurally diverse and can be found in nature as monazite [(Ce,La,Nd,Th,U)PO4], xenotime [(Y,HREE,U,Th)PO4; HREE – heavy rare-earth elements], fluorapatite [(Ca,Ce)5(PO4)3F], vitusite [Na3(Ce,La,Nd)(PO4)2], rhabdophane [(Ce,La)PO4.H2O], and brockite [(Ca,Th,Ce)PO4.H2O].110 Of the various rare-earth phosphate minerals that exist in nature, the monazite and xenotime minerals contain the highest weight percent of rare-earth elements and these minerals are used commercially for the extraction of rare-earth elements.111,112 Monazite and xenotime are abundant rare-earth minerals that exist as an accessory phase in rocks such as granitoids, rhyolites, and gneisses.110–113 The mineral monazite is also found in alluvial deposits and beach sands as a result of the weathering of rocks.110,113 In addition to accommodating rare-earth elements, the monazite and xenotime minerals also contain varying amounts of radioactive elements such as thorium and uranium which makes these minerals slightly radioactive.114–116 Due to the presence of thorium and uranium, these minerals have been exposed to radiation events over geological time scales and yet the monazite mineral, in particular, is never found completely in a metamict state (i.e., amorphous).117–120 This observation suggests that these minerals are structurally resistant to radiation damage events.121 However, radiation studies on synthetic and natural monazite samples have shown that these minerals are easily damaged by
radiation events but could also recover from structural damage upon annealing at lower temperatures.122–124 Unlike the monazite mineral, radiation studies of xenotime minerals are not well documented in the literature. Monazite and xenotime minerals are also highly insoluble and possess high chemical durability in aqueous environments and this is demonstrated by the fact that the monazite minerals are still present in beach sands and placer deposits wherein the minerals are frequently exposed to aqueous conditions.113 Based on this mineralogical evidence, synthetic analogues of monazite and xenotime minerals are being proposed as potential host candidates for nuclear waste sequestration applications.88,121,125
Another important mineral that is chemically related to monazite is the rhabdophane mineral.126,127 Rhabdophane is a naturally occurring hydrous rare-earth phosphate mineral (REPO4.nH2O; RE = La to Dy) that is formed during the aqueous alteration of monazite minerals and could play a role in controlling the solubility of actinides.128–130 A few studies have shown the formation of rhabdophane on the surface of synthetic phosphate ceramics as a result of chemical alteration of the latter and have proposed that the rhabdophane material could act as a protective barrier by either delaying or stopping the release of actinides to the environment.126,128–
134 In terms of structural stability, the rhabdophane phase is considered to be metastable and can undergo structural transformations to the monazite- or xenotime- type structure at higher temperatures.126 It is for this reason that the synthetic analogues of the rhabdophane mineral have not received attention as a nuclear wasteform in the literature. Nevertheless, a detailed investigation of the structure and properties of rhabdophane-type materials is required in order to assess the long term chemical performance of monazite wasteforms in geological repository based conditions.127
1.5.1 Crystal Structures of Monazite and Xenotime
The general formula of compounds adopting the monazite and xenotime structure is REPO4, where RE represents a rare-earth element.135–137 In the case of monazites, the RE site is occupied by the larger, light rare earth elements (LREE – La to Gd) whereas in xenotime, the RE site is occupied by smaller, heavy rare earth elements (HREE – Tb to Lu and Y).135 Materials adopting the monazite- and xenotime-type structure crystallize in a monoclinic (space group – P21/n) or tetragonal (space group – I41/amd) crystal system, respectively.135,136,138
The crystal structures of monazite (e.g., CePO4) and xenotime (e.g., YPO4) are shown in Figures 1.3a and 1.3b. The RE ion in the monazite structure is coordinated to nine oxygen atoms and can be visualized as an equatorial pentagon of oxygen atoms interpenetrated by a tetrahedron of oxygen atoms (Figure 1.3a).138–140 Nine different RE-O bond distances (e.g., In CePO4, Ce-O = 2.460 Å – 2.776 Å) are reported for this structure, thus indicating a significant distortion of the REO9 polyhedra.135,140 As a result of this distortion, the monazite structure offers greater structural flexibility and can accommodate cations of differing sizes and charges.78,140 The ability of a crystalline structure to accommodate various cations is an important property for the nuclear waste form mainly because the nuclear waste stream obtained from the reprocessing of spent fuel and dismantling of nuclear weapons contains a wide variety of radioactive elements.78 A slightly distorted PO4 tetrahedron provides the link for connecting the individual REO9 polyhedra, which leads to the formation of infinite chains of edge sharing REO9 and PO4 polyhedra along the c-axis.135 These chains are connected laterally by edge-sharing of adjacent REO9 polyhedra.135
The xenotime structure (e.g., YPO4), which is isostructural with the zircon phase (ZrSiO4; Space group: I41/amd), places the RE ions in an eight fold coordination environment of oxygen
atoms (Figure 1.3b).135,138 Similar to the monazite structure, edge-sharing chains of alternating REO8 polyhedra and PO4 tetrahedra exist in the xenotime structure.135 The presence of two unique RE-O bond distances (e.g., In YPO4, 2 x [4 Y-O] = 2.309 Å and 2.381 Å) in the xenotime structure, as opposed to the nine different RE-O bond distances in monazites, suggests that the REO8 polyhedron in the xenotime structure is more symmetric.135,138 The presence of a more regular coordination environment around the RE ion could suggest that the xenotime structure will impose severe symmetry, size, and charge restrictions on the cations that could be incorporated in the structure.78 Therefore, unlike the monazite structure, the xenotime structure is not expected to accommodate a wide variety of cations.
1.5.2 Crystal Structure of Rhabdophane
Evidence for the existence of the rhabdophane-type structure was first reported by Mooney.
Mooney found that the phosphates of La, Ce, and Nd crystallized in two forms: monoclinic (monazite) or hexagonal (REPO4.nH2O) crystal systems.141,142 The hexagonal structure (Space group – P6222 or P3121) of the hydrated rare-earth phosphates was proposed based on the analysis of diffraction photographs taken from powder samples.142 Recently, Mesbah et al.
reported that the use of a hexagonal model for the Rietveld refinement of synchrotron powder X-ray diffraction data (XRD) data collected from polycrystalline rhabdophane (SmPO4.0.667H2O) led to a poor refinement.126,142 The crystal structure of rhabdophane (SmPO4.0.667H2O) was solved ab-initio by Mesbah et al., with the results indicating that rhabdophane-type materials adopt a monoclinic (Space group – C2) structure.126 It should be noted that a large body of work is available on the luminescent properties of rhabdophanes in which the crystal structure is considered to be hexagonal.143–147
Figure 1.3 Crystal structures of (a) CePO4 (Monazite-type; Space group: P21/n) and (b) YPO4
(Xenotime-type; Space group: I41/amd). The crystal structures were generated using the VESTA (Visualization for Electronic and Structural Analysis) software program.148
Figure 1.4 (a) Crystal structure of SmPO4.0.667H2O (rhabdophane-type; Space group: C2). (b) Ball and stick representation of the alternate arrangement of Sm and P atoms in chains 1 (Ch1) and 2 (Ch2). The crystal structure was generated using the VESTA software program.148
The crystal structure of monoclinic rhabdophane (e.g., SmPO4.0.667H2O) is shown in Figure 1.4a.126 The structure is comprised of two chains referred to as Ch1 and Ch2 (See Figure 1.4b).126 There is an alternating sequence of Sm-O polyhedra and PO4 tetrahedra along the ‘a’
direction in both chains (Figure 1.4b).126 The alternating PO4 tetrahedra in Ch1 have four different P-O bond distances (e.g., In SmPO4.0.667H2O, P-O = 1.535, 1.536, 1.547, 1.549 Å) and P-O-P bond angles whereas in Ch2, the level of distortion in the alternating PO4 tetrahedra is not uniform (i.e., four distinct P-O bond distances) [e.g., In SmPO4.0.667H2O, P-O = 1.529, 1.538, 1.545, 1.548 Å] and P-O-P bond angles in the first tetrahedra followed by two distinct P-O bond distances [e.g., In SmPO4.0.667H2O, P-O = 1.534 Å and 1.537 Å] and P-O-P bond angles in the second tetrahedra).126 The Sm ions in chains Ch1 and Ch2 differ in terms of their coordination environment. The Sm ions in Ch1 are bonded to nine oxygen atoms (eight provided by the PO4 groups and one from water) and in Ch2, the Sm atoms are coordinated to only eight oxygen atoms provided by the PO4 groups (Figure 1.4b).126 Both SmO9 and SmO8 polyhedra are distorted. The connection of the two chains leads to the formation of infinite channels running along the ‘a’ direction.126 Each channel is formed by the connection of six chains (4 Ch1 and 2 Ch2) and water resides inside these channels (Figure 1.4a).126