Placida Rodríguez Hernandez

The stability of the competing orthorhombic Pnma and monoclinic P2₁/c phases of Praseodymium pentaphosphate (PrP₅O₁₄) have been studied using density functional theory (DFT). At 0 K, the Pnma structure is found to be preferred over the P2₁/c one with the enthalpy change with pressure of both phases highlighting a shift in phase preference from Pnma to P2₁/c at ∼2.5 GPa. Independently of the predicted high-pressure structural phase transition at 0 K, our computed elastic properties and phonon dispersion bands as a function of pressure indicate a phonon instability at ∼4.5 GPa due to the appearance of imaginary frequencies, followed by a dynamical instability at 8.5 GPa due to the violation of the Born criteria on the Pnma structure of PrP₅O₁₄. These results eliminate the orthorhombic structure as a possible high-pressure candidate for the monoclinic P2₁/c polymorph. Furthermore, the relative stability of the orthorhombic and monoclinic polymorphs has been evaluated at ambient pressure and as a function of temperature by means of vibrational free-energy calculations. The results indicate a free-energy crossing at 42 K, with the Pnma phase being energetically favored from 0 K to 42 K, after which the P2₁/c phase becomes preferred. These results demonstrate why PrP₅O₁₄ can only be obtained at ambient pressure in the monoclicnic P2₁/c polymorph, different to other rare earth pentaphosphates.

CsI ₃ : pressure-induced change from electron-rich multicenter bonds (ERMBs) in finite linear I ₃ ⁻ units towards electron-deficient multicenter bonds (EDMBs) in infinite linear iodine chains, I _∞ , explains the properties of infinite linear halide chains.

ABX ₃ and BX ₃ perovskites and their distorted variants are solidstate systems with exceptional properties, which allow them to be used in a plethora of potential technological applications. This notwithstanding, the nature of the chemical B–X bonding, which forms the framework where the A atoms can be inserted, is still under debate. Through a joint experimental and theoretical study of AIO₃ (A = K, Rb, Cs, Tl, NH₄) compounds and in particular in cesium iodate (CsIO₃) under compression, we show how the IO₃ ^– polyanions, present in these compounds at room pressure, undergo a gradual pressure-induced polymerization (PIP) process in three dimensions (3D). This results in a pressure-induced symmetrization of the crystalline structure that leads to a tetragonal perovskite structure, with IO₅₊₁ units, in CsIO₃ and eventually to a cubic perovskite, with IO₆ units, in other AIO₃ compounds. We demonstrate that the PIP process induces a change in the chemical bonding from the resonant delocalized I–O bonds in IO₃ ^– polyanions toward the unconventional I–O electron-deficient multicenter bonds (EDMBs) in AIO₃ cubic perovskites. The process of EDMB formation in the cubic perovskites agrees with the recently proposed unified theory of multicenter bonding and contradicts previous assumptions that considered these bonds to be impossible in valence electron-rich elements, such as chalcogens and halogens. Interestingly, our results suggest that (i) the formation of the cubic and slightly distorted ABX ₃ and BX ₃ perovskites, with A, B, and X being main-group elements, at high pressure is driven by the formation of 3D EDMBs due to the PIP process of the BX ₃ units (monomers) leading to the formation of regular BX ₆ units; and (ii) unconventional EDMBs could be already present at room conditions in the cubic or slightly distorted ABX ₃ and BX ₃ perovskites, with A, B, and X being main-group elements. The presence of unconventional EDMBs could explain the extraordinary properties of these perovskites.

The application of high pressure to orthorhombic columbite CoV₂O₆ resulted in two phase transitions, characterized by single-crystal X-ray diffraction (XRD) and Raman spectroscopy up to 54.9(1) GPa. The first phase transition, at 20.0(1) GPa, is isostructural, preserving the space group (Pbcn) but implies a sudden volume reduction (4.7%) and a symmetrization of VO₆ octahedra, as well as a group shift of the vibrational modes toward higher wavenumbers. The named columbite-II structure remains stable up to 45.6(1) GPa. Beyond this pressure, a different monoclinic polymorph (space group P2₁/c) was deduced from the high-quality diffraction data. The two observed phase transitions are reversible, but below 4.9(3) GPa, the sample mainly transforms into the previously known brannerite polymorph. The anisotropic compressibility of the lattice axis and the Birch–Murnaghan equation of state were obtained from the XRD analysis. Our experimental results are supported by density functional theory calculations.

ABX3 and BX3 perovskites and their distorted variants are solid-state systems with exceptional properties, which enable them for a plethora of potential technological applications. This notwithstanding, the nature of the chemical B‒X bonding, which forms the framework where the A atoms can be inserted, is still under debate. Through a joint experimental and theoretical study of AIO3 (A= K, Rb, Cs, Tl, NH4) compounds and in particular in cesium iodate (CsIO3) under compression, we show how the IO3‒ polyanions, present in these compounds at room pressure, undergo a gradual pressure-induced polymerization (PIP) process in three dimensions (3D). This results in a pressure-induced symmetrization of the crystalline structure that leads to a tetragonal perovskite structure, with IO5+1 units, in CsIO3 and eventually to a cubic perovskite, with IO6 units, in other AIO3 compounds. We demonstrate that the PIP process induces a change in the chemical bonding from the resonant delocalized I‒O bonds in IO3‒ polyanions towards the unconventional I‒O electron-deficient multicenter bonds (EDMBs) in AIO3 cubic perovskites. The process of EDMB formation in the cubic perovskites agrees with the recently proposed unified theory of multicenter bonding and contradicts previous assumptions that considered these bonds to be impossible in valence electron-rich elements, such as chalcogens and halogens. Interestingly, our results suggest that: i) the formation of the cubic and slightly distorted ABX3 and BX3 perovskites with A, B, and X being main-group elements at high pressure is driven by the formation of 3D EDMBs due to the PIP of BX3 units leading to the cubic skeleton of regular BX6 units; and ii) unconventional EDMBs could be already present at room conditions in the cubic or slightly distorted ABX3 and BX3 perovskites with A, B, and X being main-group elements perovskites. The presence of unconventional EDMBs could explain the extraordinary properties of these perovskites.

Polyhalides are molecular systems that defy conventional views of chemical bonding, being infinite linear halide chains the most challenging systems. By studying CsI3 under compression, we show how I3‒ polyanions, with electron-rich multicenter bonds, polymerize giving rise to infinite linear iodine chains, I∞, and demonstrate that these chains, and, by extension, infinite linear halide chains, feature electron-deficient multicenter bonds. This result is in sharp contrast with previous assumptions that considered this type of bond to be impossible in valence electron-rich elements as halogens. Importantly, the process of formation of electron-deficient multicenter bonds in CsI3 at high pressure agrees with the recently proposed unified theory of multicenter bonding.

Here, we propose a multicenter bond theory that addresses the origin and mechanisms behind the formation of electron-rich multicenter bonds (ERMBs) and electron-deficient multicenter bonds (EDMBs), with special emphasis on molecules and solids primarily composed of electron-rich elements. We show that both types of multicenter bonds have the same origin, but a different mechanism of formation upon increasing electronic density; e.g. upon reduction, increase of pressure, or chemical substitution of elements by their heavier analogs. In addition to our recent demonstration of the formation of EDMBs in electron-rich systems, such as pnictogens, chalcogens, and chalcogenides related to phase change materials (see H. H. Osman et al., J. Mater. Chem. C, 2024, 12, 10447–10474), here we present other examples of solids with electron-rich elements forming EDMBs and ERMBs. We conclude that EDMBs can occur not only as three-center–two-electron (3c–2e) bonds in molecules but also as linear or quasi-linear two-center–one-electron (2c–1e) bonds in extended solids. In addition, we propose that pure ERMBs can only occur as linear or quasi-linear three-center–four-electron (3c–4e) bonds. All these claims suppose a change of paradigms regarding the current understanding of ERMBs and EDMBs. To understand the formation of ERMBs and EDMBs in electron-rich elements, we show some of the simplest geometries of linear EDMBs and ERMBs along one (1D), two (2D), and three (3D) dimensions that can be found in the hypercoordinated multicenter units of molecules and solids with electron-rich elements and propose a new way of notation of these hypercoordinated units. Finally, we show that both types of multicenter bonds, in general, do not violate the doublet/octet rule.

This study investigates the high-pressure structural and vibrational properties of nano-Sc₂O₃ using a combination of X-ray diffraction, Raman spectroscopy, and theoretical calculations. Nano-Sc₂O₃ maintains its cubic bixbyite structure up to 26.4 GPa, without evidence of phase transitions, contrasting with bulk Sc₂O₃, which transitions to a monoclinic phase around 25–28 GPa. Raman spectroscopy reveals a pressure-induced blue shift in the vibrational modes, indicating lattice compression, and the absence of new modes confirms the retention of the cubic symmetry. Theoretical predictions using density functional theory (DFT) closely match the experimental data, validating the computational approach we use to model the pressure-dependent vibrational behavior of nano-Sc₂O₃. Comparisons with previous studies seem to show that the nanoscale material exhibits enhanced structural stability compared to its bulk counterpart, likely due to size effects and surface energy contributions. These findings provide new insights into the behavior of nanomaterials under extreme conditions and highlight the potential applications of nano-Sc₂O₃ in high-pressure environments.

Here, we propose a multicenter bond theory that addresses the origin and mechanisms behind the formation of electron-rich multicenter bonds (ERMBs) and electron-deficient multicenter bonds (EDMBs), with special emphasis on molecules and solids primarily composed of electron-rich elements. We show that both types of multicenter bonds have the same origin, but a different mechanism of formation upon increasing electronic density; e.g., upon reduction, increase of pressure, or chemical substitution of elements by their heavier analogs. In addition to our recent demonstration of the formation of EDMBs in electron-rich systems, such as pnictogens, chalcogens, and chalcogenides related to phase change materials (see H.H. Osman et al., J. Mater. Chem. C 2024, 12, 10447), here we present other examples of solids with electron-rich elements forming EDMBs and ERMBs. We conclude that EDMBs can occur not only as three-center-two-electron (3c-2e) bonds in molecules but also as linear or quasi-linear two-center-one-electron (2c-1e) bonds in extended solids. In addition, we propose that pure ERMBs can only occur as linear or quasi-linear three-center-four-electron (3c-4e) bonds. All these claims suppose a change of paradigms regarding the current understanding of ERMBs and EDMBs. To understand the formation of ERMBs and EDMBs in electron-rich elements, we show some of the simplest geometries of linear EDMBs and ERMBs along one (1D), two (2D), and three (3D) dimensions that can be found in the hypercoordinated multicenter units of molecules and solids with electron-rich elements and propose a new way of notation of these hypercoordinated units. Finally, we show that both types of multicenter bonds, in general, do not violate the doublet/octet rule.