Many of the fundamental questions regarding the solid-state chemistry of boron are still unsolved, more than 200 years after its discovery. Recently, theoretical work on the existence and stability of known and new modifications of the element combined with high-pressure and high-temperature experiments have revealed new aspects. A lot has also happened over the last few years in the field of reactions between boron and main group elements. Binary compounds such as B(6)O, MgB(2), LiB(1-x), Na(3)B(20), and CaB(6) have caused much excitement, but the electron-precise, colorless boride carbides Li(2)B(12)C(2), LiB(13)C(2), and MgB(12)C(2) as well as the graphite analogue BeB(2)C(2) also deserve special attention. Physical properties such as hardness, superconductivity, neutron scattering length, and thermoelectricity have also made boron-rich compounds attractive to materials research and for applications. The greatest challenges to boron chemistry, however, are still the synthesis of monophasic products in macroscopic quantities and in the form of single crystals, the unequivocal identification and determination of crystal structures, and a thorough understanding of their electronic situation. Linked polyhedra are the dominating structural elements of the boron-rich compounds of the main group elements. In many cases, their structures can be derived from those that have been assigned to modifications of the element. Again, even these require a critical revision and discussion.

Single crystals of the ternary borides Cr2AlB2, Cr3AlB4, Cr4AlB6, MoAlB, WAlB, Mn2AlB2, and Fe2AlB2 were grown from the elements with an excess of Al. Structures were refined by X-ray methods on the basis of single crystal data. All compounds crystallize in orthorhombic space groups. In each case boron atoms show the typical trigonal prisms BM6. The BM6-units are linked by common rectangular faces forming B-B-bonds. Thus, zigzag chains of boron atoms are obtained for MoAlB, WAlB, and M2AlB2 (M = Cr, Mn, Fe); chains of hexagons for Cr3AlB4; and double chains of hexagons for Cr4AlB6. The same subunits are known for the binary borides CrB, Cr3B4, Cr2B3, and β-WB, too. The boride partial structures are separated by single layers of Al-atoms in the case of the chromium compounds and double layers for WAlB, i.e., W2Al2B2. All crystal structures can be described using a unified building set principle with quadratic 4(4)-nets of metal atoms. The different compositions and crystal structures are obtained by different numbers of metal layers in the corresponding parts according to the formula (MB)2Aly(MB2)x. This principle is an extension of a scheme which was developed for the boridecarbides of niobium. Furthermore, there is a close similarity to the group of ternary carbides MAl(MC)n, so-called MAX-phases. Therefore, they might be named as "MAB-phases". The pronounced two-dimensionality and the mixture of strong covalent and metallic interactions make MAB-phases to promising candidates for interesting material properties. All compositions were confirmed by EDX measurements. Additionally, microhardness measurements were performed.

Using differential scanning calorimetry (DSC) measurements in combination with structural and optical characterization we have investigated the formation conditions of different phases of tris(8‐hydroxyquinoline)aluminum (Alq3). We have identified the δ‐phase as a high‐temperature phase of Alq3 being composed of the facial stereoisomer, and report an efficient method to obtain blue luminescent Alq3 by a simple annealing process. This allows the preparation of large amounts of pure δ‐Alq3 by choosing appropriate annealing conditions, which is necessary for further characterization of this blue‐luminescent phase, and offers the possibility of fabricating blue organic light‐emitting devices (OLEDs) from this material.

The perovskite phase (CH3 NH3 )2 Pb(SCN)2 I2 with a structure closely related to the K2 NiF4 -type was identified as the product of the reaction of CH3 NH3 I and Pb(SCN)2 by single-crystal X-ray analysis. This extends the range of suitable dyes for solar cell applications to a class of perovskite-related structures of the general composition (AMX3 )n (AX)m .

We report on two different crystalline phases of tris͑8-hydroxyquinoline͒ aluminum (Alq 3) which were obtained by thermal sublimation in a horizontal glass tube. These phases are investigated by x-ray powder diffraction, Raman and infrared spectroscopy, and low temperature photoluminescence measurements. Apart from the already known ␣ phase we could identify a new crystalline phase of Alq 3 (␦-Alq 3) showing blueshifted fluorescence. As compared to the ␣ phase this new phase is characterized by a larger unit cell volume, a reduced number of Raman lines in the energy range between 70 and 700 cm Ϫ1 , a blueshift of the photoluminescence maximum by about 0.2 eV, and a decreased intersystem crossing to the triplet state. These differences are interpreted in terms of the isomery of the Alq 3 molecule. It is supposed that the new phase contains the facial isomer, whereas in the other phases only the meridianal isomer was reported. Low temperature photoluminescence spectra show a well-resolved vibronic progression with about the same spacing of 550 cm Ϫ1 for both crystalline phases of Alq 3. Site-selective photoluminescence measurements reveal the existence of an additional redshifted featureless emission, which is ascribed to energy relaxation into low-lying states.