Crystal Structures of Inorganic Oxoacid Salts Perceived as Cation Arrays: A Periodic-Graph Approach - Inorganic 3D Structures - page 42

Fig. 4 The NiAs net ( P 6 3 /
mmc ) and some related
structures of oxoacid salts.
Anti -NiAs: CaCO 3 (aragonite)
and isostructural MCO 3 ,MBO 3
( Pnma ); normal NiAs: MgSeO 3
and isostructural MSeO 3 ,NaIO 3
( Pnma ); FeSO 4 and isostruc-
tural MPO 4 ,MSO 4 ,MSeO 4
( Cmcm )

Thus, MgCO 3 exists only in the calcite form; for CaCO 3 this form is stable at
ambient conditions, while MCO 3 (M Sr, Ba, Pb) crystallize in the aragonite type.

This tendency is kept for carbonates of d - and f -metals: MCO 3 (M

¼

Mn, Fe, Co,
Ni, Zn) belong to the calcite type, while EuCO 3 has the aragonite-like structure.
Calcite-like disordered high-temperature phases are known for SrCO 3 and BaCO 3
but they are not quenchable [ 40 ]. High-pressure phases of CaCO 3 (post-aragonite
[ 41 ]) and BaCO 3 [ 42 ] have topologically denser 8,8-coordinated CsCl (bcu) and a
novel binodal 9,9-coordinated underlying net, respectively. One more metastable
phase of CaCO 3 , the mineral vaterite [ 43 ] , represents an antitype to aragonite, i.e.,
M 2+ and CO 3 2- ions are topologically equivalent to Ni and As atoms in the NiAs-
like underlying net.
The differences in topologies of the calcite and aragonite underlying nets are
reflected by different coordination types of carbonate ions: T 6 and T 33 , respectively
(Fig. 5 ). In both cases, the carbonate ion is connected to six M cations but forms
different numbers of M-O bonds: 6 and 9, respectively. As a result, the coordina-
tion numbers of cations are larger in the latter case that conforms to large sizes of
the cations in the aragonite type. Since the oxoanion has very flexible coordination,
it can fit any coordination numbers of cations. An example is the crystal structure of
CuCO 3 , where Cu atoms have CN

¼

¼ 7 but the resulting topology is 5-coordinated