SPINEL.docx

SPINEL & INVERSE SPINEL STRUCTURES The spinels spinels have  have the general formula AB2X4. Where: AII = a divalent cation like Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, n !III = a trivalent cation like Al, "a, In, Ti, #, Cr, Mn, Fe, Fe, Co, Ni $ = %, , e etc. Structure Stru cture of Norm Norml l Spi Spinels nels !AB 2"4#$ The dival divalent ent A II ion& occu'( the tetra tetrahedr hedral al void void&, &, )herea& the trivalent !III ion& occu'( the octahedral void& in the clo&e 'acked arrangement of o*ide ion&. A normal &'inel can +e re're&ented a&: !AII#tet!BIII#2oct"4 .g. MgAl-% /kno)n a& &'inel0, Mn1%, ZnFe-%, FeCr-% /chromite0 etc. Structures of In%erse spinels !B!AB#"4#$ The AII  ion& occu'( the octahedral void&, )herea& half of !III ion& occu'( the tetrahedral void&. It can +e re're&ented a&: /! III0tet/AII!III0oct% .g. Fe1% /ferrite0, CoFe-%, NiFe-% etc. The a+ove inver&e &'inel& can al&o +e )ritten a&: Fe1% = FeIII/FeIIFeIII0% CoFe-% = FeIII/CoIIFeIII0% NiFe-% = FeIII/NiIIFeIII0% The num+er of octahedral &ite& occu'ied ma( +e ordered or random. The random occu'ation lead& to defected &'inel&. .g. NiAl-% for )hich the formula can +e )ritten a& /Al2.34Ni2.-40tet 5Ni2.34Al6.-47octa%. Another defected &'inel i& 89Al -%1

ACT"RS AECTIN' T(E STRUCTURE " SPINELS )# T*e relti%e si+es of A n, B$  B$   In general, the &maller cation 'refer& to occu'( the &ite of  lo)er coordination i.e., tetrahedral &ite. o)ever, in the &'inel, MgAl-% it&elf thi& factor i& out)eighed +( greater lattice energ( of &maller cation, Al1;, )hich occu'ie& the octahedral &ite and thu& +( giving normal &'inel &tructure. Otherwise the "spinel" will have the inverse spinel stucture!

2# T*e -,elun. constnts for t*e norml n, in%erse structures$  It i& o+&erved that the Madelung con&tant& are &ame for +oth normal and inver&e &'inel& and hence are not that much im'ortant in arriving at the &tructure.

/# Li.n,0lel, St1ili+tion Ener.ies$ A''lica+le )henever there are tran&ition metal ion& and i& di&cu&&ed +elo). The &tructure& of &'inel& are affected +( the relative There i& a tendenc( of formation of inver&e &'inel &tructure in &ome ca&e& /not all the ca&e&0 )hich contain tran&ition metal ion&. Thi& i& +ecau&e, the tran&ition metal ion ma( get e*tra &ta+ilit( / %ther ion& )ith d6, d-, d, d, d3, dD too have &lightl( more 'reference for octahedral &(mmetr(. > That mean&, if AII  *s ,/ or , confi.urtion and the !III  ion ha& configuration other than the&e, then the spinel is in%erte,. > If the divalent AII i& a tran&ition metal /)ith configuration& other than d2? high&'in d4  d 620 and the !III ion i& a non tran&ition metal, there i& a tendenc( to form inver&e &'inel. !ut there are e*ce'tion& like FeAl-% )hich ha& normal &'inel &tructure. > A+ove generaliation& are valid for high &'in &(&tem& a& the o*ide ion i& e*'ected to +e a )eak field ligand. For e*am'le, Co 1; i& a lo) &'in &(&tem even in 're&ence of o*o ligand& due to high charge on the ion. AII Non tran&ition metal or

BIII 

Structure

Non tran&ition

'inel &tructure

metal

d2 or d4 or d62 tran&ition metal A tran&ition metal Non tran&ition metal

d6 or d- or d1 or

or 2

4

)ith

d or d or d3 or dB 62

d  or d  or d

'inel &tructure

or dD

tran&ition metal configuration& A tran&ition metal

Non tran&ition

)ith

metal

d6 or d- or d1 or

or

d or d or d3 or dB

tran&ition meta

or dD

)ith

Inver&e &'inel

d2 or d4 or d62 configuration&

configuration&

Tran&ition metal

Tran&ition metal

)ith higher CF

)ith lo)er CF

value

value

Inver&e &'inel

EXA-PLES "R SPINEL AN3 INVERSE SPINEL STRUCTURES 60 -.Al2"4 is  norml spinel &ince +oth the divalent and trivalent ion& are non tran&ition metal ion&. There i& no Eue&tion of CF. -0 -n/"4  is  norml spinel  &ince the Mn-;  ion i& a high &'in d4 &(&tem )ith ero