TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Crystal Structure : At least some of the atoms in structure (1) are of carbon and/or silicon. The icosahedrons of structure (1) are linked together e.g. in linear chains within the packing layer or via (especially four) covalent bonds (4), especially with two atoms or atom groups of structure (1) bonded to two atoms or atom groups of adjacent structures (1) via covalent two- electron-two-center bonds (4).At least some of the atoms in structure (2), which is present in ionic form, are metals, especially magnesium, aluminum, lithium, scandium, titanium, iron or beryllium, and structure (1) is stabilized by structure (2) by electron transfer.At least a part of structure (2) occupies the corresponding octahedron gaps. Structure (1) is also bonded to the corresponding structure (3) and structure (3) has each of three structure units (1B) in one layer bonded to a structure unit (1A) in an adjacent layer.The crystal structure is predominantly monoclinic and corresponds to (or is derived from) a space group C2/c model with lattice constants a = 7.288+/-0.001A, b = 8.795+/-0.001A and c = 7.296+/-0.001A and crystal angles alpha = gamma = 90degrees and beta = 105.29+/-0.02degrees.Preferred Stoichiometric Composition: S112S2uS3v;S1 = elements in structure (1) or boron alone or together with carbon and/or silicon;S2 = elements in structure (2) or magnesium, aluminum, lithium, scandium, titanium, iron or beryllium;S3 = elements in structure (3) or carbon, nitrogen, boron, oxygen, silicon or phosphorus; andu and v = real numbers 0-2, In particular, u = 1 and v = 2.Preferred Manufacture: Production of the material or of the composites is by:(i) an auxiliary agent process, especially using Mg and optionally also another material as the auxiliary together with a source for the desired structures;(ii) powder metallurgy; or(iii) a sol-gel process from the required elements or their sources.Methods (i) and (ii) use the required elements directly or sources for the elements by solid synthesis, heat energy being supplied especially by microwave radiation or resistance heating. EXAMPLE - None given in the specification.