EuNi2P4, the first magnetic unconventional clathrate prepared via a mechanochemically assisted route†
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) Å3 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.