Energetics and Thermodynamic Stability of the Mixed Valence Ytterbium Germanides

Abstract

The results of an experimental study concerning the thermodynamic stability of the Yb germanides, described as intermediate valence compounds, complemented by a computational investigation for the Yb3Ge5 compound are reported. These compounds belong to the rare earth (RE) tetrelides (tetrel = Si, Ge, i.e., group 14 elements), a class of intermetallic materials showing unusual and promising physical properties (giant magnetocaloric effect, magnetostriction, and magnetoresistence). The high-temperature decomposition reactions of the Yb-Ge intermediate phases were studied experimentally by means of the KEMS (Knudsen effusion mass spectrometry) and KEWL (Knudsen effusion weight loss) techniques. From the reaction enthalpies derived by measuring the Yb(g) decomposition pressures as a function of temperature, the heats of formation of five out of six of the intermediate phases in the Yb-Ge system were calculated. From the computational side, the stability of the Yb3Ge5(s) compound has been investigated by DFT-LCAO-B3LYP (density functional theory-linear combination of atomic orbitals-hybrid b3lyp exchange-correlation functional) first principles calculations deriving its equilibrium geometry and the enthalpy of formation at 0 K in relation to the intermediate valence state of Yb in the lattice.

Yb4LiGe4 – A Yb mixed valent Zintl phase with strong electronic correlations

“…Whereas the lattice parameters of Yb 5 Ge 4 obtained in the present investigation are comparable to the literature values within a few e.s.d. [38][39][40][41]. The cell dimensions for our lithiumcontaining samples show some significant differences from the different literature data [21,28].…”

“…Whereas the lattice parameters of Yb 5 Ge 4 obtained in the present investigation are equal to the literature values within a few e.s.d. [37][38][39][40] the cell dimensions for our lithium-containing samples show some significant differences from the different literature data [37][38][39][40]. In all three cases, the lattice volume is smaller than that of Yb Assuming that the Li content in the sample investigated in Ref.…”

“…6-, and detailed studies of Yb 5 Ge 4[37][38][39][40] revealed mixed-valence behavior of ytterbium, and the formula can be written as [Yb 2+ ] 5-x [Yb 3+ ] x [Ge 2 ] 6-[Ge 2 ] 6-. One position (Yb1) shows notably larger distances to the germanium ligands (first four shortest distances in the coordination sphere are 3.06, 3.14, 3.16 and 3.17 Å) if compared to the Yb2 position (the first four shortest distances are 2.86, 2.92, 2.92 and 2.93 Å).…”

Thermodynamic properties of RE2Co17 intermetallics at high temperatures (RE = Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, Lu)

“…Since d 200 feature is measured and a = 2d 200 , dV = 6a 2 dd 200 . Combining equations (15) and (14), one obtains:…”

“…In order to derive the thermodynamic properties, measurements in reversible conditions are mandatory. To do this, the measurements of the vapor pressure [15][16][17] and electromotive force, emf, of solid-state galvanic cells [18][19][20][21] are the most suited techniques. The emf method when applicable is the most accurate.…”