Solid Solutions Ln_xU_1−xC₂ with Ln = Tb, Dy, Ho, Tm, and Lu Showing Ideal Vegard Behavior

Round 1

Reviewer 1 Report

Manuscript U. Ruschewitz et al. described the synthesis and characterization of LnxU1-xC2. This study is interesting from the point of view of fun science. And before this work is recommended for publication I have several questions. The authors have analyzed the phase using PXRD, can the data for the original graphite be cited for comparison? The introduction talks about carbides, how is carbide formed from graphite? Can Raman spectroscopy be done to rule out intercalation of graphite by metal ions? It would be important for the authors to emphasize the difference between the product and the initial graphite.

Author Response

Dear reviewer,

thank you for your comments. The strongest reflection of graphite is the (002) reflection with d = 3.3555 Å (=> 1/d = 0.298 Å^-1). In Fig. 5 of the manuscript, there is not intensity found at 1/d = 0.298 Å^-1. This clearly indicates that we do not have larger amounts of graphite impurities in this sample. As stated in the manuscript (line 257/258) in some samples we find very weak (002) reflections of graphite (e.g. Fig. S3.2 for x_nom = 0.3, green curve). One has to take into account that carbon is compared to uranium and the lanthanides a weak scatterer for X-rays. Therefore, smaller amounts of graphite might not be observable in PXRD patterns. The visual inspection of our samples however clearly shows that the materials are (mostly) homogeneous and impurities of unreacted graphite can be (mainly) excluded.

The formation of C₂ units in the carbides of this work from graphite is a well-known process since the first investigations in the 19^th century. However, to the best of our knowledge a mechanistic description of this process is mainly unknown and far beyond the scope of this work.

Our materials show metallic behavior, e.g. metallic reflectivity. Therefore, Raman spectroscopy is not applicable.

Kind regards,

Uwe Ruschewitz

Reviewer 2 Report

The manuscript presents a comprehensive study across five LnxU1-xC2 solid solutions. The authors have collected rich XRD and XANES data from samples with various compositions. They observe a clear Vegard behavior in all five solid solutions. The XANES results suggest an electron transfer from uranium to the lanthanides with increasing x. I recommend the publication of this study in Inorganics after minor revision:

1. In the abstract, the authors concluded the study with an ambiguous conclusion. I recommend ensuring that the abstract presents a clear and confident summary, saving more in-depth discussion for the Results and Discussion or Conclusion and Prospects sections.

2. I found some confusion when reading lines 132 to 138. The authors mentioned that U4+ leads to the elongation of the C-C bond. However, wouldn't the decrease in U4+ content (increasing x) lead to a shorter c-axis? Moreover, how does this effect offset the volume shrinkage caused by the smaller ionic radius of Tm and Lu? By referring to Figures S4.1 and S5.1, it appears that the lattice parameters for TmxU1-xC2 and LuxU1-xC2 continue to increase with x.

3. The titles and tick labels on the x and y-axes in Figure 4 are too small to read. Please increase the font size.

Author Response

Dear reviewer,

Thank you very much for your helpful advices.

Ad 1.: This is a very good comment. We changed the abstract by omitting its last sentence starting with “However, …”.

Ad 2.: This is a very interesting comment! Actually, the relationship between the radii of U⁴⁺/Ln³⁺ and the unit cell volume V_cell is not straightforward. E.g. for r(U⁴⁺) = 89 pm V_cell(UC₂) = 74.0 Å is observed, for r(Tb³⁺) = 92.3 pm
V_cell(TbC₂) = 84.3 Å and for r(Tm³⁺) = 88 pm V_cell(TmC₂) = 78.7 Å. Taking into account that the anion C₂^4- should be larger than C₂^3-, the unit cell volumes simply do not follow the radii of the respective cations. What is the conclusion? The given radius for U⁴⁺ according to Shannon might be too large or the given valence state for Uranium (U⁴⁺) in UC₂ is wrong, it should have a higher valence state. It is also possible that a strong overlap of U- and C-orbitals (stronger bonding) leads to a reduction of the unit cell volume of UC₂. In the manuscript, I have tried to find a wording that describes these unclear aspects best. Thank you for bringing this to our attention.

Ad 3.: We have provided a new Figure 4 with an enlarged font size. Thank you for the advice!

Kind regards,

Uwe Ruschewitz