Synthesis and crystallochemical characterization of the new intermetallic phases La(AgxMg1-x)12 (0.11≤x≤0.21), LaAg4+xMg2-x (-0.15≤x≤1.05) and LaAg2+xMg2-x (0

The crystal structures of three new LaAgMg ternary phases were solved and refined from X-ray diffraction data recorded on single crystals. All the studied phases show wide homogeneity regions extending at a constant La concentration. The Mg-rich compound La(AgxMg1x)12 (0.11 = x = 0.21) crystallises with a large tetragonal unit cell [I, I41/amd, tI208, own str. type, Z = 16, a = 10.1686(10), c = 46.539(5) angstrom, R1 = 0.013, wR2 = 0.025, x = 0.12]. The Ag-rich phase LaAg4+xMg2x (0.15 = x = 1.05) shows two structural modifications: body-centred tetragonal for an Ag content lower than ca. 70 at.-% [II, I4/mmm, tI14, YbAl4Mo2-type, Z = 2, a = 7.1995(3), c = 5.5996(2) angstrom, R1 = 0.031, wR2 = 0.081, x = 0.4] and primitive tetragonal for a higher Ag-content [III, P4/nmm, tP14, own str. type, Z = 2, a = 7.2227(4), c = 5.4433(9) angstrom, R1 = 0.013, wR2 = 0.030, x = 0.9]. These two structural models are group-subgroup related (LaAg5Mg is the k2 subgroup of LaAg4Mg2) as usual for second-order transformations. The LaAg2+xMg2x phase (0 < x = 0.45) crystallises with an orthorhombic unit cell [IV, Cmcm, oS20, own str. type, Z = 4, a = 4.805(1), b = 17.258(4), c = 5.527(1) angstrom, R1 = 0.029, wR2 = 0.081, x = 0.25]. In order to gain insights into their bonding peculiarities, calculations of the electronic structures (LMTO approach) were performed on idealised stoichiometric models of the studied compounds representing a chemical modelling of mixed occupations. We also computed the structural parameters of the Ag-rich phases and the vibrational properties of LaAg4+xMg2x within the pseudopotential approach in generalised-gradient approximation (GGA).