Nitrides, lanthanide

The chemistry of lanthanide nitridoborates was developed by the more flexible and more efficient solid-state metathesis route by using nitridoborate salts and a lanthanide trichloride, at reaction temperatures as low as 600 °C. This type of reaction has been previously established and studied in some detail for reactions, such as the synthesis of lanthanide nitrides (LnN). Following this concept, lanthanum nitridoborates are obtained from reactions of lanthanum trichloride and lithium nitridoborate (or calcium nitridoborate), performed in a salt-balanced manner with respect to the formation of the co-produced liCl ... 

The homoleptic Ln" bis(trimethylsilyl)amides are among the most volatile Ln compounds, and have featured in the doping of Ge/Ln semiconductors, or in the formation of lanthanide nitrides. ... 

During the last five years lanthanide amides, and in particular the silylam-ides, have attracted considerable attention under the assumed name precursors for electronic and ceramic materials [16,17], Herein the systems Ln[N(SiMe3)2]3, for their part, provide essential precursors for the generation of, e.g., pure alkoxides. However, while the steric environment of the alkoxide ligand declined excessively [18], variation of the amide ligand seemed to stick. This goes hand in hand with the very recent start of the exploration of organometallic lanthanide amides themselves as molecular precursors to, e.g., lanthanide metals or lanthanide nitrides (Sect. 7.2). 

Low temperature processes are desirable in material sciences. Lanthanide nitrides which possess intrinsic electronic and magnetic properties are typically synthesized by ammonolysis of the metals at temperatures near 1000 °C. Inorganic amides of type Ln(NH2)s(x = 2,3) are intermediates in this process (Sect. 2.1). A versatile method starting from molten Ln(btsa)3 precursors was reported by LaDuca and Wolczanski [286]. The formation of low crystalline LnNj x according to Scheme 15 was observed at about 210 °C after 3 h. 

FIGURE 7 Total energy differences (in eV) between divalent and trivalent scenarios for lanthanide nitrides (triangles). The results for elemental lanthanides (diamonds) are repeated for an easy comparison (including 43 mRy correction, but not for the nitrides). 

FIGURE 8 Lattice parameters of the lanthanide nitrides. The open circles are the experimental values, whereas the solid circles are the calculated lattice parameters. 

TABLE 2 Spin-resolved band gaps (in Ry) and densities of states (in states/Rydberg/formula unit) at the Fermi energy for the lanthanide nitrides... 

In the case of lithium nitride, YCI3 and LaCls were the first rare-earth chlorides investigated (Fitzmaurice et al. 1993). The highly exothermic reactions, once initiated at 400°C, were extremely rapid, leading to crystalline YN and LaN nitrides which were isolated from LiCl after tetrahydrofuran trituration. The same type of rapid exothermic solid-state metathesis reaction was then applied to produce all the RN rare-earth nitrides (R=Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) as well as mixed lanthanide nitrides (RR )N (RR =PrNd, DyHo, TbDy) (Fitzmaurice et al. 1994). Note that the reaction requires initiation via a conventional or microwave oven but is self-sustaining. The prepared nitrides have a stoichiometry LnN x (x = 0.1-0.2). 

The R4NS3CI3 compounds, like the lanthanide nitride sulfides, are not sensitive to hydrolysis. They were obtained from appropriate molar ratios of R metal, sulfur and NaN3, in the presence of NaCl. Their symmetry is hexagonal (P63mc) and they are isostructural with oxychloride Ba40Cl6 (Frit et al. 1970) or oxide sulfide chlorides R4OS4CI2 (R=La-Nd) (Schleid 1991, Schleid and Lissner 1994),... 

Many of the binary compounds of the lanthanides, such as oxides, nitrides, and carbides, can exist as non stoichiometric compounds. These form crystals where some of the anions ate missing from the sites the anions normally occupy. 

As nitridoborates are known as A3(BN2) for alkaline elements (A = Li, Na) and as AE3(BN2)2 for alkaline-earth elements (AE = Ca, Sr, Ba) it would be interesting to find methods for the synthesis of nitridoborates of transition or lanthanide elements. Can they be made straightforward Hke Ii3(BN2) and AE3(BN2)2 from metal nitrides and layer-like a-BN, or do they require new preparative strategies - if they can be made at all ... 

Nitridoborates of lanthanum and the lanthanides were obtained from reactions of lanthanide metal or lanthanide metal nitride with layer-like (a-)BN at elevated temperatures (3>1200°C). These reactions require elaborated techniques in the inert gas sample-handling and the use of efficient heating sources, such as induction heating. Only some compounds remain stable in this high-temperature segment, and the yields of such reactions are often limited due to the competing stability of binary phases, allowing only the most (thermodynamically) stable compounds to exist. 

Structures of the lanthanide nitridoborates appear as layered structures with approximate hexagonal arrangements of metal atoms, and typical coordination preferences of anions. As in many metal nitrides, the nitride ion prefers an octahedral environment such as in lanthanum nitride (LaN). As a terminal constituent of a BNx anion, the nitrogen atom prefers a six-fold environment, such as B-N Lns, where Ln atoms form a square pyramid around N. Boron is typically surrounded by a trigonal prismatic arrangement of lanthanide atoms, as in many metal borides (Fig. 8.10). All known structures of lanthanide nitridoborates compromise these coordination patterns. 

No, the editor didn t know what this name meant either.) It means salts of the triva-lent anions of Group V, restricted in [1] to arsenides, antimonides and bismuthides and prepared by reaction of sodium pnictides with anhydrous halides of transition and lanthanide metals. This violently exothermic reaction may initiate as low as 25°C. Avoidance of hydrated halides is cautioned since these are likely to react uncontrollably on mixing. Another paper includes a similar reaction of phosphides, initiated by grinding [2], Nitrides are reported made from the thermally initiated reaction of sodium azide with metal halides, a very large sealed ampoule is counselled to contain the nitrogen [3],... 

A common precursor to LnN are the simple inorganic amides Ln(NH2)x (x = 2, 3) which can be placed between the nitrides and the alkyl substituted amides. Their main use lies in the synthesis of other solid materials like lanthanide hydroxides [21,33], carbides [34] or above-mentioned nitrides. Very recently solutions of europium and ytterbium in liquid ammonia have been rediscovered as synthetic tools (Sect. 7.1). 

The use of lanthanide amides in synthetic chemistry is meanwhile established and fine tuning of the ligands provides new targets. Examination of small ligands started recently and their exploration as precursors to, e.g., moisture stable, ternary nitrides might be promising. However, the search for low coor-... 

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