Magnesium cations as templates for the self-assembly of supramolecular luminescent {Mg@[B18φ34-35]}-clusters.

Solvothermal reaction of magnesium nitrate and boron oxide in N,N-dimethylformamide produced a number of particularly complex supramolecular magnesium borates. Five topologically different types of negatively charged {Mg@[B18φ34-35]}-clusters, φ = O, OH, were observed with the magnesium cation as a core and octadecaborate anions as shells. The clusters assemble via common borate polyhedra forming 1D chains, a 2D mesoporous layer, and 3D mesoporous frameworks with an effective channel width of up to 16 Å. Topological analysis of the clusters in combination with the modular crystallography approach indicates that numerous new functional materials can be obtained by varying their assembly mode. At least one compound containing such clusters exhibits a very strong luminescence.

Crystal structure refinement, low- and high-temperature X-ray diffraction and Mössbauer spectroscopy study of the oxoborate ludwigite from the Iten'yurginskoe deposit.

This paper reports an investigation of the chemistry, crystal structure refinement and thermal behavior (80-1650 K) of ludwigite from the Iten'yurginskoe deposit (Eastern Chukotka, Russia). Its chemical composition was determined by electron microprobe analysis, giving an empirical formula (Mg1.70Fe2+0.29Mn0.01)Σ2.00(Fe3+0.90Al0.08Mg0.02)Σ1.00O2(BO3). A refinement of the crystal structure from single-crystal X-ray diffraction data (SCXRD) was provided for the first time for ludwigite from this deposit (R = 0.047). The structure can be described as a framework composed of [MO6]n- octahedra and isolated [BO3]3- triangles located in triangular interstices of the framework. Based on a comprehensive analysis of SCXRD and Mössbauer spectroscopy data, the M1 site is occupied by Mg, M2 and M3 by Mg and Fe2+, M4 by Fe3+, Mg and Al. There are also oxo-centered [O4M4]n+ and [O2M5]n+ polyhedra building up a framework with the [BO3]3- triangles located in its hexagonal interstices. No indications of magnetic ordering are found in the temperature range investigated. The Fe2+ → Fe3+ oxidation occurs above 600 K, and is accompanied by a decrease of the unit-cell parameters and subsequent incomplete solid-phase decomposition with the formation of hematite, warwickite and magnetite. The mineral melts at temperatures above 1582 K. The thermal expansion of ludwigite is slightly anisotropic, which is explained by a dense packing of the [MO6]n- octahedra as well as a virtually perpendicular orientation of the oxo-centered double chains to each other. At room temperature, maximum expansion is along the c axis (αc = 9.1 × 10-6 K-1) and minimum expansion is in the ab plane (αa = 8.6 × 10-6, αb = 7.6 × 10-6 K-1), which is due to the preferred orientation of the [BO3]3- triangles. A comparison of the thermal behavior of three oxoborates of the ludwigite group, namely azoproite (Mg,Fe2+)2(Fe3+,Ti,Mg,Al)O2(BO3), vonsenite (Fe2+,Mg)2(Fe3+,Mn2+,Sn,Al)O2(BO3) and ludwigite (Mg,Fe2+,Mn)2(Fe3+,Al,Mg)O2(BO3), is provided.

Ag4B7O12X (X = Cl, Br, I) Heptaborate Family: Comprehensive Crystal Chemistry, Thermal Stability Trends, Topology, and Vibrational Anharmonicity.

Using glass crystallization and solid-state techniques, we were able to complete the family of salt-inclusion silver halide borates, Ag4B7O12X, by the X = Cl and I members. The new compounds are characterized by differential scanning calorimetry, single-crystal and high-temperature powder X-ray diffraction, optical spectroscopy, and density functional theory calculations. In all structures, the silver atoms exhibit strong anharmonicity of thermal vibrations, which could be modeled using Gram-Charlier expansion, and its asymmetry was characterized by the skewness vector. The topology of the silver halide and borate sublattices has been analyzed separately for the first time. Along the I → Br → Cl series, we observe a decrease of the melting point and configuration entropy and an increase of thermal expansion and its anisotropy and thermal vibration anharmonicity, which indicates decreasing stability.

The first bismuth borate oxyiodide, Bi4BO7I: commensurate or incommensurate?

The first bismuth borate oxyiodide, Bi4BO7I, has been prepared by solid-state reaction in evacuated silica ampoules. Its crystal structure [space group Immm(00γ)000] comprises litharge-related layers of edge-sharing OBi4 tetrahedra; the interlayer space is filled by I- and [BO3]3- anions. The wavevector, q = 0.242 (3)c*, is very close to the rational value of c*/4, yet refinement based on commensurate modulation faces serious problems indicating the incommensurate nature of the modulation. The I-/[BO3]3- anions are ordered in a complex sequence along [001], i.e. -<-BO3-BO3-I-I->n = 28-I-I-I-<-BO3-BO3-I-I->n = 28-BO3-BO3-BO3-, leading to a structural modulation. The principal feature of the latter is the presence of -I-I-I- and -BO3-BO3-BO3- sequences that cannot be accounted for in the a × b × 4c supercell. The thermal expansion of Bi4BO7I is weakly anisotropic (αa = 8, αb = 15 and αc = 17 × 10-6 K-1 at 500 K) which is caused by preferential orientation of the borate groups.

Investigation of thermal behavior of mixed-valent iron borates vonsenite and hulsite containing [OM4]n+ and [OM5]n+ oxocentred polyhedra by in situ high-temperature Mössbauer spectroscopy, X-ray diffraction and thermal analysis.

The investigation of elemental composition, crystal structure and thermal behavior of vonsenite and hulsite from the Titovskoe boron deposit in Russia is reported. The structures of the borates are described in terms of cation-centered and oxocentred polyhedra. There are different sequences of double chains and layers consisting of oxocentred [OM4]n+ tetrahedra and [OM5]n+ tetragonal pyramids forming a framework. Elemental composition was determined by energy-dispersive X-ray spectroscopy (EDX). Oxidation states and coordination sites of iron and tin in the oxoborates are determined using Mössbauer spectroscopy and compared with EDX and X-ray diffraction data (XRD). According to results obtained from high-temperature Mössbauer spectroscopy, the Fe2+ to Fe3+ oxidation in vonsenite and hulsite occurs at approximately 500 and 600 K, respectively. According to the high-temperature XRD data, this process is accompanied by an assumed deformation of crystal structures and subsequent solid-phase decomposition to hematite and warwickite. It is seen as a monotonic decrease of volume thermal expansion coefficients with an increase in temperature. A partial magnetic ordering in hulsite is observed for the first time with Tc ≃ 383 K. Near this temperature, an unusual change of thermal expansion coefficients is revealed. Vonsenite starts to melt at 1571 K and hulsite melts at 1504 K. Eigenvalues of thermal expansion tensor are calculated for the oxoborates as well as anisotropy of the expansion is described in comparison with their crystal structures.

Bridging the Salt-Inclusion and Open-Framework Structures: The Case of Acentric Ag4B4O7X2 (X = Br, I) Borate Halides.

An acentric borate family, Ag4B4O7X2 (X = Br, I), has been prepared by slow cooling stoichiometric melts in evacuated silica ampules. Their crystal structure is comprised of two porous interpenetrating frameworks and demonstrates a further development of the "salt-inclusion" architecture toward a "covalent-inclusion" structure. The (Ag2X)+ sublattice shows strong anharmonic vibrations. Thermal expansion is strongly anisotropic because of the presence of condensed rigid kernite boron-oxygen chains aligned perpendicular to the c axes.

The first silver bismuth borate, AgBi2B5O11.

The first silver bismuth borate, AgBi2B5O11 (silver dibismuth pentaborate), has been prepared via glass crystallization in the Ag2O-Bi2O3-B2O3 system and characterized by single-crystal X-ray diffraction. Its structure is derived from that of centrosymmetric Bi3B5O12 by ordered substitution of one Bi3+ ion for Ag+, which results in the disappearance of the mirror plane and inversion centre. Second harmonic generation (SHG) measurements confirm the acentric crystal structure. It is formed by [Bi2B5O11]∞ layers stretched along c and comprised of vertex-sharing B5O10 and BiO3 groups which incorporate the Ag+ cations. The new compound was characterized by thermal analysis, high-temperature powder X-ray diffraction, and vibrational and UV-Vis-NIR (near infrared) spectroscopy. Its thermal expansion is strongly anisotropic due to the presence of rigid B5O10 groups aligned in a parallel manner. The minimal value is observed along their axis [parallel to c, αc = 3.1 (1) × 10-6 K-1], while maximal values are observed in the ab plane [αa = 20.4 (2) and αb = 7.8 (2) × 10-6 K-1]. Upon heating, AgBi2B5O11 starts to decay above 684 K due to partial reduction of silver; incongruent melting is observed at 861 K. According to density functional theory (DFT) band-structure calculations, the new compound is a semiconductor with an indirect energy gap of 3.57 eV, which agrees with the experimental data (absorption onset at 380 nm).

The novel borate Lu5Ba6B9O27 with a new structure type: synthesis, disordered crystal structure and negative linear thermal expansion.

Single crystals of Lu5Ba6B9O27 were obtained by cooling from a melt and polycrystals of the borate were prepared using a multi-step solid-state synthesis. The crystal structure was determined from single-crystal X-ray diffraction data. The borate crystallizes in a new structure type in the monoclinic crystal system in space group C2/c, with cell parameters a = 13.0927 (3), b = 9.9970 (2) and c = 20.4884 (4) Å, ß = 106.827 (1)°, V = 2566.86 (9) Å3 and Z = 4. It is described as a framework composed of rings consisting of vertex-sharing [BO3] triangles and [LuO6] octahedra. The Ba atoms are in the cavities of the framework. The structure is disordered: one of the B atoms is surrounded by six O atoms with partial occupancies of 0.5. The thermal properties of Lu5Ba6B9O27 were investigated by thermal analysis and high-temperature X-ray powder diffraction. Its thermal expansion is highly anisotropic. The negative expansion (contraction) is along the b axis, i.e. parallel to the planes of the largest number of [BO3] triangles. The coefficient of negative linear expansion ranges from -1.42 (at 20°C) to -5.57 × 10-6 °C-1 (at 1000°C). Thermal deformation of the ac plane is described in terms of the theory of shear deformation of monoclinic crystals. The Lu5Ba6B9O27 sample melts at 1170°C.

Synthesis and Characterization of the High-Pressure Nickel Borate γ-NiB4O7.

γ-NiB4O7 was synthesized in a high-pressure/high-temperature experiment at 5 GPa and 900 °C. The single-crystal structure analysis yielded the following results: space group P6522 (No. 179), a = 425.6(2), c = 3490.5(2) pm, V = 0.5475(2) nm3, Z = 6, and Flack parameter x = -0.010(5). Second harmonic generation measurements confirmed the acentric crystal structure. Furthermore, γ-NiB4O7 was characterized via vibrational as well as single-crystal electronic absorption spectroscopy, magnetic measurements, high-temperature X-ray diffraction, differential scanning calorimetry, and thermogravimetry. Density functional theory-based calculations were performed to facilitate band assignments to vibrational modes and to evaluate the elastic properties and phase stability of γ-NiB4O7.

Structural Evolution from 0D Units to 3D Frameworks in Pb Oxyhalides: Unexpected Strongly Corrugated Layers in Pb7O6Br2.

Novel Pb7O6Br2 (1) lead oxybromide was prepared from Pb oxybromide melt by the "rapid quenching" route. Bonding scheme, thermal expansion, and structural properties were studied. The structural features of this unexpectedly complex phase are described on the basis of lone electron pair stereochemical activity and Pb-Br versus Pb-O bonding scheme. The structure of 1 contains a number of cavities, which can be assigned to the self-containments of the lone electron pairs on Pb(2+) cations. "Empty" □Pb4 chains are observed in between of the folding sides of the adjacent strongly corrugated oxocentered [Pb7O6](2+) layers. Highly isotropic thermal expansion of 1 appeared to be unexpected. The possible explanations of such a behavior in 1 are given. The structure of 1 is an interesting example of tetrahedral framework with mixed chemical bonding and is the densest known among Pb oxyhalides with the density of 18.4 tetrahedra/1000 Å(3). Current study shows that oxocentered layers derivatives from α-PbO can be very flexible and form rather dense three-dimensional structural topologies. The properties and structure are compared to other phases crystallizing in the anhydrous PbO-PbX2 (X = F, Cl, Br, I) systems, illustrate the complexity of lead oxyhalides, and reveal new and general pathways for the targeted synthesis of new phases with the Pb-O units of desired dimensionality. The indirect gap value of ∼ 2.04 eV obtained from generalized gradient approximation calculations demonstrates potentially good photocatalytic properties of 1.

Incommensurate modulation and thermal expansion of Sr3B(2†+†x)Si(1†-†x)O(8†-†x/2) solid solutions.

Crystal structures of Sr3B(2 + x)Si(1 - x)O(8 - x/2) solid solutions with nominal compositions x = 0.28, 0.53, 0.78 in the Sr3B2SiO8-Sr2B2O5 section of the SrO-B2O3-SiO2 system are refined using single-crystal X-ray diffraction data. Incommensurate structure modulations are mainly associated with various orientations of corner-sharing (B,Si)-polyhedra. Preference is given to the (3 + 2)-dimensional symmetry group Pnma(0ßγ)000(0ßγ)000 for a single crystal compared with an alternate model of a twin formed by monoclinic components, each of them corresponding to the (3 + 1)-dimensional symmetry group P2(1)/n(0ßγ). Single-phase polycrystalline samples of solid solutions are investigated by high-temperature X-ray powder diffraction in air. Orientation preferences of the BO3 units lead to a strong anisotropy of thermal expansion. Negative expansion is observed along the a axis over the temperature range 303-753 K. Anisotropy decreases both on heating and decreasing of the boron content.