Eu_xSr_1–xC₂ (0 ≤ x ≤ 1): A Dicarbide Solid Solution with Perfect Vegard Behavior

Abstract

A solid solution EuxSr1–xC2 (0 ≤ x ≤ 1) was synthesized by direct reaction of the elements at 1123 K. The crystal structures of these compounds, investigated by synchrotron powder diffraction, depend upon x. For x > 0.5 the monoclinic ThC2 type structure (C2/c, Z = 4) is observed and for x ≤ 0.5 the ThC2 type structure coexists with the tetragonal CaC2 type structure (I4/mmm, Z = 2). The unit cell volumes per formula unit of all EuxSr1–xC2 compounds show perfect Vegard behavior, which is due to the almost identical ionic radii of Eu2+ and Sr2+. Mössbauer spectroscopic investigations indeed reveal that europium is in the divalent state over the whole composition range. EuxSr1–xC2 exhibits several temperature dependent phase transitions that were studied by synchrotron powder diffraction and differential thermal analysis. The transition to a cubic high‐temperature modification (Fmm, Z = 4) is of special interest, as it contains information about strain effects appearing inside the modifications with ordered C2 dumbbells (ThC2 and CaC2 type structures). The linear temperature dependence of the obtained transition temperatures TPh shows that no observable strain exists in EuxSr1–xC2, which is again due to the almost identical radii of Eu2+ and Sr2+. EuxSr1–xC2 may therefore be described as a strain free dicarbide solid solution with perfect Vegard behavior.

Valence Changes in Dicarbide Solid Solutions Induced by Structural Changes

“…As a result, all solid solutions crystallize in different modifications depending on x. In addition, solid solutions containing Eu show a clear tendency towards a perfect vegard behaviour as reported for Eu x Sr 1-x C 2 [5] , whereas solutions containing Yb show unusual effects that point to a change of Yb valence stimulated by composition, temperature and different crystal structures. .…”

Structure Induced Yb Valence Changes in the Solid Solution YbxCa1-xC2

“…Looking at the mixed occupancy in solid solutions, one would expect a linear relationship of the transition temperature and the heat of transition on x between the two boundary compounds with x = 0 and x = 1. In fact, this is only the case for cations that show comparable interactions with the C 2 dumbbells. ,,, A significant lattice strain that should be proportional to the differences in the ionic radii as well as to the amount of mixing with a theoretical maximum at x = 0.5 reduces the expected values, because a concerted alignment of the C 2 dumbbells gets hampered by different cations in the dumbbell’s immediate crystallographic surrounding . Therefore, a characteristic minimum of the transition temperatures occurs, whose value mainly depends on the differences of the cations’ radii. ,, …”

“…The first system we investigated was Eu x Sr 1‑ x C 2 with 0 ≤ x ≤ 1 . As Eu 2+ (135 pm, CN = 10) and Sr 2+ (136 pm, CN = 10) have very similar ionic radii, the formation of a solid solution was very likely.…”

“…However, the change of the crystal structure from monoclinic (like obtained for pure EuC 2 ) to tetragonal did not result in a change of the Eu valence state. This was investigated by Mössbauer spectroscopy, which showed the exclusive presence of Eu 2+ cations for the whole composition range …”

“…This was investigated by Mossbauer spectroscopy, which showed the exclusive presence of Eu 2+ cations for the whole composition range. 24 In this current Article we report the results of our investigations on the solid solution Yb x Ca 1-x C 2 with 0 ≤ x ≤ 1. This time we have chosen calcium as the alkaline earth metal, as the ionic radius of Yb 2+ (112 pm, CN = 10) is very similar to that of Ca 2+ (114 pm, CN = 10).…”