Comparative Optical Absorption Studies of K2NiF4 and KNiF3

Abstract
The absorption spectrum of K2NiF4 crystal has been measured at 77°K and compared with that of KNiF3 from the point of view of the magnetic ordering in the crystals. The smaller shifts of band maxima with temperature in K2NiF4 are attributed to the smaller Néel temperature of the crystal. The extra line observed in the 1 E g a band is compared with similar line in KNiF3 and the molecular field approximation is found to give a reasonably good account of the observed results. The absorption bands in the ultraviolet region arising from the simultaneous electronic excitation of two nickel ions are discussed.