Order/disorder in YbNi1±xGa2∓x (x ≤ 0.08): Crystal structure, thermal expansion and magnetic properties

Abstract

The ternary phase YbNi1±xGa2∓x (x ≤ 0.08) has been synthesised from the elements by high-frequency melting in argon atmosphere with subsequent annealing at 600 °C or 800 °C. From X-ray powder and single crystal diffraction investigations the phase is found to be isotypic with MgCuAl2: space group Cmcm, Z = 4, a = 4.0987–4.1147 Å, b = 9.8516–9.8620 Å, c = 6.5995–6.5916 Å for the specimens annealed at 800 °C. The quantum chemical analysis of atomic interactions in YbNiGa2 shows Ga and Ni forming a network polyanion with covalent bonds, while ytterbium cations are located within the cavities of the polyanion. From the magnetic susceptibility data and LIII XAS spectra, the 4f13 electronic configuration of ytterbium (Yb3+) was established in samples with nickel excess (comparing to x = 0), whereas an additional small contribution of the 4f14 configuration is found for compositions with the gallium excess. No change in the ytterbium valence state was detected down to 5 K. In situ low- and high-temperature powder diffraction investigations applying both X-ray synchrotron radiation and neutrons revealed no change in structure within the broad temperature range 3.4–1173 K. Both synchrotron and neutron diffraction data show smooth, but strongly anisotropic variation of lattice parameters below 200 K: the thermal contraction ratio in [100], [010] and [001] directions is 4.5:3.2:1. The temperature dependence of atomic displacement parameters indicates a certain amount of static disorder for all atomic species in the crystal. At least two distinct sites with varying charge distributions in the vicinity of the Ga atoms are confirmed by 69,71Ga NMR spectroscopy.