All atomically laminated MAB phases
(M = transition metal, A =
A-group element, and B = boron) exhibit orthorhombic or tetragonal
symmetry, with the only exception being hexagonal Ti
2
InB
2
. Inspired by the recent discovery of chemically ordered hexagonal
carbides, i-MAX phases, we perform an extensive first-principles study
to explore chemical ordering upon metal alloying of M
2
AlB
2
(M from groups 3 to 9) in orthorhombic and hexagonal symmetry.
Fifteen stable novel phases with in-plane chemical ordering are identified,
coined i-MAB, along with 16 disordered stable alloys. The predictions
are verified through the powder synthesis of Mo
4/3
Y
2/3
AlB
2
and Mo
4/3
Sc
2/3
AlB
2
of space group
R
3̅
m
(no. 166), displaying the characteristic in-plane chemical order
of Mo and Y/Sc and Kagomé ordering of the Al atoms, as evident
from X-ray diffraction and electron microscopy. The discovery of i-MAB
phases expands the elemental space of these borides with M = Sc, Y,
Zr, Hf, and Nb, realizing an increased property tuning potential of
these phases as well as their suggested potential two-dimensional
derivatives.