The vibrational signatures of the calcium oxalate polyhydrates are investigated using a combination of Density Functional Theory‐Dispersion corrected, Fourier Transform‐ Raman and ‐Infrared (IR) spectroscopies. Most vibrational bands were assigned and the theoretical predictions were compared with in‐house and other experimental data, for both, IR and Raman spectroscopies. Such an approach allowed a more accurate analysis of vibrational spectra helping in the completion of the band assignments of the vibrational bands of the mono, di, and tri hydrate calcium oxalate (COM, COD, and COT). Particular attention has been paid to the degree of hydration of COD, the low Raman wavenumbers, and the presence of oxalic acid in natural calcium oxalate polyhydrates. The obtained results are expected to be supportive in the detection of the different polyhydrates in natural samples, such as in kidney stones.

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We report first-principle atomistic simulations on the effect of local strain gradients on the nanoscale domain morphology of free-standing PbTiO3 ultrathin films. First, the ferroelectric properties of free films at the atomic level are reviewed. For the explored thicknesses (10 to 23 unit cells), we find flux-closure domain structures whose morphology is thickness dependent. A critical value of 20 unit cells is observed: thinner films show structures with 90 • domain loops, whereas thicker ones develop, in addition, 180 • domain walls, giving rise to structures of the Landau-Lifshitz type. When a local and compressive strain gradient at the top surface is imposed, the gradient is able to switch the polarization of the downward domains, but not to the opposite ones. The evolution of the domain pattern as a function of the strain gradient strength consequently depends on the film thickness. Our simulations indicate that in thinner films, first the 90 o domain loops migrate towards the strain-gradient region, and then the polarization in that zone is gradually switched. In thicker films, instead, the switching in the strain-gradient region is progressive, not involving domain-wall motion, which is attributed to less mobile 180 o domain walls. The ferroelectric switching is understood based on the knowledge of the local atomic properties, and the results confirm that mechanical flexoelectricity provides a means to control the nanodomain pattern in ferroelectric systems.