NiO has the rock salt structure but Ni As has the nickel arsenide structure. Bri

Question

NiO has the rock salt structure but Ni As has the nickel arsenide structure. Briefly describe the two structures in terms of close packing and space-filling polyhedra. Compounds with the rock salt structure are usually electrical insulators whereas those with the NiAs structure are often semiconducting or metallic. Why is this? The nature of the silicate anion in silicate structures is usually given by the Si:O ratio. Why is this? What silicate anion do you expect in the following: Ca_2SiO_4 MgSiO_3 Ca_6Si_2O_7 Li_2Si_2O_5

Explanation / Answer

a. In NiO we have octahedral Ni(II) with O2- sites which is similar to the structure adopted by NaCl with Na having octahedral structure with Cl- sites while in NiAs the cations are in octahegral closed packing so each cation is in coordination with six anions and the anions are in coordination with six cations but take up trigonal prismatic sites. In terms of packing they take up ABAB type structure.

In case of solid crystalline structure the atoms are fixed inthe crystal lattice structure thus making NiO (rock salt structure) an insulator while in NiAs structure as explained above they have face sharing octahedral layers which allows the transfer of electrons from one layer to the next like we have in silicon semiconductors.

b. because depending on the Si:O ratio the structure of the molecule (lattice) changes. Thus Si:O ratio gives the structure of the crystal as well as give their properties.

i. SiO4-4

ii. SiO3-2

iii. Si2O7-12

iv. Si2O5-2

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