Solid-State Chemistry with Nonmetal Nitrides By Wolfgang Schnick* Among the nonmetal nitrides, the polymeric binary compounds BN and Si3N4 are of partic­ ular interest for the development of materials for high-performance applications. The out­ standing features of both substances are their thermal, mechanical, and chemical stability, coupled with their low density. Because of their extremely low reactivity, boron and silicon nitride are hardly ever used as starting materials for the preparation of ternary nitrides, but are used primarily in the manufacture of crucibles or other vessels or as insulation materials. The chemistry of ternary and higher nonmetal nitrides that contain electropositive elements and are thus analogous with the oxo compounds such as borates, silicates, phosphates, or sulfates was neglected for many years. Starting from the recent successful preparation of pure P3N5, a further binary nonmetal nitride which shows similarities with Si3N4 with regard to both its structure and properties, this review deals systematically with the solid-state chemistry of ternary and higher phosphorus(v) nitrides and the relationship between the various types of structure found in this class of substance and the resulting properties and possible applications. From the point of view of preparative solid-state chemistry the syntheses, structures, and properties of the binary nonmetal nitrides BN, Si3N4, and P3N5 will be compared and con­ trasted. The chemistry of the phosphorus(v) nitrides leads us to expect that other nonmetals such as boron, silicon, sulfur, and carbon will also participate in a rich nitride chemistry, as initial reports indeed indicate. 1. Introduction tors: extremely strong bonds between the elements and the presence of highly crosslinked covalent structures. Nitrogen, the main component of the atmosphere, is om­ Besides these two criteria, the extremely high bond energy nipresent. The lightest element in the fifth main group plays of N2 (941 kJ mol"*) as a possible decomposition product of an important role in chemical compounds, in particular in the nitrogen compounds is also of importance when dis• the oxidation states ν and πι (N03 and NO J, respectively) cussing the thermal stability of nitridic materials. Compared as well as — HI (NH3, —NH2, -NH-, and "N-). The with the corresponding oxides, the thermal dissociation of oxidation state of nitrogen in the nitrides is also — in; only many nitrides with evolution of N2 (for the oxides 02, bond a few hundred nitrides have so far been characterized, al­ energy: 499 kJmol"1) occurs at much lower temperatures. though for example the neighboring element oxygen has Thus, the elimination of N2 from Si3N4 occurs at atmospher• been shown to form more than ten thousand oxides. In spite ic pressure at about 1900 °C, while Si02 can be heated to of this relatively small number, the nitrides include some over 2000 °C without any noticeable decomposition. Alu• extremely useful compounds: silicon nitride, Si3N4, has be­ minum nitride decomposes above about 1800°C, while the come an important nonoxidic material, whose applications extrapolated boiling point of A1203 is about 3000 °C. range from ceramic turbochargers to integrated semiconduc­ The affinity of most elements for oxygen is larger than that 11,21 tor modules. Because of its unusually high thermal con­ for nitrogen, thus the bond energies for element-oxygen _1 ll3] ductivity (285 Wm K" ), aluminum nitride is predes­ bonds are generally higher than those of the corresponding tined for use as a substrate material in semiconductor element-nitrogen bonds (single bond energies: Si-O = 444, manufacture. Boron nitride is used as a high-temperature Si-N = 335; P-O = 407, P-N = 290 kJmol"1[41). Similarly crucible material, as a lubricant (hexagonal (Λ)-ΒΝ), and in the bond enthalpies of the oxides are significantly higher the abrasives sector (cubic (c)-BN). In recent years Λ-ΒΝ has than those of the corresponding nitrides (AH?(S\02) = also become increasingly important in the manufacture of -911, V3[A/fJ>(Si3N4)] = - 248; V2[A//?(B203)] = - 637, composite materials. Δ//?(ΒΝ) = - 254; ι/2[Δ//?(Α1203)] = - 838, Δ//?(ΑΙΝ) = The extreme stability of the substances, which are used as — 318 kJ mol"1151), although a quantitative comparison is high-performance ceramics, is due in part to the strengths of difficult because of their differing compositions. the bonds joining the constituent elements; a second impor­ The formation of oxides is thus an important side reaction tant factor is the presence of highly crosslinked structures in in the syntheses of nitrides. Thus, the preparation of nitrides the solid state. When the electronegativity difference is only in a pure state requires the complete exclusion of oxygen and small, heteronuclear bonds with a high degree of covalent water. This precondition has certainly previously played an character are formed. The high chemical, thermal, and me­ important role in hindering a detailed investigation of ni­ chanical stability of nitridic materials such as silicon nitride trides. or boron nitride results from the interplay of these two fac- Among the theoretically possible binary main group ele­ ment nitrides with nitrogen in the oxidation state — πι and [*] Prof. Dr. W. Schnick the electropositive elements in the maximum oxidation state Laboratorium für Anorganische Chemie der Universität Postfach 101251, D-W-8580 Bayreuth (FRG) corresponding to their group number, many are either Telefax: Int. code (921)55-2535 nonexistent or have until now not been obtained in a pure 806 (φ VCH Verlagsgesellschaft mbH, W-6940 Weinheim, 1993 0570-0833/93/0606-0806 $ 10.00 + .25/0 Angew. Chem. Int. Ed. Engl. 1993, 52, 806-818 and well-defined form because of their low stability (Fig. 1). gen atoms is decreased by electron-withdrawing groups or 173 Li3N exhibits an unusually high tendency for formation; the by mesomeric effects, a requirement which is hardly realiz­ reaction between lithium metal and molecular nitrogen able in the speculative compound Sb3N5. In the case of other starts at room temperature and atmospheric pressure with­ main group elements (e.g. carbon and sulfur) the reasons for out any additional activation. In contrast, there is no reliable the hitherto nonexistence of corresponding binary nitrides evidence for the existence and stability of analogous com­ (C3N4 and SN2) appear to be more complex or to be due to pounds of the heavier alkali metals. Apparently in the ni­ preparative problems. trides M3N (M = Na, K, Rb, Cs) the high formal charge of The binary nonmetal nitrides BN, Si3N4, and P3N5 will be the nitride ion (N3~) and the unfavorable molar ratio of main subjects of the following discussion. These are the only cations to anions (3:1) make it impossible to form a stable nonmetal nitrides so far studied in detail in which the elec­ ionic structure in which the electrostatic, coordinative, and tropositive elements have the maximum possible oxidation lattice-energetic requirements of a stable solid material are state corresponding to their group number. The syntheses, fulfilled. For the alkaline earth metals, binary nitrides with structure, and properties of BN and Si3N4 were described the composition M3N2 are, however, known for all the ele­ many years ago. However, both compounds had been stud­ ments. ied because of their application in the area of high-perfor­ mance materials. Because of preparative difficulties, pure phosphorus(v) nitride has only recently been obtained. Al­ though P3N5 is thermodynamically appreciably less stable Li Be Β than BN or Si3N4, it has still been possible to develop a Mg AI Si Ρ multifaceted solid-state chemistry of the phosphorus(v) ni­ Ca Ga Ge trides. This success has provided the impetus for a further systematic search for new ternary and higher nonmetal ni­ Sr In trides. Ba Fig. 1. Binary nitrides are only known for a fraction of the main group elements in the maximum oxidation state corresponding to their group numbers. 2. The Binary Nonmetal Nitrides BN, Si3N4, and P3N5 In contrast to the ionic structures of the nitrides of lithium Binary nitrides form the basis for the syntheses of ternary and the alkaline earth metals, the decreasing electronegativ­ or higher nonmetal nitrides containing electropositive ele­ ities from the third group onwards lead to the formation of ments. Innumerable methods for the preparation of these compounds with more covalent character (e.g. BN, A1N, compounds are known; however, only a limited number of Si3N4). As the group number increases, the heavier homo- these procedures which afford pure, well-defined products logues in their highest oxidation state show a clearly decreas­ can be readily carried out under laboratory conditions. Such ing tendency to form stable binary nitrides: In the third main methods must be applied when the binary nitrides are to be group TIN is "missing", while in the neighboring group tin used in the syntheses of new compounds. and lead form no stable nitrogen compounds with the ex­ [61 pected stoichiometry M3N4. This trend continues up to the sixth main group; here no compounds with the composi­ 2.1. Boron Nitride BN VI tion M N2 (M = S, Se, Te) are known. In some cases the instability of the element-nitrogen bond appears to be re­ Within the family of known ceramic materials boron ni­ sponsible for the nonexistence of the corresponding binary tride has the lowest density (ρ — 2.27 gem"3). It is colorless nitrides. Thus stable molecular compounds of antimony(v) in the pure state and sublimes at about 2330 °C under a and nitrogen are only formed when the basicity of the nitro- nitrogen pressure of one atmosphere.181 Its decomposition Wolfgang Schnick, born in 1957 in Hannover, studied chemistry at the Universität Hannover and received his doctorate there in 1986, having worked with Martin Jansen on alkali metal ozonides. After a postdoctoral year with Albrecht Rabenau at the Μ ax-Planck-Institut für Festkörperfor• schung in Stuttgart he moved to the Universität Bonn.