Answer
$(1,1,-2,1) or (1,1,\bar2,1)$
$(0,-1,1,2)or(0,\bar1,1,2)$
Work Step by Step
For converting the $(1,1,1)$ and $(0,\bar1,2)$ planes into the four index Miller–Bravais scheme for hexagonal unit cells.
The four index Miller–Bravais scheme for hexagonal unit cells be (h,k,i,l)
and we have given the value of (h,k,l)
to find the value of i
$i=-(h+k)$
So for the value $(1,1,1)$
$i=-(1+1)=-2$
The four index Miller–Bravais scheme for hexagonal unit cells is $(1,1,-2,1) or (1,1,\bar2,1)$
For value $(0,-1,2)$
$i=-(0-1)=1$
The four index Miller–Bravais scheme for hexagonal unit cells is $(0,-1,1,2)or(0,\bar1,1,2)$
