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Understanding the properties of molecular solids at high pressure is a key element in the development of new solid-state theories. However, the most commonly used generalized-gradient approximation (GGA) of the density functional theory (DFT) often fails to correctly describe the behavior of these systems at high pressure. Here we utilize the hybrid DFT approach to model the properties of elemental bromine at high pressure. The calculations reproduce in very good agreement with experiment the properties of the molecular phase I (Cmca symmetry) and its pressure-induced transition into the non-molecular phase II (Immm). The experimentally yet unobserved transition into phase III (I4/mmm) is predicted to occur at 128 GPa, followed by subsequent formation of an fcc lattice at 188 GPa. Analysis of the structure and electronic properties of the modelled phases indicates that the molecular Cmca phase becomes metallic just at the borderline of its stability, and that both Immm and I4/mmm phases are metallic and quasi-2D. Finally, we show that the incommensurate phases of bromine postulated from experiment are transient species which can be viewed as intermediates in the dissociation process occurring at the boundary of the transition from phase I to phase II.

Benzoic acid (BA) is a model system for studying proton transfer (PT) reactions. The properties of solid BA subject to high pressure (exceeding 1 kbar = 0.1 GPa) are of particular interest due to the possibility of compression-tuning of the PT barrier. Here we present a simulations aimed a evaluating the value of this barrier in solid BA in the 1 atm - 15 GPa pressure range. We find that pressure-induced shortening of the O...O contacts within the BA dimers leads to a decrease in PT barrier, and subsequent symmetrization of the hydrogen bond. However, this effect is obtained only after taking into account zero-point energy (ZPE) differences between BA tautomers and the transition state. The obtained results shed light on previous experiments on compressed benzoic acid, and indicate that a common scaling behavior with respect to O...O distance might be applicable for hydrogen-bond symmetrization in both organic and inorganic systems.

Solid-state systems whose properties at high pressure (exceeding 1 GPa) mimic those of MgSiO3 are of large importance in the study of the interior of planets. By means of Density Functional Theory (DFT) calculations we studied the high pressure properties of a MgSiO3 analogue, NaZnF3. We reproduce the phase-transition sequence previously reported for this compound (GdFeO3 -> CaIrO3 -> La2S3), and predict that it should undergo a two-step dissociation: decomposition into a equimolar mixture of Na2ZnF4 and NaZn2F5 at 25.4 GPa, followed by a breakdown into ZnF2 and NaF at 66.8 GPa. These processes are analogous to those predicted for compressed MgSiO3. Moreover, both Na2ZnF4 and NaZn2F5 are isostructural with analogous phases from the Mg-Si-O system. We also find that both these novel compounds are thermodynamically stable at ambient conditions (Na2ZnF4) or at low pressure of 19 GPa (NaZn2F5). Our study indicates that NaZnF3 could serve as a good low-pressure analogue of MgSiO3 exhibiting the same sequence of phase transitions, and pressure induced decomposition, but at pressures an order of magnitude lower.

Topological Insulators (TI) are materials with novel quantum states which exhibit a bulk insulating gap while the edge/surface is conducting. This has been extensively explored in several Half-Heusler (HH) compounds hosting the exotic TI behaviour. In the present work we employ, first-principles based Density Functional Theory to perform thorough investigations of pressure induced topological phase transition (TPT) in HH AgSrBi which belongs to the F-43m space group. AgSrBi is intrinsically semi-metallic in nature which, under isotropic pressure exhibits semi-metal to trivial insulator transition retaining the cubic symmetry whereas, on breaking the cubic symmetry we observe the much sought after non-trivial semi-metal to TI phase transition. We also explore the effect of lowering crystal symmetry in realizing TI nature. Following this we perform qualitative analysis of the electronic properties to understand the origin of this non-trivial behavior followed by the quantitative analysis of the Z$_2$ classification which indicates that AgSrBi is a strong TI (i.e., Z$_2$ = (1, 101)). We thus, propose AgSrBi as a dynamically stable TI which can be used as ultra-thin films thermoelectric and nanoelectronic applications., Comment: 6 pages, 3 figures

Tellurium trioxide, TeO3, is the only example of a trioxide adopting at ambient conditions the VF3-type structure (a distorted variant of the cubic ReO3 structure). Here we present a combined experimental (Raman scattering) and theoretical (DFT modelling) study on the influence of high pressure (exceeding 100 GPa) on the phase stability of this compound. In experiment the ambient-pressure VF3-type structure (R-3c symmetry) is preserved up to 110 GPa. In contrast, calculations indicate that above 66 GPa the R-3c structure should transform to a YF3-type polymorph (Pnma symmetry) with the coordination number of Te6+ increasing from 6 to 8 upon the transition. The lack of this transition in the room-temperature experiment is most probably connected with energetic barriers, in analogy to what is found for compressed WO3. The YF3-type phase is predicted to be stable up to 220 GPa when it should transform to a novel structure of R-3 symmetry and Z = 18. We analyse the influence of pressure on the band gap of TeO3, and discuss the present findings in the context of structural transformations of trioxides and trifluorides adopting an extended structure in the solid state

The silver-fluorine phase diagram has been scrutinized as a function of external pressure using theoretical methods. Our results indicate that two novel stoichiometries containing Ag+ and Ag2+ cations (Ag3F4 and Ag2F3) are thermodynamically stable at ambient and low pressure. Both are computed to be magnetic semiconductors at ambient pressure conditions. For Ag2F5, containing both Ag2+ and Ag3+, we find that strong 1D antiferromagnetic coupling is retained throughout the pressure-induced phase transition sequence up to 65 GPa. Our calculations show that throughout the entire pressure range of their stability the mixed valence fluorides preserve a finite band gap at the Fermi level. We also confirm the possibility of synthesizing AgF4 as a paramagnetic compound at high pressure. Our results indicate that this compound is metallic in its thermodynamic stability region. Finally, we present general considerations on the thermodynamic stability of mixed valence compounds of silver at high pressure.

As-grown AgF2 has a remarkably similar electronic structure as insulating cuprates, but it is extremely electronegative, which makes it hard to handle and dope. Furthermore, buckling of layers reduces magnetic interactions and enhances unwanted self-trapping lattice effects. We argue that epitaxial engineering can solve all these problems. By using a high throughput approach and first principle computations, we find a set of candidate substrates which can sustain the chemical aggressiveness of AgF2 and at the same time have good lattice parameter matching for heteroepitaxy, enhancing AgF2 magnetic and transport properties and opening the possibility of field-effect carrier injection to achieve a new generation of high-Tc superconductors. Assuming a magnetic mechanism and extrapolating from cuprates we predict that the superconducting critical temperature of a single layer can reach 195 K., Comment: 18 pages, 8 figures, 2 tables, and supplementary information

Raman scattering measurements indicate that cis-bridged chains are retained in AuF3 even at a compression of 45 GPa - in contrast to meta-GGA calculations suggesting that structures with such motifs are thermodynamically unstable above 4 GPa. This metastability implies that novel gold fluorides (e.g. AuF2) might be attainable at lower pressures than previously proposed., Comment: arXiv admin note: text overlap with arXiv:1912.01850

Studying the effect of high pressure (exceeding 10 kbar) on the structure of solids allows to gain deeper insight in the mechanism governing crystal structure stability. Here we report a study on the high-pressure behaviour of zinc difluoride (ZnF2) - an archetypical ionic compound which at ambient pressure adopts the rutile (TiO2) structure. Previous investigations, limited to a pressure of 15 GPa, revealed that this compound undergoes two pressure-induced phase transitions: TiO2 - CaCl2 at 4.5 GPa, and CaCl2 - HP-PdF2 at 10 GPa. Within this joint experimental-theoretical study we extend the room temperature phase diagram of ZnF2 up to 55 GPa. By means of Raman spectroscopy measurements we identify two new phase transitions: HP-PdF2 - HP1-AgF2 at 30 GPa and HP1 AgF2 - PbCl2 at 44 GPa. These results are confirmed by Density Functional Theory calculations which indicate that in the HP1-AgF2 polymorph the coordination sphere of Zn2+ undergoes drastic changes upon compression. Our results point to important differences in the high-pressure behaviour of ZnF2 and MgF2, despite the fact that both compounds contain cations of similar size. We also argue that the HP1-AgF2 structure, previously observed only for AgF2, might be observed at large compression in other AB2 compounds.

Meta-GGA calculations of the ambient and high-pressure polymorphs of silver difluoride indicate that the compression-induced structural changes lead to a 3.5-fold increase in the strength of antiferomagnetic spin-spin interactions resulting in coupling constant values higher than those found for record-holding oxocuprates(II)., Comment: 22 pages, 3 figures for main manuscript, 7 Supplementary Figures, 4 Supplementary Tables

The parent compound of high-Tc superconducting cuprates is a unique Mott state consisting of layers of spin-1/2 ions arranged on a square lattice and with a record high antiferromagnetic coupling within the layers. Compounds with similar characteristics have long been searched for. Nickelates and iridates had been proposed as cuprate analogs but so far have not reached a satisfactory similarity. Here we use a combination of experimental and theoretical tools to show that the commercial compound AgF2 is an excellent cuprate analog with remarkably similar electronic parameters to La2CuO4 but larger buckling of planes. Two-magnon Raman scattering reveals a superexchange constant which reaches 70% of that of a typical cuprate. We argue that structures that reduce or eliminate the buckling of the AgF2 planes could have an antiferromagnetic coupling that matches or surpasses the cuprates., Comment: 15 pages, 3 figures plus Supplementary Information in ancillary folder

Hybrid density functional calculations are performed for a variety of systems containing d9 ions (Cu2+, Ag2+), and exhibiting quasi-one-dimensional magnetic properties. In particular we study fluorides containing these ions in a rarely encountered compressed octahedral coordination which forces the unpaired electron in the d(z2) orbital. We predict that such systems should exhibit magnetic anisotropies surpassing that of Sr2CuO3 - one of the best realizations of an one-dimensional system known to date. In particular we predict that the inter-chain coupling in the Ag2+ containing [AgF][BF4] should be nearly five orders of magnitude smaller than the intra-chain interaction. Our results indicate that quasi-one-dimensional spin-1/2 systems containing chains with spin sites in a d(z2)1 local ground state could constitute a versatile model for testing modern theories of quantum many-body physics in the solid state., Comment: 20 pages, 8 figures, and supplemental material

On account of the rapid development of noble gas chemistry in the past half-century both xenon and krypton compounds can now be isolated in macroscopic quantities. The same though does not hold true for the next lighter group 18 element, argon, which forms only isolated molecules stable solely in low-temperature matrices or supersonic jet streams. Here we present theoretical investigations into a new high-pressure reaction pathway which enables synthesis of Ar-bearing compounds in bulk and at room temperature.