Magnesium-containing polyanion and intermediate europium valence: crystal structure, chemical bonding, and properties of EuMg_1−xTl_1+x(x= 0.013−0.058)

Abstract

The ternary europium-magnesium-thallium phase EuMg_1-xTl_1+x (x = 0.013-0.058) was synthesized from the elements by induction melting in sealed tantalum tubes in a water-cooled quartz sample chamber. The new phase crystallizes with the orthorhombic TiNiSi type of structure (space group Pnma, Z = 4). The crystal structures were investigated using powder and single-crystal X-ray diffraction, which gave the following results (21 variables per refinement): a = 805.3(1) pm, b = 493.5(1) pm, c = 874.6(1) pm, and wR2 = 0.058, based on 673 F² values for EuMg_0.942(4)Tl_1.058(4); a = 806.2(1) pm, b = 493.39(8) pm, c = 873.9(1) pm, and wR2 = 0.047, based on 814 F² values for EuMg_0.987(2)Tl_1.013(2); and a = 806.6(2) pm, b = 493.65(8) pm, c = 874.0(1) pm, and wR2 = 0.033, based on 675 F² values for EuMg_0.983(2)Tl_1.017(2). All of the crystals revealed a small amount of Mg/Tl mixing on the magnesium site. Chemical bonding analysis using an electron localization function approach revealed a three-dimensional, covalently bonded [Mg_1-xTl_1+x] network in which the europium atoms fill slightly distorted hexagonal prismatic channels. Temperature-dependent magnetization measurements of a EuMgTl sample out of the homogeneity range detected Curie-Weiss paramagnetism above 100 K with an effective magnetic moment of 7.84(5) μ_B/Eu atom. EuMgTl ordered ferromagnetically at 23.1(5) K. ¹⁵¹Eu Mössbauer spectroscopy measurements showed two signals at 78 K [δ = -10.14(1) and 0.62(5) mm/s, corresponding to Eu²⁺ and Eu³⁺, respectively] in a ratio of 86/14, indicating the presence of a significant amount of trivalent europium in EuMgTl. At 4.2 K, full magnetic hyperfine field splitting with a hyperfine field of 23.4(4) T was observed for divalent europium. The partial oxidation of europium to the trivalent state was corroborated by Eu L_III X-ray absorption spectroscopy data.