  The coordination polyhedra around the cations are the building blocks of
ionic solids. In context of amorphous InGaZn oxide (a-IGZO), even though the
coordination polyhedra are irregularly arranged, it will be beneficial to
identify them. In this work, we address the questions, (a) is it possible to
classify all the polyhedra that occur in a-IGZO into only a few distinct
groups? and (b) are these the same polyhedral motifs as those observed in the
crystalline indium gallium zinc oxide (c-IGZO) or other related crystalline
oxides of indium, gallium and zinc? Therefore, in this first principles based
study, a large number (ten) of equivalent samples of a-IGZO were prepared by ab
initio melt-and-quench molecular dynamics, so that several distinct samples of
the amorphous landscape are obtained. The combination of all these structures
thus obtained is a better representation of a real a-IGZO sample. For the ten
samples containing 360 cations, we propose a simpler and more accurate method
for determining the coordination number of each polyhedron, which was verified
by charge density plots. Based on a method of comparing bond angles between
metal and oxygen atoms, the identified polyhedra were matched to the polyhedral
motifs present in the related crystalline systems, such as, InGaZnO4, In2O3,
Ga2O3 and ZnO. We find, the a-IGZO primarily consists of the following
polyhedra: a tetrahedron from space group 199 and an octahedron from space
group 206 of In2O3; a tetrahedron from space group 12 and an octahedron from
space group 167 of Ga2O3; a tetrahedron from space group 186 of ZnO; zinc and
gallium trigonal bipyramids from c-IGZO; and one zinc 4-fold, one zinc 5-fold
and one indium 5-fold coordination polyhedra that occur only in the amorphous
phase. We are able to reduce the description of structure from 360 to 10 groups
of polyhedra. The benefits of this identification could be enormous.
