X-ray diffraction study of phonon and magnon properties of Eu2Cu6P5 ferromagnet

Abstract

Ternary europium copper phosphide Eu₂Cu₆P₅ was synthesized from the elements by the standard ampule method as a phase-pure product and studied by means of X-ray powder diffraction. Temperature dependencies of the tetragonal unit cell parameters and volume revealed pronounced anisotropy. The observed anomalies on the a(T) and c(T) curves at about TC ≈ 36 K were related to magnetic phase transition in the phosphide under study. The linear spontaneous magnetostrictions in the basal plane and in the direction of the c axis have different signs that indicates a pronounced anisotropy of the thermal expansion at the temperatures of magnetic ordering. Temperature changes of the unit cell volume Vu(T) were analyzed within the Debye-Einstein approximation. The parameters of the model applied (Debye and Einstein characteristic temperatures, shares of the correspondence contributions) were determined. It was shown that the ratio of the shares of the Debye and Einstein contributions to the thermal characteristics are 0.85/0.15; consequently, in the region of moderately low temperatures, the thermal properties of Eu2Cu6P5 are determined mainly by the Debye vibrations of the Cu/P framework. Experimental data on thermal expansion and calculated heat capacity were used to calculate the temperature-dependent Grüneisen parameter, γ(T), for the further analysis of the lattice dynamics of Eu2Cu6P5. The revealed monotonic growth of γ(T), which is a measure of the anharmonicity of lattice vibrations, indicates an increasing influence of anharmonicity on the thermal properties of Eu₂Cu₆P₅. The anisotropy of the vibrations of europium atoms, as well as local regions of structural heterogeneity of the sample that leads to the appearance of local stresses in the crystal structure can be the reasons for the increasing influence of anharmonicity.