Synthesis, Crystal Structure, and Enthalpies of Formation
of Churchite-type REPO4·2H2O (RE = Gd to
Lu) Materials
Version 2 2019-07-16, 14:11
Version 1 2019-07-15, 13:03
Posted on 2019-07-16 - 14:11
Monazite
(REPO4; RE = La to Gd) and xenotime (REPO4;
RE = Tb to Lu & Y) materials have been proposed as host
matrices for the immobilization of actinides. Aqueous alteration of
monazite and xenotime minerals could result in the formation of rhabdophane
(REPO4·0.667H2O; RE = La to Dy) and churchite
(REPO4·2H2O; RE = Gd to Lu & Y) phases,
respectively. Among these structure types, the structure and properties
of churchite materials are not well-understood, and this study aims
to bridge this gap by providing a comprehensive insight into the structure
and thermochemical properties of churchite materials. Churchite materials
(REPO4·2H2O; RE = Gd to Lu) were synthesized
by a low-temperature precipitation route, and their crystal structures
were determined by powder X-ray diffraction (XRD). Examination of
the powder XRD data showed that the churchite materials crystallize
in the monoclinic crystal system (space group C2/c). The enthalpies of formation (ΔH°f,ox) of churchite-type REPO4·2H2O (RE = Gd to Yb) determined by high-temperature oxide melt
solution calorimetry are more negative than their anhydrous counterparts
(i.e., xenotime structure) and indicate that the formation of churchite
is more exothermic than the xenotime phase. However, the churchite
materials are likely to have a more negative entropy of formation
(ΔS°f,ox) due to the presence
of water molecules, resulting in a less negative Gibbs free energy
of formation (ΔG°f,ox) than
the xenotime structure. Therefore, churchite materials are expected
to be stable at lower temperatures. For the unique case of GdPO4 and GdPO4·nH2O materials, which could adopt all the above-discussed structure
types, the ΔG°f,ox of monazite
from oxides was observed to be more negative than those of xenotime,
rhabdophane, and churchite thereby suggesting the following order
of stability: Gd-churchite < Gd-rhabdophane < Gd-xenotime <
Gd-monazite.
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Subramani, Tamilarasan; Rafiuddin, Mohamed Ruwaid; Shelyug, Anna; Ushakov, Sergey; Mesbah, Adel; Clavier, Nicolas; et al. (2019). Synthesis, Crystal Structure, and Enthalpies of Formation
of Churchite-type REPO4·2H2O (RE = Gd to
Lu) Materials. ACS Publications. Collection. https://doi.org/10.1021/acs.cgd.9b00524
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AUTHORS (11)
TS
Tamilarasan Subramani
MR
Mohamed Ruwaid Rafiuddin
AS
Anna Shelyug
SU
Sergey Ushakov
AM
Adel Mesbah
NC
Nicolas Clavier
DQ
Danwen Qin
SS
Stephanie Szenknect
EE
Erik Elkaim
ND
Nicolas Dacheux
AN
Alexandra Navrotsky