A study on AB 2O 6compounds, Part IV: on segregation, electrostatic imbalance and connectivity formulas

Abstract

In continuation of earlier parts of this series on AB 2 O 6 compounds we discuss the influence of electronegativity differences on the formation of rosaite -or columbite-type structures. We also compare volume increments in the structures of several AB 2 O 6 compounds with those of the constituting binaries. We apply an advanced treatment of the co-ordination rule to derive a measure of segregation of A and B ions within the oxygen matrix and finally also connectivity formulas giving insight into structural details beforehand, and we venture to use such procedures for structure prediction when only co-ordination numbers can be "guessed on reasonable grounds" from our crystal chemical intuition.

The co-ordination number rule and the rule of hardness, powerful tools to rationalize inorganic structures

“…In SrV 2 O 6 (r = 3.16) we find also O [2] instead of only [3] and [4]. (HgV 2 O 6 and o-BaNb 2 O 6 -I are further examples in this family (see [30,31] leads us to the formulation of another rule of crystal chemistry. The r value 2.89 is realized by fewer O [2] than expected and an O [4] instead.…”

“…In this paper we examine the co-ordination rule and the rule of hardness in their consequences in very detail, and we find that this gives good insight into complex structures a posteriori. Small steps a priori may also be taken as shown in our earlier papers [30,32]. One may even think of developing algorithms to calculate possible combinations of p,q, etc.…”

“…In the case of FeNb 2 O 6 we will then speak of a type of segregation which is not caused by "numeric problems" (they could be solved otherwise as in the rosiaite structure) but by a "chemical" effect, i.e., Σ chem . (We have discussed reasons for the choice of different structures among AB 2 O 6 compounds in earlier papers [30,31]. )…”

“…However, the coordination is different in the series Ca [6] V 2 [5] O 6 , Sr [9] V 2 [5] O 6 and Ba [8,10] V 2 [4] O 6 . We have dwelled on this subject in previous papers dealing with such AB 2 O 6 compounds [30,31,32] and coined what we call the rule of mutual influence of different cations on their seeming size and polarity. The "size" of an ion -and therefore its seemingly structure determining property -depends on the chemical relations with its neighbors.…”

A study on AB2O6compounds, Part V: a DFT study on charge balance as driving force for structural organisation

“…Compounds of composition AB 2 O 6 exhibit a variety of different structures, in previous papers [1][2][3] we have described more than 30 of them. Some of them may be grouped into a so-called hcp family since they represent variants of filling part of the octahedral voids in a hexagonal close-packed array of oxide ions.…”