High-Pressure And High-Temperature Behavior Of Typical Main Group Multivalent Oxide Molecular Crystals

Oxide molecular crystals specifically denote solid materials wherein non-metallic oxides are intricately bonded through intermolecular forces,including hydrogen bonds or van der Waals forces.High-pressure and high-temperature（HPHT）can significantly alter the chemical properties of elements,leading to the formation of high-pressure phases with new structures and atypical stoichiometric compounds.As hydrogen bonds and van der Waals forces,along with other intermolecular forces,are acknowledged as weak interactions,even slight external forces can induce substantial alterations in intermolecular distances,leading to structural reconstructions.Hence,pressure emerges as a highly appropriate thermodynamic parameter for investigating molecular crystals.When subjected to pressure,molecules undergo the transition from liquid or gas to solid,evolving from their initial disordered state into a well-organized crystalline structure,thus giving rise to molecular crystals.Molecular crystals exhibit exceptionally rich structural evolution behaviors under HPHT conditions,including polymerization,amorphization,ionization,and decomposition.In-depth research into these behaviors not only contributes to expanding our general understanding of molecular crystals but also deepens our understanding of the fundamental material world.Furthermore,it provides crucial clues for the discovery of new materials and novel properties.Here we have performed systematically investigation of three typical variable-valence elemental oxide molecular,SO₂,Se O₂,and NO₂,by employing synchrotron X-ray diffraction techniques and in situ Raman spectroscopy,integrating advanced crystal structure prediction methods with first-principles computational methods,resulting in the following innovative achievements.1.Sulfur is a typical multivalent element,exhibiting different oxidation states（-2 to+6）under various redox conditions,and actively participating in various chemical processes within the Earth’s interior.Being the primary desulfurization gas during volcanic eruptions,the investigation into its structural morphology and stability of SO₂under HPHT conditions relevant to the Earth’s interior is indispensable for comprehending the origin of SO₂ in volcanic activities,which has been a longstanding focal point in the realms of high-pressure physics,high-pressure chemistry,and Earth sciences.At atmospheric pressure,sulfur in the SO₂ molecular exhibits+4 oxidation state.Earlier studies suggest that under low pressure,SO₂undergoes a sequence of structural phase transitions,culminating in a transformation into a polymeric amorphous phase at approximately 26 GPa,which can persist up to at least 60 GPa.However,in the actual conditions of the lower mantle,the impact of pressure alone has not been the sole consideration,temperature,as another fundamental thermodynamic parameter,is notably underestimated in prior studies.In this study,using first-principles calculations,we demonstrate that under HPHT conditions corresponding to the lower mantle,SO₂ is no longer stable but decomposes into SO₃ and S.Experimental validation of this predicted disproportionation reaction is conducted through laser-heated Diamond Anvil Cell（DAC）experiments,combined with Raman spectroscopy and synchrotron X-ray diffraction（XRD）measurements,under conditions of 95.2 GPa and2700 K.Additionally,this paper successfully synthesizes,for the first time,an extended framework structure of SO₃.These resuls suggest that the SO₂ gas emitted during volcanic eruptions does not directly originate from the deep mantle.Further calculations reveal that under the pressure conditions of the deep mantle,subducted sulfate minerals undergo spontaneous decomposition,producing this newly identified SO₃,which subsequently returns to the Earth’s surface alongside geological movements such as mantle plumes.Our findings extend the phase diagram of S-O compounds over a broader range of temperature and pressure,indicating that although SO₂ is widespread in the Earth’s atmosphere and other planets,it is not a stable compound under the HPHT conditions associated with the planet’s interior.Simultaneously,the paper proposes incorporating SO₃ into the deep sulfur cycle of the Earth,providing valuable insights for elucidating the mechanisms of sulfur cycling within the Earth’s interior.2.The disproportionation of SO₂ fundamentally stems from the modulation of the potential energy surface by HPHT,while with the prerequisite that the sulfur in SO₂being in an intermediate oxidation state.Building upon this,our attention is directed towards another similarly intermediate-valent member of the same family of oxides,Se O₂.As a typical lone pair compound,the crystal structure of Se O₂ exhibits significant lone pair stereoactivity and coordination polyhedra asymmetry.Lone pair electrons are highly susceptible to temperature and pressure,holding crucial significance for the reactivity of molecules.Previous studies,conducted at a relatively low pressure and temperature ranges（<10 GPa,<1000°C）,discovered four W-shaped one-dimensional infinite chain structures of Se O₂ with comparable configurations,where temperature and pressure lead to different phase transition pathways.Here,through CALYPSO structure prediction method,we predict a novel crystal structure of Se O₂ with V-shaped one-dimensional infinite chain:Pnma-Se O₂.The calculated phonon spectra indicate its dynamic stability at room temperature and up to 2500 K.Further calculations reveal that the lattice response with pressure along the V-shaped chain direction in the Pnma structure is significantly weaker than that along the W-shaped chain direction in those low-pressure phases,suggesting that the rigidity of the one-dimensional covalent chain in Se O₂ can be appropriately modulated by pressure.Furthermore,we conduct systematic HPHT experiments and successfully synthesized the predicted orthorhombic structure at conditions of 48.5 GPa,2200 K,or 87 GPa,2500 K.Our results establish a typical paradigm for understanding the pressure modulation mechanism of lone pair compounds.3.Owing to its unique half-filled electron configuration,Group V oxides exhibit the most diverse chemical stoichiometries at atmospheric pressure.Similar to SO₂ and Se O₂,the non-oxygen elements in NO₂ also exhibit an intermediate valence state,endowing it with a significant degree of structural evolution freedom at elevated temperatures and pressures.NO₂ crystallizes into N₂O₄ at low temperatures or low pressures.Numerous studies have confirmed the autoionization of N₂O₄ under various conditions,with a particular focus on the triggering factors for its ion isomer NO⁺NO₃.In order to elucidate the triggers for its ionization transitions and to explore further potential structural evolution beyond ionization,we have systematically investigated the structural evolution of solid N₂O₄ condensed from gaseous NO₂/N₂O₄ applying Raman spectroscopy in a diamond anvil cell（DAC）.Results show that both irradiation and high temperature can induce ionization transition,as evidenced by the pronounced Raman vibrations of NO⁺NO₃.Conversely,the features of N₂O₄ can persist up to at least 80 GPa in the absence of laser irradiation or heating by our Raman and XRD measurements,suggesting that pressure is not a direct trigger for ionization.Furthermore,we also uncover an unexpected two-step decomposition reaction of N₂O₄solid,where the ionized product NO⁺N O₃ is demonstrated to undergo further decomposition,yielding O₂ and a NO_x（_x<2）compound composed of N-O single,N-N single and N=O double bonds.Our findings significantly expand the phase diagram of N₂O₄ and clarify its ionization in response to photons or temperature,as well as further decomposition under extreme conditions.These results also proposes an rare process of oxygen release in oxide molecular crystals.