The crystal structure of a K2NiF4-type oxides, such as LaSrAlO4 and Sr2TiO4, consists of an alternate stacking of rock-salt AO- and perovskite ABO3-layers. It can also be described as a Ruddelsden-Popper phase An-1A’2BnO3n+1 with n=1.

Where can I found the CIF file of K2NiF4?

try the ICSD database at 

https://cds.dl.ac.uk/cds/datasets/crys/icsd/llicsd.html

Thank you very much, Thomas Walther!