  Using molecular dynamics simulation we have investigated the influence of
random pinning on the phase diagram and melting scenarios of a two-dimensional
(2D) system with the Hertz potential for $\alpha=5/2$. For the first time it
has been shown that random pinning can cardinally change the mechanism of
first-order transition between the different crystalline phases (triangular and
square) by virtue of generating hexatic and tetratic phases: a triangular
crystal to hexatic transition is of the continuous
Berezinskii-Kosterlitz-Thouless (BKT) type, a hexatic to tetratic transition is
of the first-order, and finally, a continuous BKT type transition from tetratic
to the square crystal.
