Crystal Structures of Inorganic Oxoacid Salts Perceived as Cation Arrays: A Periodic-Graph Approach - Inorganic 3D Structures - page 43

Fig. 5 Most typical
coordination types of
triangular and trigonal-
pyramidal oxoanions

Table 3 Space-group symmetries for M 2 CO 3 polymorphs
M atom

a -phase b -phase g -phase

Na P 6 3 / mmc C 2/ m C 2/ m

P 6 3 / mmc C 2/ c P 2 1 / c

K

P 6 3 / mmc Pnma P 2 1 / c

Rb

P 6 3 / mmc Pnma P 2 1 / c

Cs

sqp (SnP) net; both Cu and carbonate ions are topologically equivalent in the
underlying net.
g ¼ 2:1. The thermal polymorphism gives three phases for M 2 CO 3 (M ¼ Na, K,
Rb, Cs) with a great variety in space-group symmetries [ 44 , 45 ] (Table 3 ). Strictly
speaking, these structures cannot be considered as oxygen-stuffed: owing to the
large g ratio and large size of the alkali atoms, not all cations are shielded by the
oxygens. This leads to abnormal coordination numbers of some alkali atoms and to
an essential distortion of their environment. As a result, in all phases there is a large
room around the abnormally coordinated alkali atoms; different size of this room
gives rise to the structural differences of the polymorphs.
However, all these phases can be described in terms of the same underlying net
of the Ni 2 In structure type (Fig. 6 ), a net of great importance for this stoichiometric
g ratio (see [ 7 ] and more examples below). For the ionic model, it is important that
there is a binary ionic compound, the high-pressure phase of BaF 2 , which has the
same topological motif [ 46 ] . Thus, all varieties of these polymorphs are caused by
different distortions of the same cation array.
The Li 2 CO 3 polymorphs show the typical trend for baric polymorphism: the under-
lying net becomes topologically denser in the high-pressure phase. Thus, at ambient
conditions the cation array Li 2 C follows the anti-fluorite (Li 2 O) 4,8-coordinated
motif, but above 10 GPa it transforms to the 5,10-coordinated AlB 2 -like net.