Magnetic structure of melilite-related centrosymmetric K_2CoP_2O_7: neutron diffraction and DFT study

Abstract

A great number of X_2(ZT_2)O_7 compositions with large X=Ln, Ca-Ba, Na, K and small Z and T=Be,Al,Si,Ga,Ge,P,V adopt the melilite crystal structure type. The structure is built of tetrahedral layers of five-member rings and X is located in the interlayer space. Typically, the materials crystallize in the non-centrosymmetric P-42_1m space group. The compositions with magnetic ions (e.g. Z=Mn"2"+,Fe"2"+,Co"2"+,Cu"2"+) are very interesting systems from the magnetic structure point of view due to the 2D character of the structure and lack of inversion center. Depending on chemistry and geometry of a particular composition, a variety of magnetic structures were reported, from simple 3D antiferromagnets to 2D spirals and multiferroics. During our investigations of melilite-type materials a K_2CoP_2O_7 composition was prepared and its magnetic property and structure characterized. The material is built of melilite-type tetrahedral layers but crystallizes in centro-symmetric P4/mnm space group. The magnetic structure has been determined using neutron powder diffraction on the Echidna diffractometer (ANSTO) and further explored with DFT. The results will be presented and discussed in the context of the broad melilite structural family.