High-Pressure Phase Transition of Coffinite, USiO₄

Abstract

Synchrotron powder diffraction patterns and Raman spectra of synthetic coffinite, USiO 4 , were collected for pressures up to 35 GPa and are complemented with DFT+U-based calculations. USiO 4 undergoes a first-order phase transition from a zircontype (space group I4 1 /amd) to a scheelite-type structure (space group I4 1 /a) at ≈15 GPa and ambient temperature. Contrary to earlier reports, the data indicate that this transition is completely reversible upon pressure release. Bulk moduli were obtained from the p−V data for the zircon-type and scheelite-type USiO 4 phases. For zircon-type USiO 4 , the value for B = 186(5) GPa, whereas, for the scheelite-type phase, B = 204(9) GPa, where the latter is significantly lower than a value proposed earlier (B = 274(16) GPa, [Zhang, F. X.; et al. Am. Mineral. 2009, 94, 916]). Lattice dynamical calculations point toward a Γ-point soft mode triggering the pressure-induced phase transition. ■ INTRODUCTIONA pressure-induced phase transition from zircon-type structures to scheelite-type structures is known for many ABO 4 (A = Sc, Y, REE; B = V, As) compounds. It has also been observed in orthosilicates that crystallize in the zircon structure type at ambient conditions, such as zircon, ZrSiO 4 , 2 and hafnon, HfSiO 4 . 3 The phase transition of ZrSiO 4 has been extensively studied under dynamic compression and hydrostatic pressures at ambient and at high temperatures. 2,4−7 At ambient temperature, the phase transition in ZrSiO 4 occurs in the pressure range between 20 and 30 GPa. This transition pressure is strongly influenced by impurities and radiation damage. 8−11 HfSiO 4 undergoes this transition at ≈19.6 GPa. 3 More recently, this pressure-induced phase transition has also been reported for coffinite, USiO 4 . 1 On the basis of force-field calculations, Bose et al. 12 suggested that thorite, ThSiO 4 , will also transform to the scheelite-type at high pressure (p > 3 GPa).The zircon structure has space group symmetry I4 1 /amd, and the high-pressure scheelite structure has space group symmetry I4 1 /a, which is a maximal subgroup of I4 1 /amd. In the case of ZrSiO 4 , the scheelite-type structure is ≈10% more dense than the zircon-type structure at ambient pressure. 6 The structural relationship of the crystal structures may be described by a shift and rotation of the SiO 4 4− tetrahedra ( Figure 1). However, during the phase transition, the cell volume abruptly decreases, the length of the c-axis nearly doubles, while the a-axis is shortened by a third, while the number of formula units per unit cell remains constant. This, as well as the fact that the highpressure phases are quenchable, implies that the phase transition is of first order. Attempts to reconcile group− subgroup relationships with the observation of first-order phase transitions are widely discussed. For the zircon-scheelite phase transition, this has been done by Smirnov et al., 6 who proposed a route via a cubic intermediate phase. Figure 1. Schematic diagram of the structural relationship between the zir...

The Role of Water and Hydroxyl Groups in the Structures of Stetindite and Coffinite, MSiO4 (M = Ce, U)

“…The symmetric internal stretching (ν1) and the asymmetric stretching (ν3) modes both increased to higher frequencies above 200°C (Figure S7b). While such a trend was not typically expected for high temperature Raman spectroscopy, 98 it was what has been found in high pressure Raman spectroscopy studies of zircon-type materials, 60,86,99 where the Si-O bond length decreased with increasing pressure, similar to the contraction of Si-O bond distance relating to ν1 and ν3 shifting to higher frequencies. The vibrational frequencies of the internal bending modes (ν2 and ν4) increased to higher frequencies from 200 °C to 600 °C, and then decreased to lower frequencies from 600 °C to 1000 °C (Figure S7a).…”

“…A 10 mol % fraction of UO2 identified at 816 °C (Table S3). This could be the result of two possible decomposition reactions: (1) USiO4  UO2 + SiO2 in which the formed SiO2 could remain amorphous and thus invisible from XRD 22,53,64,71,[83][84][85][86][87][88] and/or (2) hydroxylated USiO4 (U(SiO4)1-x(OH)4x•nH2O)  anhydrous USiO4 + UO2. 11 These two reactions assume different coffinite starting types: the hydrated form, the hydroxylated form, or a combination of both, which will be discussed later.…”

Temperature-Dependent Gibbs Free Energies of Reaction of Uranyl-Containing Materials Based on Density Functional Theory

“…25 In fact, becquerelite phase in the presence of high concentrations of hydrogen peroxide also transforms into studtite within eight hours. 43 Note that while studtite and metastudtite have been observed to decompose in air at temperatures of about 330 K and 513 K, respectively, [44][45]18 these temperatures may be much higher in the presence of large concentrations of hydrogen peroxide. As shown in a previous experimental study of the dehydration of studtite, 29 the lifetime of this metastable phase is also very large in the presence of water, this phase being observed at least up to 90ºC (363 K).…”

Oxyhydroxy Silicate Colloids: A New Type of Waterborne Actinide(IV) Colloids

“…U IV oxyhydroxy silicate colloids form a candidate class of colloids accomplishing the colloid‐chemical processes necessary for the formation of coffinitic uranium ore. Furthermore, An IV oxyhydroxide colloids may also be involved in the “coffinitization” of UO 2 ‐based spent fuel, a phenomenon expected in nuclear waste repositories after the access of water which occurs via the dissolution and the reprecipitation of An IV[110, 111] and which is of significance for the behavior of spent fuel.…”