Condensed phosphates crystal structure

Structures of ternary condensed phosphates The structures of alkali rare earth condensed phosphates can be divided into fourteen different types (table 1). The formation of a particular structure depends on the crystallization conditions as well as on the alkali and rare earth metals concerned (fig. 21). Characteristic of all alkali rare earth ternary polyphosphates is that the structural framework consists of a helical chain of (P03) , formed by corner sharing of PO4 tetrahedra along some axis. Besides the polyphosphates, there are cyclic condensed phosphates containing the cyclic P40 2 group. 

Molecules of PNO can be obtained by co-condensation of O3 and PN in a noble gas matrix at low temperature [23]. Like PN(NH), (Chapter 7.2) it has a cristobalite-type crystal structure and can be obtained from P3N5 and ammonia. Solid oxynitrides are of interest because of their relationship to the condensed phosphates (Chapter 5.7). 

Metaphosphate glasses are of particular interest in the development of glass-ceramics and glass-ceramic fibers (Griffith 1995) by the controlled crystallization of phosphate base glasses. These glasses are composed of (PO ) tetrahedra chains. In chemical terms, these structural units must be called condensed phosphates. 

Phosphate chemistry and silicate chemistry are much alike, but they are also very dissimilar. It was the expressed purpose of this work to prepare crystals of phosphates with properties similar to silicates referred to as asbestos, by substituting phosphorus atoms for silicon atoms in molecular structures like those found in serpentine minerals. It was hoped that many useful properties of silicates could be gained while retaining the long safety record of condensed phosphates. 

The crystal structure of an ammonium cerium condensed phosphate, whose composition corresponds either to NH4Ce(P03)5 (Ce(IV)) or to NH4HCe(P03)5, has been determined by Palkina et al. (1981e). Complex infinite PO4 tetrahedra chains are held together by the cations and cerium has the coordination number eight. 

The addition of phosphoric acid to freshly made antimonic acid (see Section 4.1.3) prevents precipitation of the pyrochlore phase and allows stabilization of phosphatoantimonic high polymers as sols [28]. Incorporation of phosphates in the high polymers alters their crystal structure [29] and leads to a significant reduction in their size and quantity, to the benefit of phosphatoantimonic polyanions. The ratio P/Sb = 0.5 is the threshold for the existence of high polymers in solution. The composition and degree of condensation of the polyanions are functions of the pH... 

The present review is mainly concerned with the crystal chemistry of compounds containing phosphorus(V) bound to oxygen and nitrogen forming P(N, 0)4 tetrahedra. Thus, ammonium phosphates will be omitted as well as hydrazine and carbon containing compounds. We will focus on the structure of molecular, molecular ionic, and condensed PON compounds. Some aspects of these compounds have been summarized in the literature in the past (13-16). 

Samples of tridimensional microporous zinc phosphates with fully connected framework with structures type FAU and CZP have been prepared. The influence of some synthesis parameters on the nature of the crystalline phases obtained, such as crystallization time and temperature, concentration of the different reactives or pH, are discussed. Physicochemical characterization of the pure samples has been performed by different techniques ICP, XRD, C and P MAS-NMR, TG, TG-MS, in-situ thermal XRD analysis and SEM. These materials were tested in the Knoevenagel condensation of different esters and benzaldehyde. They have demonstrated a good selectivity and a higher activity than the basic zeolites previously described in the literature with these reactions. 

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