A closer look into close packing : Pentacoordinated silicon in a high-pressure polymorph of danburite

Due to their high technological and geological relevance, silicates are one of the most studied classes of inorganic compounds. Under ambient conditions, the silicon in silicates is almost exclusively coordinated by four oxygen atoms, while high-pressure treatment normally results in an increase in the coordination from four- to sixfold. Reported here is a high-pressure single-crystal X-ray diffraction study of danburite, CaB₂Si₂O₈, the first compound showing a step-wise transition of Si coordination from tetrahedral to octahedral through a trigonal bipyramid. Along the compression, the Si₂O₇ groups of danburite first transform into chains of vertice-sharing SiO₅ trigonal bipyramids (danburite-II) and later into chains of edge-sharing SiO₆ octahedra (danburite-III). It is suggested that the unusual formation of an SiO₅ configuration is a consequence of filling up the pentacoordinated voids in the distorted hexagonal close packing of danburite-II

Se–Cl Interactions in Selenite Chlorides: A Theoretical Study

The Se–Cl interactions in five selenite chlorides (α,β-Zn2(SeO3)Cl2 (sofiite and its polymorph), α,β-Cu5O2(SeO3)2Cl2 (georgbokiite and parageorgbokiite), and KCdCu7O2(SeO3)2Cl9 (burnsite)) have been investigated by means of the analysis of their theoretical electron density distributions. The analysis reveals the existence in the structures of two basic types of interactions: intermediate interactions with essential covalent contribution and closed-shell interactions. In Zn2(SeO3)Cl2 polymorphs and burnsite, all metal-oxide and metal-chloride interactions are of the first type, whereas in georgbokiite and parageorgbokiite, the Jahn–Teller distortion results in the elongation of some of the Cu–X bonds and their transition to the closed-shell type. All anion–anion interactions are of the closed-shell type. The energy of the closed-shell Se–Cl interactions can be estimated as 1.4–2.6 kcal.mol−1, which is comparable to weak hydrogen bonds. Despite their weakness, these interactions provide additional stabilization of structural architectures. The Se4+–Cl− configurations are localized inside framework channels or cavities, which can be therefore be viewed as regions of weak and soft interactions in the structure

Thermal Expansion and Polymorphism of Slawsonite SrAl2Si2O8

Slawsonite’s (SrAl2Si2O8) structure evolutions depending on temperature (27–1000 °C) have been studied by in situ single-crystal X-ray diffraction. The SrO7 polyhedron expands regularly with the temperature increase. Silicon and aluminum cations are ordered in tetrahedral sites of the studied slawsonite; no significant changes in their distribution as temperature increases were observed. Slawsonite demonstrates a relatively high volume thermal expansion (αV = 23 × 10−6 °C−1) with high anisotropy, typical for framework feldspar-related minerals and synthetic compounds. It was found that, contrary to previously published data, the crystal structure of slawsonite is stable in the studied temperature range and no phase transitions occur up to 1000 °C. The role of Ca and Ba substitution for Sr and Al/Si ordering on polymorphism of natural MAl2Si2O8 (M = Ca, Sr, Ba) is herein discussed

Paracelsian, BaAl2Si2O8BaAl_{2}Si_{2}O_{8}, under high pressure: three new phase transitions and three new high-pressure phases

Minerals of the feldspar group are the most common minerals in the Earth’s crust. Due to their high geological relevance, numerous X-ray diffraction studies at non-ambient conditions (high temperature (HT) and / or pressure (HP)) have been performed with detailed studies focused on orthoclase, sanidine, microcline, albite, and anorthite. Paracelsian, BaAl2Si2O8, is considered as a member of the feldspar group, despite the fact that its structure topology is different from that of the feldspar topology. Its crystal structure was first investigated by Smith (1953) in the Pnam space group, and later re-considered by Bakakin and Belov (1960) in the monoclinic P21/c structure type (Bakakin, Belov, 1960). The structure is based upon a three-dimensional framework formed by corner-sharing SiO4 and AlO4 tetrahedra forming four- and eight-member rings and channels running along the a axis. The channels outlined by the eight-membered rings are occupied by Ba atoms. The framework topology of paracelsian is identical to that observed in danburite, CaB2Si2O8, a borosilicate mineral, which is known as an ore mineral for boron. Recently, Pakhomova et al. (2017) investigated high-pressure evolution of danburite and found the HP-modification of danburite with Si in exclusively fivefold coordination. This prompted us to investigate the HP-behaviour of other minerals with the same structure topology and, in particular, paracelsian.The HP-study of paracelsian was undertaken using in situ single-crystal X-ray diffraction in a diamond anvil cell at the Extreme Conditions Beamline P02.2 of PETRA III (DESY, Hamburg) using the wavelength of 0.2896 Å. For the study, we have used crystals of natural paracelsian from the Benallt Mine, Gwynedd, Wales, UK (from a private collection of the fifth author) with the size of 0.010×0.010×0.005 mm3. Experiment was performed at ambient temperature in the pressure range from 0.0001 to 32 GPa with the pressure step no more than 4 GPa (10 pressure points in total).The study yielded surprising results with HP-induced phase transitions and three new modifications of paracelsian. The first phase transition (from paracelsian-I to paracelsian-II) occurs between 3 and 6 GPa. The transition is isosymmetric, i.e. the space group remains the same (P21/c). The structure of paracelsian-II is obtained from that of paracelsian-I by the formation of additional Al–O bond and the change of coordination number of Al from 4 to 5. As a result, the AlO4 tetrahedra transform into AlO5 trigonal bipyramids. 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. The third phase transition is observed between 28 and 32 GPa and results in the symmetry decreasing from Pna21 to Pn. The structure of paracelsian-IV is a distorted version of the structure of paracelsian-III.This study was supported by the Russian Foundation for Basic Research

Pentacoordinated silicon in the high-pressure modification of datolite, CaBSiO4(OH)\mathrm{CaBSiO_{4}(OH)}

A new modification of borosilicate datolite, CaBSiO$_4$(OH), has been discovered using synchrotron-based in situ high-pressure single-crystal X-ray diffraction. The phase transition from low (I) to high (II) pressure modification is isosymmetric and occurs between 27 and 33 GPa. The crystal structure of datolite-II contains pentacoordinated Si atoms forming SiO$_5$ triangular bipyramids that share edges to form Si2O$_8$ dimers. The dimers are linked through BO$_4$ tetrahedra, resulting in the [B(SiO$_4$)OH]$^{2−}$ layers of a novel topology that has not previously been observed in inorganic compounds. Datolite-II is only the second inorganic structure that contains Si in purely fivefold coordination. The results obtained shed new light on the high-pressure behaviour of silicates and demonstrate that cold compression can be considered as a low-energy pathway to metastable structures, which might possess unusual and unexpected coordination geometries and topologies

Topologically prone or cation compression restricted phase transition: An example of feldspar-related SrGe2_2B2_2O8_8

The stability and phase transformation processes of SrGe$_2$B$_2$O$_8$ were studied under extreme conditions (up to 960 ºC / 40 GPa) using in situ single-crystal and powder X-ray diffraction. The compound demonstrated stability up to 900 °C, when it starts to decompose to SrGe$_4$O$_9$ and Sr$_2$B$_2$Ge$_4$O$_{14}$. Under high-pressure (HP) conditions SrGe$_2$B$_2$O$_8$ undergoes two phase transitions, accompanied by formation of B$_2$O$_7$ dimers and stepwise increasing of Ge coordination number from 4 to 6 through the 5-coordinated polyhedron. This unusual stepwise increase in the coordination number of the framework cation is consistent with the behavior of silicon in isostructural CaSi$_2$B$_2$O$_8$, suggesting that certain topologies are prone to similar effects. We also assume that observed differences in HP behavior of isostructural paracelsian-like phases is caused by different compression capacities of tetrahedral cations (Si, Al, B)

Temperature-Induced Phase Transition in a Feldspar-Related Compound BaZn₂As₂O₈∙H₂O

The high-temperature (HT) behavior of BaAs2Zn2O8∙H2O was studied by in situ single-crystal X-ray diffraction (SCXRD) and hot stage Raman spectroscopy (HTRS) up to dehydration and the associated phase transition. During heating, the studied compound undergoes the dehydration process with the formation of BaAs2Zn2O8, which is stable up to at least 525 °C. The evolution of the fourteen main Raman bands was traced during heating. The abrupt shift of all Raman bands in the 70–1100 cm−1 spectral region was detected at 150 °C, whereas in the spectral region 3000–3600 cm−1 all the bands disappeared, which confirms the dehydration process of BaAs2Zn2O8∙H2O. The transition from BaAs2Zn2O8∙H2O to BaAs2Zn2O8 is accompanied by symmetry increasing from P21 to P21/c with the preservation of the framework topology. Depending on the research method, the temperature of the phase transition is 150 °C (HTRS) or 300 °C (HT SCXRD). According to the HT SCXRD data, in the temperature range 25–300 °C the studied compound demonstrates anisotropic thermal expansion (αmax/αmin = 9.4), which is explained by flexible crankshaft chains of TO4 (T = As, Zn) tetrahedra. Additionally, we discussed some crystal-chemical aspects of minerals with both (ZnOn) and (AsOm) polyhedra (n = 4, 5, 6; m = 3, 4) as main structural units

Compressibility of hingganite-(Y): high-pressure single crystal X-ray diffraction study

Behaviour of hingganite-(Y), Y$_2\square$Be$_2$Si$_2$O$_8$(OH)$_2$, on compression to 47 GPa has been studied by synchrotron-based in situ high-pressure single-crystal X-ray diffraction at room temperature in a diamond anvil cell. In the studied pressure range no obvious phase transitions have been observed. The compression of hingganite-(Y) crystal structure is anisotropic, with b axis showing the maximal compressibility. A fit of the experimental pressure–volume data by the Birch-Murnaghan third-order equation of state yielded the bulk modulus of 131(2) GPa and its pressure first derivative of 3.5(2). The difference between high-pressure behaviour of hingganite-(Y) and structurally related datolite is governed by the different chemical nature of interlayer cations