Collapsing cristobalitelike structures in silica analogues at high pressure

A DFT-Based Quantitative and Geometric Analysis of the Effect of Pressure on Boron Arsenate

Boron arsenate, BAsO₄, is a β-cristobalite-like crystal which has been reported to exhibit the rather unusual property of negative linear compressibility behaviour at elevated pressures, that is expanding rather than shrinking in a linear dimension when subjected to pressure. This work proposes a ‘geometry—deformation mechanism’-based mathematical model to aid the discernment of the manner how this anomalous pressure behaviour is achieved. The model makes use of data obtained from DFT simulations over an extended range of pressures, including extreme pressure conditions, and rigorously explains the macroscopic properties of this material in terms of the nanoscale deformations. More specifically, through this model, it was possible to decipher the different contributions to the deformation mechanism and compressibility properties of BAsO₄. Moreover, for the first time, it was shown that a rule related to the sum of angles of tetrahedrally coordinated atoms is so robust that it applies at the extreme pressures studied here.

Na4B8O9F10: A Deep-Ultraviolet Transparent Nonlinear Optical Fluorooxoborate with Unexpected Short Phase-Matching Wavelength Induced by Optimized Chromatic Dispersion

Exploring significant ultraviolet/deep-ultraviolet nonlinear optical (NLO) materials is hindered by rigorous and contradictory requirements, especially, possessing a moderate optical birefringence to meet phase-matching (PM). Except for suitable birefringence, small chromatic dispersion is also crucial to blue-shift the PM wavelength. Here, the introduction of fluorinated tetrahedral boron-centred chromophore strategy was proposed to optimize the chromatic dispersion. Herein, [BF4]- unit with large HOMO-LUMO band gap was introduced to Na-B-O-F system and Na4B8O9F10 was designed and synthesized successfully for the first time. Na4B8O9F10 with optimized chromatic dispersion can achieve a short second harmonic generation PM wavelength of 240 nm with a relatively small birefringence (cal. 0.036@1064 nm). Notably, Na4B8O9F10 is the first acentric crystal with [BF4]- unit among reported metal-fluorooxoborate system, involving isolated [BF4]- and novel [B7O10F6]5- fundamental building blocks.

Shearing Deformations of β-Cristobalite-Like Boron Arsenate

Boron arsenate, BAsO4, is crystalline material (I4¯ group) that was recently shown to be auxetic in its (001) plane for loading in any direction in this plane, and, which exhibits negative linear compressibility at elevated pressured in its [001] direction. This work presents and discusses the results of extensive density functional theory (DFT) based simulations aimed at studying deformations that such crystals undergo when subjected to shear loading in an attempt to obtain a better insight into the manner in which this material responds to mechanical loads. The deformations for shearing in the (001) plane are described in terms of the ‘rotating squares’ model, which was used to explain the auxeticity in the same plane where it was shown that shear loading results primarily in deformations which make the ‘squares’ become ‘parallelogram-like’ rather than rotate. This lack of rigidity in projected ‘squares’ was discussed by looking at changes in bond lengths and bond angles.

Borates or phosphates? That is the question

Chemical nomenclature is perceived to be a closed topic. However, this work shows that the identification of polyanionic groups is still ambiguous and so is the nomenclature for some ternary compounds. Two examples, boron phosphate (BPO₄) and boron arsenate (BAsO₄), which were assigned to the large phosphate and arsenate families, respectively, nearly a century ago, are explored. The analyses show that these two compounds should be renamed phosphorus borate (PBO₄) and arsenic borate (AsBO₄). Beyond epistemology, this has pleasing consequences at several levels for the predictive character of chemistry. It paves the way for future work on the possible syntheses of SbBO₄ and BiBO₄, and it also renders previous structure field maps completely predictive, allowing us to foresee the structure and phase transitions of NbBO₄ and TaBO₄. Overall, this work demonstrates that quantum mechanics calculations can contribute to the improvement of current chemical nomenclature. Such revisitation is necessary to classify compounds and understand their properties, leading to the main final aim of a chemist: predicting new compounds, their structures and their transformations.

Stimuli-responsive structural changes in metal–organic frameworks

This feature article mainly summarizes how the structure of MOFs changes under external stimuli.

Zero Linear Compressibility in Nondense Borates with a "Lu-Ban Stool"-Like Structure

Discovering materials that exhibit zero linear compressibility (ZLC) behavior under hydrostatic pressure is extremely difficult. To date, only a handful of ZLC materials have been found, and almost all of them are ultrahard materials with densified structures. Here, to explore ZLC in nondense materials, a structural model analogous to the structure of the "Lu-Ban stool," a product of traditional Chinese woodworking invented 2500 years ago, is proposed. The application of this model to borates leads to the discovery of ZLC in AEB₂ O₄ (AE = Ca and Sr) with the unique "Lu-Ban stool"-like structure, which can obtain a subtle mechanical balance between pressure-induced expansion and contraction effects. Coupled with the very wide ultraviolet transparent windows, the ZLC behavior of AEB₂ O₄ may result in some unique but important applications. The applications of the "Lu-Ban stool" model open a new route for pursuing ZLC materials in nondense structural systems.

Negative thermal expansion and electronic structure variation of chalcopyrite type LiGaTe2

The LiGaTe₂ crystals were grown by the Bridgman–Stockbarger technique and the cell parameter dependence on temperature in the range of 303–563 K was evaluated by the X-ray diffraction analysis and first principles calculations.

M3+(H2AsO4)(H2As2O7) (M3+ = Al, Ga) and In2(H2AsO4)2(H2As2O7)2: a new layer structure type and a new framework structure type containing the rare H2As2O72- group

The crystal structures of hydrothermally synthesized aluminium dihydrogen arsenate(V) dihydrogen diarsenate(V), Al(H₂AsO₄)(H₂As₂O₇), gallium dihydrogen arsenate(V) dihydrogen diarsenate(V), Ga(H₂AsO₄)(H₂As₂O₇), and diindium bis[dihydrogen arsenate(V)] bis[dihydrogen diarsenate(V)], In₂(H₂AsO₄)₂(H₂As₂O₇)₂, were determined from single-crystal X-ray diffraction data collected at room temperature. The first two compounds are representatives of a novel sheet structure type, whereas the third compound crystallizes in a novel framework structure. In all three structures, the basic building units are M³⁺O₆ octahedra (M = Al, Ga, In) that are connected via one H₂AsO₄^- and two H₂As₂O₇^2- groups into chains, and further via H₂As₂O₇^2- groups into layers. In Al/Ga(H₂AsO₄)(H₂As₂O₇), these layers are interconnected by weak-to-medium-strong hydrogen bonds. In In₂(H₂AsO₄)₂(H₂As₂O₇)₂, the H₂As₂O₇^2- groups link the chains in three dimensions, thus creating a framework topology, which is reinforced by weak-to-medium-strong hydrogen bonds. The three title arsenates represent the first compounds containing both H₂AsO₄^- and H₂As₂O₇^2- groups.

High mechanical strength in Zn4B6O13 with an unique sodalite-cage structure

Borate crystal Zn₄B₆O₁₃ exhibits high mechanical strength, originating from the sodalite-cage structure.

Chemically driven negative linear compressibility in sodium amidoborane, Na(NH2BH3)

Over the past few years we have been witnessing a surge of scientific interest to materials exhibiting a rare mechanical effect such as negative linear compressibility (NLC). Here we report on strong NLC found in an ionic molecular crystal of sodium amidoborane (NaAB) – easily-accessible, optically transparent material. In situ Raman measurements revealed abnormal elongation of B-N and N-H bonds of NaAB at pressure about 3 GPa. Ab initio calculations indicate the observed spectroscopic changes are due to an isostructural phase transition accompanied by a stepwise expansion of the crystal along c axis. Analysis of calculated charge density distribution and geometry of molecular species (NH₂BH₃) univocally points to a chemically driven mechanism of NLC – pressure-induced formation of hydrogen bonds. The new H-bond acts as a “pivot screw” coupling N-H covalent bonds of neighbor molecular species – a system resembling a two-lever “jack device” on a molecular scale. A mechanism based on formation of new bonds stands in apparent contrast to mechanisms so far reported in majority of NLC materials where no significant alteration of chemical bonding was observed. The finding therefore suggests a qualitatively new direction in exploration the field towards rational design of incompressible materials.

H3O(+) tetrahedron induction in large negative linear compressibility

Despite the rarity, large negative linear compressibility (NLC) was observed in metal-organic framework material Zn(HO₃PC₄H₈PO₃H)∙2H₂O (ZAG-4) in experiment. We find a unique NLC mechanism in ZAG-4 based on first-principle calculations. The key component to realize its large NLC is the deformation of H₃O⁺ tetrahedron. With pressure increase, the oxygen apex approaches and then is inserted into the tetrahedron base (hydrogen triangle). The tetrahedron base subsequently expands, which results in the b axis expansion. After that, the oxygen apex penetrates the tetrahedron base and the b axis contracts. The negative and positive linear compressibility is well reproduced by the hexagonal model and ZAG-4 is the first MOFs evolving from non re-entrant to re-entrant hexagon framework with pressure increase. This gives a new approach to explore and design NLC materials.

A non-topological mechanism for negative linear compressibility

When exposed to high pressure, the framework material UTSA-16 expands in one direction as the result of distortions localised in soft Co(ii)-based tetrahedra, rather than topological flexing of the network.

Structural studies of metal–organic frameworks under high pressure

Over the last 10 years or so, the interest and number of high-pressure studies has increased substantially. One area of growth within this niche field is in the study of metal–organic frameworks (MOFs or coordination polymers). Here we present a review on the subject, where we look at the structural effects of both non-porous and porous MOFs, and discuss their mechanical and chemical response to elevated pressures.

Isotropic Negative Area Compressibility over Large Pressure Range in Potassium Beryllium Fluoroborate and its Potential Applications in Deep Ultraviolet Region

Isotropic negative area compressibility, which is very rare, is observed in KBBF and the related mechanism is investigated by combined high-pressure X-ray diffraction (XRD) experiments and first-principles calculations. The strong mechanical anisotropy leads to a large Poisson's ratio and high figure of merit for the acoustic-optics effect, giving KBBF potential applications as smart strain converters and deep-ultraviolet (DUV) acoustic-optic devices.

Negative linear compressibility in a crystal of α-BiB3O6

Negative linear compressibility (NLC), a rare and important mechanical effect with many application potentials, in a crystal of α-BiB₃O₆ (BIBO) is comprehensively investigated using first-principles calculations and high-pressure synchrotron X-ray diffraction experiments. The results indicate that the BIBO crystal exhibits the second largest NLC among all known inorganic materials over a broad pressure range. This unusual NLC behaviour is due to the rotation and displacement of the rigid [BO₃] and [BO₄] building units that result in hinge motion in an umbrella-like topology. More importantly, the parallel-polar lone-pair electrons on the Bi³⁺ cations act as “umbrella stands” to withstand the B-O hinges, thus significantly enhancing the NLC effect. BIBO presents a unique example of a “collapsible umbrella” mechanism for achieving NLC, which could be applied to other framework materials with lone-pair electrons.

Negative Linear Compressibility in Organic Mineral Ammonium Oxalate Monohydrate with Hydrogen Bonding Wine-Rack Motifs

Negative linear compressibility (NLC) is a relatively uncommon phenomenon and rarely studied in organic systems. Here we provide the direct evidence of the persistent NLC in organic mineral ammonium oxalate monohydrate under high pressure using synchrotron X-ray powder diffraction, Raman spectroscopy and density functional theory (DFT) calculation. Synchrotron X-ray powder diffraction measurement reveals that ammonium oxalate monohydrate shows both positive and negative linear compressibility along b-axis before 11.5 GPa. The red shift of the external Raman modes and abnormal changes of several selected internal modes in high-pressure Raman spectra further confirmed the NLC. DFT calculations demonstrate that the N-H···O hydrogen bonding "wine-rack" motifs result in the NLC along b-axis in ammonium oxalate monohydrate. We anticipate the high-pressure study of ammonium oxalate monohydrate may represent a promising strategy for accelerating the pace of exploitation and improvement of NLC materials especially in organic systems.

Negative linear compressibility

While all materials reduce their intrinsic volume under hydrostatic (uniform) compression, a select few actually expand along one or more directions during this process of densification.

Triple Guest Occupancy and Negative Compressibility in Hydrogen-Loaded β-Hydroquinone Clathrate

The molecular interactions and structural behavior of a previously unexplored clathrate system, hydrogen-loaded β-hydroquinone (β-HQ+H2), were investigated under high pressure with synchrotron X-ray diffraction and Raman/infrared spectroscopies. The β-HQ+H2 system exhibits coupling of two independently rare phenomena: multiple occupancy and negative compressibility. The number of H2 molecules per cavity increases from one to three, causing unit cell volume increase by way of unique crystallographic interstitial guest positioning. We anticipate these occupancy-derived trends may be general to a range of inclusion compounds and may aid the chemical and crystallographic design of both high-occupancy hydrogen storage clathrates and novel, variable-composition materials with tunable mechanical properties.

Carbon enters silica forming a cristobalite-type CO2-SiO2 solid solution

Extreme conditions permit unique materials to be synthesized and can significantly update our view of the periodic table. In the case of group IV elements, carbon was always considered to be distinct with respect to its heavier homologues in forming oxides. Here we report the synthesis of a crystalline CO2-SiO2 solid solution by reacting carbon dioxide and silica in a laser-heated diamond anvil cell (P = 16-22 GPa, T>4,000 K), showing that carbon enters silica. Remarkably, this material is recovered to ambient conditions. X-ray diffraction shows that the crystal adopts a densely packed α-cristobalite structure (P4(1)2(1)2) with carbon and silicon in fourfold coordination to oxygen at pressures where silica normally adopts a sixfold coordinated rutile-type stishovite structure. An average formula of C0.6(1)Si0.4(1)O2 is consistent with X-ray diffraction and Raman spectroscopy results. These findings may modify our view on oxide chemistry, which is of great interest for materials science, as well as Earth and planetary sciences.

Partially collapsed cristobalite structure in the non molecular phase V in CO2

Non molecular CO(2) has been an important subject of study in high pressure physics and chemistry for the past decade opening up a unique area of carbon chemistry. The phase diagram of CO(2) includes several non molecular phases above 30 GPa. Among these, the first discovered was CO(2)-V which appeared silica-like. Theoretical studies suggested that the structure of CO(2)-V is related to that of β-cristobalite with tetrahedral carbon coordination similar to silicon in SiO(2), but reported experimental structural studies have been controversial. We have investigated CO(2)-V obtained from molecular CO(2) at 40-50 GPa and T > 1500 K using synchrotron X-ray diffraction, optical spectroscopy, and computer simulations. The structure refined by the Rietveld method is a partially collapsed variant of SiO(2) β-cristobalite, space group I42d, in which the CO(4) tetrahedra are tilted by 38.4° about the c-axis. The existence of CO(4) tetrahedra (average O-C-O angle of 109.5°) is thus confirmed. The results add to the knowledge of carbon chemistry with mineral phases similar to SiO(2) and potential implications for Earth and planetary interiors.

Rational design of materials with extreme negative compressibility: selective soft-mode frustration in KMn[Ag(CN)2]3

We show that KMn[Ag(CN)(2)](3) exhibits the strongest negative linear compressibility (NLC) effect over the largest pressure range yet observed. Variable pressure neutron powder diffraction measurements reveal that its crystal lattice expands along the c axis of its trigonal cell under increasing hydrostatic pressure, while contracting along the a axis. This corresponds to a "wine-rack"-like mechanism for NLC that we find also results in anisotropic negative thermal expansion (NTE) in the same material. Inclusion of extra-framework K(+) counterions has minimal effect on framework flexibility (and hence the magnitude of NTE/NLC) but selectively frustrates the soft phonon modes responsible for destroying NLC in the related material Ag(3)[Co(CN)(6)].

Crystal structures of α-quartz homeotypes boron phosphate and boron arsenate: structure-property relationships

The structures of single crystals of the α-quartz homeotypes boron phosphate and boron arsenate prepared under high-pressure, high-temperature conditions in a belt-type apparatus were determined by X-ray diffraction. Both structures are more distorted with respect to the β-quartz structure type than α-quartz itself with average tetrahedral tilt angles δ of 19.7° and 24.6° for BPO4 and BAsO4, respectively. The structure of BAsO4 is one of the most highly distorted found among α-quartz homeotypes. It is shown that for the known α-quartz homeotypes, the density, which can be related to the frequency constant for piezoelectric resonators, scales with the intertetrahedral bridging angle θ.

Compounds with a 'stuffed' anti-bixbyite-type structure, analysed in terms of the Zintl-Klemm and coordination-defect concepts

Compounds with a ‘stuffed anti-bixbyite’ structure, such as Li₃AlN₂, were analysed in terms of both the extended Zintl–Klemm concept and the coordination-defect concept. For the first time, inorganic crystal structures are seen as a set of ‘multiple resonance structures’ (Klemm pseudo-structures) which co-exist as the result of unexpected electron transfers between any species pair comprising either like or unlike atoms, cations or anions. If this is the driving force controlling crystal structures, the conventional oxidation states assigned to cations and anions lose some of their usefulness.

The bixbyite structure (Mn₂O₃) () is often described as a distorted face-centered cubic (f.c.c.) array of Mn atoms, with O atoms occupying 3/4 of the tetrahedral holes. The empty M₄ tetrahedra are centred at 16c. In anti-bixbyite structures (Mg₃N₂), cation vacancies are centred in empty N₄ tetrahedra. If 16 hypothetical atoms were located at this site they would form the structure of γ-Si. This means that anti-bixbyite structures are ideally prepared to accommodate Si(Ge) atoms at these holes. Several compounds (Li₃AlN₂ and Li₃ScN₂) fully satisfy this expectation. They are really anti-bixbyites ‘stuffed’ with Al(Sc). The presence of these atoms in 16c is illuminated in the light of the extended Zintl–Klemm concept (EZKC) [Vegas & García-Baonza (2007 ▶). Acta Cryst. B63, 339–345], from which a compound would be the result of ‘multiple resonance’ pseudo-structures, emerging from electron transfers between any species pair (like or unlike atoms, cations or anions). The coordination-defect (CD) concept [Bevan & Martin (2008) ▶. J. Solid State Chem.181, 2250–2259] is also consistent with the EZKC description of the pseudo-structures. A more profound insight into crystal structures is gained if one is not restricted to the contemplation of classical anions and cations in their conventional oxidation states.

Large negative linear compressibility of Ag3[Co(CN)6]

Silver(I) hexacyanocobaltate(III), Ag(3)[Co(CN)(6)], shows a large negative linear compressibility (NLC, linear expansion under hydrostatic pressure) at ambient temperature at all pressures up to our experimental limit of 7.65(2) GPa. This behavior is qualitatively unaffected by a transition at 0.19 GPa to a new phase Ag(3)[Co(CN)(6)]-II, whose structure is reported here. The high-pressure phase also shows anisotropic thermal expansion with large uniaxial negative thermal expansion (NTE, expansion on cooling). In both phases, the NLC/NTE effect arises as the rapid compression/contraction of layers of silver atoms--weakly bound via argentophilic interactions--is translated via flexing of the covalent network lattice into an expansion along a perpendicular direction. It is proposed that framework materials that contract along a specific direction on heating while expanding macroscopically will, in general, also expand along the same direction under hydrostatic pressure while contracting macroscopically.