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

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

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

Pressure-Induced Phase Transitions in Danburite-Type Borosilicates

The high-pressure behaviors of two isotypic borosilicates (maleevite, BaB2Si2O8, and pekovite, SrB2Si2O8) have been studied using in situ single-crystal X-ray diffraction and Raman spectroscopy. Maleevite undergoes one reconstructive phase transition between 36 and 38 GPa with the formation of a triclinic phase, maleevite-II, featuring octahedrally coordinated silicon. In contrast, pekovite undergoes two phase transitions: first, an isosymmetric order–disorder phase transition to pekovite-II (between 18 and 23 GPa) and then a reconstructive phase transition with the formation of triclinic pekovite-III (between 29 and 33 GPa). The structure of pekovite-II is characterized by the splitting of the Si site into two sites. The results have been confirmed by Raman spectroscopy and density functional theory (DFT) calculations. Raman spectra indicate that the reconstructive phase transitions of both borosilicates are irreversible. Upon decompression, the triclinic phases persist metastably at least down to 12 and 17 GPa, for pekovite and maleevite, respectively. The comparison of the high-pressure behavior of danburite-group minerals with the general formula MB2Si2O8 (M = Ca, Sr, Ba) reveals that increasing size of an extraframework cation for M = Sr and Ba governs the stability of the danburite-type structure and prevents the formation of pentacoordinate silicon species observed in danburite (M = Ca)

Discovery of terrestrial allabogdanite (Fe,Ni)2P(Fe,Ni)_{2}P, and the effect of Ni and Mo substitution on the barringerite-allabogdanite high-pressure transition

Minerals formed at high pressures are sensitive indicators of extreme pressure-temperature conditions that occur in nature. The discovery of the high-pressure polymorph of (Fe,Ni)$_2$P, allabogdanite in the surficial pyrometamorphic rocks of the Hatrurim Formation (the Mottled Zone) surrounding the Dead Sea basin in Israel is the first terrestrial occurrence of a mineral previously only found in iron meteorites. Stepwise annealing experiments demonstrate that allabogdanite is metastable at ambient pressure and that it irreversibly transforms into its low-pressure polymorph, barringerite, upon heating to 850±50°C. High-pressure high-temperature diamond-anvil cell (DAC) experiments confirm the results of annealing experiments. The DAC data indicate that Hatrurim allabogdanite is metastable below 7.4 GPa, and the low- to high-pressure phase transition (barringerite→allabogdanite) occurs at 25±3 GPa and 1400±100°C. The observed transition pressure of Hatrurim allabogdanite is significantly higher than that of pure synthetic Fe$_2$P (8 GPa), due to partial substitution of Fe for Ni (4 wt.%) and Mo (2.5 wt.%). Because the influence of substituting impurities on the conditions of phase transitions can be unexpectedly strong, our findings confirm that caution should be exercised when extrapolating data from experiments on synthetic compounds to natural systems. Based on the discovery of terrestrial allabogdanite (Fe,Ni)$_2$P coupled with experiments probing the phase transitions in this natural composition, we contend that terrestrial allabogdanite formed via transformation from barringerite and posit potential scenarios of its formation