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Question

For the spinel structure MgAl2O4, the correct statement is/are -

This question has multiple correct options


A

50% of the octahedral voids are occupied by ions

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B

Al3+ ions are equally distributed in tetrahedral voids and octahedral voids.

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C

Oxide ions occupy CCP lattice

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D

12.5% of the tetrahedral voids are occupied by ions.

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Solution

The correct option is D

12.5% of the tetrahedral voids are occupied by ions.


Explanation for correct Answers:

Option (A) 50% of the octahedral voids are occupied by ions

  • Cubic close packing is a type of packing of atoms in which the atoms are so closely packed. ‘
  • The unit cell in cubic close packing (CCP) is face-centered cubic (FCC).
  • FCC unit cell contributes four atoms, one-eighth of the atom from the atoms placed in the corner, one atom which is placed in the center of the cube and is not shared by a nearby lattice, and half of an atom from the six atoms present in the faces.
  • The general formula of spinel structure is AB2O4 where the A atoms occupy one-eighth of the tetrahedral voids and B atoms occupy half of the octahedral voids
  • In MgAl2O4, the Al3+ atoms occupy half of the octahedral voids.
  • The percentage occupied by Al3+is 12×100=50%.
  • Hence, the correct option.

Option (C) Oxide ions occupy CCP lattice

  • The Oxide ions are arranged in CCP and have an FCC unit cell.
  • Hence, the correct option

Option (D) 12.5% of the tetrahedral voids are occupied by ions.

  • In MgAl2O4, the Mg2+ occupy one-eighth of the tetrahedral voids.
  • Therefore, the percentage occupied by Mg2+is18×100=12.5%
  • Hence, the correct option.

Explanation for Incorrect Answers:

Option (B) Al3+ ions are equally distributed in tetrahedral voids and octahedral voids.

  • In MgAl2O4, the Al3+ atoms occupy half of the octahedral voids and the Mg2+ occupy one-eighth of the tetrahedral voids.
  • Hence, incorrect option.

Therefore, the options (A),(C), and (D) are correct.


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