Revisiting the magnetic and crystal structure of multiferroic KNiPO$_4$

The magnetic, dielectric and structural properties of type-I multiferroic KNiPO 4 have been inves-tigated by neutron powder diffraction, magnetization, dielectric and high temperature synchrotron-XRD measurements. Below the N´eel transition of T N = 25 K, KNiPO 4 displays a weakly non-collinear antiferromagnetic (AFM) structure with the orientation of the Ni 2+ magnetic moments mainly along a axis. The compound crystallizes in the polar orthorhombic Pna 2 1 space group at room temperature. A second-order structural phase transition corresponding to the onset ferroelectricity is observed at around T C ∼ 594(3) ◦ C, above which the crystal structure of KNiPO 4 adopts the centrosymmetric Pnma space group. The compound also displays another structural phase transition at T 0 ∼ 469 – 488 ◦ C, with a first-order character, which is attributed to the rearrangement of oxygen ligands, resulting in a change in the nickel ion co-ordination from four to five.