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Angewandte Chemie 130(29), 9186 - 9191 (2018). doi:10.1002/ange.201805152, A nitrogen‐rich compound, $\mathrm{ReN_{8} ⋅ x N_{2}}$, was synthesized by a direct reaction between rhenium and nitrogen at high pressure and high temperature in a laser‐heated diamond anvil cell. Single‐crystal X‐ray diffraction revealed that the crystal structure, which is based on the ReN8 framework, has rectangular‐shaped channels that accommodate nitrogen molecules. Thus, despite a very high synthesis pressure, exceeding 100 GPa, $\mathrm{ReN_{8} ⋅ x N_{2}}$ is an inclusion compound. The amount of trapped nitrogen (x) depends on the synthesis conditions. The polydiazenediyl chains $\mathrm{[−N=N−]_∞}$ that constitute the framework have not been previously observed in any compound. Ab initio calculations on $\mathrm{ReN_{8} ⋅ x N_{2}}$ provide strong support for the experimental results and conclusions., Published by Wiley-VCH, Weinheim

Journal of the American Ceramic Society 100(10), 4893 - 4901 (2017). doi:10.1111/jace.14936, The compressional behavior and the P-induced deformation mechanisms at the atomic scale of (Cs,K)Al$_{4}$Be$_{5}$B$_{11}$O$_{28}$ (londonite, a ~7.31 Å and space group $P^{-}_4$3m) were investigated by in situ single-crystal synchrotron X-ray diffraction with a diamond anvil cell up to 26 GPa. No phase transition was observed within the P-range investigated: this material exhibits isotropic compression (i.e., with cubic symmetry) in response to the applied pressure. Fitting the P–V data with a Birch-Murnaghan isothermal equation of state, we obtained: V$_0$=390.8(3) Å$^3$, K$_{P0}$=212(7) GPa (β$_0$=1/K$_{P0}$=0.0047(1) GPa$^{−1}$) and K′=4.6(6). A series of structural refinements, based on the high-pressure intensity data, were performed. The stiffness of londonite (similar to that of carbides) is governed by its close-packing structure, and in particular by the very low compressibility of B- and Be-tetrahedra and the modest compressibility of the Al-octahedra. The Cs-polyhedra are the most compressible units of the structure. The effects of pressure can be accommodated by intrapolyhedral compression or deformation, leading to a modest bulk compression. The high amount of boron in londonite (B$_2$O$_3$ ~50 wt%) makes its synthetic counterpart a potential neutron absorber. In addition, the high content of Cs makes londonite-type materials as potential hosts for nuclear waste., Published by Wiley-Blackwell, Oxford [u.a.]

SIMP-SGI-SOGEI-AIV joint conference, Pisa, University of Milano, Italy, 3 Sep 2017 - 6 Sep 2017, Londonite is a rare Cs-bearing mineral with ideal chemical formula (Cs,K)Al4Be4(B,Be)12O28 (with Cs > K). The building block units of the structure of londonite are represented by clusters of four edge-sharing Al-octahedra linked to B- and Be-tetrahedra. Gatta et al. (2011) investigated the phase stability and the elastic behavior of londonite up to 4.85(5) GPa (at room-T) and up to 1000°C (at room-P) by in situ X-ray powder diffraction data, but no structure refinements were possible. Whether no phase transition was observed within the pressure-range investigated, londonite proved to have an extremely high bulk modulus: KP0 = 280(12) GPa, similar to those of carbides (e.g., B4C with KP0 ~ 245-306 GPa; Lazzari et al., 1999; Fujii et al., 2010). Considering the thermo-elastic properties and the significantly high fraction of boron (B2O3 ~50 wt%), the synthetic counterparts of londonite could be considered a potential inorganic host for 10B in composite neutron-absorbing materials. Furthermore the high content of Cs makes londonite-type materials potential host for nuclear waste. However, to date, because of the absence of structural data at high pressure and to the modest P-range investigated by Gatta et al. (2011), a comprehensive description of the P-induced deformation mechanisms at the atomic scale is still missing. In this study, the isothermal compressional behaviour of londonite is studied by in situ single-crystal synchrotron X-ray diffraction experiment with a diamond anvil cell up to 25 GPa. The compressional behavior and the deformation mechanisms at the atomic scale are described. Londonite does not experience any phase transition or change of the compressional behavior within the P-range investigated.Fujii, T., Mori, Y., Hyodo, H., Kimura, K. (2010): X-ray diffraction study of B4C under high pressure. J. Phys. Conf. Ser., 215, 012011. Gatta, G.D., Vignola, P., Lee, Y. (2011): Stability of (Cs,K)Al4Be5B11O28 (londonite) at high pressure and high temperature: a potential neutron absorber material. Phys. Chem. Miner., 38, 429-434. Lazzari, R., Vast, N., Besson, J.M., Baroni, S., Dal Corso, A. (1999): Atomic structure and vibrational properties of icosahedral B4C boron carbide. Phys. Rev. Letters, 83, 3230-3233.

The 25th Annual Conference of the Italian Synchrotron Light Society, Trieste, Italy, 5 Oct 2017 - 6 Oct 2017, Colemanite, CaB3O4(OH)3*H2O, is one of the most important mineral commodities for the extraction of boron. Despite a recent interest on its potential applications in ceramic processes, very few was known on the behavior of the colemanite crystal structure at non-ambient conditions of temperature and pressure. An in situ low-temperature X-ray diffraction experiment was performed down to 104 K at the XRD1 beamline of the Elettra synchrotron source. A displacive phase transition from the centrosymmetric P21/a colemanite to a ferroelectric polymorph with P21 symmetry was long time known to occur in the T-range between 273 and and 263 K (e.g. [1]). Thermal analysis and in situ single-crystal X-ray diffraction data confirmed the transition, which was found to occur between 275 and 263 K. Single crystal X-ray and neutron diffraction data (down to 104 and 20 K, respectively) showed that the asymmetric distribution of ionic charges along the b crystallographic axis is likely responsible for the observed ferroelectric behavior. On the other hand, in situ high-pressure single-crystal X-ray diffraction experiments, performed up to 24 GPa at the P02.2 beamline of the Petra-III synchrotron source (Hamburg, Germany), disclosed a much more complex scenario, with a first-order reconstructive phase transition occurring between 13.95 and 14.91 GPa, toward a denser polymorph with a = 3*aCOL, b = bCOL and c = 2*cCOL. Despite reconstructive, the transition is single crystal-to-single crystal and involves an increase in the average coordination number of both the Ca and B sites. The tripling of the a-axis and the doubling of the c-axis imply the split of every independent atomic site of colemanite in six new independent positions in the high-P polymorph. In particular, three of the six new sites, generated from the parent triangularly coordinated B, increase their coordination number from three to four, gaining a bond with a H2O oxygen. The elastic behavior of colemanite and of the high-P polymorph have been described by means of III- and II-order Birch-Murnaghan equations of state, respectively, yielding the following bulk moduli: 67(4) GPa (colemanite, KV' = 5.5(7)) and 50(8) GPa (high-P colemanite). [1] F.N. Hainsworth, H.E. Petch Can. J. Phys. 1966, 44, 3083. [2] P. Lotti, G.D. Gatta, D. Comboni, G. Guastella, M. Merlini, A. Guastoni, H-P. Liermann J. Am. Cer. Soc. 2017, in press, DOI: 10.1111/jace.14730.

The journal of physical chemistry / C 121(3), 1546 - 1551 (2017). doi:10.1021/acs.jpcc.6b12095, In this work, the hydrogen absorption and desorption properties of the system K$_2$[Zn(NH$_2$)$_4$]-8LiH are investigated in detail via in situ synchrotron radiation powder X-ray diffraction (SR-PXD), Fourier transform infrared spectroscopy (FT-IR), and volumetric methods. Upon milling, K$_2$[Zn(NH$_2$)$_4$] and 8LiH react to form 4LiNH$_2$-4LiH-K$_2$ZnH$_4$, and then 4LiNH$_2$-4LiH-K$_2$ZnH$_2$ releases H$_2$ in multiple steps. The final products of the desorption reaction are KH, LiZn$_{13}$, and Li2NH. During rehydrogenation, KH reacts with LiZn$_{13}$ under 50 bar of hydrogen producing K$_3$ZnH$_5$. This phase appears to enhance the hydrogenation process which after its formation at ca. 220 °C takes place in only 30 s. The system 4LiNH$_2$-4LiH-K$_2$ZnH$_2$ is shown to be reversible under the applied conditions of vacuum at 400 °C for desorption and 50 bar of H2 at 300 °C for absorption., Published by ACS, Washington, DC

IUCrJ 4(5), 671 - 677(2017). doi:10.1107/S2052252517010612, 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, CaB$_2$Si$_2$O$_8$, the first compound showing a step-wise transition of Si coordination from tetrahedral to octahedral through a trigonal bipyramid. Along the compression, the Si$_2$O$_7$ groups of danburite first transform into chains of vertice-sharing SiO$_5$ trigonal bipyramids (danburite-II) and later into chains of edge-sharing SiO$_6$ octahedra (danburite-III). It is suggested that the unusual formation of an SiO$_5$ configuration is a consequence of filling up the pentacoordinated voids in the distorted hexagonal close packing of danburite-II., Published by Chester

Angewandte Chemie / International edition International edition 128(48), 15277 - 15281 (2016). doi:10.1002/ange.201608530, Titanium(III) phosphate, TiPO$_4$ , is a typical example of an oxyphosphorus compound containing covalent P−O bonds. Single‐crystal X‐ray diffraction studies of TiPO$_4$ reveal complex and unexpected structural and chemical behavior as a function of pressure at room temperature. A series of phase transitions lead to the high‐pressure phase V, which is stable above 46 GPa and features an unusual oxygen coordination of the phosphorus atoms. TiPO4‐V is the first inorganic phosphorus‐containing compound that exhibits fivefold coordination with oxygen. Up to the highest studied pressure of 56 GPa, TiPO$_4$‐V coexists with TiPO$_4$‐IV, which is less dense and might be kinetically stabilized. Above a pressure of about 6 GPa, TiPO$_4$‐II is found to be an incommensurately modulated phase whereas a lock‐in transition at about 7 GPa leads to TiPO$_4$‐III with a fourfold superstructure compared to the structure of TiPO$_4$‐I at ambient conditions. TiPO$_4$‐II and TiPO$_4$‐III are similar to the corresponding low‐temperature incommensurate and commensurate magnetic phases and reflect the strong pressure dependence of the spin‐Peierls interactions., Published by Wiley-VCH, Weinheim

MISCA2016 - The 4th joint conference of the Italian and Spanish Crystallographic Associations, MISCA2016, Tenerife, Spain, 21 Jun 2016 - 25 Jun 2016, Colemanite, CaB3O4(OH)3 x H2O, is a common constituent of natural borate deposits and a relevant mineral commodity, for its applications in the B-extraction processes and in the production of lightweight cements and ceramics [1,2]. Its crystal structure is monoclinic (P21/a, a = 8.712 Å, b = 11.247 Å, c = 6.091 Å, β = 110.12°, V = 560.4 Å3), characterized by infinite chains of Bpolyhedra based on a fundamental ring-unit made by one triangular BO3 and two B(O,OH)4 tetrahedra. These chains are alternated with chains of Ca-polyhedra (coordination number, CN =8), giving rise to sheets parallel to (010). Adjacent sheets are mainly interconnected through an H-bonding system involving hydroxyl groups and H2O molecules. The links among structural polyhedra, both B and Ca, are all corner sharing in type [3]. Despite its relevance in industrial applications, the behavior of colemanite at high pressure has never been investigated. In this work, we have characterized the elastic behavior, the phase stability, the phase transition and the structural evolution with pressure of a natural colemanite single crystal based on an in-situ synchrotron X-ray diffraction study, performed at the P02.2 beamline of PETRA-III, Hamburg, Germany. Colemanite, in its ambient conditions polymorph, is found to be stable up to 13.95 GPa. A IIIorder Birch-Murnaghan (III-BM) EoS fit leads to a refined bulk modulus (at ambient conditions) (KV0) of 76(8) GPa [K = 4.4(10)]. Comparative structure analysis, based on the refinements of the structure model at different pressures, shows that the B-polyhedra act as quasi-rigid units and the bulk compression is mainly accommodated by the compression of Ca-polyhedra and by polyhedral tilting. Between 13.95 and 14.91 GPa, colemanite undergoes a reconstructive phase transition toward a monoclinic polymorph (P21/n, a = 11.726(11) Å, b = 10.206(1) Å, c = 23.45(3) Å, β = 95.07(9)°, V = 2796(4) Å3, at 14.91 GPa). The average coordination number of boron is found to increase: of the 18 independent B sites, 3 are triangularly coordinated and 15 are tetrahedra. Two independent infinite chains of B-polyhedra are alternated with two independent chains of Capolyhedra (CN = 8 or 9), by means of corner- and edge-sharing links. A II-BM EoS fit leads to a refined KV0 of 60(6) GPa for the high-P polymorph of colemanite. X-ray diffraction patterns collected during P-release show that the phase transition is completely reversible and colemanite fully recovers its starting structural features. [1] K. Okuno Radiat. Prot. Dosim. 2005, 115, 258-261. [2] A. Christogerou, T. Kavas, Y. Pontikes, S. Koyas, Y. Tabak, G.N. Angelopoulos Ceram. Int. 2009, 35, 447-452. [3] P.C. Burns, F.C. Hawthorne Can. Mineral. 1993, 31, 297-304.

The 2nd European Mineralogical Conference, Rimini, Italy, 11 Sep 2016 - 15 Sep 2016, Phillipsite is a low Si/Al natural zeolite, often found as autogenic mineral in both “close” and “open” hydrologic system or in vugs of basalt, as an alteration product of volcanic glass. Along with laumontite, it is one of the most common zeolites found in oceanic basalts. In order to investigate the high-pressure behavior of phillipsite and its structural evolution at the atomic scale, we performed an in situ single-crystal synchrotron X-ray diffraction experiment up to 10 GPa with a diamond anvil cell, using a nominally penetrating pressure-transmitting fluid (methanol:ethanol:H2O = 16:3:1 mix) (Gatta, 2010). The unit-cell parameters and the structure refinements within the P-range investigated show that: 1) phillipsite does not adsorb further H2O molecules from the penetrating-transmitting fluid within the P-range investigated; 2) the configuration of the extra-framework population changes with pressure (between 2 and 3 GPa), affecting the elastic behavior of the mineral. More in details, two distinct compressional regimes have been observed, in which the bulk moduli differs drastically (i.e., KV = 89(8) GPa between 0 and 2 GPa, KV = 18.8(7) GPa between 3 and 9 GPa); 3) phillipsite is crystalline at least up to 10 GPa, and this is surprising if we consider its microporous nature; 4) all the P-induced effects are completely reversible in decompression. The structural refinements allowed us to describe the mechanisms, at the atomic scale, that govern its elastic behavior, which are mainly governed by inter-tetrahedral tilting. The relatively low compressibility of phillipsite at room-P and its relatively wide P-stability shown in this experiment suggests that this zeolite is a potential H2O carrier during the first phase of the oceanic crust subduction or, toward the industrial front, its potential use in systems for the mechanical energy storage/dissipation (Eroshenko et al., 2001; Soulard et al., 2004). Acknowledgements: The author acknowledges the Italian Ministry of Education, MIUR-Project: “Futuro in Ricerca 2012 - ImPACT- RBFR12CLQD”.