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

TL;DR

This study investigates the magnetic, dielectric, and structural properties of multiferroic KNiPO$_4$, revealing its antiferromagnetic order, ferroelectric transition at high temperature, and complex structural phase transitions.

Contribution

It provides detailed insights into the magnetic and structural phase transitions of KNiPO$_4$, including the first observation of a high-temperature ferroelectric transition and ligand rearrangement.

Findings

Antiferromagnetic order below 25 K with non-collinear structure.

Ferroelectric transition occurs around 594°C, changing crystal symmetry.

Structural phase transition at 469-488°C involving ligand rearrangement.

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 N\'{e}el 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 $Pna2_1$ 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 $T_\mathrm{0}\sim$ 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.