Björn Blaschkowski

Nitridoborate der Lanthanoide: Synthesewege, Strukturprinzipien und Eigenschaften einer neuen Verbindungsklasse

Untersuchungen auf dem Gebiet der Nitridoborate der Lanthanoide (Ln) haben in den vergangenen Jahren zu neuartigen Verbindungen gefuhrt, die hier zusammen mit einigen ihrer Eigenschaften vorgestellt werden. Nitridoborate werden auf zwei prinzipiell unterschiedliche Arten hergestellt: Enweder wird hexagonales Bornitrid durch Reaktionen mit LnN fragmentiert oder Dinitridoborat-Ionen werden in andere Nitridoborat-Ionen umgewandelt. Ln-Nitridoborate enthalten Molekulanionen wie [BN]n−, [BN2]3−, [B2N4]8−, [B3N6]9− und [BN3]6−, die kombiniert und gemeinsam mit Nitrid-Ionen auftreten konnen. Im Kristall sind die Anionen in Schichten angeordnet und in typischen Formationen von Metallatomen umgeben. Endstandige N-Atome sind von Metallatomen in Form einer quadratischen Pyramide uberdacht, bei den B-Atomen dominieren trigonal-prismatische Umgebungen durch Metallatome. Nitridoborate bilden sowohl salzartige als auch metallreiche Verbindungen und haben das Potenzial fur vieles, was man sich unter interessanten Festkorpereigenschaften vorstellt, was ihnen einen festen Platz in den Materialwissenschaften verschaffen sollte.

Eine Festkörper-Metathese-Route zur Synthese von Nitridoboraten des Lanthans

Festkorperreaktionen zwischen Selten-Erd-Elementen und hexagonalem BN erfordern hohe Reaktionstemperaturen und liefern in der Regel keine reinen Produkte. Durch exotherme Festkorper-Metathese-Reaktionen konnen Nitridoborate des Lanthans effektiv hergestellt werden. Die Metathese-Reaktion zwischen LaCl3 und Li3BN2 zundet bei 560 °C. Einkristalle werden bei etwas hoheren Temperaturen aus dem bei der Reaktion entstandenen LiCl-Flux gebildet. In modifizierten Metathese-Reaktionen konnen unterschiedliche N-Gehalte von Zielverbindungen durch Li3N eingestellt werden. Der Zusatz von Lithium ermoglicht Synthesen von metallreichen Nitridoboraten. Nach diesen Methoden wurden Nitridoborate wie La3B3N6, La6B4N10, La3+x(B2N4)Nx (x = 0, 1, 2) und die supraleitfahige Verbindung La3Ni2B2N3 hergestellt. A Solid State Metathesis Route for the Synthesis of Lanthanum Nitridoborates Solid state reactions between rare earth elements and hexagonal BN afford high reaction temperatures and often do not yield pure products. Exothermic solid state metathesis reactions provide an effective route for the synthesis of nitridoborates of lanthanum. An ignition of the metathesis reaction between LaCl3 and Li3BN2 was obtained at 560 °C. Single-crystal growth was obtained at slightly higher temperatures by the aid of a LiCl flux, that is formed during the reaction. Modified metathesis reactions with additional Li3N may be used to adjust certain N contents for desired compositions. Additional lithium metal is used to synthesize metal-rich nitridoborates. The preparative route presented here, was not only used for the synthesis of nitridoborates such as La3B3N6, La6B4N10 or La3+x(B2N4)Nx (x = 0, 1, 2), but also for the superconductive compound La3Ni2B2N3.

Rare-earth metal transition metal borocarbide and nitridoborate superconductors

Abstract Few years after the discovery of superconductivity in high-Tc cuprates, borocarbides and shortly after nitridoborates with reasonably high Tcs up to about 23 K attracted considerable attention. Particularly for the rare-earth metal series with composition RNi2[B2C] it turned out, that several members exhibit superconductivity next to magnetic order with both Tc above or below the magnetic ordering temperature. Therefore, these compounds have been regarded as ideal materials to study the interplay and coexistence of superconductivity and long range magnetic order, due to their comparably high ordering temperatures and similar magnetic and superconducting condensation energies. This review gathers information on the series RNi2[B2C] and isostructural compounds with different transition metals substituting Ni as well as related series like RM[BC], RM[BN], AM[BN] and R3M2[BN]2N (all with R = rare-earth metal, A = alkaline-earth metal, M = transition metal) with special focus on synthesis, crystal structures and structural trends in correspondence to physical properties.

About the air- and water-stable copper(I) dicyanamide: synthesis, crystal structure: vibrational spectra and DSC/TG analysis of Cu[N(CN)2]

Abstract Light-yellow microcrystalline samples of Cu[dca] ([dca]−≡[N(CN)2]−≡dicyanamide anion) were obtained by blending an in-situ generated aqueous Cu+ brine with stoichiometric amounts of Na[dca] dissolved in water. The crystal structure of Cu[dca] was solved and refined from powder X-ray diffraction (PXRD) data. Cu[dca] crystallizes in the orthorhombic space group Cmcm (no. 63) with the lattice parameters of a=356.28(3), b=611.10(9) and c=1525.87(10) pm. The crystal structure contains undulated chains of alternating Cu+ and boomerang-shaped [N≡C–N–C≡N]− ions with C2v symmetry running along [100]. It is closely related to that of Ag[dca] crystallizing in space group Pnma (no. 62). The vibrational spectra for Cu[dca] and Cu[dca]2 were recorded exhibiting modes typical for the dicyanamide anion. Comparative DSC/TG measurements were performed for both copper dicyanamides and the cyanide Cu[CN].

An unexpected lanthanum chloride carbonate hydrate: synthesis, crystal structure, vibrational spectra and thermal degradation of LaCl[CO3]·3H2O

Abstract Microcrystalline powders of LaCl[CO3]·3H2O were precipitated and isolated from stoichiometric aqueous solutions of LaCl3·7H2O and Na2[CO3]·10H2O. This confirmed the surprising emergence of corresponding single crystals during a reaction of equimolar aqueous solutions of LaCl3·7H2O and Na[N3] under ambient conditions by the uptake of atmospheric CO2. According to the X-ray structure analysis of colorless, transparent and rectangular single-crystals, LaCl[CO3]·3H2O adopts an orthorhombic structure, space group Pbca with the unit-cell parameters a=856.82(5), b=1598.57(9) and c=967.68(6) pm for Z=8. The tenfold coordination polyhedron around La3+ consists of two monodentate as well as two bidentate [CO3]2− anions together with three oxygen atoms from coordinating water molecules and one chloride anion. According to DSC/TG studies and X-ray powder diffraction, LaCl[CO3]·3H2O loses first its water and then carbon dioxide under thermal treatment up to 900°C to leave LaOCl behind. Vibrational infrared and Raman spectra confirmed the presence of H2O and [CO3]2− in LaCl[CO3]·3H2O and its stepwise degradation at elevated temperatures.