Nitridoborates

A remarkable variety of compounds in the Ca-(B,C,N) system has opened a window for research in related fields. With the elements boron, carbon and nitrogen, substance classes such as borocarbides, boronitrides, and carbonitrides can be considered to contain anionic derivatives of binary compounds B4C, BN, and C3N4. Until now, most compounds in these substance classes have been considered to contain alkali, alkaline-earth, or lanthanide elements. Lanthanide borocarbides are known from the work of Bauer [1]. Lanthanide boronitrides represent a younger family of compounds, also assigned as nitridoborates [2] following the nomenclature of oxoborates. 

The compounds that are being reported here are mostly those with combined anions, such as BN anions in nitridoborates. On the other hand, we know of compounds with mixed anions, like the well-known Ti(N,C) in which anions share the same lattice site, or NbBN with separate B and N sublattices. How can these compounds be explained in comparison with compounds having C-N or B-N bonding Are there separate strategies for their synthesis, what are the differences in stability, what are their properties, and what are their structures like These and some other questions will be discussed, mainly for Ca-(B,C,N) and La-B-N systems. 

Ca3(BN2)2 is readily formed when (distilled) calcium metal is melted in the presence of (layer-type) boron nitride. This reaction provides some insight on how alkaline-earth metals like calcium may act as a catalyst in the phase transformation of layered a-BN into its cubic modification. Instead of metals, nowadays alkaline-earth (Ca, Sr, Ba) nitridoborates can be used as a flux catalyst in high-pressure and high-temperature transformation reactions to produce cubic boron nitride [15]. 

Since a separation process for Ca3(BN2)2 and CaBg is unknown, Ca3N2 is better employed as a starting material for the synthesis of calcium nitridoborate nitride [16], and calcium nitridoborate ... 

Reactions suggest that Ca3(BN2)N can be viewed as an intermediate in the reaction (4). The substitutional effect of nitride by nitridoborate ions reflects that the linear [N=B=N] ion (with a typical B-N distance around 135 pm) can be viewed as a pseudo-nitride. 

A long time passed between the first discovery of AE3(BN2)2 phases [17] (AE = Ca, Sr) and their structural characterizations [18, 19]. Two distinct phases are known to exist for both calcium and strontium nitridoborate, denoted as low-temperature y9-AE3(BN2)2 and high-temperature a-AE3(BN2)2 [20]. Their phase transitions have been studied by temperature-dependent XRD, thermo-... 

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. 

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 ... 

For the generation of more nitrogen-rich compounds, a nitride is included in the reaction. This synthesis allows the formation of different nitridoborate (8) ions as well as nitridoborate nitrides (9) [26] ... 

Previously, all these variations led to the discovery of lanthanide nitridoborates. Products were obtained as powders or single crystals, and crystal structures... 

In this younger field of chemistry, some examples of anions may be still missing. One possible candidate is the [BN4] ion with a tetrahedral coordination of boron as in cubic BN, others may be adopted from oxoborates. Another interesting feature is the existence of structures with condensed nitridoborate anions derived from portions of BN structures. Until now the only example with a condensed anion structure is U(BN), containing kinked B-B bonded chains of (BN)x with B-B distances near 188 pm, slightly longer than in [B2N4] . ... 

Nitridoborate anions are surrounded by metal atoms in a typical pattern that allows the prediction of new structures, to some degree. In any case, this typical pattern helps one to understand the construction scheme of structures and to memorize the particular structures. 

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. 

Lanthanide nitridoborates can be divided into three classes salt-like compounds, semiconductors, and conductors or superconductors, as already shown in Fig. 8.7. Salt-like structures are usually transparent materials, marked by the typical color of the lanthanide ion. Here we discuss only nitridoborate compounds of lanthanum. The compounds La3(B3N, ) [27], La5(B3N, )(BN3) [28], Lag(B3N6)(BN3)N [29], and La3(BN3)N all count as salt-like materials, with La, ... 

B3N6] A [BN3] and N (Fig. 8.11). Band-structure calculations performed for La3(B3N5) revealed a band gap in the order of 4 eV. The corresponding nitridoborate oxide La5(BN3)Og [30] is also salt-Hke, owing the typical nitridoborate structure pattern regarding the environment of the [BN3] ion with lanthanum... 

Based on the results of our band-structure calculations we assume that the metal-like properties of lanthanum nitridoborates are related by B-B interactions between adjacent BNx units in structures. 

Mixed B-C-N compounds of lanthanum maybe subdivided into La-(BNx), La-(BCx), and La-(CNx) compounds (Fig. 8.15). The chemistry of lanthanide nitridoborates has been developed in some detail and some properties were studied. But still more work is necessary, especially in the field of quaternary Ln-metal-(BNx) compounds. 

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