Crystal Structures of Inorganic Oxoacid Salts Perceived as Cation Arrays: A Periodic-Graph Approach - Inorganic 3D Structures - page 61

5. In some cases, the cation array in an oxoacid salt corresponds to the underlying
net of a metal-organic compound; no similar inorganic binary compound has
been found so far. This could be a challenge to synthetic chemists to obtain such
analog. This fact also shows that underlying nets in inorganic and metal-organic
compounds have common topological properties.
The eutactic distribution of cations in the array is a consequence of their
tendency to a uniform arrangement in the crystal space. This trend is influenced
by the following features of the array:
1. The most preferable topological motifs (packing nets) for the cation arrays are
b.c.c. and close packings; in most cases, the b.c.c. motif is the most uniform.
Hence, the main structure-forming role in the oxoacid salts belongs to cations;
the eutactic oxygen arrays are rather rare.
2. Increase in size and charge of the cations results in an increase in the array
uniformity.
3. High temperature and pressure favor increase in the uniformity of the structure-
forming (most uniform) cation array.
4. Increase in the uniformity changes the topological motif of the cation array: it
tends to be b.c.c. The increase in the uniformity of the structure-forming array
accompanies decrease in the uniformity of other ion sublattices [ 17 ] .
These regularities seem to be valid not only for oxygen-stuffed cation arrays but
also for ionic compounds containing other anions [ 86 ] .
5 Concluding Remarks
The periodic-graph approach now is well developed for metal-organic coordination
compounds [ 19 ] and can give fruitful results when analyzing typical inorganic
substances. The main advantages of this approach over visual (even computer-
aided) exploration of crystal structures are as follows: (1) its algorithms are formal,
strict, and universal, and they are used for any crystal structure according to a
common scheme; (2) the topology of an atomic net is unambiguously determined
by a set of numerical indices; (3) the concept of the underlying net allows one to
easily classify crystal structures of complex compounds by reducing them to
simpler topologies and using computer databases of reference nets; (4) the concept
of packing net formalizes the analysis of cation arrays: they are classified with the
topological indices; and (5) the topological software provides fast processing
hundreds and thousands of structures in a reasonable time.
The recent results of the topological analysis of cation arrays in the inorganic
crystal structures described in this review underline the importance of the model of
cation array and show that the number of preferable (“default”) topological motifs
is rather small. We have good cause to believe that this fact, as well as the
developed software, opens the door to a comprehensive taxonomy of inorganic