Silica Phases and Stuffed Derivatives

Question 1

Stuffed Derivative

Answer 1

Structure where there is a partial substitution of some atoms along with additional ions “inserted” into the cavities of the structure

Question 2

Cristobalite

Formula, structure, and coordination

Answer 2

(polymorph of) SiO2

Si coordinated to 4 O atoms

Cubic; Derivative of the cubic diamond structure; Si atoms occupy C sites, and O located halfway along the Si bonds

Si-O Bond length = 1.6 A
r(Si) = 0.26 A
r(O) = 1.35 A; ratio=.193 but CN = 4

Question 3

Displacive Transition

Answer 3

Distortion of the structure without the breaking of primary bonds.

This rapid transition cannot be quenched.

Ex. Transition between high (beta) and low (alpha) quartz at 573K (300 C)

Question 4

Reconstructive Transformation

Answer 4

Reassembly of the structure with the breakage of primary bonds.

This very slow transition can be quenched.

Ex. Transition from tridymite to cristobalite (hexagonal-cubic)

Question 5

Tridymite

Answer 5

SiO2 polymorph (4-coordination)

Wurtzite hexagonal; superimposed puckered hexagonal rings of Si with O located halfway between Si bonds

Question 6

Stuffed Derivative

Answer 6

During the formation of a silica structure, a certain porportion of the Si(4+) atoms is replaced by Al(3+) atoms (also has CN=4 wrt O)

Excess negative charge can be counterbalanced by the addition of another ion to the structure (i.e. K(1+))

Looking at the structure, charge-balancing species will likely occupy the largest interstice

Question 7

Beta-quartz

Answer 7

Hexagonal silica polymorph

High T

Question 8

Alpha-quartz

Answer 8

Trigonal SiO2 polymorph

Low T