Structure Induced Yb Valence Changes in the Solid Solution Yb_xCa_1-xC₂

Abstract

The solid solution Yb(x)Ca(1-x)C2 (0 ≤ x ≤ 1) was synthesized by reaction of the elements at 1323 K. The crystal structures within this solid solution, as elucidated from synchrotron powder diffraction data, depend on x and exhibit some interesting features that point to a structure dependent valence state of Yb. Compounds with x ≥ 0.75 crystallize in the tetragonal CaC2 type structure (I4/mmm, Z = 2) and obey Vegard's law; for x ≤ 0.75 the monoclinic ThC2 type structure (C2/c, Z = 4) is found, which coexists with the monoclinic CaC2-III type structure (C2/m, Z = 4) for x ≤ 0.25. The monoclinic modifications show a strong deviation from Vegard's law. Their unit cell volumes are remarkably larger than expected for a typical Vegard system. HERFD-XANES spectroscopic investigations reveal that different Yb valence states are responsible for the observed volume anomalies. While all tetragonal compounds contain mixed-valent Yb with ∼75% Yb(3+) (similar to pure YbC2), all monoclinic modifications contain exclusively Yb(2+). Therefore, Yb(x)Ca(1-x)C2 is a very rare example of a Yb containing compound showing a strong structure dependence of the Yb valence state. Moreover, temperature dependent synchrotron powder diffraction, neutron TOF powder diffraction, and HERFD-XANES spectroscopy experiments reveal significant Yb valence changes in some compounds of the Yb(x)Ca(1-x)C2 series that are induced by temperature dependent phase transitions. Transitions from the tetragonal CaC2 type structure to the monoclinic ThC2 or the cubic CaC2-IV type structure (Fm3m, Z = 4) are accompanied by drastic changes of the mean Yb valence from ∼2.70 to 2.0 in compounds with x = 0.75 and x = 0.91. Finally, the determination of lattice strain arising inside the modifications with ordered dumbbells (ThC2 and CaC2 type structures) by DSC measurements corroborated our results concerning the close relationship between crystal structure and Yb valence in the solid solution Yb(x)Ca(1-x)C2.

Solid Solutions LnxU1−xC2 with Ln = Tb, Dy, Ho, Tm, and Lu Showing Ideal Vegard Behavior

“…For Eu x Sr 1−x C 2 , we observed an ideal Vegard behavior [15], which is not surprising due to the very similar ionic radii of Sr 2+ (118 pm) and Eu 2+ (117 pm) for CN = 6 [16]. Using XANES, we observed interesting valence changes of ytterbium in Yb x Ca 1−x C 2 depending upon the composition x and the crystal structure (CaC 2 vs. ThC 2 type structures) [17]. Notably, these valence changes led to a drastic volume decrease of ~12%.…”

“…As mentioned in the introduction, XANES has been shown to be a versatile tool for the analysis of electronic states of lanthanide [17,18,24] as well as uranium [25,26] cations. Figure 3 shows a typical spectrum at the U-L III edge of a solid solution investigated in this work.…”

“…Notably, these valence changes led to a drastic volume decrease of ~12%. Taking the ionic radii (CN = 6) for Ca 2+ (100 pm), Yb 2+ (102 pm), and Yb 3+ (87 pm) [16] into account, it is evident that at the Ca-rich side, Yb is forced into its divalent state, whereas at the Yb-rich side, an almost trivalent Yb dominates [17], similar to pristine YbC 2 [18]. Further investigations on Ca x Eu 1−x C 2 , Ba x Eu 1−x C 2 , Sr x Yb 1−x C 2 , and Ba x Yb 1−x C 2 revealed the following [19]:…”

“…Additionally, we have included XANES investigations to shed some light on the electronic states of the constituting metal cations in the solid solutions Ln x U 1-x C 2 with Ln = Tb, Dy, Ho, Tm, and Lu. XANES has been shown to be a versatile tool for the analysis of lanthanide [17,18,24] as well as uranium [25,26] cations.…”

Festkörperchemie 2013

“…Wie aber sind sie in einem gemischten Acetylid wie YbxCa1‐xC2 verteilt? Ruschewitz und Mitarbeiter3 dokumentieren einen Valenz‐ und Strukturwechsel vom monoklinen ThC2‐ (C2/c mit Yb2+) zum orthorhombischen CaC2‐Typ (I4/mmm mit Yb2+δ) in Abhängigkeit von x. Dazu nutzten sie energieaufgelöste Nahkantenspektroskopie. Ab der Zusammensetzung Yb0,75Ca0,25C2 koexistieren beide Strukturen und deuten auf einen weiteren Valenzwechsel bei hohen Temperaturen hin.…”