Preparation and Crystallographic Properties

The preparation of Pr3Se4 is analogous to that of La3 e4, Dernier et al. [6]. Values for the lattice constant a of Pr3Se4 from Dernier et al. [6], Bucher et al. [1,7], and Yarembash [2] are  

A cubic to tetragonal first-order structural transition is detected at Ttr = 30 K by resistivity measurements [1] and at 40 K by determination of the elastic stiffness constant Bucher et al. [7]. The probable space group of the tetragonal form is I42d-Dj, a = 8.844(3), 

The mechanism of the structural transition is not clear. Since Pr3Se4 as well as La3X4 (X = chalcogenide) show the transition, it cannot be due to a cooperative Jahn-Teller effect. For a discussion, see p. 83. Tt is a sensitive function of stoichiometry. The introduction of vacancies according to Pr2.96 00.04 64 reduces Tt from 40 to 30 K, Bucher et al. [7]. 

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C 4 UO2, Preparation and Crystallographic Properties C 5 UO2, Physical Properties. Electrochemical Behavior C 6 UO2, Chemical Properties (in preparation)... 

Intermediate Compositions of Pr3Se4 Pr2Se3 Solid Solutions Preparation and Crystallographic Properties... 

The most extensively studied oxide selenides belong to the M202Se type data on preparation and crystallographic properties dominate this section. Main topics of the sections on selenites, selenates, and associated alkali double compounds are the preparation of the compounds and their thermal decomposition. The selenide halides, especially the selenide fluorides of the type MSeF, show very interesting crystallographic properties. Main topics of this section are the polytypes of YSeF. 

Griessen, R. and Riesterer, T. (1988) Heat of formation models, in Hydrogen in Intermetallic Compounds I Electronic, Thermodynamic and Crystallographic Properties, Preparation, Vol. 63 (ed. L. Schlapbach), Springer Series Topics... 

The compounds (Et4N)4[M(NSCe)a] (M = Pa, U) have been characterized by comparing their spectral and crystallographic properties with the corresponding thiocyanate complexes (26). Some thorium(IV) complexes have been prepared containing DMF and N-bonded seleno-cyanate groups (326). Although various uranyl selenocyanate complexes have been reported, no structural data are available (686). 

Progress in the preparative and structural fields of protactinium chemistry has been rapid during the past 6 years and there is now sufficient information available, particularly in the halide and oxide fields, to permit a more balanced comparison than has previously been possible with the properties of the actinide elements, on the one-hand, and those of niobium and tantalum, on the other. In this connection one must, of course, bear in mind the fact that the ionic radii of protactinium in its various valence states [Pa(V), 0.90 A and Pa(IV), 0.96 A] are appreciably larger than those of niobium or tantalum and this itself will have a considerable influence on the chemical and crystallographic properties of the elements. 

Heat of formation models, in Hydrogen in Intermetallic Compounds I Electronic, Thermodynamic, and Crystallographic Properties, Preparation (ed. 

Almost all of the materials which have been discussed in this chapter share one main characteristic their intrinsic properties are closely connected to the numerous parameters of the experimental conditions of the preparation. Compared with pure element thin films, this sensitivity is enhanced by the fact that the samples possess at least two components. We have noticed that for some systems the large variation of the results has not always permitted one to reach a final conclusion. So, some of the physical, chemical and crystallographical properties and other phenomena are difficult to explain. Some of them are characteristic ... 

Griessen R, Riesterer T (1988) Heat of formation models. In Schlapbach L (ed) Hydrogen in intermetallic compounds I electronic, thermodynamic, and crystallographic properties, preparation, vol 63, Series topics in applied physics. Springer, Berlin, pp 219-284... 

An acyclic azide structure was rejected since the compounds show no azide IR absorption. A less likely three-membered ring was also considered. Comparison with other potential dihydro-SIV-thiatriazoles was made. Simple a-azido thioethers show the spectroscopic and chemical properties of azides rather than of dihydro-Slv-thiatriazoles. o-(Methylthio)phenyl azide was prepared but also showed the characteristics of an azide.70 These results cast doubt on the suggested structure, since the only essential difference in composition is an amino group that is not in a position to stabilize the suggested heterocyclic ring. Structure R1(R3R4N)C=N—N=N—SR2 is an alternative, but. further consideration must await X-ray crystallographic analysis. 

Work is well under way on the high resolution structure of the uninhibited nuclease as are attempts to prepare crystals with other mono-, di-, and trinucleotides of various types so that more insight may be gained into the actual workings of the nuclease. As described elsewhere in this volume (25), some of the most exciting prospects for the future are the correlations to he made between the crystallographic studies of the three-dimensional structure and the properties of the fragmented and synthetic versions of this nuclease. 

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