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Polytypism of AlPO4•2H2O variscite
No full textVariscite (orthorhombic), a hydrated aluminophosphate with formula Al(PO4)•2H2O, and metavariscite (the monoclinic polymorph) are uncommon secondary minerals that are relevant in environmental applications and particularly in technological applications when synthesized in a dehydrated form. Although two orthorhombic modifications are known (so-called "Lucin-type" and "Messbach-type"), the fine-grained nature of the "Messbach-type" variscite has hampered determination of its crystal structure.In this contribution, the crystal structure of the latter (a natural specimen from Tooele County, Utah) has been solved and refined using X-ray powder diffraction data via ab initio charge-flipping methods and the Rietveld method. Our results, structural interpretations, and topological analysis demonstrate that the two orthorhombic structural modifications are polytypes, and we refer to them as variscite1O ("Lucin-type") and as variscite2O ("Messbach-type"), to be consistent with modern polytype terminology. The structure of variscite2O is similar to that of the 1O polymorph, with a doubling of the b unit-cell parameter. The variscite2O crystal structure contains two crystallographically independent Al3+ cations coordinated by two H2O molecules and four oxygen atoms of the PO4 groups. Two crystallographically independent PO4 tetrahedra share their corners with four adjacent AlO4(OH2)2 octahedra. Both orthorhombic polymorphs belong to the family of framework 3D MT structures in which octahedra (M) and tetrahedra (T) are linked by bridging O atoms, and topological analysis suggests that these two structures may be considered polytypes. Similarities between these polytypes, along with observed broadening of diffraction peaks of the Tooele material, suggest that interstratifications of the two forms may exist in nature. Besides the long-range characterization of the crystal structure by X-ray diffraction, information on the short-range structural properties of this mineral have been gained through 31P and 27Al MAS/NMR measurements. Results from NMR corroborate the crystal structure determination and they show distinct signals for each of the two independent P and Al positions in variscite2O. In addition, high-temperature XRD, thermal analyses, and NMR measurements clarified the nature of the transformation of variscite2O to the derivative AlPO4 structure. The crystal structure of this new anhydrous AlPO4 phase (AlPO4-variscite2O in analogy to its parent structure) can be described as a 3D framework of alternating AlO4 and PO4 tetrahedra linked by bridging O atoms. Thermogravimetric analyses revealed almost complete dehydration above ~450K, and NMR results were consistent with Al and P atoms located at tetrahedral sites
Polytypism of AlPO4•2H2O variscite
No full textVariscite (orthorhombic), a hydrated aluminophosphate with formula Al(PO4)•2H2O, and metavariscite (the monoclinic polymorph) are uncommon secondary minerals that are relevant in environmental applications and particularly in technological applications when synthesized in a dehydrated form. Although two orthorhombic modifications are known (so-called "Lucin-type" and "Messbach-type"), the fine-grained nature of the "Messbach-type" variscite has hampered determination of its crystal structure.In this contribution, the crystal structure of the latter (a natural specimen from Tooele County, Utah) has been solved and refined using X-ray powder diffraction data via ab initio charge-flipping methods and the Rietveld method. Our results, structural interpretations, and topological analysis demonstrate that the two orthorhombic structural modifications are polytypes, and we refer to them as variscite1O ("Lucin-type") and as variscite2O ("Messbach-type"), to be consistent with modern polytype terminology. The structure of variscite2O is similar to that of the 1O polymorph, with a doubling of the b unit-cell parameter. The variscite2O crystal structure contains two crystallographically independent Al3+ cations coordinated by two H2O molecules and four oxygen atoms of the PO4 groups. Two crystallographically independent PO4 tetrahedra share their corners with four adjacent AlO4(OH2)2 octahedra. Both orthorhombic polymorphs belong to the family of framework 3D MT structures in which octahedra (M) and tetrahedra (T) are linked by bridging O atoms, and topological analysis suggests that these two structures may be considered polytypes. Similarities between these polytypes, along with observed broadening of diffraction peaks of the Tooele material, suggest that interstratifications of the two forms may exist in nature. Besides the long-range characterization of the crystal structure by X-ray diffraction, information on the short-range structural properties of this mineral have been gained through 31P and 27Al MAS/NMR measurements. Results from NMR corroborate the crystal structure determination and they show distinct signals for each of the two independent P and Al positions in variscite2O. In addition, high-temperature XRD, thermal analyses, and NMR measurements clarified the nature of the transformation of variscite2O to the derivative AlPO4 structure. The crystal structure of this new anhydrous AlPO4 phase (AlPO4-variscite2O in analogy to its parent structure) can be described as a 3D framework of alternating AlO4 and PO4 tetrahedra linked by bridging O atoms. Thermogravimetric analyses revealed almost complete dehydration above ~450K, and NMR results were consistent with Al and P atoms located at tetrahedral sites
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Distribution of industrial minerals in Sardinia (Italy): clinoptilolite bearing rocks of the Logudoro region
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