Nickel-lanthanum alloys are the basis of a large number of bulk amorphous metallic materials, and their intermetallic compounds are of interest as hydrogen storage materials. To analyze the kinetics and equilibria of reactions with hydrogen and establish the criteria predetermining the possibility for a melt to produce glasses, its stability characteristics, and further transformations, including the formation of nanocrystals, accurate data on the thermodynamic properties of nickel-lanthanum crystalline compositions are essential. Nevertheless, the thermodynamic characteristics of the intermediate phases in the Ni-La system have not hitherto been found with the required accuracy [1][2][3][4][5][6][7][8][9]. Available data are in conflict concerning even the number and composition of the compounds that occur in the crystalline state [1,2].In the present work, the vapor composition and thermodynamic properties of crystalline nickel-lanthanum alloys were studied by Knudsen mass spectrometry in a wide temperature range (685-1547 K) and over the entire composition range. Saturated vapor pressures were measured using Knudsen double cells made of tantalum, niobium, or molybdenum. To prevent an interaction of alloys and the standard used with the material of effusion cells, zirconium or titanium diboride was deposited on their surfaces by plasma spraying. The references were Ag (99.99%), Cu (99.999%), Mn (99.99%), or Ca (99.9%). The samples were prepared from nickel (99.9%) and lanthanum (99.9%). Experimental procedures were analogous to those described elsewhere [10][11][12].Ni-La crystalline alloys have extremely low partial pressures of the components in a major portion of the temperature and composition range. The routine strategy of Knudsen mass spectrometry allowed us to determine only p (Ni) for the heterogeneous equilibrium fields Ni 5 La + Ni 17 La 4 and Ni 17 La 4 + β Ni 7 La 2 (Table 1) of the phase diagram of the Ni-La system [1]. The measured vapor pressures (ion current intensities) of nickel over alloys and the metal [12] and Gibbs energies of Ni phase transitions taken from [13] allowed us to calculate nickel partial thermodynamic functions relative to the completely paramagnetic fcc modification (Table 2).To extend the measuring range to lower temperatures, a procedure [10, 11] based on initiating and studying equilibria that involve volatile reaction products was used. The activity (partial Gibbs energy) of lanthanum in the crystalline alloys with nickel varies in a wide range. This gives no possibility to select a single universal additive to investigate these alloys over the entire concentration range, as was shown in the calculations and preliminary runs performed. This can be done by initiating and studying equilibria of reactions with LiF and MgF 2 . These compounds interact with Ni-La alloys by the reactions (1) (2) Equilibrium (1) was explored earlier in studying the lanthanum-aluminum alloys [10]. The reaction of MgF 2 with lanthanum metal was extremely violent and resulted in p (Mg) values esse...