High-Pressure Synthesis of the β-Zn3N2 Nitride and the α-ZnN4 and β-ZnN4 Polynitrogen Compounds

High-pressure nitrogen chemistry has expanded at a formidable rate over the past decade, unveiling the chemical richness of nitrogen. Here, the Zn-N system is investigated in laser-heated diamond anvil cells by synchrotron powder and single-crystal X-ray diffraction, revealing three hitherto unobserved nitrogen compounds: β-Zn₃N₂, α-ZnN₄, and β-ZnN₄, formed at 35.0, 63.5, and 81.7 GPa, respectively. Whereas β-Zn₃N₂ contains the N^3- nitride, both ZnN₄ solids are found to be composed of polyacetylene-like [N₄]_∞^2- chains. Upon the decompression of β-ZnN₄ below 72.7 GPa, a first-order displacive phase transition is observed from β-ZnN₄ to α-ZnN₄. The α-ZnN₄ phase is detected down to 11.0 GPa, at lower pressures decomposing into the known α-Zn₃N₂ (space group Ia3̅) and N₂. The equations of states of β-ZnN₄ and α-ZnN₄ are also determined, and their bulk moduli are found to be K₀ = 126(9) GPa and K₀ = 76(12) GPa, respectively. Density functional theory calculations were also performed and provide further insight into the Zn-N system. Moreover, comparing the Mg-N and Zn-N systems underlines the importance of minute chemical differences between metal cations in the resulting synthesized phases.

Polymorphs of the Gadolinite-Type Borates ZrB2 O5 and HfB2 O5 Under Extreme Pressure

Based on the results from previous high‐pressure experiments on the gadolinite‐type mineral datolite, CaBSiO₄(OH), the behavior of the isostructural borates β‐HfB₂O₅ and β‐ZrB₂O₅ have been studied by synchrotron‐based in situ high‐pressure single‐crystal X‐ray diffraction experiments. On compression to 120 GPa, both borate layer‐structures are preserved. Additionally, at ≈114 GPa, the formation of a second phase can be observed in both compounds. The new high‐pressure modification γ‐ZrB₂O₅ features a rearrangement of the corner‐sharing BO₄ tetrahedra, while still maintaining the four‐ and eight‐membered rings. The new phase γ‐HfB₂O₅ contains ten‐membered rings including the rare structural motif of edge‐sharing BO₄ tetrahedra with exceptionally short B−O and B⋅⋅⋅B distances. For both structures, unusually high coordination numbers are found for the transition metal cations, with ninefold coordinated Hf⁴⁺, and tenfold coordinated Zr⁴⁺, respectively. These findings remarkably show the potential of cold compression as a low‐energy pathway to discover metastable structures that exhibit new coordinations and structural motifs.

Face-Centered Cubic Refractory Alloys Prepared from Single-Source Precursors

Three binary fcc-structured alloys (fcc–Ir_0.50Pt_0.50, fcc–Rh_0.66Pt_0.33 and fcc–Rh_0.50Pd_0.50) were prepared from [Ir(NH₃)₅Cl][PtCl₆], [Ir(NH₃)₅Cl][PtBr₆], [Rh(NH₃)₅Cl]₂[PtCl₆]Cl₂ and [Rh(NH₃)₅Cl][PdCl₄]·H₂O, respectively, as single-source precursors. All alloys were prepared by thermal decomposition in gaseous hydrogen flow below 800 °C. Fcc–Ir_0.50Pt_0.50 and fcc–Rh_0.50Pd_0.50 correspond to miscibility gaps on binary metallic phase diagrams and can be considered as metastable alloys. Detailed comparison of [Ir(NH₃)₅Cl][PtCl₆] and [Ir(NH₃)₅Cl][PtBr₆] crystal structures suggests that two isoformular salts are not isostructural. In [Ir(NH₃)₅Cl][PtBr₆], specific Br…Br interactions are responsible for a crystal structure arrangement. Room temperature compressibility of fcc–Ir_0.50Pt_0.50, fcc–Rh_0.66Pt_0.33 and fcc–Rh_0.50Pd_0.50 has been investigated up to 50 GPa in diamond anvil cells. All investigated fcc-structured binary alloys are stable under compression. Atomic volumes and bulk moduli show good agreement with ideal solutions model. For fcc–Ir_0.50Pt_0.50, V₀/Z = 14.597(6) ų·atom⁻¹, B₀ = 321(6) GPa and B₀’ = 6(1); for fcc–Rh_0.66Pt_0.33, V₀/Z = 14.211(3) ų·atom⁻¹, B₀ =259(1) GPa and B₀’ = 6.66(9) and for fcc–Rh_0.50Pd_0.50, V₀/Z = 14.18(2) ų·atom⁻¹, B₀ =223(4) GPa and B₀’ = 5.0(3).

A Room-Temperature Verwey-type Transition in Iron Oxide, Fe5 O6

Functional oxides whose physicochemical properties may be reversibly changed at standard conditions are potential candidates for the use in next‐generation nanoelectronic devices. To date, vanadium dioxide (VO₂) is the only known simple transition‐metal oxide that demonstrates a near‐room‐temperature metal–insulator transition that may be used in such appliances. In this work, we synthesized and investigated the crystals of a novel mixed‐valent iron oxide with an unconventional Fe₅O₆ stoichiometry. Near 275 K, Fe₅O₆ undergoes a Verwey‐type charge‐ordering transition that is concurrent with a dimerization in the iron chains and a following formation of new Fe−Fe chemical bonds. This unique feature highlights Fe₅O₆ as a promising candidate for the use in innovative applications. We established that the minimal Fe−Fe distance in the octahedral chains is a key parameter that determines the type and temperature of charge ordering. This model provides new insights into charge‐ordering phenomena in transition‐metal oxides in general.

Raman Spectroscopy Study on Chemical Transformations of Propane at High Temperatures and High Pressures

This study is devoted to the detailed in situ Raman spectroscopy investigation of propane C₃H₈ in laser-heated diamond anvil cells in the range of pressures from 3 to 22 GPa and temperatures from 900 to 3000 K. We show that propane, while being exposed to particular thermobaric conditions, could react, leading to the formation of hydrocarbons, both saturated and unsaturated as well as soot. Our results suggest that propane could be a precursor of heavy hydrocarbons and will produce more than just sooty material when subjected to extreme conditions. These results could clarify the issue of the presence of heavy hydrocarbons in the Earth’s upper mantle.

High pressure phase transitions of paracelsian BaAl2Si2O8

Three new polymorphs of aluminosilicate paracelsian, BaAl₂Si₂O₈, have been discovered using synchrotron-based in situ high-pressure single crystal X-ray diffraction. The first isosymmetric phase transition (from paracelsian-I to paracelsian-II) occurs between 3 and 6 GPa. The phase transition is associated with the formation of pentacoordinated Al³⁺ and Si⁴⁺ ions, which occurs in a stepwise fashion by sequential formation of Al-O and Si-O bonds additional to those in AlO₄ and SiO₄ tetrahedra, respectively. The next phase transition occurs between 25 and 28 GPa and is accompanied by the symmetry change from monoclinic (P2₁/c) to orthorhombic (Pna2₁). The structure of paracelsian-III consists of SiO₆ octahedra, AlO₆ octahedra and distorted AlO₄ tetrahedra, i.e. the transition is reconstructive and associated with the changes of Si⁴⁺ and Al³⁺ coordination, which show rather complex behaviour with the general tendency towards increasing coordination numbers. The third phase transition is observed between 28 and 32 GPa and results in the symmetry decreasing from Pna2₁ to Pn. The transition has a displacive character. In the course of the phase transformation pathway up to 32 GPa, the structure of polymorphs becomes denser: paracelsian-II is based upon elements of cubic and hexagonal close-packing arrangements of large O²⁻ and Ba²⁺ ions, whereas, in the crystal structure of paracelsian-III and IV, this arrangement corresponds to 9-layer closest-packing with the layer sequence ABACACBCB.

Breakdown of Magnetic Order in the Pressurized Kitaev Iridate β-Li_{2}IrO_{3}

Temperature-pressure phase diagram of the Kitaev hyperhoneycomb iridate β-Li_{2}IrO_{3} is explored using magnetization, thermal expansion, magnetostriction, and muon spin rotation measurements, as well as single-crystal x-ray diffraction under pressure and ab initio calculations. The Néel temperature of β-Li_{2}IrO_{3} increases with the slope of 0.9  K/GPa upon initial compression, but the reduction in the polarization field H_{c} reflects a growing instability of the incommensurate order. At 1.4 GPa, the ordered state breaks down upon a first-order transition, giving way to a new ground state marked by the coexistence of dynamically correlated and frozen spins. This partial freezing in the absence of any conspicuous structural defects may indicate the classical nature of the resulting pressure-induced spin liquid, an observation paralleled to the increase in the nearest-neighbor off-diagonal exchange Γ under pressure.

Pentacoordinated silicon in the high-pressure modification of datolite, CaBSiO4(OH)

A new modification of borosilicate datolite, CaBSiO₄(OH), has been discovered using synchrotron-basedin situhigh-pressure single-crystal X-ray diffraction.

Charge-ordering transition in iron oxide Fe4O5 involving competing dimer and trimer formation

Phase transitions that occur in materials, driven, for instance, by changes in temperature or pressure, can dramatically change the materials' properties. Discovering new types of transitions and understanding their mechanisms is important not only from a fundamental perspective, but also for practical applications. Here we investigate a recently discovered Fe4O5 that adopts an orthorhombic CaFe3O5-type crystal structure that features linear chains of Fe ions. On cooling below ∼150 K, Fe4O5 undergoes an unusual charge-ordering transition that involves competing dimeric and trimeric ordering within the chains of Fe ions. This transition is concurrent with a significant increase in electrical resistivity. Magnetic-susceptibility measurements and neutron diffraction establish the formation of a collinear antiferromagnetic order above room temperature and a spin canting at 85 K that gives rise to spontaneous magnetization. We discuss possible mechanisms of this transition and compare it with the trimeronic charge ordering observed in magnetite below the Verwey transition temperature.

The Pressure-Induced Polymorphic Transformations in Fluconazole

The structural properties and Raman spectra of fluconazole have been studied by means of X-ray diffraction and Raman spectroscopy at pressures up to 2.5 and 5.5 GPa, respectively. At a pressure of 0.8 GPa, a polymorphic phase transition from the initial form I to a new triclinic form VIII has been observed. At higher pressure of P = 3.2 GPa, possible transformation into another new polymorphic form IX has been detected. The unit cell parameters and volumes, and vibration modes as functions of pressure have been obtained for the different forms of fluconazole.

The polymorphic phase transformations in the chlorpropamide under pressure

The crystal structure and vibrational spectra of the chlorpropamide have been studied by means of the X-ray diffraction and Raman spectroscopy at pressures up to 24.6 and 4.4 GPa, respectively. Two polymorphic phase transitions, between initial orthorhombic form-A and a monoclinic form-AI at P ∼ 1.2 GPa and, in additional, to another monoclinic form-AII at P ∼ 3.0 GPa, were observed. At pressures above 9.6 GPa, a transformation to the amorphous phase of chlorpropamide was revealed. The lattice parameters, unit cell volumes, and vibration modes as functions of pressure were obtained for the different polymorphic modifications of chlorpropamide.

Unusual compression behavior of anatase TiO2 nanocrystals

The size-dependent stiffness variations in nanocrystalline anatase, a leading material for applications in photovoltaics, photocatalysis, photoelectrochromics, sensors, and optical coatings, were determined using in situ synchrotron x-ray diffraction and Raman scattering. An unusual, abrupt change in the compression curve at approximately 10 GPa and subtle breaks in the pressure shifts of the intense E(g) Raman band at approximately 10 and approximately 15 GPa have been correlated with approximately 2 A-scale disordering of nanocrystalline anatase structure that fully amorphizes under high compression.

Size-dependent pressure-induced amorphization in nanoscale TiO2

We investigated the size-dependent high-pressure phase transition behavior of nanocrystalline anatase TiO2 with synchrotron x-ray diffraction and Raman spectroscopy to 45 GPa at ambient temperature. Pressure-induced amorphization results in a high-density amorphous (HDA) form when the starting crystallite size is < 10 mm. The HDA-TiO2 transforms to a low-density amorphous form at lower pressures. Harnessing the nanometer length scale thus provides a new window for experimental investigation of amorphization in poor glass formers and a synthesis route for new amorphous materials.

Pressure-induced magnetization in FeO: evidence from elasticity and Mössbauer spectroscopy

The complete elastic tensor of Fe0.94O (wüstite) has been determined to 10 GPa using acoustic interferometry at GHz frequencies inside a diamond-anvil cell. The soft mode (C44) elastic constant of FeO is reduced by 20% over the experimental pressure range. An unusual discontinuity in the pressure derivatives of C11 and C12 at 4.7+/-0.2 GPa corresponds to the pressure at which the onset of a magnetic ordering transition is observed by high-pressure Mössbauer spectroscopy and neutron powder diffraction. Our new results combined with literature structural high P-T data suggest that there is a magnetic, although still cubic, phase of FeO between approximately 5 and approximately 17 GPa.

Experimental and theoretical identification of a new high-pressure TiO2 polymorph

Our combined theoretical and experimental investigations have led to the discovery of a new polymorph of titanium dioxide, where titanium is seven-coordinated to oxygen in the orthorhombic OI ( Pbca) structure. The zero-pressure bulk modulus of the new phase measured in the pressure range 19 to 36 GPa is 318(3) GPa. We demonstrate that the group IVa dioxides (TiO2, ZrO2, HfO2) on compression at ambient temperature all follow the common path: rutile -->alpha-PbO2-type --> baddeleyite-type (MI) --> orthorhombic OI (Pbca) structure --> cotunnite-type (OII).

Aggregate sound velocities and acoustic Grüneisen parameter of iron up to 300 GPa and 1,200 K

Successful interpretation of available geophysical data requires experimental and theoretical information on the elasticity of solids under physical conditions of Earth's interior. Because iron is considered as major component in Earth's core, elastic properties of iron at high pressures and temperatures are very important for modeling its composition and dynamics. We use in situ x-ray diffraction data on epsilon-iron at static pressures up to 300 GPa and temperatures to 1,200 K to determine the Debye-Waller temperature factors and calculate aggregate sound velocities and Grüneisen parameter of epsilon-iron by using an approach that is based on Rietveld refinement at high pressures and temperatures.

Materials science. The hardest known oxide

A material as hard as diamond or cubic boron nitride has yet to be identified, but here we report the discovery of a cotunnite-structured titanium oxide which represents the hardest oxide known. This is a new polymorph of titanium dioxide, where titanium is nine-coordinated to oxygen in the cotunnite (PbCl2) structure. The phase is synthesized at pressures above 60 gigapascals (GPa) and temperatures above 1,000 K and is one of the least compressible and hardest polycrystalline materials to be described.

Stability of Ferropericlase in the Lower Mantle

We have heated ferropericlases (Mg(0.60)Fe(0.40))O and (Mg(0.50)Fe(0.50))O to temperatures of 1000 kelvin at pressures of 86 gigapascals, simulating the stability of the solid solution at physical conditions relevant to Earth's lower mantle. The in situ x-ray study of the externally heated samples in a Mao-Bell-type diamond anvil cell shows that ferropericlase may dissociate into magnesium-rich and iron-rich oxide components. The result is important because the decomposition of ferropericlase into lighter and heavier phases will cause dynamic effects that could lead to mantle heterogeneity.

In situ X-Ray study of thermal expansion and phase transition of iron at multimegabar pressure

The density of varepsilon-iron has been calculated at pressures and temperatures up to 300 GPa and 1300 K, respectively. We observe varepsilon to beta phase transition at pressures between 135 and 300 GPa and temperature above 1350 K; the pattern can be interpreted in terms of double hexagonal close-packed structure. The density calculated at high pressure and temperature (330-360 GPa and 5000-7000 K) closely matches with preliminary reference Earth model density, thereby imposing constraint on the composition of the Earth's inner core.

Stability of Perovskite (MgSiO3) in the Earth's Mantle

Available thermodynamic data and seismic models favor perovskite (MgSiO3) as the stable phase in the mantle. MgSiO3 was heated at temperatures from 1900 to 3200 kelvin with a Nd-YAG laser in diamond-anvil cells to study the phase relations at pressures from 45 to 100 gigapascals. The quenched products were studied with synchrotron x-ray radiation. The results show that MgSiO3 broke down to a mixture of MgO (periclase) and SiO2 (stishovite or an unquenchable polymorph) at pressures from 58 to 85 gigapascals. These results imply that perovskite may not be stable in the lower mantle and that it might be necessary to reconsider the compositional and density models of the mantle.

Synchrotron X-Ray Study of Iron at High Pressure and Temperature

X-ray synchrotron experiments with in situ laser heating of iron in a diamond-anvil cell show that the high-pressure epsilon phase, a hexagonal close-packed (hcp) structure, transforms to another phase (possibly a polytype double-layer hcp) at a pressure of about 38 gigapascals and at temperatures between 1200 and 1500 kelvin. This information has implications for the phase relations of iron in Earth's core.