Volume 228, Issue 9 - Borates. Part I

The paper presents a brief review of the present state of high-temperature borate crystal chemistry. This review summarizes the results of high- and low-temperature single crystal X-ray diffraction studies for more than 10 borate structures and high-temperature powder Xray diffraction data for about 65 borates. Thermal behavior of their crystal structures, thermal expansion, polymorphic transitions and their relationship to borate glasses are presented. These studies allow to formulate the basic principles of high-temperature borate crystal chemistry and to reveal the regularities of thermal behavior of borates. On heating, the BO 3 and BO 4 polyhedra and rigid groups consisting of these polyhedra, practically maintain their configuration and size, but they are able to rotate like hinges exhibiting highly anisotropic thermal expansion, including linear negative expansion. Based on these results, we generalize the term “rigid group” and render thermal vibrations as the key ingredient for the self-assembly of borate rigid groups.

Three new uranyl borates, K(UO 2 )(BO 3 ) (1), Rb(UO 2 )(BO 3 ) (2) and Cs(UO 2 )(BO 3 ) (3) have been prepared using B 2 O 3 fluxes at 1000 ºC. 1 and 3 can also be synthesized using potassium tetraborate tetrahydrate as a molten flux at 290 ºC, which provides a new low temperature route to prepare actinide borates. 1 possesses an a-uranophane anion sheet topology identical to that in the previously reported compounds Li(UO 2 )(BO 3 ) and Na(UO 2 )(BO 3 ). 2 and 3 display a new [UO 5 ]1 ∞ anion topology with a basic building block of edge-sharing dimers comprised of (UO 7 ) 2 polyhedra. The A(UO 2 )(BO 3 ) (A ¼ alkali metal) series shows a systematic structural evolution, which will be discussed in the main text.

Single crystals of novel metastable borate Ba 3 Bi 2 (BO 3 ) 4 , were obtained from melt with stoichiometric composition. The compound is structurally related to the A3Ln2(BO 3 ) 4 family of luminescent borates (A = Ca, Sr, Ba; Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb). The crystal structure was solved by direct methods and refined to R = 0.041 (wR = 0.045), orthorhombic, Pnma, a = 7.9508(5), b = 17.399(1), c = 8.9791(5) A˚ , V = 1242.1(1) A˚ 3, Z = 4. The structure contains isolated BO 3 triangles linked through Ba 2+ and Bi 3+ cations distributed among three cation sites, M1, M2 and M3. The mixture of the Ba 3 Bi 2 (BO 3 ) 4 and BaBiBO 4 phases in the ~1 : 1 ratio was investigated by means of high-temperature X-ray powder diffraction in air. Ba 3 Bi 2 (BO 3 ) 4 demonstrates an anisotropic thermal expansion with the following coefficients: a a = 16, a b = 11, a c = 11, a V = 38 x 10 -6 K -1 at 25 ºC. The anisotropy increases on heating: a a = 32, a b =-2, ac = 7, a V = 37 x 10 -6 K -1 at 700 ºC. The reason for the strong anisotropy of thermal expansion is the preferable orientation of the BO 3 triangles.

Two new ternary borates, Rb 3 MB 5 O 10 (M = Zn, Cd) have been synthesized by solid-state reactions, and their structures were determined by single-crystal X-ray diffraction. They crystallize in the same monoclinic space group P2 1/n with lattice constants a = 10.449(11) A˚ , b = 9.414(10) A˚ , c = 11.690(12) A˚ , and b = 105.888(11)º for Rb3ZnB5O10, while a = 7.6391(5)A˚ , b =19.2304(13)A˚ , c = 7.6905(5)A˚ , and β = 116.698(4)º for Rb 3 CdB 5 O 10 . And their crystal structures both contain the interesting two-dimension (2D) infinite [MB 5 O 10 ]1 ∞ (M = Zn, Cd) layers. However, owing to the different configurations and stacking directions of the [MB 5 O 10 ]1 ∞ (M = Zn, Cd) layers, Rb 3 ZnB 5 O 10 and Rb 3 CdB 5 O 10 are not isostructural. The IR spectra further confirm the presence of both BO 3 and BO 4 groups and UV-Vis-NIR diffuse reflectance spectra show their band gaps are 2.66 and 2.34 eV for Rb 3 ZnB 5 O 10 and Rb 3 CdB 5 O 10 , respectively.

The new cerium polyborate Ce 2 B 8 O 15 was synthesized under high-pressure/high-temperature conditions of 6 GPa and 1050 ºC in a Walker-type multianvil apparatus. The single-crystal structure determination revealed that the new compound crystallizes in the space group P2/c with the lattice parameters a = 916.6(2), b = 421.0(1), c = 1248.9(3) pm, β = 116.7(1)º, V = 0.4303(2) nm 3 , R1 = 0.0356, and wR2 = 0.0504. The crystal structure of Ce 2 B 8 O 15 exhibits a new fundamental building block (FBB) in borate chemistry that consists of four BO 4 tetrahedra and can be written as 4□: [Φ] 〈3□〉|□|. These FBB are interconnected via common corners, forming a complex threedimensional network that contains the Ce 3+ cations. Ce 2 B 8 O 15 represents the most boron rich rare-earth borate synthesized under high-pressure/high-temperature conditions so far. We report about the synthetic conditions, structural details, thermal behaviour, and the IR/Raman spectra of Ce 2 B 8 O 15 .

The crystal structure of boron-doped mullite (B-mullite) was studied by means of neutron powder diffraction and 11 B solid state, magic-angle-spinning (MAS) NMR spectroscopy. The samples were prepared from single- phase gels consisting of aluminum nitrate nonahydrate, tetraethoxysilane, and boric acid, annealed at 1200 ºC. Boron resides in a planar, trigonally coordinated position crosslinking the octahedral chains perpendicular to the caxis. The B position was derived from a combination of Rietveld refinements with difference Fourier calculations and gridsearch analyses applied to a series of samples with varying B contents. Based on the sample with the highest B content the local BO 3 configuration was determined by distance least squares (DLS) refinements resulting in split positions for two O atoms in the AlO 6 octahedron which leads to a local distortion of the octahedral geometry. This model was verified by subsequent Rietveld refinements of several samples, details of the crystal structure are given for the sample with the highest B content. The model for B incorporation is corroborated by the MAS NMR results which clearly show that the 11B isotropic chemical shift (δ iso ) and quadrupolar coupling constant and asymmetry paramater (C Q , η) describe that of threecoordinate B in a near-trigonal planar environment. Furthermore this crystallographic model provides an explanation for the anisotropic behavior of the lattice parameters upon B-incorporation.

Among the borates in the Middle European Zechstein Salt Succession boracite Mg 3 [B 7 O 13 Cl] is the most common mineral in quantity and local distribution. An exceptional enrichment is observed in Stassfurt Serie (Z2) in the Stassfurth seam K 2 H. Boracite is to be found in two varieties: individual crystals in cubic, tetrahedral or dodecahedral habit on the one hand and fibrous crystals so-called “stassfurtite” on the other hand. The formation conditions such widely spread borates in the salt succession are ambiguous in two respects. First of all the synthetic formation of boracites is to be made by hydrothermal or melt conditions. Both processes can be suspended for the salt succession. Furthermore the cubic modification is stable above 265 ºC for the Mg-boracite. The cubic, tetrahedral or dodecahedral habit could be used as a geothermometer, but such conditions can be exclude by the paragenetic minerals, esp. carnallite (MgKCl 3 ⋅ 6 H 2 O). The chemical composition of orthorhombic, pseudo-cubic boracite depends on the location. Pure Mg-boracite in hexahedral habit and in fibrous habit, so-called “stassfurtite”, occurs in the North Harz region, whereas the Fe-, Mn-, Mg-boracite appears in the South Harz region. Until now the source of boron, the time of formation of crystals, but also the reasons for the differences in habit of the single hexahedral crystals are still unclear. The formation during a diagenetic/metamorphic process is evident. However, the preferred formation in Stassfurt seam could be an indication for the boron enrichment in an early diagenetic process. Furthermore permit the determination of the thermal stability and the volatile content of crystals conclusions to the chemical composition of the fluid. The observed variation suggests that the condition of crystal growth as well as the chemical composition of fluid repeatedly changed over the time. Randomly occuring xenomorpheous anhydrite and magnesite inclusions within single boracite crystals have been interpreted as an indication to factors of chemical milieu during the formation of crystals. The reversible phase transition temperature of the boracite is a linearly function of the iron and manganese content and varies from 265 ºC for Mg-boracite to 330 ºC for Fe(Mn)-boracite. The thermal decomposition of boracite is determined by two processes. The decomposition started with a boronchlorine release (BOCl?), having a maximum rate at 1050 ºC. Additionally to this release one observes a simultaneous emission of H 2 O, HCl, HF, CO 2 , N 2 , SO 2 , H 2 , and hydro carbons. The results give evidence for the aged approach of a secondary formation of boracite within the complete Stassfurt seam, possibly in connection with the formation of salt diapirs in the Jura and Cretaceous period. The wider environmental distribution of borates is an indication of chemical transport processes within the salt succession. This should be a more important issue in the discussion about the utilisation of salt diapirs for the storage of nuclear waste.

Na 3 B 2 PO 5 F 4 has been synthesized by an allsolid- state reaction between Na 3 PO 4 and NaBF 4 and, alternatively by solid-gas reaction between Na 3 PO 4 and BF 3 . The new fluoridoboratophosphate is dimorphic. Monoclinic Na 3 B 2 PO 5 F 4 -II (P2 1 /n, a = 6.5230(6), b = 12.174(1), c = 8.8279(8) Å, β= 102.322(5)º) and orthorhombic Na 3 B 2 PO 5 F 4 -II (Cmcm, a = 12.175(2), b = 6.719(1), c = 9.077(1) Å) were obtained at 623 and 673 K, respectively. The latter is a dynamically or statically disordered polymorph of the former. Both modifications contain a cyclic six membered anion build up by three interconnected tetrahedra, i.e. one PO 4 and two BF 2 O 2 units. These complex cyclo-anions feature the same number of valence electrons as cyclo-triphosphate [P 3 O 9 ] 3- and fluoridooxidoborate [B 3 O 3 F 6 ] 3- .

The here reported compounds of the isotypic series Ln 4 O 4 Cl[BO 3 ] with Ln = Eu-Tm represent the first type of oxide chloride oxoborates with trivalent lanthanoid cations. They crystallize monoclinically with the space group P2 1 /n and four formula units per unit cell. The cell dimensions range from a = 878.21(6), b = 818.42(5), c = 1091.50(7) pm, β = 101.49(1)º for Eu 4 O 4 Cl[BO 3 ] to a = 847.08(6), b = 795.23(5), c = 1083.16(7) pm, β = 103.64(1)º for Tm 4 O 4 Cl[BO 3 ] and are in good agreement with the lanthanoid contraction. The basic building blocks of this structure consist of O 2- -centered (Ln 3+ ) 4 tetrahedra ([OLn 4 ] 10+ ) linked first via common edges to 1 ∞ {[O 12 Ln 10 ] 6+ } double chains running along the [010] direction. Their further fusion again via shared edges leads to highly corrugated 2 ∞ {[O 4 Ln 4 ] 4+ } layers spreading out parallel to the (101) plane. These layers are held together by isolated [BO 3 ] 3- triangles and Cl - anions. The four crystallographically different Ln 3+ cations show seven or eight O2 - and Cl - anions in their coordination spheres.