Isomorphism and allotrophy in compounds of the type A₂XO₄

Abstract

The solubility of silver acetate at 25°C. in aqueous solutions containing various amounts of silver nitrate and silver perchlorate has been determined.2. The results obtained are interpreted by means of the theory of Debye and Hückel and by assuming the formation of diacetato-argentate ion and of acetatodisilver ion.3. The dissociation constants of these complex ions have been determined. 4. It is suggested that the stability of one of these ions is due to resonance. 5. The true activity product for silver acetate has been calculated. REFERENCES

Structure of the low-temperature variety of calcium sodium orthophosphate, NaCaPO4

“…The existence of low-temperature varieties of NaCaPO4 and NaSrPO 4 has been previously reported, but no precise information has been given concerning their structural characteristics (Klement & Kresse, 1961;Bredig, 1942;Paques-Ledent, 1972). Bredig (1942) had proposed for NaCaPO4 a fl-K2SO4-type structure, but Engel & Aalen (1976) observed superlattice reflections on the powder pattern and indexed them by isotypy with a~-CaESiO 4, the cell parameters of which are related to those of fl-KESO 4 by the following relations: a -~ 2C~_K2SO 4, b _~ 2b/~_K2SO 4, c ~_ afl_K2SO 4.…”

“…Bredig (1942) had proposed for NaCaPO4 a fl-K2SO4-type structure, but Engel & Aalen (1976) observed superlattice reflections on the powder pattern and indexed them by isotypy with a~-CaESiO 4, the cell parameters of which are related to those of fl-KESO 4 by the following relations: a -~ 2C~_K2SO 4, b _~ 2b/~_K2SO 4, c ~_ afl_K2SO 4.…”

Structure reinvestigation of the high-temperature form of K2SO4

“…Two modifications of potassium sulphate, K2SO4, were reported by Bredig (1942), Bernard & Hocart (1961) and Fischmeister (1962). The first-order phase transition between them occurs at 860 K. The space group and structure of the low-temperature form are well established (Robinson, 1958;McGinnety, 1972).…”

“…The space groups reported for the high-temperature form of K2SO 4 were inconsistent, Bredig (1942) and Eysel & Hahn (1970) proposed the space groups P3m 1 and P63mc, respectively, from powder X-ray diffraction analyses. Iwai, Sakai & Watanab+ (1973) carried out a structure analysis using a high-temperature Weissenberg technique.…”

Dicalcium silicate solid solutions

“…The possibility of a solid solution of N a20 and Al20 a or of N a20 and Fe20a in a 2CaO.Si02 had been considered, but the sharp disagreement with the optical properties of "a 2CaO.Si02", as reported by Rankin and Wright [1j1 and with the diffraction pattern as given by Hansen [2], was unexplained. The clue to the answer appeared in a recent paper by Bredig [3] on compounds of the type A2X04, to which 2CaO.Si02 belongs. Using data obtained on solid solutions of 2CaO.Si02 and 3CaO.P205, and by analogy to other similar structures among compounds such as the alkali sulfates, he presents strong evidence for the existence at high temperatures of a hexagonal crystal structure of 2CaO.Si02• Bredig assumes that this form is unstable at room temperature, and when in a pure state, always inverts upon cooling to one of the lower-temperature forms of 20aO.Si02• He contends that this hexagonal form may be maintained at room temperature by foreign material in solid solution.…”