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

Abstract

The magnetic, dielectric and structural properties of type-I multiferroic KNiPO${}_4$ have been investigated by neutron powder diffraction, magnetization, dielectric and high temperature synchrotron-XRD measurements. Below the Néel transition of $T_{\mathrm{N}}$ = 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 $T_{\mathrm{C}}\sim$ 594(3)${}^{\circ }$C, above which the crystal structure of KNiPO${}_4$ adopts the centrosymmetric $Pnma$ space group. The compound also displays another structural phase transition at $T_0\sim$ 469 -- 488${}^{\circ }$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.