Three-dimensional corner-sharing tetrahedral

Crystalline rhombohedral NZP, has an open three-dimensional channel-type structure built from corner-sharing tetrahedral PO4 and octahedral ZrOg units. It contains two possible sites for the Na cations, only one of which is occupied. The mineral kosnarite KZr2(P04)3 also has a similar type structure [40,41]. The sodium salt, which has a very low thermal expansion coefficient, can be prepared by heating together appropriate quantities of sodium metaphosphate and ZrOj at 1200°C [42]. 

In the borates, the boron atoms are coordinated to oxygen atoms forming either a tetrahedral or a triangular configuration. [BO4] or [BO3] units can exist individually in the borate. However, it is more common for them to form a one-dimensional chain, two-dimensional plane, or three-dimensional network by sharing corners and by sharing edges in a few instances. 

Methane has four shared pairs of electrons. Here, minimum electron repulsion is achieved by arranging the electrons at the corners of a tetrahedron (Figure 4.6). Each H—C—H bond angle is 109.5°. Methane has a three-dimensional tetrahedral structure. Silicon, in the same group as carbon, forms compounds such as SiCl4 and SiH4 that also have tetrahedral structures. 

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