Monoclinic Form of the Rhabdophane Compounds: REEPO₄·0.667H₂O

Abstract

Hydrated rhabdophane with a general formula REEPO 4 •nH 2 O (REE: La → Dy) has been always considered to crystallize in the hexagonal system. A recent re-examination of this system by the use of synchrotron powder data of the SmPO 4 •0.667 H 2 O compound led to a structure crystallizing in the monoclinic C2 space group with a = 28.0903(1) Å, b = 6.9466(1) Å, c = 12.0304(1) Å, β = 115.23(1)°, and V = 2123.4 (1) Å 3 with 24 formula units per unit cell. The structure consists of infinite channels oriented along the [101] direction and formed by the connection of Sm-polyhedra and P-tetrahedra by sharing O-edges. The water molecules filling the space have been localized for the first time. The monoclinic form of the hydrated rhabdophane was confirmed by studying the series with REE: La → Dy. Moreover, the dehydration of SmPO 4 •0.667H 2 O led to the stabilization of an anhydrous form SmPO 4 in the C2 space group with a = 12.14426 (1) Å, b = 7.01776(1) Å, c = 6.34755(1) Å, β = 90.02 (1)°, V = 540.97(1) Å 3 , and Z = 6.

Crystal Chemistry and Stability of Hydrated Rare-Earth Phosphates Formed at Room Temperature

“…The calculated lattice volume displays a positive linear correlation with the effective ionic radius, indicating that the crystal lattice shrinks uniformly as the ionic radius decreases (Figure 3a). The lattice volumes calculated in the present experiment are larger than that of well-crystalline rhabdophane structure reported in [8], most likely owing to the presence of more water molecules in the structure of LREPO4,hyd.…”

“…The peak positions of REiPO4,hyd (RE = La-Gd except Pm) correspond well, with a slight peak shift, to the structure of rhabdophane [8]. The patterns for REiPO4,hyd (RE = Tb-Lu) and REallPO4,hyd could not be identified due to the broadened peaks and lack of matching structures.…”

“…Based on the structural characteristics indicated by XRD analysis in this study, the structure of the REiPO4,hyd precipitates are classified as LREPO4,hyd for phases containing RE = La-Gd, MREPO4,hyd containing RE = Tb-Ho, and HREPO4,hyd containing RE = Er-Lu, respectively. In LREPO4,hyd, which has a rhabdophane structure, the coordination numbers (CNs) of RE 3+ ions are one third eight-fold and two-thirds nine-fold [8], resulting in an average CN of 8.7. RE 3+ in MREPO4,hyd and HREPO4,hyd can be considered to have eight-fold coordination because the CN of xenotime and churchite is eight-fold.…”

“…Transition in XRD patterns of REiPO4,hyd (RE = Tb, Dy, Ho, Yb) and REallPO4,hyd formed at pH 3 for 3 days at room temperature during annealing up to 500 °C. The patterns of each REPO4,hyd are accompanied by the spectrum of the ideal RE phosphate minerals; rhabdophane, monazite, and xenotime, based on [5,8], where r.t. represents room temperature. A circle indicates peaks exhibiting structural transformation.…”

“…Crystalline RE phosphates occur in four different structures: monazite (light REPO 4 ; P2 1 /n), rhabdophane (light REPO 4 ·nH 2 O; P6 2 22 or C2), xenotime (heavy REPO 4 ; I4 1 /amd), and churchite (heavy REPO 4 ·nH 2 O; I2/a) [5][6][7][8]. In the crust (granite, metamorphic and/or metasomatic rocks, and pegmatite), monazite and xenotime are the stable phases for light REE and heavy REE, respectively, while rhabdophane and churchite occurrences are almost exclusively encountered in soil [9][10][11][12][13].…”