Theoretical and Experimental Studies of the Structural Chameleon EuYCuTe₃

Layered orthorhombic single crystals of EuYCuTe₃ are synthesized using the ampoule method from the elemental precursors taken in the ratio of 1 Eu:1 Y:1 Cu:3 Te by heating up to 1120 K with an excess of CsI as flux. The orthorhombic structure of EuYCuTe₃ is established, and structural parameters are obtained using X-ray diffraction. At ambient conditions, the sample crystallizes in the space group Pnma with the unit cell parameters a = 11.2730(7) Å, b = 4.3214(3) Å, c = 14.3271(9) Å. The structure is composed of vertex-connected [CuTe₄]^7- tetrahedra, which form chains along the [010] direction, and of edge-connected [YTe₆]^9- octahedra, which form layers parallel to the (010) plane. The Eu²⁺ cations are found in a capped trigonal prismatic coordination of Te^2- anions. The structural phase transition from the α to the β phase is discovered upon heating the sample to 323 K, which comes accompanied with a decrease of [CuTe₄]^7- tetrahedral distortion. The symmetry of the high-temperature phase is established as ordered in the space group Cmcm (a = 4.3231(3) Å, b = 14.3328(9) Å, c = 11.2843(7) Å). The nature and microscopic mechanism of the phase transition is discussed. By cooling it down below 3 K, the soft ferromagnetic properties of EuYCuTe₃ are discovered. The correlation of the ferromagnetic transition temperature in the series of chalcogenides EuYCuCh₃ (Ch = S, Se, Te) with the ionic radius of the chalcogenide anion is established. The structural dynamical elastic properties of α- and β-EuYCuTe₃ were calculated within the ab initio approach. The vibrational mode frequencies and decomposition on irreducible representations, as well as the degree of ion involvement in each mode, were determined. The calculations reveal an imaginary mode in the Y-point of the Brillouin zone in the high symmetry β-EuYCuTe₃ phase. This finding explains the nature of structural reconstruction in EuYCuTe₃ crystal as a second-order phase transition induced by soft mode condensation at the edge of the Brillouin zone. The exfoliation of a single layer is simulated theoretically. The exfoliation energy is estimated, and the dynamical properties of EuYCuTe₃ single layers are studied.