Mössbauer Spectroscopy

590B S09

Sergey L. Bud’ko

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Sergey L. Bud’ko(Сергей Леокадьевич Будько)

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Nobel Prize in 1961 for PhD work of 1958

Mössbauer spectroscopy and its sensitivity

Mössbauer spectroscopy is a technique in which interactionbetween the electromagnetic moment of the nuclear charge andelectromagnetic field produced by the extra-nuclear electrons arestudied. This interaction gives splitting/shifting of the nuclearenergy levels.

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For the most common Mössbauer isotope, 57Fe, the linewidth is

5x10-9eV. Compared to the Mössbauer gamma-ray energy of

14.4keV this gives a resolution of 1 in 1012 (or the equivalent of a

small speck of dust on the back of an elephant or one sheet of

paper in the distance between the Sun and the Earth).

Emission Absorption

Free emitting and absorbing atoms

Recoil

mc

E=E

2

2

R

2

γEnergy of recoil

γ-ray energy

Mass of atomafter Enver Murad, Workshop on Martian Phyllosilicates, 2008

Emission Absorption

Emitting and absorbing atoms fixed in a lattice

No recoil

Mc

E=E 2

2

R

2

γ

Mass of particle

Mössbauer spectroscopy is the recoil-free emission and absorption of γ-rays

(VERY LARGE)

after Enver Murad, Workshop on Martian Phyllosilicates, 2008

Experimentally:

Experimental

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Experimental

Mostly in transmission mode

Source (emitted radiation: resonant γ-rays (recoil-free), non-resonant γ-rays (involving recoil), radiation from all other transitions, secondary radiation (mainly X-rays) produced in the matrix)

57Co in Rh matrix: activity: 5 mCi to 150 mCi

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57Co in Rh matrix: activity: 5 mCi to 150 mCi

119mSn in Ca-stannate matrix: 2-25 mCi

Absorber – your sample (“usually” powder, can use single crystals, thickness is an issue)

Experimental

Drive system (mechanical, electromechanical, piezoelectric, hydraulic, etc.) Constant-velocity and sweep-velocity systems. Usually the source makes the motion.

Up to 300 mm/s (1000 mm/c);

Resonance frequency ~ 25 Hz

Different modes allowed

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Different modes allowed

~25 mm/s per Volt

---------------------------------------------------

(and if you are really cheap you can try to use old speaker)

Experimental

Detector (scintillation detectors, proportional counters, lithium-drifted germanium or silicon detectors).

Proportional counter – Ar/Kr/Xe filled, ~ 2 kV operating voltage.

Also need a lot of electronics, cryostat/furnace, ME fitting software

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cryostat/furnace, ME fitting software

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Hyperfine Parameters

Chemical Isomer Shift (IS) (δδδδ): Arises out of the interactionbetween nuclear charge density and the surrounding ‘s’ electroncharge cloud. IS can give information about the spin state as wellas the co-ordination number.

Quadrupole Splitting (QS) (∆∆∆∆): Arises due to interaction between the

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Quadrupole Splitting (QS) (∆∆∆∆): Arises due to interaction between the

electric quadrupole moment of the nucleus and Electric Field

Gradient created by the electrons. QS can give information about

the charge symmetry around the nucleus.

Hyperfine field (Hint) It gives the internal magnetic field of a

magnetic material

Isomer Shift

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Quadrupolar Splitting

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Magnetic Hyperfine Field

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Magnetic Dipole + Electron Quadrupole Interactions

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can get some idea about the magnetic moment direction

“IDEAL” Mössbauer spectra - summary

Depending on the local environments of the Fe atoms and the magnetic properties, Mössbauer spectra of iron oxides can consist of a singlet, a doublet, or a sextet.

Phyllosilicates, 2008

Symmetric charge

No magnetic field

Asymmetric charge

No magnetic fieldSymmetric or asymmetric charge

Magnetic field (internal or external)

∆ Bhfδ Isom

er

shift

Quadru

pole

split

ting

Magnetic h

yperf

ine f

ield

after EnverMurad, Workshop on Martian Phyllosilicates

e T

ran

sm

issio

n

∆

Fe3+

-4 -2 0 2 4

Velocity (mm/s)

Rela

tive

δFe2+

-4 -2 0 2 4

Velocity (mm/s)

after Enver Murad, Workshop on Martian Phyllosilicates, 2008

Use of Mössbauer spectroscopy as a “fingerprinting” technique

3

4

[6]Fe2+

[8]Fe2+

[5]Fe3+

Isomer shifts and quadrupole splittings of Fe-bearing phases vary systematically as a function of Fe

1.0 1.5

1

-0.5 0.5

Isomer shift (mm/s)

0

2

0.0

[6]Fe(II)

[6]Fe(III)[6]Fe3+

[4]Fe3+

[4]Fe2+

[sq]Fe2+

[5]Fe3+

[5]Fe2+

as a function of Fe oxidation, Fe spin states, and Fe coordination.

Knowledge of the Mössbauer parameters can therefore be used to “fingerprint” an unknown phase.

after Enver Murad, Workshop on Martian Phyllosilicates, 2008

ME applications: Fe – containing minerals on Mars

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Mars

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Mars surface geology

Examples

Pressure-induced magnetic order

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J.P.Sanchez et al., Hyperfine Interactions…

Examples

TbNi2B2C (actually, Tb(Ni0.9957Fe0.01)2B2C)

TWFM

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TN

D. Sanchez et al, PRB

Examples

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RNi2B2C

Fe replaces Ni, both are non-magnetic in these materials

No HF field on 57Fe

Possibly collinear AFM (consistent with earlier neutron data)

Examples

CeFeAs(O/F)

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neutron scattering

Examples

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D.Sanchez et al. + Ames Laboratory

unpublished

Mössbauer Spectroscopy

“cheap”

“fast”

sensitive to magnetic transitions, phase purity

can give some information about the nature of the magnetic phase

local probe

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local probe

reasonably easy to use T, P, H as variables

limited number of isotopes

interpretation somewhat model/fit dependent

NEED TO USE IN CONJUNCTION WITH OTHER MEASUREMENTS

timescales associated with some spectroscopic techniques

Energy of excited state Typical relaxation Typical

Technique (Hz) time (seconds) linewidth (Hz)

NMR (solution) 108 10 10-1

ESR (solution) 1010 10-5 105

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rotationalspectroscopy (gas) 1011 10-4 104

vibrationalspectroscopy (gas) 1014 10-8 108

electronicspectroscopy (solution) 1016 10-15 1015

Mössbauerspectroscopy (solid) 1019 10-8 108

Reading materials

Mössbauer Spectroscopy and its Applications, T E Cranshaw, B W Dale, G O Longworth and C E Johnson, (Cambridge Univ. Press: Cambridge) 1985

Mössbauer Spectroscopy, D P E Dickson and F J Berry, (Cambridge Univ. Press: Cambridge) 1986

The Mössbauer Effect, H Frauenfelder, (Benjamin: New York) 1962Principles of Mössbauer Spectroscopy, T C Gibb, (Chapman and Hall: London) 1977

Mössbauer Spectroscopy, N N Greenwood and T C Gibb, (Chapman and Hall:

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Mössbauer Spectroscopy, N N Greenwood and T C Gibb, (Chapman and Hall: London) 1971

Chemical Applications of Mössbauer Spectroscopy, V I Goldanskii and R H Herber ed., (Academic Press Inc: London) 1968

Mössbauer Spectroscopy Applied to Inorganic Chemistry Vols. 1-3, G J Long, ed., (Plenum: New York) 1984-1989

Mössbauer Spectroscopy Applied to Magnetism and Materials Science Vol. 1, G J Long and F Grandjean, eds., (Plenum: New York) 1993