EuNi2P4, the first magnetic unconventional clathrate prepared via a mechanochemically assisted route

Abstract

For the first time, a magnetic unconventional clathrate EuNi₂P₄ has been prepared from the elements via a combined mechanochemical and solid-state route. Its crystal and electronic structure, magnetic and transport properties as well as lattice dynamics were elucidated. It crystallizes in the orthorhombic space group Fddd (Z = 8) with unit cell parameters a = 5.1852(1) Å, b = 9.4834(1) Å, and c = 18.9893(2) Å, V = 933.78(1) ų at room temperature. Its crystal structure can be described as a covalent Ni–P framework forming a twisted Kelvin cell with Eu²⁺ guest cations occupying the voids of this framework, which is supported by chemical bonding analysis based on the electron localization function topology. Eu²⁺ guests sit inside the oversized cages with a coordination number of 24 and exhibit strong rattling, which manifests in both thermodynamic properties and Raman spectra. The compound is metallic and exhibits a rather low thermal conductivity of ∼4 W K⁻¹ m⁻¹ at high temperature along with a positive Seebeck coefficient. The magnetism of EuNi₂P₄ is predetermined by Eu²⁺ (4f⁷) with dominant antiferromagnetic interactions. According to the magnetic susceptibility, heat capacity and resistivity measurements, EuNi₂P₄ undergoes three phase transitions at ∼2.5 K, 6.1 K and 11.3 K in zero magnetic field.