8.2) Inorganic solids: non-close packing structures

Describe the structure of CsCl

primitive cubic; Cl at corners and Cs at centre (or vice versa); not body centred as centre and corner atoms are different; not cp as CN=8 (cp requires 12) as atoms are connected octahedrally

What two groups of materials crystallise with the CsCl structure

halides of large monovalent cations (CsI, CsBr, NH4Cl); intermetallics (CuZn, AuMg, AlNi)

Describe the structure of most silicates

cation + complex silicate anion; based on SiO4 tetrahedra sharing corners only; no more than two SiO4 share one corner (oxygen); Si/O ratio can vary

Label the oxygen atoms in a silicate

bridging; non bridging

SiO2 (eg quartz or cristobalite) structure

1:2 ratio; four bo per Si (zero nbo); diamond network of Si with O inserted onto each linkage

Mg2SiO4 (eg olivine)

1:4 ratio; zero bo per Si (four nbo); 3d solid based on hexagonal close packed O ions with Si occupying 1/8 of tetrahedral sites and Mg occupying ½ octahedral sites

Chain and sheet silicates

formed by intermeditae Si:O ratios; eg 1:3 with 2 bo and 2 nbo

What are the trends in oxide bonding? Use Group IV oxides as an example (MO2)

going down the group (C→Si→Pb) go from molecular covalent to polymeric 3d structures (not cp) to ionic 3d structures mostly based on cp; CN also increases down the group