# Stabilization Of The CN35– Anion In Recoverable High-pressure Ln3O2(CN3) (Ln=La, Eu, Gd, Tb, Ho, Yb) Oxoguanidinates

Andrey Aslandukov, Pascal L. Jurzick, Maxim Bykov, Alena Aslandukova, Artem Chanyshev, Dominique Laniel, Yuqing Yin, Fariia I. Akbar, Saiana Khandarkhaeva, Timofey Fedotenko, et al.

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Andrey Aslandukov, Pascal L. Jurzick, Maxim Bykov, Alena Aslandukova, Artem Chanyshev, et al. Stabilization Of The CN35–Anion In Recoverable High-pressure Ln3O2(CN3) (Ln=La, Eu, Gd, Tb, Ho, Yb) Oxoguanidinates. Angewandte Chemie International Edition, 2023, 62 (47), pp.e202311516. 10.1002/anie.202311516. [https://doi.org/10.1002/anie.202311516](https://doi.org/10.1002/anie.202311516). [hal-05077987](https://HAL.inhal-05077987)

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# Stabilization Of The CN₃<sup>5-</sup> Anion In Recoverable High-pressure Ln₃O₂(CN₃) (Ln = La, Eu, Gd, Tb, Ho, Yb) Oxoguanidinates

Andrey Aslandukov,* Pascal L. Jurzick, Maxim Bykov, Alena Aslandukova, Artem Chanyshev, Dominique Laniel, Yuqing Yin, Fariia I. Akbar, Saiana Khandarkhaeva, Timofey Fedotenko, Konstantin Glazyrin, Stella Chariton, Vitali Prakapenka, Fabrice Wilhelm, Andrei Rogalev, Davide Comboni, Michael Hanfland, Natalia Dubrovinskaia, and Leonid Dubrovinsky

**Abstract:** A series of isostructural Ln₃O₂(CN₃) (Ln=La, Eu, Gd, Tb, Ho, Yb) oxoguanidinates was synthesized under high-pressure (25–54 GPa) high-temperature (2000–3000 K) conditions in laser-heated diamond anvil cells. The crystal structure of this novel class of compounds was determined via synchrotron single-crystal X-ray diffraction (SCXRD) as well as corroborated by X-ray absorption near edge structure (XANES) measurements and density functional theory (DFT) calculations. The Ln₃O₂(CN₃) solids are composed of the hitherto unknown CN₃<sup>5-</sup> guanidinate anion—deprotonated guanidine. Changes in unit cell volumes and compressibility of Ln₃O₂(CN₃) (Ln=La, Eu, Gd, Tb, Ho, Yb) compounds are found to be dictated by the lanthanide contraction phenomenon. Decompression experiments show that Ln₃O₂(CN₃) compounds are recoverable to ambient conditions. The stabilization of the CN₃<sup>5-</sup> guanidinate anion at ambient conditions provides new opportunities in inorganic and organic synthetic chemistry.

Inorganic ternary metal-C–N compounds with covalently bonded C–N anions encompass important classes of solids. The most investigated classes are cyanides (CN⁻)¹,² and carbodiimides (NCN²⁻),³⁻⁹ although more complex anions (i.e. dicyanamides,¹⁰,¹¹ tricyanomethanides,¹² and acetonitriletriide¹³) are known. Inorganic cyanides have applications in gold mining, metal finishing, electroplating¹⁴ and also can be used as reactants or/and catalysts in organic syntheses.¹⁵ Inorganic carbodiimides exhibit interesting optical properties, making them useful in optoelectronics and photonics³⁻⁵ and have a potential for energy storage and conversion, e.g. as electrode materials for lithium-ion

batteries, fuel cells, and supercapacitors.⁶⁻⁹ While CN⁻ and CN₂²⁻ anions are well-known, the next members of this anionic series—i.e. the CN₃<sup>5-</sup> anion, a derivative of guanidine, and CN₄<sup>8-</sup>, a derivative of hypothetical tetraaminomethane—have not been discovered yet, although discussed in the literature.¹⁶,¹⁷

Here, we present the first stabilization of the CN₃<sup>5-</sup> guanidinate anion in the Ln₃O₂(CN₃) (Ln=La, Eu, Gd, Tb, Ho, Yb) family of compounds. Unexpectedly, these compounds were discovered while studying the polynitride chemistry of rare earth elements. A series of Ln₃O₂(CN₃) oxoguanidinates were synthesized at pressures and temper-

[*] A. Aslandukov, A. Aslandukova, Dr. A. Chanyshev, F. I. Akbar, Dr. S. Khandarkhaeva, Prof. L. Dubrovinsky Bayerisches Geoinstitut, University of Bayreuth Universitätstrasse 30, 95440 Bayreuth (Germany) E-mail: andrii.aslandukov@uni-bayreuth.de

A. Aslandukov, Dr. Y. Yin, Prof. N. Dubrovinskaia Material Physics and Technology at Extreme Conditions, Laboratory of Crystallography, University of Bayreuth, Universitätstrasse 30, 95440 Bayreuth (Germany)

P. L. Jurzick, Dr. M. Bykov Institute of Inorganic Chemistry, University of Cologne, Greinstrasse 6, 50939 Cologne (Germany)

Dr. D. Laniel Centre for Science at Extreme Conditions and School of Physics and Astronomy, University of Edinburgh EH9 3FD Edinburgh, United Kingdom

Dr. T. Fedotenko, Dr. K. Glazyrin
Deutsches Elektronen-Synchrotron DESY
Notkestrasse 85, 22607 Hamburg (Germany)

Dr. S. Chariton, Prof. V. Prakapenka
Center for Advanced Radiation Sources, University of Chicago
Chicago, Illinois 60637 (USA)

Dr. F. Wilhelm, Dr. A. Rogalev, Dr. D. Comboni, Dr. M. Hanfland
European Synchrotron Radiation Facility
BP 220, 38043 Grenoble Cedex (France)

Prof. N. Dubrovinskaia
Department of Physics, Chemistry and Biology (IFM), Linköping University
SE-581 83 Linköping (Sweden)

© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
atures of 25 to 54 GPa and 2000–3000 K, respectively, from molecular nitrogen and metals (La, Gd, Tb, Ho) contaminated by oxygen, as well as from O-contaminated azides $Eu(N_3)_2$ and $Yb(N_3)_2$ in a laser-heated diamond anvil cell (DAC). The crystal structures of $Ln_3O_2(CN_3)$ oxoguanidinates were determined and refined on the basis of single-crystal X-ray diffraction (SCXRD) and corroborated by X-ray absorption near edge structure (XANES) measurements and by density functional theory (DFT) calculations. The family of $Ln_3O_2(CN_3)$ compounds and $CN_3^{5-}$ anion were found to be quenchable to ambient conditions.

Among all synthesized compounds, the best SCXRD data quality was obtained for $La_3O_2(CN_3)$, therefore we will first discuss the crystal structure of $La_3O_2(CN_3)$ and its evolution on decompression and then discuss other $Ln_3O_2(CN_3)$ ($Ln$=Eu, Gd, Tb, Ho, Yb) family members and the trends in their crystal-chemistry.

For the high-pressure high-temperature synthesis of the $La_3O_2(CN_3)$ compound, pieces of lanthanum were loaded in air (leading to the partial oxidization of the metal) into the sample chamber of DACs, the latter then were gas-loaded with molecular nitrogen (see Methods section for details). In two independent experiments, the samples were compressed to 54(1) and 25(1) GPa and laser-heated to 2500(200) K. According to synchrotron SCXRD data, in both experiments the same novel solid, adopting an unexpected $La_3O_2(CN_3)$ composition, was synthesized, and its crystal structure was determined. It was found to have the orthorhombic space group *Pnma* (#62) and lattice parameters $a=9.2875(7)$ Å, $b=6.8259(5)$ Å, $c=6.149(2)$ Å at 54(1) GPa and $a=9.795(2)$ Å, $b=7.0062(11)$ Å, $c=6.3699(11)$ Å at 25(1) GPa (see Tables S1–S2 and the CIFs for the full crystallographic data[¹⁸]). The carbon atom in $La_3O_2(CN_3)$ comes from the diamond anvil; the latter being able to act as a carbon source and participating in the chemical reactions is well known.[¹⁹–21]

After the $La_3O_2(CN_3)$ synthesis at 54(1) GPa, the sample was decompressed in a few pressure steps down to ambient pressure, when the DAC was opened in air. SCXRD reflections from $La_3O_2(CN_3)$ crystallites could be traced down to ambient conditions and persisted after air exposure (Table S3 and CIF for the full crystallographic data[¹⁸]). Thus, $La_3O_2(CN_3)$ is recoverable to ambient conditions and, at least for some time, resistant to atmospheric oxygen and moisture. One can assume that the presence of $O^{2-}$ anions in the phase plays an important role in its stability at ambient conditions.

Although it is often difficult to distinguish C/N/O atoms in the crystal structure from the SCXRD data, in the present study it was successfully done based on the value of the $R_1$ agreement factor, the ADPs ratio, interatomic distances, and the charge balance. A detailed justification of the structure model is provided in Supplementary Discussion 1 of the Supporting Information. The structure model was confirmed by supporting DFT calculations using the Vienna ab initio simulation package.[²²] DFT calculations show that the relaxed structural parameters for $La_3O_2(CN_3)$ (Table S4) closely reproduce the corresponding experimental values at 54(1) GPa, as well as at 1 bar. Phonon dispersion relations

calculated in the harmonic approximation show that the $La_3O_2(CN_3)$ phase is dynamically stable at 54 GPa (Figure S1a), as well as at ambient pressure (Figure S1b).

Under high pressure, the elements can behave differently than under ambient conditions, featuring exotic chemistry and unusual oxidation states.[²³⁻²⁵] Therefore, in this study, to be sure in charge balance considerations, the +3 oxidation state of lanthanum in $La_3O_2(CN_3)$ was confirmed by synchrotron La $L_{II}$ edge XANES measurements at the ID12 beamline at ESRF. The position of the lanthanum white line in $La_3O_2(CN_3)$ matches the white line position of the reference sample ($La_3O_3$), indicating the same +3 oxidation state of La in these compounds (Figure 1). The structure of $La_3O_2(CN_3)$ (Figure 2a–c) has two La, one C, two N, and two O distinct crystallographic atomic positions. Carbon and nitrogen atoms form planar trigonal $CN_3$ units (Figure 2d, e), while both oxygen atoms are isolated distinct atoms. O1 atoms are surrounded by six La atoms at the apexes of a distorted octahedron. The O1$La_6$ octahedra are interconnected with each other by common vertices, forming a three-dimensional framework, whereas the $CN_3$ triangles and isolated O2 atoms are positioned in the voids between the octahedra (Figure 2b, c).

La1 atoms are coordinated by five N atoms and four O atoms (the coordination number $CN=9$, the coordination polyhedron is a distorted capped square antiprism), while La2 atoms are eight-fold coordinated by four N atoms and four O atoms (coordination number $CN=8$, the coordination polyhedron is irregular) (Figure S2).

Compared to the HNC($NH_2)_2$ guanidine molecule, which has two types of C–N bonds: two single C–N bonds ($d_{C-N}=1.36$ Å) and one double C=N bond ($d_{C-N}=1.30$ Å), the deprotonated $CN_3^{5-}$ unit (Figure 2d, e) in $La_3O_2(CN_3)$ has three almost equal C–N bonds indicating the delocalization

<figure><img src="image_5.png" /><figcaption>**Figure 1.** La $L_{II}$ edge XANES spectra of $La_3O_2(CN_3)$ sample at 25 GPa and $La_2O_3$ reference sample at ambient conditions. The position of white lines is 5897 eV in both spectra. The low signal-to-noise ratio in the spectrum of $La_3O_2(CN_3)$ is because the measurements were carried out in a DAC, and 2 mm-thick diamonds absorb 98% of the X-ray radiation at these energies.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Crystal structure of La₃O₂(CN₃). (a) A general view. (b) A view along the *b*-axis. (c) A view along the *a*-axis. (d) CN₃ unit at 54 GPa. (e) CN₃ unit at 1 bar. La atoms are green, C atoms are brown, N atoms are gray, and O atoms are red. Red distorted octahedra correspond to O1La₆. Thin grey lines outline the unit cell. The bond length values obtained from experiments are shown in black, while those obtained from the DFT calculations are shown in red.</figcaption></figure>

of π-bond. The average length of the C–N bond in CN₃ 5⁻ at 1 bar is d<sub>C–N</sub>≈1.40 Å, which is longer than an average C–N distance in guanidine (d<sub>C–N</sub>=1.34 Å). At the same time, the average length of the C–N bond in CN₃ 5⁻ at 1 bar is between those of a typical single C–N bond (d<sub>C–N</sub>≈1.47 Å) and a bond of the order of 1.5 (in pyridine d<sub>C–N</sub>=1.35 Å), suggesting an expected bond order of 1.33 (Figure 2d). The C–N bond in CN₃ 5⁻ is longer than C–O and N–O bonds in the well-known trigonal CO₃ 2⁻ carbonate (d<sub>C–O</sub>=1.27–1.29 Å) and NO₃⁻ nitrate anions (d<sub>N–O</sub>=1.24–1.26 Å) at 1 bar, respectively.

There are known at ambient conditions quaternary
Ln–C–N–O compounds with a stable stoichiometry
Ln₂O₂CN₂: *I*₄/*mmm*-La₂O₂CN₂[²⁶] and *P*-3*m*1-Ln₂O₂(CN₂)
(Ln=Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb).[²⁷,²⁸]
The crystal structures of Ln₂O₂CN₂ compounds consist of
Ln₂O₂²⁺ layers and linear CN₂²⁻ anions at the interlayer
positions oriented whether along[²⁶] or perpendicular[²⁷,²⁸] to
the layers. Thus, the structure motif of Ln₂O₂CN₂ com-
pounds completely differs from those of La₃O₂(CN₃).

The crystal structure of the La₃O₂(CN₃) compound has similarities with that of the La₃(SiN₃O)O oxonitridosilicate[²⁹] (Figure 3a, b). Both structures are based on OLa₆ octahedra framework. While Si atoms form [SiN₃O]⁷⁻ tetrahedra in La₃(SiN₃O)O, carbon atoms in La₃O₂(CN₃) prefer a trigonal coordination, and the (CN₃O) ensemble of atoms thereby exists as a CN₃ 5⁻ unit and a distinct O²⁻ anion (Figure 3c, d).

The calculated electron localization function for
La₃O₂(CN₃) confirms the expected covalent bonding be-
tween carbon and nitrogen atoms within the CN₃ units
(Figure 4a). There is no electron localization between C1
and O2 atoms (Figure 4b) that corroborates the absence of
covalent interactions between the CN₃ unit and the O2

atom, and thus there is no CN₃O tetrahedron in the studied
material. A reason for that important difference in the
crystal chemistry of carbon and silicon is the smaller radius
of carbon, which therefore prefers a lower coordination
number than its group neighbor silicon at ambient con-
ditions. It is a well-known difference between carbonates
and silicates at ambient conditions: carbon forms trigonal
CO₃ 2⁻ groups, while silicon prefers SiO₄ tetrahedra. Only
under sufficient compression (at least to 20 GPa) carbon
starts to behave like silicon and form CO₄ 4⁻ tetrahedral
anions.[³⁰⁻³⁴] The recent high-pressure high-temperature syn-
thesis of C₃N₄ polymorphs[²¹] shows that, above 70 GPa, CN₄
tetrahedra can also be formed. Our experimental results
demonstrate that 54 GPa is not a sufficient pressure for the
formation of tetra-coordinated carbon with C–N bonds.
However, one can expect the stabilization of CN₄ 8⁻ units or/
and the formation of polycarbonitrides built of corner/edge-
sharing CN₄ tetrahedra in ternary M–C–N systems at
pressures above 70 GPa.

The structure evolution and compressibility of
La₃O₂(CN₃) were studied experimentally upon the decom-
pression from 54(1) GPa down to 1 bar, as well as using
DFT by variable-cell structure relaxation in the pressure
range of 0 to 100 GPa. The lattice parameters and unit cell
volume of La₃O₂(CN₃) extracted from the SCXRD data
collected upon the decompression are closely reproduced by
DFT (Figure 5).

The evolution of the *a*, *b*, and *c* lattice parameters (Figure 5a) shows an anisotropic response of La₃O₂(CN₃) to pressure: its compressibility along the three main directions increases in the following order: *a* > *c* > *b* (it is also clear from the evolution of the *a*/*a*₀, *b*/*b*₀, *c*/*c*₀ ratio with pressure, Figure S3). One can explain the greater compressibility along the *a* direction due to the larger spacing between
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Comparison of the structures of La₃O₂(CN₃) and La₃(SiN₃O)O oxonitridosilicate. (a) Crystal structure of La₃O₂(CN₃) viewed along the c-axis. (b) Crystal structure of La₃(SiN₃O)O[29] viewed along the c-axis. (c) and (d) The arrangements of CN₃O and SiN₃O ensembles of atoms in both compounds. La atoms are green, C atoms are brown, N atoms are gray, O atoms are red, and Si atoms are blue. Red distorted octahedra correspond to O1La₆ octahedra. Thin grey lines outline the unit cell.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Cross sections of the electron localization function calculated for La₃O₂(CN₃). (a) In the plane of a CN₃⁻ unit. (b) In the (010) plane containing C1, N2, and O2 atoms.</figcaption></figure>

O1La₆ octahedra in that direction (Figure 1b). The coordination numbers and coordination polyhedra of La1 and La2 atoms do not change in the studied pressure range.

The fitting of the La₃O₂(CN₃) experimental pressure-volume data with a 2nd order Birch-Murnaghan equation of state yields a bulk modulus of K₀=146(3) GPa (V₀=493.95 Å³ was fixed), a value that agrees well with the bulk modulus K₀=139.1(5) GPa (V₀=498.16 Å³ was fixed) obtained by fitting the pressure-volume points of La₃O₂(CN₃) DFT-relaxed structures from 0 to 100 GPa with the 2nd order Birch-Murnaghan equation of state (Figure 5b).

The synthesis of the other Ln₃O₂(CN₃) (Ln=Eu, Gd, Tb, Ho, Yb) family members was done under similar high-pressure high-temperature conditions (pressures of 25 to 54 GPa and temperatures of 2000–3000 K) from partially oxidized metals (Gd, Tb, Ho) and nitrogen as well as from oxygen-contaminated azides Eu(N₃)₂ and Yb(N₃)₂ in a laser-heated DAC (Table S1). The crystal structures of Ln₃O₂(CN₃) (Ln=Eu, Gd, Tb, Ho, Yb) oxoguanidinates were also determined based on SCXRD (Tables S6–S13).

The fact that Ln₃O₂(CN₃) phases were obtained for different lanthanides at different pressures and from different precursors demonstrates the stability of such a structure type and chemical composition.

It should be noted that contrary to the case of La₃O₂(CN₃), the crystal structure refinements for Ln₃O₂-(CN₃) (Ln=Eu, Gd, Tb, Ho, Yb) indicate a possible splitting of the C1 atom between two crystallographic positions (see Supplementary Discussion 2). While models with split C1 atom position resulted in a slightly lower R₁ agreement factor, we cannot exclude that this is an artifact of the limited quality of X-ray diffraction data collected in the DACs. The investigation of this phenomenon requires additional studies. Here we will consider the structure models without splitting for all Ln₃O₂(CN₃) (Ln=Eu, Gd, Tb, Ho, Yb) compounds.

The structural evolution of these compounds with pressure was also studied by DFT calculations (Figures S4–S8). For Tb₃O₂(CN₃) and Ho₃O₂(CN₃), the decompression down to 1 bar was done, and the experimentally observed P–V dependence is well-reproduced by DFT calculations (Figs. S6 and S7). For Ln=Gd, Tb, Ho the recoverability of the synthesized compounds was examined, and they were found to be recoverable, just like La₃O₂(CN₃) (Tables S7, S10, and S12). The volume of the Ln₃O₂(CN₃) unit cell at the same pressure decreases when going through the sequence La–Eu–Gd–Tb–Ho–Yb, as expected due to the lanthanide contraction (Figure 6a). Moreover, the correlation between the unit cell volume and ionic radii³ is linear, which is similar to known classes of lanthanide compounds (e.g. nitrides, Figure S9).

The behavior of the CN₃⁻ anion under compression was investigated both experimentally and theoretically for all
<figure><img src="image_9.png" /><figcaption>**Figure 5.** Compressional behavior of the La₃O₂(CN₃) structure. (a) Experimental (red squares, triangles, and diamonds) and calculated (black squares, triangles, and diamonds) pressure dependence of the La₃O₂(CN₃) lattice parameters. (b) Experimental (red squares) and calculated (black squares) pressure dependence of the La₃O₂(CN₃) unit cell volume and the fit of the experimental P–V data (red curve) and calculated P–V data (black curve) using a 2nd order Birch-Murnaghan equation of state. Errors in the experimental data are within the symbol size. The calculated P–V data also can be fitted using the 3rd order Birch-Murnaghan equation of state, yielding K₀ = 131.3(9) GPa, K' = 4.36(4) with fixed V₀ = 498.16 Å³.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 6.** Comparison of crystallographic characteristics of the studied Ln₃O₂(CN₃) compounds at 1 bar and in the range up to 100 GPa. (a) The volume of the Ln₃O₂(CN₃) unit cell at 1 bar versus the volume of lanthanide ion (ionic radii are taken from http://abulafia.mt.ic.ac.uk/shannon/ptable.php (original reference[35]) for CN=8). (b) The average C–N bond length within the CN₃⁻ unit in Ln₃O₂(CN₃) (Ln=La, Eu, Gd, Tb, Ho, Yb) at different pressures.</figcaption></figure>

Ln₃O₂(CN₃) (Ln=La, Eu, Gd, Tb, Ho, Yb) phases (Figure 6b). Due to significant uncertainties in C–N distances extracted from the experimental data, no remarkable differences in the C–N bond lengths dependent on a lanthanide cation at the same pressure was noted, but there is a general trend of a small decrease of the C–N bond length upon compression. The analysis of theoretical data provides a more resolved picture: regardless of a cation, C-N bonds become shorter by ≈0.04 Å upon compression from 1 bar to 100 GPa, and furthermore, a small and monotonous contraction of the C–N bond length in CN₃⁻ is noted when going from lanthanum to ytterbium. At the same time, metal-

nonmetal distances change significantly depending on both the pressure and the cation (Figure S10), indicating that the decrease in the unit cell volume is primarily attributed to the decrease in the volume of the Ln coordination polyhedra. Due to this reason, the calculated bulk moduli of Ln₃O₂(CN₃) solids decrease along the Ln=La–Eu–Gd–Tb–Ho–Yb lanthanide row (Table S14, Figure S11).

To conclude, a series of recoverable Ln₃O₂(CN₃) (Ln=La, Eu, Gd, Tb, Ho, Yb) oxoguanidinates was synthesized under high-pressure high-temperature conditions. The fact that the Ln₃O₂(CN₃) phases were obtained for different
lanthanides, under different pressures, and from different precursors, demonstrates the stability of such a structure type and chemical composition. Despite the significant difference in ionic radii of La³⁺ and Yb³⁺, all studied Ln₃O₂(CN₃) compounds are isostructural and the only difference in their crystal chemistry is the unit cell volumes and interatomic distances decreasing due to the lanthanides' contraction. The lanthanide contraction also dictates the slight decrease in compressibility of Ln₃O₂(CN₃) compounds going from Ln=La to Ln=Yb. These solids all feature the hitherto unknown CN₃⁻ guanidinate anion—i.e. deprotonated guanidine. This discovery extends the list of carbon-nitrogen inorganic anions and may open up new synthesis routes in inorganic and organic chemistry. Also, based on the analysis of carbon crystal-chemical behavior under pressure and its comparison with silicon crystal chemistry, one can expect the stabilization of currently unknown CN₄⁻ unit or/and formation of polycarbonitrides built of corner/edge-sharing CN₄ tetrahedra in ternary M–C–N systems at pressures above 70 GPa.

*Note:* At the same time, the high-pressure synthesis of SbCN₃ antimony nitridocarbonate was carried out by another group,[³⁶] which demonstrates the universality of a high-pressure approach for the stabilization of CN₃⁻ guanidinate anion.

## Acknowledgements

The authors thank Prof. Björn Winkler for useful discussions. The authors acknowledge the Deutsches Elektronen-Synchrotron (DESY, PETRA III) for the provision of beamtime at the P02.2 beamline, the European Synchrotron Radiation Facility (ESRF) for the provision of beamtime at the ID15b and ID12 beamlines. Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation – Earth Sciences (EAR – 1634415). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Computations were performed at the Leibniz Supercomputing Center of the Bavarian Academy of Sciences and the Humanities, and the research center for scientific computing at the University of Bayreuth. M.B. acknowledges the support of Deutsche Forschungsgemeinschaft (DFG Emmy-Noether project BY112/2-1). D.L. thanks the UKRI Future Leaders Fellowship (MR/V025724/1) for financial support. N.D. and L.D. thank the Deutsche Forschungsgemeinschaft (DFG projects DU 954-11/1, DU 393-9/2, DU 393-13/1; DU 945/15-1) for financial support. N.D. also thanks the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971). For the purpose of open access, the authors have applied a Creative Commons Attribution (CC BY) license to any Author Accepted Manuscript version

arising from this submission. Open Access funding enabled and organized by Projekt DEAL.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article. Additional references are cited within the Supporting Information.[³⁷⁻⁶⁰]

**Keywords:** Diamond Anvil Cell • Guanidinate • High Pressure • Lanthanides • Single-Crystal X-Ray Diffraction

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Manuscript received: August 8, 2023

Accepted manuscript online: September 28, 2023

Version of record online: October 16, 2023