Spinel Structures

Question 1

What are spinels

Answer 1

1. Gemstones that are quite rare
2. Many colours but most common is red
3. Often mined with rubies and would be confused

Question 2

Describe difference in strcuture between rubies and spinels

Answer 2

1. Rubies based on hexagonal structures- Al2O3

2. Spinel parent structure is cubic MgAl2O3

Question 3

What is the general composition of a spinel

Answer 3

1. A2+ B3+2 O4

Question 4

What is the formulation of a normal spinel

Answer 4

1. (A2+tet)(B3+2oct)O4

Question 5

Which spinel ions are in a tetrahedral environment

Answer 5

1. Mg2+= A2+ = tetrahedral

Question 6

Which spinel ions are in an octahedral environment

Answer 6

1. Al3+ = B3+ = octahedral

Question 7

Why is the Al in an octahedral

Answer 7

1. It has greatest charge
2. Surrounded by 6 (rather than 4) negative charges
3. D values are generally bigger for M3+ than M2+

Question 8

Which spin are A2+ and B3+

Answer 8

1. High-spin configurations
2. O2- is low in spectrochemical series
3. So causes weak splitting

Question 9

Describe where each ion is located

Answer 9

1. A fcc array of A2+ ions which can be divided into 8 smaller cubes
2. Each of these 8 fractions houses an AO4 or a B4O4 mini-cube in an alternating pattern
3. Overall the A2+ ions are located in tetrahedral holes and the B3+ ions are located in octahedral holes

Question 10

What is the ratio of A and B ions

Answer 10

1. 1:2

2. Twice as many ions in octahedral coordination environments as there are in tetrahedral coordination environments

Question 11

What are inverse spinels and describe structure

Answer 11

1. Ion distribution (B3+tet)(A2+B3+Oct)O4
2. The divalent A2+ ions swap places with half of the trivalent (B3+2) ions
3. A2+ ions now in oct and half of the B3+ ions sit in tetrahedral sites

Question 12

When will an inverse spinel arise

Answer 12

1. If the inverse structure has a CFSE advantage

Question 13

Which spin configurations are relevant

Answer 13

1. High-spin only in oxide containing spinels

Question 14

How would you work out if there is a CFSE advantage in having an inverse rather than normal spinel structure

Answer 14

1. First calculate CFSEnormal based on Doct values for both ions
2. Then calculate CFSEinverse based on Doct values for both ions
3. Whichever has most stabilisation - most negative value is the favoured

Question 15

What is the formula to calculate CFSEnormal

Answer 15

1. 4/9CFSE(A2+) + 2(CFSE(B3+)

Question 16

What is the formula to calculate CFSEinverse

Answer 16

1. = (1+4/9)CFSE(B3+) + CFSE(A2+)

Question 17

What are problems with using CFSE calculations to predict spinel structures

Answer 17

1. Based on approximation that Dtet=around*4/9Doct
2. Although CFSEM3+>CFSEM2+ the exact difference between the 2 values depends in the ions types present
3. In cases where CFSEnormal is close to CFSEinverse the predictions are least reliable
4. Better predictions if action Dvalues for A2+ and B3+ are known