Crystal Structures of Inorganic Oxoacid Salts Perceived as Cation Arrays: A Periodic-Graph Approach - Inorganic 3D Structures - page 45

Fig. 7 The corundum ( -

Al 2 O 3 ) net ( R 3 c ) and some
related structures of oxoacid
salts. Anti -Corundum:
Hg 3 (BO 3 ) 2 and isostructural
M 3 (BO 3 ) 2 ( R 3 c ); normal
corundum: Al 2 (SO 4 ) 3 and
isostructural M 2 (SO 4 ) 3 ( R 3)

a

Fig. 8 Topological
resemblance between K 2 Te 3
and Mg 3 (BO 3 ) 2 crystal
structures

3:2. The orthoborates of the Ca 3 (BO 3 ) 2 family can be considered as based
on a distorted anti-corundum (cor) motif M 3 B 2 if one takes into account the four
strongest M-O bonds out of eight oxygens surrounding the M 2+ cation (Fig. 7 ) [ 48 ] .
The four remaining M-O contacts give rise to additional connections M-B in the
cation array and to a new 6,9-coordinated underlying net derived from cor. The
series M 3 (BO 3 ) 2 (M

g ¼

Mg, Mn, Ni, Cd) with octahedrally coordinated M atoms is
related to the K 2 Te 3 crystal structure [ 49 ] if one ignores Te-Te bonds (Fig. 8 ) . The
a

¼

-Zn 3 (BO 3 ) 2 polymorphs [ 50 ], being unique structure types, are topologically
close to the cor net since their topological indices are similar (Table 5 ) . One can
conclude that the differences are caused by the appearance of eight-cycles in the
underlying nets of the polymorphs compared to the cor net.
g ¼

, b

3:1. These cation-rich structures of M 3 BO 3 compounds are also tend to have
antitypes as underlying nets; thus, the cation array Tl 3 BinTl 3 BO 3 have the
topology of 3,9-coordinated UCl 3 net (Fig. 9 ). The Ag 3 B arrays in two Ag 3 BO 3
phases (obtained at ambient and high pressure) are the first examples of 4,12-
coordinated epinet sqc 908.

3.1.3 Nitrates

g ¼

1:1. At ambient conditions the MNO 3 nitrates also show a strong dependence
of the underlying net topology on the size of the M + cation. Like carbonates