Mossbauer spectroscopy - Principles and applications

Mossbauer spectroscopy an effective tool to study Mossbauer active nuclei.

1. MOSSBAUER
SPECTROSCOPY
V.SANTHANAM
DEPARTMENT OF CHEMISTRY
SCSVMV
PRINCIPLES AND APPLICATIONS

2. MOSSBAUER SPECTROSCOPY
•AlsoknownasNuclearGammaResonance
Spectroscopy.
•Inthismethodnucleusabsorbsangamma
rayphotonandundergoestransition.
•Firsttheconceptofϒphotonresonant
absorptionwassuggestedbyKuhn-1929
•FirstobservedbyMossbauerin1958
•AwardedNobelprizeforthiswork
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3. 3SANTHANAM SCSVMV

4. Why it is difficult?
•Theonlysuitablesourceofϒradiationisthe
excitednucleiofthesameisotopeinthe
courseofradioactivedecay.
•Nowayoftuningtheenergyoftheemittedϒ
photon.
•Theenergiesinvolvedaremuchhigherand
intheorderofkeV.
•Recoileffect
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5. What is recoil?
•Wheneverahighenergyparticle/projectile
isreleasedfromabodyatrest,thereleasing
bodyfeelsaback-kicki.e.itispushed
backwards,whichiscalledrecoileffect.
•Thisistrueforabsorptionofhighenergy
particlesalso.
•Therecoilhappenstoconservethe
momentum.
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6. Recoil continued…
•Whenagaseousatomormoleculeemitsa
quantumofenergyE,theemittedquantum
willalwayshavethemomentumE/C
WhereC–isthevelocityoflight
•Toconservemomentumtheemitterrecoils
withmomentumPwhichisequalandin
oppositedirection.
P=M.V
R=-E/C
whereV
Rrecoilvelocity
-signshowsthatitsdirectionisopposite 6SANTHANAM SCSVMV

7. E
R= P
2
/2m = E
2
/2MC
2
= (E
t-E
R)
2
/2MC
2
≈ E
t/2MC
2
Since E
Ris small
•IfMincreasestherecoilenergycanbestill
lower.Sothesourceandtheabsorberare
fixedonalargerlatticetoincreasemass
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9. What happens because of recoil effect ?
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10. E
ϒ E
ϒ
E
R
E
R
E
tE
t
E.S E.S
G.S G.S
Emission [Eγ= E
t–E
R] Absorption [Eγ= E
t+ E
R]
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11. Why recording MB spectrum is difficult?
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12. What is there in X axis? λor νor E?
•Itisalreadyknownthat,forspectroscopic
techniqueweneedamonochromatic
radiation.
•Butgammaraysarecomingoutbecauseof
theenergydifferencebetweenthenuclear
levels.
•Wecannotalterit,tochangethegamma
photon’senergy.
•SoweuseDopplereffecttochangethe
energyofthephoton.
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13. Doppler Effect
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14. Tuning the energy by using Doppler Effect
Source moves towards absorber, νincreases
Source moves away from absorber, νdecreases 14SANTHANAM SCSVMV

15. Mössbauer Effect
•RecoillessNuclearResonanceAbsorptionof
gamma-Radiation.
•TheMössbauerEffecthasbeenobservedfor
about100nucleartransitionsinsome80
nuclidesinnearlyfiftyelements.
•Notallofthesetransitionsaresuitablefor
actualexploitation.
•valuablecontributionstothephysical,
chemical,biological-andearthsciences
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16. 16SANTHANAM SCSVMV

17. Conditions for MB spectra
•Theenergyofnucleartransitionmustbe
largeenoughtogive,usefulϒrayphoton;
butnotlargeenoughtocauserecoileffect.
•Theenergyoftheϒrayphotonmustbein
therangeof10–150keV.
•Asubstantialamountofthenucleardecay
mustbewithϒrayemission.
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18. …..Conditions continued….
•Thelifetimeoftheexcitedstatemustbelong
enoughgiveareasonablybroademission
range.Since,extremelynarrowlinesarenot
useful(τmustbe001–100nS)
•Theexcitedstateoftheemittershouldbe
havingalong-livedprecursor,andeasyto
handle.
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19. …..Conditions continued….
•Thegroundstateoftheisotopeshouldbe
stable.Itsnaturalabundanceshouldbehigh
oratleasttheenrichmentofthatisotope
shouldbeeasy.
•Thecrosssectionofforabsorptionshouldbe
high.
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20. 57
Co
270
days
57
Fe*
127.90 k.eV/ 91%
57
Fe* (99.3 nS)
14.41 k.eV
57
Fe
136.32 k.eV /
9 %
Why Fe is the most studied?
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21. Why Fe is the most studied?
•Theprecursorof
57
Feis
57
Cowhichdecaysto
57
Fe*withahalflifeof270days(highly
stableprecursor)
•9%of
57
Fe*decaystogroundstatedirectly
withemissionofϒphotonofenergy136.32
keV.
•91%of
57
Fe*decaystoanotherexcitedstate
withemissionof121.91keVenergy
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22. •The lower excited state is having a life time of 99.3nS
(more stable)
•This decays to ground state with emission of 14.41
keV.
•This transition satisfies all the conditions for MB
spectra except 2
nd
condition.
•But it is compensated by larger absorption cross
section.
•Other elements that can be studied-are
119
Sn,
121
Sb,
125
Te,
129
I,
129
Xe and
197
Au
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23. Recording the MB spectrum
•Usuallythestandardemitterisusedasthe
source.
•Sampleunderinvestigationistheabsorber.
•Boththesampleandabsorberareembedded
onacrystallatticetominimizetherecoil
effect.
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24. INSTRUMENTATION
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25. •Identicalsourceandabsorbermaterial;maximum
overlapoccursatzeroDopplervelocity.
•Thesourcefor
57
Fespectroscopyiscommercially
available
57
Co/Rh,ismountedontheshaftofa
vibrator.
•sourceisgenerallykeptatroomtemperature.
•Absorber(sampleunderstudy)maybecooled
downtoliquidnitrogenorliquidhelium
temperaturesinacryostat,orforcontrolledheating
inanoven.
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26. •γ-raysaredetectedbyascintillationcounter,gas
proportionalcounterorasemi-conductordetector.
•Aconstantfrequencyclocksynchronisesavoltage
waveformwhichservesasareferencesignaltothe
servo-amplifiercontrollingtheelectro-mechanical
vibrator.
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27. •Thedifferencebetweenthemonitoredsignaland
thereferencesignalisamplifiedanddrivesthe
vibratoratthesamefrequency(typically50s
-1
)as
thechanneladdressadvance.
•Eachchannelcorrespondstoacertainrelative
velocityandisheldopenforafixedtimeinterval
dependingonthefrequencyandnumberof
channelsused.
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28. •Theincomingγ-countsarecollectedintheir
correspondingchannelsduringthesequential
accessinge.g.50timespersecond,untilsatisfactory
resolutionisreached.
•Thecryostatcanbefurnishedwithasuper-
conductingsolenoidformeasuringthesampleinan
appliedmagneticfield.
•Itisalsopossibletomountapressurecellinsidethe
cryostatforstudyingthesamplepropertiesunder
pressure.
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29. ISOMER SHIFT / CENTRE SHIFT / CHEMICAL SHIFT
•TheE
tvalueisaffectedbytheinteraction
betweenthenucleusandthee
-
spresent
aroundit.
•Thisarisesbecauseofthedifferentsizesof
nucleusingroundandexcitedstates.
•Thechangeinnuclearradiuswhengoing
fromg.stoe.sisΔR
•Zistheatomicnumber
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30. Isomer shift
•The change in electrostatic energy on decay is
given by (chemical shift / Isomer shift)
δ= (ε
0/5) (Ze
2
R
2
)(ΔR/R)[|ψ
s(abs)|
2
-|ψ
s(source)|
2
]
where
ε
0 –Permittivity of free space
Z -atomic number of the nucleus
e -electronic charge
ψ
s(abs) -s orbital wave function of absorber
ψ
s(source)-s orbital wave function of source
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31. 31SANTHANAM SCSVMV

32. •Sinceselectronwavefunctionshavetheir
maximaatthenucleus,selectrondensity
affectstheisomershifttoagreatextent.
•Butchangesinp&dorbitaloccupancies
affecttheselectronthroughscreeninghence
haveasmallereffectonisomershift.
•WhenΔR/Rispositivetheisomershiftisalso
positiveandnegativeΔR/Rreflectedas
negativeshift
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33. •Isomershiftisrelatedtotheoxidationstate
ofthemetal.
•In
119
SnMBspectraSn(II)showsapositive
shift(ΔR/R)w.r.toSnwhereasforSn(IV)itis
negative
Species configuration ΔR/R δ
Sn(IV) 5s
0
5p
0
-ive -ive
Sn 5s
2
5p
2
0 0
Sn(II) 5S
2
5p
0
+ive +ive
s -electron density decreased so
negative shift
s -electron density increased –p screening
not present, so positive shift
33

34. •In
119
Sncompounds,theshiftvaluesdepend
ontheligandspresentandthecoordination
numberoftin.
•Theisomershiftvaluesareusefulin
characterizingtinderivatives.
•ManySncompoundsappeartocontainSn(II)
fromtheformulaebutonanalyzingwithMB
spectraSn(IV)ispresentinthem.
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35. •Manyofthemarepolymericandcontain
Sn(IV).
•GenerallySn(II)compoundsshowshift>than
2.1mmS
-1
andSn(IV)showshift<2.1mmS
-1
(WithrelativetoSnO
2)
•Thepointofchangeoverisadisputebut
shiftslessthan2areforSn(IV)andabove2.5
areclearlyforSn(II)
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36. Organotin compounds
•Theisomershiftof(SnPh
2)
nis1.5mm/S
clearlyshowingthepresenceofSn(IV).
•AsimilarformulaSn(C
5H
3)
2ishavinga
monomericstructureinthesolidstateand
showsashiftof3.74mm/S.
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37. Chemical shift values for Fe compounds
•Feisomershiftcannotbeusedfor
determiningtheO.SofFeinamolecule.
•FeO.Sfrom0to4,oftendifferinunitcharge
only.
•Theelectronsinvolvedarefromdorbital.So
effectontheselectronsissmaller.
•Varyingspinstates(whichdependsonthe
ligandspresent)alsoaffecttheshiftvalue.
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38. •Anywaysomeusefulcorrelationshadbeen
drawn.
•Fe-porphyrincomplexesareveryimportant
biologically.
•Fecanbepresentin+2or+3stateinthem.
•Thecomplexescanbeeasilyreduced.The
electronmaybetotheFeortotheligand.In
thatambiguouscaseMBspectraisusefulin
determiningtheO.S
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39. •EventhoughassignmentofO.Sisoftenpossible,in
somecaseslike
197
Aumanyoxidationstatesshow
shiftvaluesconsiderablyoverlapping.
•Inthatcase,MBspectrumcanonlybeusedasa
supportingevidenceandcannotbeusedfor
ascertainingtheoxidationstate.
•Inaddition,theisomershiftsarewithrelativetoa
standardonly.Differentstudieshaveuseddifferent
standards.Soitisimportanttomentionwhich
standardwasused.For
57
Fe–[Fe(CN)
5NO],
119
Sn-
SnO
2
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40. Electric Quadruple Interactions
•Nucleiinstateswithanangularmomentum
quantumnumberI>1/2haveanon-sphericalcharge
distribution.Thisproducesanuclearquadruple
moment.
•Inthepresenceofanasymmetricalelectricfield
(producedbyanasymmetricelectroniccharge
distributionorligandarrangement)thissplitsthe
nuclearenergylevels.
•Thechargedistributionischaracterizedbyasingle
quantitycalledtheElectricFieldGradient(EFG).
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41. SPHERICAL Q IS 0 PROLATE Q IS +IVEOBLATE Q IS -IVE
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42. Oblate and Prolate nuclei
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43. •In the case of an isotope with a I=3/2 excited
state, such as
57
Fe or
119
Sn, the excited state
is split into two sub states m
I=±1/2 and
m
I=±3/2.
•This gives a two line spectrum or 'doublet'.
•The magnitude of splitting, Delta, is related
to the nuclear quadrupole moment, Q, and
the principle component of the EFG, V
zz, by
the relation Δ=eQV
zz/2
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44. Quadruple Splitting for
57
Fe and
119
Sn
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45. 5/2
3/2
1/2
7/2
5/2
3/2
1/2
I
e= 5/2
I
g= 7/2
Selection Rule
for MB spectra
m
I= 0,1
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46. 46SANTHANAM SCSVMV

47. Informations from quadruple splitting
•Appearanceofquadruplesplittingshowsthe
presenceofefgatthenucleus.
•Theefgmaybecreatedbytheligandfieldor
bytheelectrondistributionaroungthe
nucleus.
•Singlecrystalorapplicationofmagneticfield
givesmoreinformation.
•Ifanucleuswithsymmetricelectron
distributionisinanO
hfield,efgisnot
expected.
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48. Stereochemical activity of lone pair
•Consider Te(IV) compounds with six ligands
•If the lone pair of electron of Te is
stereochemically active , it will occupy one
vertex of a pentagonal bipyramide(pbp)
•This will create an efg at the nucleus,
resulting in q.s
•But no q.s in these class of compounds.
•So the lone pair is stereochemically inactive
•It occupies the 5s orbital
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49. •StructuresoftheTeX
4Y
2-
hadnotbeen
crystallographicallystudied.
•TheMBspectraofthosespeciesshoenoq.S,
butthisisnotaconclusiveevidencefor
absenceofefgatnucleus.
•Theefgmaynotbeenoughtocauseq.s.
•ButinCsIF6substantialq.sshowsthe
stereochemicalactivityoflonepair.
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50. Magnetic interactions
•:Inthepresenceofamagneticfieldthe
nuclearspinmomentexperiencesadipolar
interactionwiththemagneticfieldieZeeman
splitting
•Therearemanysourcesofmagneticfields
thatcanbeexperiencedbythenucleus.
•Thetotaleffectivemagneticfieldatthe
nucleus,B
effisgivenby
B
eff= (B
contact+ B
orbital+ B
dipolar) + B
applied
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51. •Thefirstthreetermsbeingduetotheatom's
ownpartiallyfilledelectronshells.
•B
contactisduetothespinonthoseelectrons
polarisingthespindensityatthenucleus
•B
orbitalisduetotheorbitalmomentonthose
electrons,andB
dipolaristhedipolarfielddue
tothespinofthoseelectrons.
Energy of the nuclear levels E
m= -gμ
NBm
I
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52. •Thismagneticfieldsplitsnuclearlevelswitha
spinofIinto(2I+1)substates.
•Transitionsbetweentheexcitedstateand
groundstatecanonlyoccurwhere
m
Ichangesby0or1.
•Thisgivessixpossibletransitionsfora3/2to
1/2transition,givingasextet,withtheline
spacingbeingproportionaltoB
eff.
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53. Effect of magnetic field on
57
Fe
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54. •Thelinepositionsarerelatedtothesplitting
oftheenergylevels,butthe
lineintensitiesarerelatedtotheangle
betweentheMössbauergamma-rayandthe
nuclearspinmoment.
•Theouter,middleandinnerlineintensities
arerelatedby:
3:(4sin
2
θ)/(1+cos
2
θ):1
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55. •Outerandinnerlinesarealwaysinthesame
proportion.
•Themiddlelinescanvaryinrelativeintensity
between0and4dependingupontheangle
thenuclearspinmomentsmaketothe
gamma-ray.
•Inpolycrystallinesampleswithnoapplied
fieldthisvalueaveragesto2,
•Butinsinglecrystalsorunderappliedfields
therelativelineintensitiescangive
informationaboutmomentorientationand
magneticordering.
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56. APPLICATIONS
OF
MOSSBAUER SPECTRA
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57. Spectra of spin free Fe(II) –
FeSO
4.7H
2O
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58. •Fromthecrystallographicdataitwasinitially
concludedthatFeSO
4.7H
2Oishavinga
perfectO
hsymmetry,withsixH
2Ooneach
vertex.
•ButtheMBspectrumrecordedshoweda
largequadruplesplitting,whichisnot
possibleinaregularO
hsymmetry.(for
perfectO
hfieldefgiszero)
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59. •Fe(II)isad
6
system.
•InweakfieldcreatedbyH
2O,anorbitally
degeneratesystemresults.
•ThisundergoesJ-Tdistortion(Z
in)toremove
thedegeneracyandformsatetragonalfield.
•Thiscreatesanefgatthenucleusandhence
quadruplesplitting.
•ThestructureassignedisandistortedO
hwith
allangles90
o
andx≠y≠z
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60. 60SANTHANAM SCSVMV

61. Prussian Blue and Turnbull’s blue
•Prussianblueisadarkbluepigmentwiththe
idealizedformulaFe
7(CN)
18.itispreparedby
addingaferricsalttoferrocyanide
Fe
3+
+[Fe
2+
(CN)
6]
4-
Fe
3+
4[Fe
2+
(CN)
6]
3
•Turnbull'sblueisthesamesubstancebutis
madefromdifferentreagents,i.e.addition
ferroussaltstoferricyanide.
Fe
2+
+[Fe
3+
(CN)
6]
3-
Fe
3+
4[Fe
2+
(CN)
6]
3
•Itslightlydifferentcolorstemsfromdifferent
impurities.
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62. •Foralongtimeonehadconsideredthemas
chemicallydifferentcompounds.
•PrussianBluewith[Fe
II
(CN)
6]
4-
anionsand
Turnbull’sBluewith[Fe
III
(CN)
6]
3-
anions,
accordingtothedifferentwayofpreparing
them.
•However,theMössbauerspectrarecordedby
Flucketal.,werenearlyidenticalforbothPB
andTBshowingonlythepresenceof
[Fe
II
(CN)
6]
4-
andFe
3+
inthehighspinstate.
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63. •Thiscouldbeconfirmedbyuseof
K
4[Fe
II
(CN)
6]andK
3[Fe
III
(CN)
6]asreference
compounds.
•ImmediatelyafteraddingasolutionofFe
2+
toasolutionof[Fe
III
(CN)
6]
3-
arapidelectron
transfertakesplacefromFe
2+
totheanion
[Fe
III
(CN)
6]
3-
withsubsequentprecipitationof
thesamematerial.
•AsingletforFe(II)andaquadrupledoublet
forFe(III)werewhichconfirmedPrussian
blueandTurnbull’sblueareidentical
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64. Fe(II) –L.S –efg= 0-
Q.S not present
Fe(III) –L.S –efg≠ 0
Q.S present
Fe(II) –H.S –efg≠ 0
Q.S present
Fe(III) –H.S –efg= 0
Q.S not present
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65. •TheFe(II)centers,whicharelowspin,are
surroundedbysixcarbonligandsin
anoctahedralconfiguration.
•TheFe(III)centers,whicharehighspin,are
octahedrallysurroundedonaverageby4.5
nitrogenatomsand1.5oxygenatoms(the
oxygenfromthesixcoordinatedwater
molecules).
•ThisintroducesanefgatFe(III)nucleusand
henceQ.S
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66. •.
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67. Sodium nitroprusside –Na
2[Fe(CN)
5(NO)]
•TheO.SofFeinnitroprussidewasamatterof
controversyforalongtime.
•InitiallyitwasassumedthatitcontainedFe(II)
andNO
+
sinceitwasdiamagnetic.[Fe(II)-d
6
;
Fe(III)–d
5
]
•ButtheMBspectrumofthesampleshoweda
doubletwithδ=-0.165mm/S
•ThisvalueistoonegativeforaFe(II)complex.
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68. 68SANTHANAM SCSVMV

69. •ThissuggeststhattheFemaybeinFe(IV)
state.
•ThemagnetismandMBspectrumarein
consistentwiththestructurewhichhasan
extensiveπ-bondingwith
+
NOligand.
•Thet
2gorbitalsofFeandthep-orbitalofN
presentin
+
NOcontainingtheodde-,toform
aπ-bond
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70. •TheFe(II)istransferringthee
-
sformfilledt
2g
leveltothevacantπ-antibondingorbitalof
NO.
•ThismakestheshiftvaluetoapproachFe(IV)
values.
•Nowbecauseofthistheshieldingofs-e
-
by
thede
-
sdecreasesandhencetheshift
becomesmore.
•ThisissupportedbythedecreaseinN-O
stretchingfrequencyinIR,sincetheanti
bondinglevelisfilled.
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71. •K
4[Fe(CN)
6]isa3d
6
LScomplexwithO
h
symmetry,whereallsixelectronsare
accommodatedinthethreet
2gorbitals.
•Bothcontributions(EFG)
valand(EFG)
lat
vanish;thereisnoquadrupolarinteraction.
•Na
2[Fe(CN)
5NO]2H
2OhasC
4vsymmetrywith
d-orbitalsplittingasshown.
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72. 72SANTHANAM SCSVMV

73. •ItsLSbehaviorrequiresthatallsixelectrons
areaccommodatedinthelowestthree
orbitalsarisingfromthetetragonalsplitting
oftheformercubict2g(Oh)subgroup.
•(EFG)
valisstillzero,but(EFG)
lat≠0arisesfrom
theligandreplacementintheiron
coordinationsphere.
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74. •Theisomershiftdataforthepentacyano
complexesofiron(II)withadifferentsixthligand
X.
•Normalizingtheisomershiftstothatofthe
pentacyanonitrosylferratecomplexaszeropoint
•Theorderingexpressesthevaryingeffectsofd
π-
p
πbackdonationforthedifferentsixthligandX.
•Theisomershiftvaluesbecomemorepositive
on goingfromNO
+
toH
2O
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75. •d
π-p
πbackdonationdecreasescausingan
increasingd-electrondensityresidingnear
theironcentre.
•Strongershieldingofs-electronsbyd-
electrons,whichfinallycreateslowers-
electrondensityatthenucleus.
•ThenuclearfactorΔR/Risnegativefor
57
Fe
explainstheincreasinglypositiveisomershift
valuesinthegivensequencefromNO
+
to
H
2O 75SANTHANAM SCSVMV

76. 76SANTHANAM SCSVMV

77. Ferredoxin
•Studyofferredoxin,aFe-Sprotein,which
assistsinin-vivoe
-
transferreactions
•Thetwo-ironcentresarenotequivalentin
thereducedform.
•TheoxidizedformwithtwoFe(III)-highspin
centrescanbedistinguishedfromthe
reducedformwithoneFe(III)-highspin
centreandoneFe(II)-highspincentreonlyby
usingMBspectrum
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78. 78SANTHANAM SCSVMV

79. Study of thermal spin-cross over
•Manycoordinationcompoundspossessing
intermediateligandfieldstrengthsshow
thermalspincrossover.[i.e.HS<-->LS]
•[Fe(phen)2(NCS)2]undergoesthermalspin
transition.
•Themainresultisthatinthetemperature
region,wheretheMASspectrareflectthe
transitiontotheLSstate,theMESspectra
stillshowthetypicalHSsignalsarisingfrom
excitedligandfieldstates 79SANTHANAM SCSVMV

80. 80SANTHANAM SCSVMV

81. 81