  The behaviour of alkaline carbonates at high pressure is poorly understood.
Indeed, theoretical and experimental investigations of general trends of
pressure induced structural changes appear in the literature only sporadically.
In this article we use a combination of ab-initio calculations and
high-pressure experiments in diamond anvil cell to determine crystal structures
of high-pressure phases of K2CO3. The comparison with experimental data on
Li2CO3 allows to reconstruct the common structural trend, which is consistent
with the simple rule that the structure of high-pressure polymorph is the same
as the ambient structure of a heavier element compound from the same group of
the periodic table. The correctness of the constructed trend is confirmed by
the perfect consistency with pressure-induced structural transformations in a
wide range of binary A2B compounds.
