Revision of the crystal structure of rhabdophane-(Ce)

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Marco E. Ciriotti
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Revision of the crystal structure of rhabdophane-(Ce)

Messaggio da Marco E. Ciriotti » sab 25 set, 2021 13:49

Referenza:

▪ Kolitsch, U. (2021): Revision of the crystal structure of rhabdophane-(Ce). Poster, MinPet 2021, Vienna, Austria, September 19-21, 2021; Mitteilungen der Österreichischen Mineralogischen Gesellschaft, 167, 116.

Abstract:
The crystal structure of rhabdophane, ideally CePO4·H2O according to the current nomenclature, was first reported by MOONEY (1950) using synthetic material (powder precipitate from aqueous solution). On the basis of powder X-ray diffraction data and a trial-and-error approach, she determined a hexagonal atomic arrangement, with the unit-cell parameters a = 7.07(2) and c = 19.06(5) Å, and the space group P6222 (no R-value given). However, she cautioned „The final assignment of the space-group symmetry must follow rather than preceed the structure determination.” In fact, an earlier study by her had suggested space group P3121 (MOONEY 1948), and a subsequent study, in which the structure of the Bi analogue was determined from Weissenberg data, led to the formula BiPO4·0.5H2O, space group P3121 (MOONEY-SLATER 1962) and the following conclusion: “In any case, the true space-group symmetry of CePO4 and of the other hexagonal rare-earth phosphates remains a question for further inquiry.“ MOONEY (1948, 1950) also noted the presence of zeolitic water in the channels of the structure [XPO4 (0-0.5H2O)]“ which „is probably necessary to stabilize the structure.” However, the idealised formula CePO4·H2O is given in the subsequent mineralogical literature (including the IMA list of approved minerals), while inorganic-chemical literature generally uses the formula CePO4·0.5H2O.
No modern study of the structure of natural rhabdophane-(Ce) or isostructural members of the rhabdophane group was ever published. The author has reinvestigated the atomic arrangement of rhabdophane-(Ce) using small but sharp, hexagonal, colourless prisms of a Nd- and La-rich specimen from the Clara mine, Black Forest, Germany. The structure solution, based on single-crystal intensity data (MoKα; 293 K) showed the correct space group to be P3121 [Flack x parameter: 0.01(4)], with a = 7.037(1), c = 6.429(1) Å, V = 275.71(7) Å3; R(F) = 1.68%. SEM-EDS analyses of both crystals and crystal fragments adjacent to the studied crystal showed only minor variations of the chemical composition. Refinement of the O(H2O) site occupancy gave 0.62(2). The bulk structural formula is ~(Ce0.41Nd0.22La0.16Ca0.06Sm0.05Pr0.05Eu0.02Gd0.01Sr0.01­U0.01)(P0.94S0.04As0.2)O4·0.62(2)H2O. Rhabdophane-(Ce) is therefore isostructural with its Bi analogue, for which a modern multi-technique study gave the formula BiPO4·0.67H2O (ROMERO et al., 1994). The asymmetric unit of the Clara mine crystal contains one metal, one semimetal and three O sites. Bond lengths of the nine-coordinate (Ce,REE) site range between 2.430(2) and 2.636(3) Å (average: 2.158 Å). The (P,S,As) site shows a bond-length range between 1.524(2) and 1.532(2) Å, with an average of 1.528 Å.

Mooney, R.C.L.. (1948): J. Chem. Phys., 16, 1003.

MOooney, R.C.L. (1950): Acta Crystallogr., 3, 337-340.

Mooney-Slater, R.C.L. (1962): Z. Kristallogr., 117, 371-385.

Romero, B., Bruque, S., Aranda, M.A.G., Iglesia, J.E. (1994): Inorg. Chem., 33, 1869-1874.
Marco E. Ciriotti

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