Estelina Lora da Silva

K ₂ Ni(IO ₃ ) ₄ (H ₂ O) ₂ extends the K ₂ M(IO ₃ ) ₄ (H ₂ O) ₂ family to Ni ²⁺ . Its non-centrosymmetric structure, wide optical band gap and structure–property relationships highlight the potential of this iodate framework.

We provide deeper insight into the crystal structures, sequential structural phase transitions (I2cm → Cmce → I4/mcm → P4/mmm), thermal expansion, and electronic properties of the n = 2 Dion–Jacobson polar oxide RbNdTa₂O₇, through X-ray powder diffraction, neutron powder diffraction, Raman studies, and density functional theory calculations. We observed a uniaxial negative thermal expansion (NTE) across the first-order transition, I2cm → Cmce, where the unit cell contracts along the c-axis, which is driven by a contraction of the NdTa₂O₆ layer. This NTE occurs within the temperature range of the first-order phase transition and contrasts with the corkscrew mechanism typically observed in Ruddlesden–Popper phases. In RbNdTa₂O₇, the I2cm (hybrid improper ferroelectric) → Cmce (antipolar) transition involves crucial changes in the bond lengths of Nd and Ta polyhedra, coupled with polar to antipolar displacement of the Nd ions, leading to a net contraction in the NdTa₂O₆ layer along the c-axis, while preserving the overall octahedral tilting magnitude. This transition highlights the intricate interplay between the Nd and Ta coordination and the associated TaO₆ distortions. Temperature-dependent Raman spectra analysis further confirms the first-order structural transition and associated NTE, providing evidence for increased bond stiffness across this transition. Additionally, using neutron powder diffraction, we have determined that the transition I4/mcm → P4/mmm occurs at approximately 1150 K. Finally, we have calculated from DFT + U, the partial density of states, the energy bandgaps, and effective masses of the charge carriers of the polar ground structure.

We present an abinitio study of the quasi-2D layered perovskite Sr₃Hf₂O₇ compound, performed within the framework of the density functional theory and lattice dynamics analysis. At high temperatures, this compound takes a I4/mmm centrosymmetric structure (S.G. n. 139); as the temperature is lowered, the symmetry is broken into other intermediate polymorphs before reaching the ground-state structure, which is the Cmc2₁ ferroelectric phase (S.G. n. 36). One of these intermediate polymorphs is the Ccce structural phase (S.G. n. 68). Additionally, we have probed the C2/c system (S.G n. 15), which was obtained by following the atomic displacements corresponding to the eigenvectors of the imaginary frequency mode localized at the Γ-point of the Ccce phase. By observing the enthalpies at low pressures, we found that the Cmc2₁ phase is thermodynamically the most stable. Our results show that the I4/mmm and C2/c phases never stabilize in the 0-20 GPa range of pressure values. On the other hand, the Ccce phase becomes energetically more stable at around 17 GPa, surpassing the Cmc2₁ structure. By considering the effect of entropy and the constant-volume free energies, we observe that the Cmc2₁ polymorph is energetically the most stable phase at low temperature; however, at 350 K, the Ccce system becomes the most stable. By probing the volume-dependent free energies at 19 GPa, we see that Ccce is always the most stable phase between the two structures and also throughout the studied temperature range. When analyzing the phonon dispersion frequencies, we conclude that the Ccce system becomes dynamically stable only around 19-20 GPa and that the Cmc2₁ phase is metastable up to 30 GPa.

We present an \textit{ab-initio} study of the quasi-2D layered perovskite Sr$_3$Hf$_2$O$_7$ com\-pound, performed within the framework of the Density Functional Theory and lattice dynamics analysis. At high temperatures, this compound takes a \textit{I4/mmm} centrosym\-met\-ric structure (S.G. n. 139); as the temperature is lowered, the symmetry is broken into other intermediate polymorphs before reaching the ground state structure, which is the \textit{Cmc2$_1$} ferroelectric phase (S.G. n. 36). One of these intermediate polymorphs is the \textit{Ccce} structural phase (S.G. n. 68). Additionally, we have probed the \textit{C2/c} system (S.G n. 15), which was obtained by following the atomic displacements corresponding to the eigenvectors of the imaginary frequency mode localized at the $\mathbfΓ$-point of the \textit{Ccce} phase. By observing the enthalpies at low pressures, we found that the \textit{Cmc2$_1$} phase is thermodynamically the most stable. Our results show that the \textit{I4/mmm} and \textit{C2/c} phases never stabilize in the 0-20 GPa range of pressure values. On the other hand, the \textit{Ccce} phase becomes energetically more stable at around 17 GPa, surpassing the \textit{Cmc2$_1$} structure. By considering the effect of entropy and the constant-volume free energies, we observe that the \textit{Cmc2$_1$} polymorph is energetically the most stable phase at low temperature; however, at 350 K the \textit{Ccce} system becomes the most stable. By probing the volume-dependent free energies at 19 GPa, we see that \textit{Ccce} is always the most stable phase between the two structures and also throughout the studied temperature range. When analyzing the phonon dispersion frequencies, we conclude that the \textit{Ccce} system becomes dynamically stable only around 19-20 GPa, and that the \textit{Cmc2$_1$} phase, is metastable up to 30 GPa.

We report an ab initio study of Sb₂S₃, Sb₂Se₃, and Bi₂S₃ sesquichalcogenides at hydrostatic pressures of up to 60 GPa. We explore the possibility that the C2/m, C2/c, the disordered Im-3m, and the I4/mmm phases observed in sesquichalcogenides with heavier cations, viz. Bi₂Se₃, Bi₂Te₃, and Sb₂Te₃, could also be formed in Sb₂S₃, Sb₂Se₃, and Bi₂S₃, as suggested from recent experiments. Our calculations show that the C2/c phase is not energetically favorable in any of the three compounds, up to 60 GPa. The C2/m system is also unfavorable for Sb₂S₃ and Bi₂S₃; however, it is energetically favorable with respect to the Pnma phase of Sb₂Se₃ above 10 GPa. Finally, the I4/mmm and the disordered body-centered cubic-type Im-3m structures are competitive in energy and are energetically more stable than the C2/m phase at pressures beyond 30 GPa. The dynamical stabilities of the Pnma, Im-3m, C2/m, and I4/mmm structural phases at high pressures are discussed for the three compounds.