Spin Crossover in Transition Metal Compounds I

324 Pages·2004·5.459 MB·English

by  Philipp Gütlich, Harold A. Goodwin (auth.), P. Gütlich, H.A. Goodwin (eds.)

TopCurrChem(2004)233:1–47
DOI10.1007/b13527
(cid:1)Springer-VerlagBerlinHeidelberg2004
Spin Crossover—An Overall Perspective
PhilippG(cid:2)tlich1())·HaroldA.Goodwin2())
1Institutf(cid:2)rAnorganischeChemieundAnalytischeChemie,
Johannes-Gutenberg-Universit(cid:3)t,StaudingerWeg9,55099Mainz,Germany
guetlich@uni-mainz
2SchoolofChemicalSciences,UniversityofNewSouthWales,2052Sydney,NSW,Australia
[email protected]
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 OccurrenceofSpinCrossover . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 DetectionofSpinCrossover . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 SpinTransitionCurves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 ExperimentalTechniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.1 MagneticSusceptibilityMeasurements. . . . . . . . . . . . . . . . . . . . . . 9
3.2.2 57FeM(cid:4)ssbauerSpectroscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2.3 MeasurementofElectronicSpectra. . . . . . . . . . . . . . . . . . . . . . . . 12
3.2.4 MeasurementofVibrationalSpectra . . . . . . . . . . . . . . . . . . . . . . . 12
3.2.5 HeatCapacityMeasurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2.6 X-rayStructuralStudies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2.7 SynchrotronRadiationStudies . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.8 MagneticResonanceStudies . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.9 OtherTechniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4 Iron(II)Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.1 [Fe(phen) (NCS)]andRelatedSystems . . . . . . . . . . . . . . . . . . . . . 19
2 2
4.2 TheInvolvementofanIntermediateSpinState . . . . . . . . . . . . . . . . . 22
4.3 Five-CoordinationandIntermediateSpinStates . . . . . . . . . . . . . . . . 23
4.4 DonorAtomSets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5 PerturbationofSCOSystems. . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.1 ChemicalInfluences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.1.1 LigandSubstitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.1.2 AnionandSolvateEffects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1.3 MetalDilution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.2 PhysicalInfluences. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
5.2.1 SampleCondition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
5.2.2 EffectofPressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.2.3 EffectofIrradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.2.4 EffectofaMagneticField . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
6 TheoreticalInterpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
7 Literature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
8 Outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2 P.G(cid:2)tlich·H.A.Goodwin
Abstract Inthischapteranoutlineispresentedoftheprincipalfeaturesofelectronicspin
crossover.Thedevelopmentofthesubjectistracedandthevariousmodesofmanifesta-
tionofspintransitionsarepresented.Theroleofcooperativityininfluencingsolidstate
behaviour is considered and the various strategies to strengthen it are addressed along
withthechemicalandphysicalperturbationswhichaffectcrossoverbehaviour.Therole
of intermediate spin states is discussed together with spin crossover in five-coordinate
systems.Thevarioustechniquesappliedtomonitoringatransitionarepresentedbriefly.
An introduction to theoretical treatments is given and likely areas for future develop-
mentsaresuggested.Relevantreviewarticlesinthefieldarelistedandreferencetolater
chaptersintheseriesisgivenwhereappropriate.
Keywords Spincrossover·Magnetism·M(cid:4)ssbauerspectroscopy· Coooperativity·
Hysteresis
ListofAbbreviations
abpt 4-Amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole
bpy 2,20-Bipyridine
btr 4,40-Bis(1,2,4-triazole)
C Heatcapacity
p
DSC Differentialscanningcalorimetry
EPR Electronparamagneticresonance
HS Highspin
LS Lowspin
LIESST Lightinducedexcitedspinstatetrapping
mephen 2-Methyl-1,10-phenanthroline
NIESST Nucleardecayinducedexcitedspinstatetrapping
NMR Nuclearmagneticresonance
ox Theoxalateion
paptH 2-(Pyridin-2-yl-amino)-4-(pyridin-2-yl)thiazole
phen 1,10-Phenanthroline
phy 1,10-Phenanthroline-2-carbaldehydephenylhydrazone
pic 2-Picolylamine
PM-BiA N-(2-Pyridylmethylene)aminobiphenyl
ptz 1-n-Propyl-tetrazole
py Pyridine
SCO Spincrossover
ST Spintransition
T Spintransitiontemperature(temperatureof505%conversion
1/2
ofall“SCO-active”complexmolecules)
TCNQ Tetracyanodiquinomethane
trpy 2,20:60,200-Terpyridine
trzH 1,2,4-Triazole
ZFS Zerofieldsplitting
SpinCrossover—AnOverallPerspective 3
1
Introduction
For about the past 80 years coordination compounds of certain transition
metal ions have been divided into two categories determined by the nature
of the bonding,whether itbe interms ofionicandcovalentbonding,inner-
and outer-orbital bonding or high spin and low spin configurations. It was
recognisedquiteearly thatthisdivisionraisedthequestionofthetransition
from one type to the other. Would this be a sharp transition, i.e. complexes
must be either one kind or the other, or would it be possible for systems to
occur in which the nature of the bonding would be subject to change de-
pending on some external perturbation? These questions were addressed in
the developmentof anunderstanding of the natureof the metal-donor atom
bond,most notablybyLinusPauling.Inhistreatmentof themagneticcrite-
rionfor bondtype, Pauling perceptively recognisedthat itwould be feasible
to obtainsystemsinwhichthetwotypescouldbepresentsimultaneously in
ratios determined by the energy difference between them [1]. In fact, this
situation had at the time just been realised. The pioneering work of Cambi
and co-workers in the 1930s on the unusual magnetism of iron(III) deriva-
tives of various dithiocarbamates led to the first recognition of the inter-
conversion of two spin states as a result of variation in temperature [2].
Work proceeded on the magnetism of various heme derivatives of iron(II)
and iron(III) and established that in these naturally occurring systems, as
wellasinrelatedporphyrinderivatives,thespinstatewasremarkablysensi-
tive to the nature of the axial ligands. For certain species, intermediate val-
ues of the magnetic moment were observed and interpreted in terms of the
bonding being in part ionic and in part covalent [3]. Later Orgel proposed
for these that there was an equilibrium between an iron(III) species with
one, and another with five unpaired electrons [4]. Remarkably, Orgel went
onto suggest that in both of the iron(II) systems [Fe(phen) ]2+ and [Fe(me-
3
phen) ]2+ the field strengthwasnear, buton opposite sides of, the crossover
3
pointintheTanabe-Suganodiagramforad6ion(showninFig.2,Chap.2).
Therapidincreaseininterestinthespincrossoversituationthatfollowed
moreorlesscoincidedwiththewidespreadacceptancebycoordinationche-
mists of the value of ligand field theory in understanding the stability, reac-
tivity and structure together with the spectral and magnetic properties of
transition metal compounds. Early in the 1960s Busch and co-workers [5]
were attempting to identify the crossover region for iron(II) and cobalt(II)
and reported the first instance of spin crossover in a complex of the latter
ion[6].Similarly,MadejaandK(cid:4)nigundertookasystematicvariationinthe
nature of the anionic groups in the iron(II) system [Fe(phen) X ] in an at-
2 2
tempt to define the crossover region [7]. Inthis period too the early studies
onthe iron(III) dithiocarbamate systems of Cambi and co-workers were be-
ingextendedandincluded,forexample,thecrucialexperimentofdetermin-
4 P.G(cid:2)tlich·H.A.Goodwin
ing the role of pressure in influencing the spin state in crossover systems.
This was the first application of this technique to the spin crossover phe-
nomenonandthepredictedeffectoffavouringof thelowspinconfiguration
with increased pressure was observed [8]. The iron(III) dithiocarbamates
have continued to attract much attention and these, together with other
iron(III) systems, are considered in detail in Chap. 10. It was at about the
time of the workof Ewald et al. [8] that the M(cid:4)ssbauer effect (first reported
in 1958 [9]) was being taken up by chemists and the application of M(cid:4)ss-
bauer spectroscopy to the study of the spin changes in the iron(III) dithio-
carbamates represents perhaps the first, albeit not the most diagnostic, in-
stance of its value in this area [10]. M(cid:4)ssbauer spectroscopy has come to
play a pivotal role in the development and understanding of the spin cross-
over phenomenonandwasthetechniquewhichwasusedto confirmtheoc-
currenceofaspintransitionastheoriginoftheunusualtemperaturedepen-
dence of the magnetism in [Fe(phen) (NCS) ], the first example of spin
2 2
crossoverinasyntheticiron(II)system[11].
2
OccurrenceofSpinCrossover
The fundamental consideration of the occurrence of spin crossover interms
of ligand field theory, for iron(II) in particular, is given by Hauser in
Chap. 2. The change in spin state exhibited by certain metal complexes un-
der the applicationofanexternalperturbationisreferredtobyanumberof
terms—spin crossover, spin transition and, sometimes, spin equilibrium.
Themostcommonperturbationresultinginachangeofspinstateforapar-
ticular complex is avariation intemperature, but pressure changes, irradia-
tionandanexternalmagneticfieldcanalsobringaboutthechange.Theor-
igin of the term “spin crossover” lies in the crossover of the energy vs field
strength curves for the possible ground state terms for ions of particular dn
configurationsinTanabe-Suganoandrelateddiagrams.Theterm“spintran-
sition” is used almost synonymously with spin crossover but the latter has
thebroaderconnotation,incorporatingtheassociatedeffects,spintransition
tending to refer to the actual physical event. Thus for a simple, complete
change in spin state, the spin transition temperature is defined as the tem-
perature at which the two states of different spin multiplicity are present in
the ratio 1:1 (g =g =0.5). As will be shown below, many transitions are
HS LS
not simple and this definition of transition temperature is not necessarily
applicable. The transition temperature is generally represented as T and
1/2
even in the less straightforward instances this can usually be readily inter-
preted.Forexample,forsystemsinwhichthetransitionisincomplete,inei-
ther thelow temperatureregion(“residualHSfraction”)or thehightemper-
ature region (“residual LS fraction”), or both, the spin transition tempera-
SpinCrossover—AnOverallPerspective 5
turecanbedefinedasthetemperatureatwhich50%oftheSCO-activecom-
plex moleculeshave changedtheir spinstate. Inthe early literature the term
“spinequilibrium”hasbeenusedtodescribethetemperaturedependenceof
the population of spin states. This term is not suited to most instances of
thespincrossover inasolidsamplesinceastraightforwardthermalequilib-
rium based on a simple Boltzmann-like distribution of the energy states is
inappropriate to account for the complex nature of the spin changes fre-
quently observed. For systems in liquid solution, however, reference to a
spin equilibrium is generally meaningful and appropriate, and is currently
used. In dilute solid solutions where the spin crossover centres are incorpo-
rated into a SCO-inactive host lattice the cooperative interactions between
the spin-changing molecules tend to disappear as the extent of dilution in-
creases and thus the situation is similar to that in liquid solution where, a
priori,cooperativeinteractionsareassumedtobeabsent.
Spincrossoverisfeasibleforderivativesofionswithd4,d5,d6andd7con-
figurations and is observed for all these in complexes of first transition se-
ries ions. Isolated examples are available for the second series, but, because
ofthelowerspinpairingenergy fortheseions,togetherwithstrongerligand
fields, it is unlikely that a large number will be found. For the d8 configura-
tion, in particular for Ni(II), change in spin multiplicity (singlet$triplet)
generally resultsinsuchamajorgeometricalrearrangementthattheprocess
is referred to as a configurational change. The difference between this and
what is normally referredto as spin crossover is one more of degreethan of
kind, but it does tend to be considered separately from spin crossover. An
early paper by Ballhausen and Liehr [12] offers some pertinent insight into
thisdistinction.
Of the ions which do show typical spin crossover behaviour the largest
number of examples is found for the configuration d6 and iron(II) accounts
for thevast majorityof these.For thisreason,muchof thediscussionwhich
follows inthis and subsequent chapters refers to transitions in iron(II). The
only other d6 ion for which crossover behaviour has been observed is co-
balt(III), but there is a very limited number of examples. The d6 configura-
tion is relatively easily obtained in the low spin configuration—the spin
pairing energy is less thanthat of comparable ions [13] and the low spin d6
configuration has maximum ligand field stabilisation energy. Thus for
Co(III),whichinducesastrongfieldinmostligands,thelowspinconfigura-
tion is almost always adopted, hence the paucityof spin crossover or purely
high spin systems for this ion. For the larger Fe(II) ion ligand fields are
weaker. Hence spin pairing is not so strongly favoured and it is possible to
obtain relatively stable high spin or low spin complexes from a broad range
of ligands. Thus it is feasible to fine-tune the ligand field with a fair degree
ofcertaintyofbringing itintothecrossover region.For thesmalleriron(III)
ion (d5) the low spin configuration is again relatively favoured, but not to
theextentobservedforCo(III),partlybecauseoftherelativelylowspinpair-
6 P.G(cid:2)tlich·H.A.Goodwin
ingenergyandhigherligandfieldstabilisationenergyof thelatter.Thusthe
occurrence of spin crossover is much more widespread for Fe(III) than for
Co(III). However, conditions are less favourable than for Fe(II), partly be-
cause of the tendency of high spin Fe(III) complexes to be readily hydrol-
ysed. For Co(II) (d7)spin crossover is well characterised, but it is much less
common than for Fe(II), possibly because of the higher spin pairing energy
and the destabilising effect of the single e electron in low spin six-coordi-
g
natecomplexes(SCOinCo(II)complexesistreatedinChap.12).ForNi(III),
also d7, SCO has been proposed in only one instance—in salts of [NiF ]3(cid:5)
6
[14].Theoccurrenceofspincrossover insystemsother thanthoseofFe(II),
Fe(III)andCo(II)isconsideredindetailinChap.13.
3
Detection ofSpinCrossover
Perhaps the two most important consequences of a spin transition are
changes inthe metal-donor atom distance, arising from a change in relative
occupanciesof thet ande orbitals(seeChap.2),andchangesinthemag-
2g g
neticproperties.Whiletheformercanbeeffectively monitored,thechanges
in magnetism are more conveniently measured. The change from low spin
to high spin results in a pronounced increase in the paramagnetism of the
system and hence the measurement of this change (as a functionof temper-
ature)wasthemeansinitiallyappliedtothedetectionofthermalspincross-
over, and remains the most common way of monitoring a spin transition.
Measurementof M(cid:4)ssbauerspectra, for iron(II)systems inparticular,offers
a more direct means of obtaining the relative concentrations of the spin
states since these give separate and well defined contributions to the overall
spectrum, each spin state having its own characteristic set of M(cid:4)ssbauer
spectral parameters (isomer shift and quadrupole splitting). Provided that
the lifetimes of the spin states are greater than the time scale of the M(cid:4)ss-
bauereffect(10(cid:5)7s)theirseparatecontributionstotheoverallspectrumcan
beidentified.Thisisthenormalsituationfor iron(II),withonereportedex-
ception for six-coordinate complexes [15]. For iron(III) the rates of inter-
conversionofthespinstatesarefrequently toorapidtoenabletheirseparate
identification in M(cid:4)ssbauer spectra. When the separate contributions are
seen their area fractions can usually be extracted with reasonable accuracy
from the M(cid:4)ssbauer spectra. The value of measurements of magnetic sus-
ceptibility and M(cid:4)ssbauer spectra in studies of SCO systems is developed
below. Their most important application is undoubtedly inthe derivationof
a spin transition curve which is a visual representation of the course of a
spintransition.
SpinCrossover—AnOverallPerspective 7
3.1
SpinTransitionCurves
A spin transition curve is conventionally obtained from a plot of high spin
fraction(g )vstemperature.Suchcurves arehighly informativeandtakea
HS
numberof forms for systems inthe solid state.The most important of these
areillustrated in Fig. 1. The varietyof manifestations of atransitionevident
inthis figure arises froma numberofsources but the most important is the
degree of cooperativity associated with the transition. This refers to the ex-
tent to which the effects of the spin change, especially the changes in the
metal-donor atom distances, arepropagated throughout the solid and is de-
termined by the lattice properties. The gradual transition (sometimes re-
ferred to as a continuous transition, but this term can have misleading con-
notations)illustratedinFig.1aisperhapsthemostcommonandisobserved
when cooperative interactions are relatively weak. This is the course of a
transition observed for a system in solution where essentially a Boltzmann
distribution of the molecular states is involved. The abrupt transition
(sometimes referred to as discontinuous, but again this can be misleading)
ofFig.1bresultsfromthepresenceofstrongcooperativity.Obviously,situa-
tions intermediate between (a) and (b) exist. Whenthe cooperativity is par-
ticularly high hysteresis may result, as shown in Fig. 1c. The appearance of
hysteresis, usually accompanied by a crystallographic phase change, associ-
atedwithaspintransitionhascometoberecognisedasoneofthemostsig-
nificant aspects of the whole spin crossover phenomenon. This confers
bistability on the system and thus a memory effect. Bistability refers to the
Fig.1a–d Representationoftheprincipaltypesofspintransitioncurves(highspinfrac-
tion (g ) (y axis) vs temperature (T) (x axis): a gradual; b abrupt; c with hysteresis; d
HS
two-step;eincomplete
8 P.G(cid:2)tlich·H.A.Goodwin
ability of a system to be observed in two different electronic states in a cer-
tainrangeofsomeexternalperturbation(usually temperature)[16].Thepo-
tentialforexploitationof thisaspect ofSCOinstorage,memoryanddisplay
devices was highlighted by Kahn and Martinez [17] and this has driven
much of the recent research in the area. The quest for stable systems which
displayawell-defined,reasonablybroadhysteresisloopspanningroomtem-
perature and an understanding of the factors which lead to such behaviour
iscontinuing.
There are two principal origins of hysteresis in a spin transition curve:
thetransitionmaybeassociatedwithastructuralphasechangeinthelattice
and this change is the source of the hysteresis; or the intramolecular struc-
tural changes that occur along with a transition may be communicated to
neighbouring molecules via a highly effective cooperative interaction be-
tween the molecules. The mode of this interaction is not always clear but
three principal strategies have been adopted in an attempt to generate it: (i)
linkageof theSCOcentresviacovalentbondsinapolymericsystem;(ii)in-
corporationof hydrogenbondingcentresintothecoordinationenvironment
allowing interaction either directly with other SCO centres or via anions or
solvate molecules; (iii) incorporation of aromatic moieties into the ligand
structure which promote p-p interactions through stacking throughout the
lattice. Partial success has been achieved for all three approaches but a full
understanding of the factors involved remains one of the major challenges
of the area. A further probable origin of cooperativity is the synergism
between an order-disorder transition and a spin transition, as has been
proposed for the systems [Fe(pic) ]Cl ·EtOH [18] and [Fe(dppen) Cl ]·
3 2 2 2
2(CH ) CO[19](dppen=cis-1,2-bis(diphenylphosphino)ethene)inwhichthe
3 2
disorder is associated with solvate molecules and for [Fe(biimidazoline) ]
3
(ClO ) where disorder in the anion orientation is considered likely [20].
4 2
Disorder involving solvate molecules and anions is relatively common so
this relatively little explored aspect to cooperativity offers scope for further
development.
Despite the relative lack of predictability, the number of systems now
knownto display a spintransitioncurveof type (c) is remarkably high, and
highestforiron(II)where,significantly,thechangeinintramoleculardimen-
sionsisthegreatestfor theionsfor whichSCOisrelativelycommon(Fe(II),
Fe(III),Co(II)).
The transitions of type (c) are defined by two transition temperatures,
one for decreasing (T #), and one for increasing temperature (T "). Two-
1/2 1/2
step transitions (Fig. 1d), first reported in 1981 for an iron(III) complex of
2-bromo-salicylaldehyde-thiosemicarbazone [21], are relatively rare and
have their origins in several sources. The most obvious is the presence of
two lattice sites for the complex molecules. There are several examples of
this [22]. In addition, binuclear systems can give rise to this effect, even
whenthe environment of each metal atom is the same—inthis instance the
SpinCrossover—AnOverallPerspective 9
spin change in one metal atom may render the transition in the twin metal
atom less favourable. The [Fe(diimine)(NCS) ] bipyrimidine series provides
2 2
theclassicexamplesofthissituation[23](Chap.7).Moregenerally,twostep
transitions can be observed in systems inwhichthere is only a single lattice
site, this being observed for example in the ethanol solvate of tris(2-picoly-
lamine)iron(II) chloride [24]. This has been interpreted in terms of short
range interactions and the preferential formation of HS/LS pairs in the pro-
gressofthetransition[25].
The retention of a significant high spin fraction (Fig. 1e) at low tempera-
tures may also arise from various sources. A fraction of the complex mole-
culesmaybeinadifferentlatticesiteinwhichthefieldstrengthissufficient-
ly reducedtopreventtheformationoflowspinspecies.Itisfeasiblethatfor
a particular lattice the major structural changes that accompany a complete
changeinspinstatemay notbeabletobeaccommodated.Thereislikely,in
addition, in some instances to be a kinetic effect involved—at sufficiently
low temperatures the rate of the high spin to low spin conversion becomes
extremely small. Because of this, it is possible in a number of instances to
freeze-in a large high spin fraction by rapid cooling of the sample [26–29].
This effect is often observed around liquid nitrogen temperature but would
obviously be more common at still lower temperatures. It occurs generally
when there is a major structural change accompanying the transition over
and above the normal intramolecular changes and hence the structural
change may proceed at a slower rate than the normal rate for the spin
change alone. The retention of a permanent low spin fraction at the upper
temperaturelimitofatransitionislesscommon,becauseofthemuchgreat-
er density of vibrational states for the high spin species and in addition ki-
neticfactorsarenotlikely tobesorelevantinthisinstance.
3.2
ExperimentalTechniques
3.2.1
MagneticSusceptibilityMeasurements
Measurement of magnetic susceptibility as a function of temperature, c(T),
has always been the principal technique for characterisation of SCO com-
pounds. The EvansNMRmethod [30] is generally applied for studies in liq-
uid solution. For measurements on solid samples SQUID magnetometers
haveprogressively replacedthetraditionalbalancemethods(Faraday,Gouy)
inmodernlaboratories,becauseoftheirmuchhighersensitivityandaccura-
cy. Alternative instruments being used are Foner-type vibrating sample and
a.c./d.c. susceptibility magnetometers. A comprehensive survey of the tech-
niques and computational methods used in magnetochemistry is given by
Palacio[31]andKahn[32].
10 P.G(cid:2)tlich·H.A.Goodwin
The transition from a strongly paramagnetic HS state to a weakly para-
magnetic or (almost) diamagnetic LS state is clearly reflected in a more or
less drasticchange inthe magnetic susceptibility.The product cTfor a SCO
material is determined by the temperature dependent contributions c and
HS
c according to c(T)=g c +(1(cid:5)g )c . With the known susceptibilities
LS HS HS HS LS
of the pure HS and LS states, the mole fractionof the HS state (or LS state),
g , at any temperature is easily derived and is plotted to produce the spin
HS
transitioncurve,asshowninFig.1.Alternatively,insteadofaplotofg (T),
HS
thespintransitioncurveisfrequentlyexpressedastheproductcTvsT,par-
ticularly inthosecaseswherethequantitiesc andc arenotaccessibleor
HS LS
not sufficiently accurately known. Expression of the spintransition curvein
terms of the effective magnetic moment m =(8cT)(cid:5)1/2 as a function of tem-
eff
peraturehasbeenwidelyusedbutisnowlesscommon.
Techniques have been developed for measurements of c(T) down to liq-
uid helium temperatures with the sample under various external perturba-
tions such as hydrostatic pressure (Chap. 22), light irradiation (Chap. 30)
andhighmagneticfields(Chap.23).
3.2.2
57FeM(cid:1)ssbauerSpectroscopy
Therecoillessnuclearresonanceabsorptionofg-radiation(M(cid:4)ssbauereffect)
has been verified for more than 40 elements, but only some 15 of them are
suitable for practical applications [33, 34]. The limiting factors are the life-
time and the energy of the nuclear excited state involved in the M(cid:4)ssbauer
transition.Thelifetimedeterminesthespectrallinewidth,whichshouldnot
exceed the hyperfine interaction energies to be observed. The transition en-
ergyoftheg-quantadeterminestherecoilenergyandthustheresonanceef-
fect [34]. 57Fe is by far the most suited and thus the most widely studied
M(cid:4)ssbauer-active nuclide, and 57Fe M(cid:4)ssbauer spectroscopy has become a
standardtechniqueforthecharacterisationofSCOcompoundsofiron.
The isomer shift d and the quadrupole splitting DE , two of the most im-
Q
portant parameters derived from a M(cid:4)ssbauer spectrum [34], differ signifi-
cantly for the HS and LS states of both Fe(II) and Fe(III). Thus, if both spin
states, LS and HS, are present to an appreciable extent (not less than ca. 3%
in anycase) andprovidedthe relaxationtime for LS$HS fluctuationis lon-
ger thanthe M(cid:4)ssbauer time window (determined by the lifetime of the ex-
cited nuclear state, which is ca. 100 ns for 57Fe), the two spin states are dis-
cernibleby theircharacteristicsubspectra.Evenincaseswherethesubspec-
tra strongly overlap, the area fractions of the resonance lines can be deter-
mined with the help of specially developed data fitting computer programs.
The area fractions t and t are proportional to the products f g and
HS LS HS HS
f g , respectively, wheref and f are the so-called Lamb-M(cid:4)ssbauer fac-
LS LS HS LS
tors of the HS and LS states. Only for f =f are the area fractions a direct
HS LS