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)

The phase diagrams of the Mg(NO 3 ) 2 –Ln(NO 3 ) 3 (Ln = Nd, Sm, Eu, Tb)–HNO 3 –H 2 O systems in the (∼20%) HNO 3 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(NO 3 ) 2 –Nd(NO 3 ) 3 –HNO 3 –H 2 O and Mg(NO 3 ) 2 –Sm(NO 3 ) 3 –HNO 3 –H 2 O systems are complicated with three equilibrium solid phases Mg(NO 3 ) 2 ·6H 2 O, Mg 3 Ln 2 (NO 3 ) 12 ·24H 2 O and Ln(NO 3 ) 3 ·6H 2 O (Ln = Nd, Sm). But the systems Mg(NO 3 ) 2 –Eu(NO 3 ) 3 –HNO 3 –H 2 O and Mg(NO 3 ) 2 –Tb(NO 3 ) 3 Tb–HNO 3 –H 2 O are simple eutonic type. The two new solid-phase compounds Mg 3 Nd 2 (NO 3 ) 12 ·24H 2 O and Mg 3 Sm 2 (NO 3 ) 12 ·24H 2 O are congruently soluble in an average medium of ∼20 mass % HNO 3 , 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 R 3 . For Mg 3 Nd 2 (NO 3 ) 12 ·24H 2 O, a  =  b  = (1.0993 ± 0.0005) nm, c  = (1.7211 ± 0.0007) nm, V  = (1.8012 ± 0.0013) nm 3 , D c  = (4.2532 ± 0.0024) g·cm −3 , Z  = 3; for Mg 3 Sm 2 (NO 3 ) 12 ·24H 2 O, ( a  =  b  = 1.1198 ± 0.0004) nm, c  = (1.7257 ± 0.0006) nm, V  = (1.8742 ± 0.0011) nm 3 , D c  = (4.1201 ± 0.0012) g cm −3 , Z  = 3. The standard molar enthalpies of solution of the compounds Mg 3 Nd 2 (NO 3 ) 12 ·24H 2 O and Mg 3 Sm 2 (NO 3 ) 12 ·24H 2 O 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.