The structure of some alkaline-earth metal uranates

Loopstra, B.O.; Rietveld, H. M.

doi:10.1107/s0567740869002974

Cited by 150 publications

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“…Fixed values of the scattering lengths were used for calcium (0.488 x 10 -12 cm) and fluorine (0.560x 10 -lz cm). That for strontium, which has been less well established, was refined giving a final value of 0.690 (+0.012)x 10 -12 cm, in excellent agreement with the value of 0-683 (+0.007)x 10 -lz cm obtained recently by Loopstra & Rietveld (1969). The final thermal parameters were Bsr=0"546 and BF=0.820 A 2 for strontium fluoride and Bca=0.330 and B~=0.505 A2 for calcium fluoride.…”

supporting

confidence: 79%

Extinction in X-ray and neutron diffraction

Cooper¹,

Rouse²

1970

Acta Cryst Sect A

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Accurate neutron-diffraction measurements from crystals suffering from severe extinction have been used to test the recent general theory of extinction of Zachariasen (Acta Cryst. (1967). 23, 558). Analysis of these measurements indicates that certain of the approximations made in the theory are not generally valid and result in systematic deviations between theory and experiment, namely a marked angle-dependent effect and an inadequacy of the theory for strong extinction. The original theory is therefore extended to take these factors into account and to give agreement with the observed data. IntroductionTheoretical formulae for the Bragg intensities of diffracted X-rays or neutrons have been derived rigorously only in the limiting cases of an ideally perfect crystal (the dynamical theory) and an ideally imperfect crystal (the kinematical theory). In general a given crystal will lie somewhere between these two extreme cases and modification of the kinematical theory is necessary to take into account the degree of perfection of the crystal. The treatment is normally based on the mosaic model for which the crystal is assumed to consist of a number of small perfect crystal domains, each slightly misoriented with respect to its neighbours. Zachariasen (1967) has recently described a general theory of X-ray diffraction in crystals, based on an approximate treatment of the coupling between incident and diffracted beams. In this theory he derives a general formula for the intensity diffracted by a finite perfect crystal and hence the intensity diffracted by a finite mosaic crystal. Because of the complexity of the problem a number of approximations are introduced and the ultimate test of this theory is therefore, as Zachariasen states, a test of its agreement with experiment. Zachariasen himself has carried out a number of experimental tests using X-ray diffraction data (see Zachariasen, 1968aZachariasen, , b, c, 1969 and has obtained excellent agreement for the crystals studied: lithium fluoride, quartz, phenakite, hambergite and calcium fluoride. In addition, other experimental X-ray diffraction tests have provided good agreement with the theory (see e.g. Chandrasekhar, Ramaseshan & Singh, 1969).The Zachariasen formulae have also been applied to accurate neutron diffraction measurements on several crystals. These include barium fluoride (Cooper, Rouse & Willis, 1968) and strontium fluoride and calcium fluoride (Cooper & Rouse, 1970). For barium fluoride only limited strongly extinguished data were collected but the theory held up to a level of extinction of about 16 per cent in intensity (at 2= 1-038 A.). For calcium fluoride complete two-dimensional hkk data were collected initially at a wavelength of 0-877 A and a preliminary analysis gave a value of r* [see equations (16)] comparable to that obtained by Zachariasen (1968b)" However, once again the theory did not correct the strongest intensities adequately and so a more extensive study was undertaken. Complete two-dimensional neu-M. J. COOPER AND K. D. ROUSE ...

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supporting

confidence: 79%

Extinction in X-ray and neutron diffraction

Cooper¹,

Rouse²

1970

Acta Cryst Sect A

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“…Although coordination seven cannot be ruled out for U the polyhedra in such cases are rather different from those described. Besides, in all alkaline-earth-metal uranates, the U atoms are always six-coordinated octahedra, with the exception of unreduced CaUO4, where they are eight-coordinated (Loopstra & Rietveld, 1969). The Ca coordination can be further reduced to six.…”

Section: Structural Considerationmentioning

confidence: 99%

“…CaUO4 belongs to the family of alkaline-earth-metal uranates (Loopstra & Rietveld, 1969;Yamashita, Fujino, Masaki & Tagawa, 1981) and the structure, first determined by Zachariasen (1948), is rhombohedral with aR = 6"263 /~, aR = 36'04 ° and space group R3m. It can be obtained from the fluorite-type structure by a slight compression along one of the threefold axes, as shown in Fig.…”

Section: Introductionmentioning

confidence: 99%

The defect structure of reduced CaUO₄

Prodan¹,

Boswell²

1986

Acta Crystallogr Sect B

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The defect structure of reduced CaUO4 was postulated on the basis of available thermogravimetric, X-ray powder and transmission electron microscope data. It is proposed that the removal of O takes place in an ordered way forming two types of planar structural units which exhibit, owing to more than one energetically equivalent possibility, a stacking disorder along the threefold axis. The basic building elements of the two different units are a central sixcoordinated U cation surrounded by six seven-coordinated Ca cations and a central six-coordinated Ca 0108-7681/86/020141-06501.50 cation surrounded by six six-coordinated U cations, respectively, where all six-coordinated polyhedra are deformed octahedra. As a consequence microdomains are formed, based on either type of ordering, with corresponding compositions CAUO3.67 and CaUO3.5o, respectively. Owing to a more convenient charge balance the two types of ordering tend to alternate with average composition CAUO3.58, where, unlike the former, nonsymmetrically reduced sixcoordinated polyhedra also occur at one of the interfaces between the two alternating domain types. Further reduction to CaUO3.50 is only possible by transforming the less-reduced and keeping from both types (~ 1986 International Union of Crystallography 142 THE DEFECT STRUCTURE OF REDUCED CaUO4 of ordering the more-reduced one only. The composition of CaUO4_x can be continuously changed between CaUO4 and CaUO3.5o by altering the ratio of reduced to unreduced regions, but there is evidence that oxidation may take place preferentially in the domains containing nonsymmetrically reduced Ca cations.

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“…For much of his scientific career, Hugo Rietveld was involved in the study of uranium compounds (Rietveld 1966;Loopstra and Rietveld 1969). In 1967, he developed a program for the refinement of neutron diffraction data (Rietveld 1967(Rietveld , 1969).…”

Section: Introductionmentioning

confidence: 99%

Rietveldite, Fe(UO2)(SO4)2(H2O)5, a new uranyl sulfate mineral from Giveaway-Simplot mine (Utah, USA), Willi Agatz mine (Saxony, Germany) and Jáchymov (Czech Republic)

Kampf¹,

Sejkora²,

Witzke³

et al. 2017

J. Geosci.

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Rietveldite (IMA2016-081), Fe(UO 2 )(SO 4 ) 2 ·5H 2 O, is a new uranyl sulfate mineral described from three localities: Giveaway-Simplot mine (Utah, USA), Willi Agatz mine (Saxony, Germany) and Jáchymov (Western Bohemia, Czech Republic). The mineral rarely occurs in blades up to 0.5 mm long, in association with other post-mining supergene uranyl sulfates and U-free sulfates. Rietveldite is orthorhombic, space group Pmn2 1 , a = 12.9577(9), b = 8. (58)(210) octahedra, which share vertices with SO 4 tetrahedra resulting in a heteropolyhedral sheet parallel to {010}; adjacent sheets are linked by hydrogen bonding only. The uranyl sulfate chains are the same as those in the structures of several other uranyl sulfate minerals. Rietveldite is named for Hugo M. Rietveld (1932Rietveld ( -2016.

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The structure of some alkaline-earth metal uranates

Cited by 150 publications

References 0 publications

Extinction in X-ray and neutron diffraction

Extinction in X-ray and neutron diffraction

The defect structure of reduced CaUO₄

Rietveldite, Fe(UO2)(SO4)2(H2O)5, a new uranyl sulfate mineral from Giveaway-Simplot mine (Utah, USA), Willi Agatz mine (Saxony, Germany) and Jáchymov (Czech Republic)

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