Negative linear compressibility in uranyl squarate monohydrate

Abstract

The mechanical properties of the uranyl squarate monohydrate material, (UO 2)(C 4 O 4) • H 2 O, were studied using theoretical solid-state methods based in Density Functional Theory employing plane waves and pseudopotentials. Very demanding calculation parameters were utilized in order to obtain a realistic description of the mechanical behavior of this material. Since the determination of the positions of the hydrogen atoms in the unit cell of uranyl squarate monohydrate was not possible from X-ray diffraction data by structure refinement, they were fully optimized theoretically. The computed lattice parameters, bond distances, angles, and Xray powder diffraction patterns of this material were in very good agreement with the experimental data. This material was found to be mechanically and dynamically stable since the corresponding stability conditions were satisfied. The values of the bulk modulus and its pressure derivatives, shear and Young moduli, Poisson ratio, ductility, hardness, and mechanical anisotropy indices of this material were reported. Furthermore, this study showed that this material exhibits the important negative Poisson ratio (NPR) and negative linear compressibility (NLC) phenomena. Uranyl squarate monohydrate is a very anisotropic brittle material characterized by a bulk modulus of 33 GPa, which shows a minimum value of the NPR of the order of −0.5. Besides, this material displays NLC values for a limited range of positive pressures, from 0.025 GPa to 0.094 GPa, applied along the direction of minimum negative Poisson ratio. The analysis of the crystal structure as a function of pressure demonstrates that the mechanism of NLC of this material is associated to the change in shape of the uranyl pentagonal bipyramids and unrelated to the wine-rack structural mechanism commonly used to rationalize this phenomenon.

Silver Oxalate: Mechanical Properties and Extreme Negative Mechanical Phenomena

“…Therefore, the dependence of the mechanical properties of Ag 2 C 2 O 4 as functions of the direction of the applied strain was computed. As may be noticed in Figure 3E and Table S6 The results obtained in previous works [1][2][3][4][5][6] show that the presence of large negative Poisson's ratios is frequently accompanied by the presence of the important negative linear compressibility phenomenon. As may be noticed in Figure 3E and Table S6 The results obtained in previous works [1][2][3][4][5][6] show that the presence of large negative Poisson's ratios is frequently accompanied by the presence of the important negative linear compressibility phenomenon.…”

“…The lowest value of the NLC found so far is −260 TPa −1 for CsH 2 PO 4 , [40,41] the next lowest values being −166 TPa −1 for oxalic acid dihydrate [3] and −76 TPa −1 for Ag 3 [Co(CN) 6 ]. [42,43] These values were determined experimentally except that of oxalic acid dihydrate, [3] which was obtained recently using the theoretical solid-state methodology.…”

“…[19] Synthetic and design approaches have been devised to obtain materials displaying specific mechanical properties. The main motivation of this study and the previous ones, [1][2][3][4][5][6] was to investigate the presence of negative mechanical phenomena in organic materials because these effects have rarely been studied in this kind of compounds, revealing a significant gap in their study. For this reason, the search for NLC materials has lost to some extent its initial impetus in recent years.…”

“…[77] The calculated elasticity matrix, C i j , is reported in Table S5, Supporting Information. As in previous studies, [1,2,5,6,[83][84][85][86] the Reuss approximation was chosen as the best approach in the case of silver oxalate because it provided the best comparison of the results for the calculated bulk modulus, B = 9.64 ± 2.51 GPa, with that determined from the EOS. For monoclinic crystals, the generic necessary and sufficient Born criterion is that all eigenvalues of the C matrix be positive.…”

“…The compressibility is a mechanical property which measures the relative volume change of a material as a response to a pressure change. Negative linear compressibility (NLC) is not related to unstable materials and may refer [1][2][3][4][5][6]19] to: a) Anisotropic negative linear compressibility (ANLC): the positive variation of the volume when the material is submitted to a uniform compression directed along a given direction; and b) Isotropic negative linear compressibility (INLC): the increase of one or two single directions of the unit cell of a material under the application of an isotropic pressure. [1,11,19,93,94] Counterexamples to the general statement of positivity of the compressibility have been found for constrained systems.…”