During the last 60 years, new high pressure techniques and their exploitation have permitted the extension of attainable pressure/volume conditions, increased versatility of the apparatus, and hydrostaticity of the attained pressure in a remarkable way. In preparative solid state chemistry, high-pressure/high-temperature synthesis always played a minor role due to technical difficulties and costs. Piston-cylinder and Belt-apparatus both were limited to the working range up to 3 and 10 GPa, respectively. New technical developments, which allow synthesis up to 25 GPa, open up an enormous field of sample synthesis in solid state chemistry. In the following, a short overview on the most important developments in multianvil-techniques is given with respect to their applications for solid state chemistry under high-pressure conditions.
The praseodymium borate-nitrate Pr[B5O8(OH)(H2O)0.87]NO3·2H2O was obtained in a hydrothermal synthesis. It crystallizes monoclinically in the space group P21/n (no. 14) with four formula units (Z=4) and unit cell parameters of a=641.9(3), b=1551.8(7), c=1068.4(5) pm, with β=90.54(2)° yielding V=1.0643(8) nm3. The defect variant constitutes the missing member in the series of isostructural, early rare earth borate-nitrates of the composition RE[B5O8(OH)(H2O)x]NO3·2H2O [RE=La (x=0; 1), Ce (x=1), Nd (x=0.85), Sm (x=0)]. In addition to powder and single-crystal X-ray diffraction data, the novel borate-nitrate was characterized through IR and Raman spectroscopy.
Ni6B22O39·H2O was synthesized in a high-pressure/high-temperature reaction at 5 GPa/900°C. It crystallizes in the orthorhombic space group Pmn21 (no. 31) with the lattice parameters a=7.664(2), b=8.121(2) and c=17.402(2) Å. The crystal structure is discussed with regard to the isotypic compounds M6B22O39·H2O (M=Fe, Co) and the structurally related phase Cd6B22O39·H2O. Furthermore, the characterization of Ni6B22O39·H2O via X-ray powder diffraction and vibrational spectroscopy is reported.
α-Y2B4O9 was synthesized in a high-pressure/high-temperature experiment at 12.3 GPa/1020°C. The crystal structure has been determined via single-crystal X-ray diffraction. α-Y2B4O9 is isotypic to the lanthanide borates α-Ln2B4O9 (Ln=Sm–Ho) and crystallizes in the monoclinic space group C2/c (no. 15) with the following lattice parameters: a=25.084(2), b=4.3913(2), c=24.726(2) Å, and β=99.97(1)°. The compound was further characterized via X-ray powder diffraction as well as IR and Raman spectroscopy.
β-Y(BO2)3 was synthesized in a Walker-type multianvil module at 5.9 GPa/1000°C. The crystal structure has been elucidated through single-crystal X-ray diffraction. β-Y(BO2)3 crystallizes in the orthorhombic space group Pnma (no. 62) with the lattice parameters a=15.886(2), b=7.3860(6), and c=12.2119(9) Å. Its crystal structure will be discussed in the context of the isotypic lanthanide borates β-Ln(BO2)3 (Ln=Nd, Sm, Gd–Lu).
Li3Y(BO3)2 was prepared by high-temperature solid state synthesis at 900°C in a platinum crucible from lithium carbonate, boric acid, and yttrium(III) oxide. The compound crystallizes monoclinically in the space group P21/c (no. 14) (Z=4) isotypically to Li3Gd(BO3)2. The structure was refined from single-crystal X-ray diffraction data: a=8.616(3), b=6.416(3), c=10.014(2) Å, β=116.6(2)°, V=494.9(3) Å3, R1=0.0211, and wR2=0.0378 for all data. The crystal structure of Li3Y(BO3)2 consists of [Y2O14] dinuclear units, which are interconnected to each other by planar B(1)O3 groups and LiO4 tetrahedra via common edges and corners along the a axis.
The mixed cation triel borate Ga4In4B15O33(OH)3 was synthesized in a Walker-type multianvil apparatus at high-pressure/high-temperature conditions of 12.5 GPa and 1300°C. Although the product could not be reproduced in further experiments, its crystal structure could be reliably determined via single-crystal X-ray diffraction data. Ga4In4B15O33(OH)3 crystallizes in the tetragonal space group I41/a (origin choice 2) with the lattice parameters a = 11.382(2), c = 15.244(2) Å, and V = 1974.9(4) Å3. The structure of the quaternary triel borate consists of a complex network of BO4 tetrahedra, edge-sharing InO6 octahedra in dinuclear units, and very dense edge-sharing GaO6 octahedra in tetranuclear units.
Fe2B2O5, synthesized under mild high-pressure / high-temperature conditions of 3 GPa and 960 ◦C, possesses a structure isotypic to the triclinic pyroborates M2B2O5 with M = Mg, Mn, Co, and Cd. Although the parameter pressure is not essential to the synthesis of Fe2B2O5, the specific conditions enhance the crystallinity of the product. Therefore, the crystal structure of the iron pyroborate Fe2B2O5 could be determined via single crystal diffraction data [space group P1̄ (Z = 4) with the parameters a = 323.1(1), b = 615.7(2), c = 935.5(2) pm, α = 104.70(3), β = 90.82(3), γ = 91.70(3)◦, V = 0.1799(1) nm3, R1 = 0.0409, and wR2 = 0.0766 (all data)]. The structure is built up from layers of isolated pyroborate units ([B2O5]4−), which are composed of two corner-sharing BO3 triangles. These pyroborate layers serve to bridge 4×1 ribbons of edge-sharing FeO6 octahedra by both edgeand corner-sharing.
Monoclinic holmium sesquioxide B-Ho2O3 and orthorhombic holmium orthogallate HoGaO3 were synthesized in a Walker-type multianvil apparatus under high-pressure / high-temperature conditions of 11.5 GPa / 1250 °C and 7.5 GPa / 1250 °C, respectively. Both crystal structures could be determined by single-crystal X-ray diffraction data, collected at r. t. The monoclinic holmium oxide crystallizes in the space group C2/m (Z = 6) with the parameters a = 1394.7(3), b = 350.83(7), c = 865.6(2) pm, β = 100.23(3)°, R1 = 0.0517, wR2 = 0.1130 (all data), and the orthorhombic compound HoGaO3 in Pnma (Z = 4) with the parameters a = 553.0(2), b = 753.6(2), c = 525.4(2) pm, R1 = 0.0222, and wR2 = 0.0303 (all data).