Phase equilibria in Mg(NO3)2–Ln(NO3)3 (Ln = Nd, Sm, Eu, Tb)–HNO3 (∼20%)–H2O systems at 298.15 K and thermodynamic properties of Mg3Ln2(NO3)12·24H2O (Ln = Nd, Sm)

Abstract

The phase diagrams of the Mg(NO3)2–Ln(NO3)3 (Ln = Nd, Sm, Eu, Tb)–HNO3–H2O systems in the (∼20%) HNO3 regions at T = 298.15 K were determined by using an isothermal solution saturation method. The Schreinemaker’s wet residues method was used to determine the compositions of solid-phases. Both Mg(NO3)2–Nd(NO3)3–HNO3–H2O and Mg(NO3)2–Sm(NO3)3–HNO3–H2O systems are complicated with three equilibrium solid phases Mg(NO3)2·6H2O, Mg3Ln2(NO3)12·24H2O and Ln(NO3)3·6H2O (Ln = Nd, Sm). But the systems Mg(NO3)2–Eu(NO3)3–HNO3–H2O and Mg(NO3)2–Tb(NO3)3Tb–HNO3–H2O are simple eutonic type. The two new solid-phase compounds Mg3Nd2(NO3)12·24H2O and Mg3Sm2(NO3)12·24H2O are congruently soluble in an average medium of ∼20 mass % HNO3, and they were identified and characterized by the methods of powder X-ray diffraction, X–ray diffraction single crystal structure analysis, and thermogravimetric/differential thermogravimetric. The XRD diffraction patterns for the two new solid-phase compounds are identical due to their isomorphic structure. They belong to hexagonal systems, with space group R3. For Mg3Nd2(NO3)12·24H2O, a = b = (1.0993 ± 0.0005) nm, c = (1.7211 ± 0.0007) nm, V = (1.8012 ± 0.0013) nm3, Dc = (4.2532 ± 0.0024) g·cm−3, Z = 3; for Mg3Sm2(NO3)12·24H2O, (a = b = 1.1198 ± 0.0004) nm, c = (1.7257 ± 0.0006) nm, V = (1.8742 ± 0.0011) nm3, Dc = (4.1201 ± 0.0012) g cm−3, Z = 3. The standard molar enthalpies of solution of the compounds Mg3Nd2(NO3)12·24H2O and Mg3Sm2(NO3)12·24H2O in water (aq) were measured to be (101.26 ± 0.77) kJ mol−1 and (80.27 ± 0.81) kJ mol−1, and their standard molar enthalpies of formation were calculated to be – (12214.13 ± 2.5) kJ mol−1 and – (12183.6 ± 2.5) kJ mol−1, respectively.