mixed bonding
The four bond types, ionic, covalent, metallic, van der Waals, are convenient idealisations, useful corners of a map. Almost every real material lives somewhere between the corners: its bonding is a blend. Mixed bonding is the honest recognition that most bonds are partly one thing and partly another, and that the proportions shape the structure.
The clearest blend is ionic-covalent. When two atoms differ in electronegativity, the bond has both a transfer character (ionic) and a sharing character (covalent), and their ratio slides smoothly with the electronegativity difference. Pauling even gave a formula estimating the percent ionic character from that difference: a large difference (sodium and chlorine) gives mostly ionic, a small one (silicon and silicon, none) gives pure covalent, and something like gallium arsenide sits in between, significantly covalent yet partly ionic. Other blends exist too: many intermetallic compounds mix metallic and covalent bonding, and molecular crystals mix strong internal covalent bonds with weak external van der Waals ones.
Mixed bonding matters because it is the rule, not the exception, and because the blend tunes real properties. The partly-ionic, partly-covalent bonding of semiconductors like GaAs or ZnS is exactly why they adopt the open, directional zinc-blende structure (inherited from the covalent side) yet show some charge separation (from the ionic side). Recognising bonding as a continuum, with the pure types as limiting cases, is the mature view, and electronegativity difference is the dial that positions any given bond along it.
Gallium arsenide (GaAs) is roughly 30 percent ionic and 70 percent covalent. The strong covalent character makes it adopt the directional, open zinc-blende structure (coordination 4, like diamond), while the ionic character separates the two atom types onto distinct sublattices, a blend you can read straight from its structure.
Real bonds blend the ideal types; the electronegativity difference sets the mix and the structure.
The percent ionic character from an electronegativity difference is a rough estimate, not a precise measurement; bonding is a continuum and any single number is a model-dependent summary. The value of mixed bonding is conceptual: stop asking which one type and start asking what blend.