Metal-insulator transition in A-site ordered perovskite oxides ACu3Fe4O12
T. Yamaguchi, K.-H. Ahn2, W. Mathias3, J. Kuneš3, and A. Hariki1
1Department of Physics and Electronics, Graduate School of Engineering, Osaka Metropolitan University, Sakai 599-8531, Japan
2Institute of Physics, Czech Academy of Sciences, 162 00 Praha, Czechia
3Institute of Solid State Physics, TU Wien, 1040 Vienna, Austria

We study metal-insulator transition (MIT) in ACu3Fe4O12 (A=Ca, La, Pr-Nd, Sm-Lu) using local density approximation (LDA) + U and dynamical mean-field theory (DMFT). We
find that ACu3Fe4O12 systems possess two instabilities towards MIT: Peierls-type instability (A=Ca) and antiferromagnetic instability (A=La, Pr-Nd). The dominant instability
can be controlled by electron doping, i.e. substitution of A-site element with different valence ions. The spin correlation function calculated by LDA+DMFT finds characteristic
spin dynamics: in CaCu3Fe4O12, a dynamical screening of the Fe eg-spins occurs in an antiferromagnetic coupling to ligand (O) holes. In LaCu3Fe4O12, on the other hand, theligand hole couples to the Cu S=1/2 spin exclusively, forming the Zhang-Rice singlet bound state on the CuO4 unit. We also discuss the different magnetic ordering in
Y Shimakawa

J. Phys. D: Appl. Phys. 48 (2015) 504006

Y Shimakawa

J. Phys. D: Appl. Phys. 48 (2015) 504006

Table 2. Cation–oxygen bond distances (Å) and bond valence sums

Table 7. Refined structural parameters of LCFO from the Rietveld

for CCFO at 300 K.
Bond
Ca–O
Cu–O
Fe–O

Distance

Bond valence sum

2.593(4) × 12

2.21

1.903(4) × 4
2.742(4) × 4
3.260(4) × 4

2.23

1.948(1) × 6

3.81

analysis of S-XRD at 300 K.

Inorganic Chemistry

Atom

Site

x

y

z

Uiso

La
Cu
Fe
O

2a
6b
8c
24g

0
0
0.25
0.3111(3)

0
0.5
0.25
0.1715(3)

0
0.5
0.25
0

0.31(2)
1.13(2)0.23(2)0.41(6)

Note: Space group; Im−3, a = 7.43413(2)Å, and Rwp = 6.23%.
ArticleLaCu3Fe4O12 (antiferro) and LuCu3Fe4O12 (ferro) in terms of the internalpressure
andstructure.
electronic phaseboundary between
AFM-Ielectronic
and FerriM-S isTable 8. Cation–oxygen bond distances (Å) and bond valence sumsfor LCFO at 300 K.

Phys. D: Appl. Phys. 48 (2015) 504006

able 2. Cation–oxygen bond distances (Å) and bond valence sumsor CCFO at 300 K.
valence sums
Bond
Distance
Bond valence sum

located between TbCu3Fe4O12
Bondand DyCu
Distance
3 Fe 4 O 12 . This Bond valence sumboundary may be interpreted asLa–O
the “zero-point”
the bond 3.17
2.641(3)of× 12
Table 7. Refined structural parameters of LCFO from the Rietveld
strains, in which no strains are included in any
metal−oxygen
1.897(2)×4analysis of S-XRD at 300 K.
bonds. This is in sharp contrast to
the electronic
phase
Cu–O
2.817(3)× 4 diagram 2.88
39
Atom
Site
x
y
z
Uiso
3.363(2)of Ca 1−x La x Cu 3 Fe 4 O 12 proposed by Chen. × 4 In the
3.06
Fe–O
LaY. Shimakawa
2a0
0.31(2)6
Ca1−xLaxCu
local ×structures
J. 0Phys. D:0 Appl.
Phys. 49 119601(2016)
3Fe4O12 phase diagram, neither2.0002(8)
Cu
6b
0
0.5
0.5
1.13(2)
nor504006
bond strains were considered, whereas carrier concenY Shimakawa
J.0.23(2)Phys. D: Appl. Phys. 48 (2015)
Fe I. Yamada
8c
0.25et al., 0.25
0.25
Inorg. Chem.
52 13751 (2013)
trationwastreatedasaprimary
parameter.
In
addition,O
24g
0.3111(3)0.1715(3)00.41(6)
Table 7. Refined structural parameters of LCFO from the Rietveld
analysisof S-XRD atin300
electronic
phase
separations
areunavoidable
a K.wide range of
Note: Space group; Im−3, a = 7.43413(2) Å, and Rwp = 6.23%.
intermediate compositions. However,Atom
Site ourx presenty study z
Uiso
Table 8. Cation–oxygen bond distances (Å) and bond valence sums
successfully reveals that bond strains
predominantly
for LCFO at 300 K.
La
2a00control 00.31(2)
Cu 3Fe6b0
0.5
0.5
1.13(2)electronic
phasetransitions
of
LnCu
O
and
a
clear
phase
4 12
Bond
Distance
Bond valence sum
Fe
8c
0.25
0.25
0.25
0.23(2)boundary is deﬁned. Therefore,O our 24g
results0.3111(3)enable 0.1715(3)precise 00.41(6)
3.17
La–O
2.641(3) ×312
4 12
tuning of electronic phases by bond strains, instead of by carrier
Note: Space group; Im−3, a = 7.43413(2) Å, and= 6.23%.3Fe4O12
cf.RwpCaCu
J. Phys. D: Appl. Phys. 48 (2015) 1.897(2)504006×4
concentration. Y Shimakawa
cf.
CaCu
3Fe4O12
2.88
Cu–O
Metal-insulatortransition
×4
2+ +Table 8. Cation–oxygen bond distances (Å) and bond valence sums
3Cu2.817(3)4Fe3.75+

Y Shimakawa

J. Phys. D: Appl. Phys. 48 (2015) 504006

Y Shimakawa

Y Shimakawa

Result: LaCu3Fe4O12Introduction: ACu3Fe4O12

alence sum
Ca–O

2.21

2.593(4) × 12

Journal of Physics D: Applied Physics

J. Phys. D: Appl. Phys. 49 (2016) 119601 (1pp)

doi:10.1088/0022-3727/49/11/119601

1.903(4) × 4
J. Phys. D: Appl. Phys. 48 (2015)
504006
Crystaland magnetic structures
2.23
Cu–O
2.742(4) Erratum:×4
3.260(4)× 4distances
Table 2. Cation–oxygen
bond
and
bond
valencesums 3Fe4O12: distinctofCaCu(Å)LaCu
3Fe
4O
12 andfor CCFO at 300 K.
3.81
Fe–O
1.948(1) charge×6transitionsof unusual high valence Fe
BondDistance
Bond valence sum

Y Shimakawa

Y Shimakawa

2.21
2.593(4) × 12
Yuichi
Shimakawa(Å) and bond valence sums
Table 2. Cation–oxygen bonddistances
×4
for CCFO at 300 K.1.903(4)Institutefor Chemical Research, Kyoto University and Japan Science and Technology Agency, CREST,
2.23
Cu–O
2.742(4)Uji,×Kyoto
4 611-0011, Japan
Bond
Distance
Bond valence sum
E-mail:shimak@scl.kyoto-u.ac.jp
3.260(4) × 4
Ca–O

CaCu3Fe4O12 : CD-type
Received 5 January 2016
Accepted for publication 13 January 2016
Published 17 February 2016

2.21
3.81
2.593(4) ×× 612
1.948(1)Figure 4. Mössbauer1.903(4)spectra ×
of 4CCFO at 300 and 4.2 K. The cross
4+
3+
5+
marks
and
the
solid
lines
respectively
theobserved spectra
2.23
Cu–O
2.742(4)× 4 versionsrepresent
Incorrectof figures were
published in the above
and the fittings. The major spectrum
at300
K
consistsofasinglepaper. The corrected figures are given below.
3.260(4)× 4K consists of Fe3+ and Fe5+.
component of Fe4+, which
at 4.2
Ca–O
Fe–O

2Fe

LDA+U energy: QR vs QB distortion

Charge disproportionation (CD) vs Charge transfer (CT) MIT(2015 J. Phys. D: Appl. Phys. 48 504006)

J. Phys. D: Appl. Phys. 48 (2015) 504006→ Fe

3.363(2) × 4

Lattice constant ( Å )7.44→ 3Cu3+ +3.064Fe3+

Fe–O

J. Phys. D: Appl. Phys. 48 (2015) 504006

2.0002(8) × 6

1-el dope

CaCu3Fe4O12

for LCFO at 300 K.

4. CONCLUSIONS
BondDistance
Bond valence sum
We investigated low-temperatureelectronicof 3.17La
La–O
2.641(3) ×phases
12
the A′-site Cu gives a bond valence sum of 2.23. The Fe
Fe–O
5+
LnCu 3Fe 4O 12 (Ln: lanthanide) perovskites.
1.897(2)All× 4 of thebond distances in the octahedron are 1.948(1) Å, giving a bond
Fe3+
+ 2+
Cu–O
2.817(3) ×state
4compounds are in an identicalvalenceof 2.88
valence sum of 3.81. The results suggest the Ca2 +Cu32 +Fe44Cu
O12
3+
3+
2+
3.75+
3.363(2)×4Cu
Ln Cu 3Fe 4O12 at high temperature. The compounds
cation charge states as seen in other 134-type perovskites like
2+
4+
2+ 4+
2+
2+
2.0002(8)× 6 induce 3.06
Ca Cu3 Mn4 O12 [26, 46] and Ca Cu3 Ti4 O12 [47–49].
with larger Ln ions (Ln = La, Pr,Fe–O
Nd, Sm, Eu,
Gd, Tb)Fe3+−1
The isomer shift of 0.15 mm s in a singlet Mössbauer specan intersite charge transfer (3Cu2+ + 4Fe3.75+ → 3Cu3+ + 4Fe3+)
trum obtained at 300 K (figure 4) indicates the Fe4+ unusual
at temperatures between 240 and 360 K, whereas allhigh valence state [50], though the value is slightly larger than
Inorganic
Chemistry4+
Figure 17. G-type antiferromagnetic
spin structureof LCFO.
Cu
compounds
withsmaller
Ln ions (Ln = Dy, Ho, Er, Tm Yb,
G-type antiferromagnetic spin structure of LCFO. Cu
those for Fe in CaFeO3 and SrFeO3 [51, 52].
Figure
15.
Field
of Fe
LCFO
300
K,at the ions at the A′ sites are nonmagnetic, and Fe (S = 5/ 2) spins at theField dependence of magnetizationof LCFO
at 300K, dependence
ions at the A′ ofsitesmagnetization
are nonmagnetic, and(S =at
5/ 2)
spins
3+
which
is
below
the
intermetallicCTtransition
temperature.
CCFO shows a transition at 210 K in the crystal structure, andB site have magnetic
which is below the intermetallic
CTcontribution.
transition temperature.
Lu) generatea charge disproportionation(8Fe3.75+→ 5Fe+and Fe–O bondFigure 14. Temperaturedependenceof Cu–O
B site have magnetic
contribution.
igure 4. Mössbauer spectra of CCFO
at 300 and transition
4.2 K. The cross
Magnetictransition
Valencetransition
Valence
Magnetictransition
Figure 13. Temperature dependenceof lattice constant (unittheThetransportK.
cross and magnetic properties as shown in figure 5. With Figure 5. Temperature dependence of lattice constant (unit cell
5+
distances and bond valence sums for A′-site Cu and B-site Fe formarks and the solid lines respectively represent the observed spectra
3Fe) below∼250−260
K. Theformer series exhibits metalcell volume for right axis),
normalized
resistivity,
magnetic
servedspectratemperature, the crystal structure symmetry changes volume for right axis), resistivity, magnetization, and isomer shift
decreasing
LCFO.
nd
the
fittings.
The
major
spectrumat300
K
consistsofasinglefor
CCFO.
susceptibility,
andisomer
shift
for
LCFO.
of
a single
4+ the transition temperature 3+
to-insulator,
and isostructural volume
fromIm−3 aboveto Pn−3 5+
below thePara- antiferromagnetic,
omponent
with the first-order nature of the transition. This intermetallic
d Fe5+. of Fe , which at 4.2 K consists of Fe and Fe .
pectrafrom a small
amount byof α-FeO3 impurity
also seen.intentemperature,
as indicatedvery2weak(1 1 1)arediffraction
magnetic
expansiontransitions
withsharply
the with
intersite
also
seen.
figure 14. The temperaturedependenceof isomer shift simultaneously
shown in CT contrasts
the CD charge
from Fe4+ to Fe3+ and Fe5+sity in the S-XRD patterns, and the unit cell volume abruptly the same as that at 300 K, and those for the two B-site Fe are
figure313
supports thechange in thevalence state.
A simulFigure 8. (a) CT transition temperature versus GII for LnCu
Fealsothe isostructural
CCFO described in a previous section. In
transfer.
InFecontrast,thelatter inshows
metal-to-semiconductor,heA′-siteCu
gives
a bond temperature.
valence sumTheof 2.23.
The
Fe–O
4O12
3.decreases
The
Fe–O
at thetransitionrefinedstructural
3.60 and 4.04, which suggests the CD transition.
taneousvalencechangeonboththeA′-siteCu
andB-siteFe
ions
LCFO transitions
both the S-XRD
and Mössbauer experiments confirm(Ln = La, Pr, Nd, Sm, Eu, Gd, Tb). The GII values are adopted2+at 400
ond
distancesin theoctahedron are
1.948(1)Å, givinga bondferrimagnetic, and charge ordering
simultaneously
givinga bond theparameters,
cation–oxygenbonddistances,and thebond
The Mössbauer spectrum at a low temperature confirms the(3Cu − 3e− → 3Cu3+ and 4Fe3.75+ + 3e− → 4Fe3+) thus leads the absence of CD at temperatures down to 4.2 K. The ilmenite
Figure4.
Mössbauer spectra of CCFO at 300 and2 +4.2 K.2 +The4cross
+
+
+
2
4
alencesumof solid
3.81.lines
The
resultssuggestthe CaCu3 Fe4spectra
K for LaCu3Fe4O12, 350 K for PrCu3Fe4O12, and 300 K for the other
12
Cu
Othe
valenceobtainedfromthestructure
analysis
forOthe2 + 3.75 +CD transition (figure 4). The spectrum at 4.2 K consists of two
12 values
the charge
disproportionation.
Structural
instability of
4 sum
3 Feandmarks
respectively
represent
the observed
3+
La3 +Cu
O12 FeTiO3 (Feto the transition from thewith
high-temperature+ Ti3+ → Fe(3−δ)+ + Ti(3+δ)+) [56] and the per3 Fe4
ation
chargeas seenin other
134-type
perovskites
likedatatheat
90
K states
areThelistedinspectrum
tables3 at
andlow-temperatureforthe
LnCu3Fe4O12 (Ln = Nd, Sm, Eu, Gd, Tb). The line is a guideandfittings.
major
3004.KThis
consistsof a single
components with almost equal intensities; one with the isomer
ovskites
like
3
3
+
+
3+ 5+ 2+
3+ 3+
LnCu
O4 12O12as. The
estimated
theBiNiO
bonddiscrepancies
andAntiferroto the low-temperature
La3 +CuFe
temperaturefrom
4+
5+
3Fe
ovskite3 4+ rock–salt-type
2 + gives
2 +Fe
4 +and3 (Bi0.5 Bi0.5 Ni O3 → Bi Ni O3) [57] are two
2+
2 + twoof4Fe, which at 4.2 ordered
K consistsofFe4c). sites. shift of 0.39 mm s−1 and the hyperfine field of 417 kOe and thestructure
B 3+
(4b
and
Oacomponent[47–49].
and
d
.
The
data
are
adopted
for
eye.
(b)
Correlation
betweend
3 Mn4 O12 [26, 46] and Ca Cu3 Ti4 O12 [47–49].
12 Cu
magneticLn
Fedependence of resistivity
in figureinstability
13 shows a metal-to-insulator
Spectrafroma small amountof −1
α-Fe2O3Cu
impurityarealso seen.
globalindices demonstratethat thattheexhibit
bondthestrains
in CT, but only underrare compounds
intermetallic
Theisomerbondvalencetheis 2.28,
whichis specnearly other with the isomer shift of 0.05 mm s−1 and the hyperfine
heshift ofsum
0.15formms A′-site
in a singlet
Mössbauersbauerspecthat
at
400
K
for
Ln
=
La,
350
K
for
Ln
=
and
300
K
for
Ln
=
Nd,
TemperaturedependenceofPr,
the refined
B-sitemagnetic
Figure4.
Mössbauer
spectra
of
CCFO
at
300
and
4.2
K.
The
cross
transitionattheintermetallicCTtransition
temperature.
The
highpressure.
LCFO
is
the
first
exampleND
patternandtheresultofthe
Rietveld
fit
for
LCFO
4+
moment in LCFO. The projected intrinsic antiferromagnetic Néel
these compounds are distinguished into two types,
which areto show temperatureum
obtained
300lines
Ka (figure4) indicates
theFe4+TheunusualFe
unusualat 50 K. The representative
magneticEu,
peaks (1Gd,
1 1) and (13 1) are Dy,temperature
theA′-site
Cuat
gives
bond
valencesum ofFe–O
marks
and thesolid
respectively
represent
the2.23.
observed
spectraofEr,about 600
K usingYb,
an S = 5/Lu.
2 Brillouinfunction(blue)
is
Sm,
Tb,
Ho,
Tm,
Red
markersAF
solutionthermal hysteresis and the jump in the resistivity are consistent induced intermetallic CT transition.
labeled in the figure. The bottom and top tick markers correspond4
shown with a dotted line.
ybond
largerthanighvalencestateThe
thoughthearevalue
is consists
slightly
well illustrated in a bond strain diagram. The ﬁrst type isdistancesin[50],
theoctahedron
1.948(1)Å, givinga thanbond
andthefittings.
major
spectrumat 300K
oflargera single
to the Bragg reflections of crystal and magnetic structures,
represent
thecompounds
transforming
to
CT
(CD)
phase
at low4+
3+
4+
respectively.
2 +5+. 4 +2+component
of
Fe
,
which
at
4.2
K
consists
of
Fe
and
Fe
hose
for
Fe
in
CaFeO
and
SrFeO
[51,
52].
ions
do
not
contributetometals)
[58,
59],
and
thus
the
Cu
3
3
valence sum of 3.81. The results suggest the Ca Cu3 Fe4 O12
overbonding in the Ln−O bondand underbonding in the Fe− Gap8temperature.
the magnetic properties.
Spectrafromamountatof210
α-Fe
O3the
impurity
arestructure,also seen.and
2instructure,
and a small
CCFO
shows
a
transitionK
crystalcation charge states as seen in other 134-type perovskites like
The magnetic ordering is also observed in the ND patterns Figure 18. Temperature dependenceof the refined
3.4. Magnetic structure of charge transferred LCFO
O bond,whileB-sitethemagnetic
other type is the opposite. Intersite charge
Figure5. Temperaturedependence
lattice5.constant(unit
cell5. Temperature dependence of lattice constantfigure5. WithFigure16. NDand theresultof the Rietveld
fit forand
LCFO
Figure(unit cellhetransport
andmagnetic
properties
asshown
figureWithcollected
below
the intermetallic
CTCu–Otransition
temperature+
2 + in4ofFigure14.pattern
Temperaturedependenceof
Fe–O bond
2 +A′-site
2 +summoment in LCFO. The projected intrinsic antiferromagnetic Néelthe
Cu44 +
givesabondvalence
of
2.23.
The
Fe–O
Ca
Cu32 +Mn
O
[26,
46]
and
Ca
Cu
Ti
O
[47–49].
12 for right axis), resistivity,
12
volumeand isomershift
4
3 magnetization,
50distances
K.inThemagneticsums
peaks for
(1diffractions
1A′-site
1) and (1
3 1)
are
The
intermetallic
CT in
LCFO causes aatchange
the representative
mag- [60]. Representative
Figuredependence ofand
latticeconstant(unit
magneticfromthe(1 1B-site1) and temperaturevolumeforright 13.
axis),Temperature
resistivity, magnetization,isomershift
metrychangesof about 600 K using
an S = 5/
2 Brillouin functionistransfertransition
temperatures
are dependent upon the global
and bond valenceCu
andFe forecreasingtemperature,
the crystal
symmetrychanges−1 structure
netic properties.
As shown in figure 11,labeled
a magnetic
splittingbondisomer
distances
inofCCFO.
the0.15octahedronarea1.948(1)Å, givinga specbond for CCFO.
3 1) planes
at and50 K top
are clearlyseen as shownin figure 16.
forin thefigure.(1The
bottomtick markers
correspond
The
shift
mm
s
in
singletMössbauer
cell
volumeforrightaxis),
normalized
resistivity,
magnetic
shown with a dotted line.
−3
LCFO.
with
a
hyperfine
field
of
about
400
kOe
is
observedintheombelow
Im−3theabove the transition temperature to Pn−3
below
the
The magnetic
analysis gives
an antiferromagnetic
2+ 4+
to the Bragg reflectionsof crystalstructure
and magnetic
structures,
2 + 4+
instability indices, whereas those of the charge disproportionasusceptibility, and 3+
isomer shift for LCFO.
valencesum ofat3.81.
results 4)
suggestthe CaCu
trum
obtained
300TheK (figureindicatestheFe 3 Feunusual
4 O12
Mössbauer spectrum at 300 K, which isrespectively.
below the transition superstructure model with a (1/2 1/2 1/2) propagation vector
ractionintenemperature,
as indicated by very weak (1 1 1) diffraction
inten3+3
4
12
witha magnetic
the first-orderof the transition.
This intermetallic
temperature, suggesting that at low temperaturesions do notare
contributeto identical regardless of the magnitude
metals) [58, 59], and thustion
the Cutransitionof the cubicnaturecell. This corresponds
to a G-type antiferromaghighvalence[50],
thoughthevalue
isslightlylargerthanalmostcationchargestate
statesas seenin other
134-type
perovskites
like
ume
the
same
as
that
at
300
K,
and
those
for
the
two
B-siteFe
areordered
state
of
Fe
spins
is
stabilizedbytheintermetallic
ty in2abruptly
the
S-XRD
patterns,
and
the
unit
cell
volumeabruptly
4+
3+
5+
netic
ordering(figure
17)
where
each
B
site
momentcouples
the
same
as
that
at
300
K,
and
those
for
the
two
B-siteFearefigure14.
The
temperaturedependenceofisomer
shift
shown
in
4+ 4 +
themagnetic
properties.
+
+
+
2
2
4
to
Fe
and
Fe
CT
contrasts
sharply
with
the
CD
from
Fe
+
2
+
those
for3 FeMn4in
andwhich
SrFeO
52].
3 46]
3 [51,
CT. The observed linear field dependence of magnetization antiferromagnetically with the six nearest neighbor moments.
Castructural
Cu
OCaFeO[26,
andCasuggests
Cu
Ti
O12structural[47–49].
ed
12and
4.04,
the4 CD
transition.
structural
ecreasesat the 3.60
transition
temperature.
The3 refined
3.60 andfigure
4.04, 13whichtransition.
The magnetic
orderingof
is alsoobserved ininstability.
the ND patternsOur bond strain analysis demonstrates
alsosuggests
supportsthe
theCDchangein theFe valence
A ansimulshownin figure 15state.
indicates
antiferromagnetic
The refinedmagnetic moment
at 50 KLCFO
isin4.03(2)µ , which issection.
theordering
isostructuraldescribed
a previous
In
3.4.inMagnetic
structureof CCFOchargetransferred
−1
CCFO
shows
a
transition
at
210
K
in
the
crystalstructure,
andFigure 14. Temperature dependence of Cu–O and Fe–O bond
nd
the
bond
The
isomer
shift
of
0.15
mm
s
in
a
singletMössbauer
specTheMössbauer
spectrumatalow
temperature
confirms
theof the confirms
magnetic moments
the intermetallic CT transi- slightly reduced from the ideal value of 5 µ for a high spin collected
the intermetallic
CToftransition
temperaturearameters, the cation–oxygen bond distances, and 4+
the bond
The 5.
Mössbauer
spectrumat aonlowtemperature
the belowthat thebondconstantstrains
controland theelectronic
phase
Figure13.belowTemperaturedependence
lattice(unit may distances
taneousvalencechangeboththeA′-siteCu cellandA B-site
Fesusceptibility
ions LCFO
FigureTemperaturedependenceoflatticeconstant(unit
bond valencesums for A′-site
Cu and B-site Fe for
both
the
S-XRD
and
Mössbauer
experiments
confirmthetransport
andmagnetic
properties
as
shown
in
figure5.
With
tion
temperature.
decrease
in
at
393
K
shown
alysis
for
the
Fe
(S
=
5/2)
due
to
covalency
effects.
Magnetic
contribution
trum
obtained
at300
K(figure4)
indicatestheFeunusualCDtransition(figure4).
The
spectrumat
4.2
K
consistsoftwo
Dy-Lu:
Ferrimagnetic(FM)
The
intermetallic
CT
in
LCFO
causesachangeinthemag[60].
Representative
magnetic
diffractions
from
the
(1
1
1)
and
2+
−
3+
3.75+
−
3+
cell volume for right axis), normalized resistivity, magnetic
alence sum values obtained from the structure analysis for the CDtransition(figure4).
The
spectrumat 4.2 K
consistsoftwo)withvolumeforrightaxis),
resistivity,
magnetization,andisomer
shift
LCFO.
in
figure13
also
agreestheleads
first-order change
from the of
(3Cu
−
3e
→
3Cu
and
4Fe
+
3e
→
4Fe
thus
from
the
A′
site
is
not
detectedintherefinement,
confirming
transitions
precisely.
decreasing
temperature,
thecrystalstructuresymmetrychangestheabsenceCDattemperatures
down
to
4.2
K.
The
ilmenite-temperaturecomponents
withthealmostintensities;
with the isomerhigh valence state[50], though
valueequalis slightly
largerone
than
netic
properties.
(13 1) planes atand
50isomer
K are clearly
as shown in figure 16.
susceptibility,
shift forseen
LCFO.
paramagnetic state to2 thestate.
In the anti- As shown in figure 11, a magnetic splitting
3.75 ++ antiferromagnetic
Spectraamount ofare also seen.
Ca
3.81
Fe–O from a small1.948(1)× α-Fe
6 2O3 impurity

AF magnetic ordering is needed to open aband gap on Fermi level in LaCu3Fe4O12

LaCu Fe O : CT-type

Metal-insulator transition+ Fe

AF instability to MIT

LaCu3Fe4O12

Y Shimakawa

450 K

7.40
7.36

FM

7.32

CCFO. Cu2+ (S = 1/ 2; green) spins at the A′ sites couple
antiferromagnetically with rock–salt ordered Fe3+ (S = 5/ 2; red)and Fe5+ (S = 3/ 2; blue) spins at the B sites.

BVS for Fe

Figure 9. Ferrimagnetic structure of charge disproportionated

3.8

BVS for Cu

7.28

2.4

3.6
3.4

2.2

1.8

VS

Figure 15.

2.00.00 0.25 0.50 0.75 1.00
x in Ca1-xLaxCu3Fe4O123+3+

Figure 17.

Article3+

3+

Quadratic
increase

AF

Figure 12. Composition dependence of lattice constant,bond valence sums for A′-site Cu and B-site Fe at 450 K in
Ca1−xLaxCu3Fe4O12 solid solution.

0022-3727/16/119601+1$33.00

1© 2016 IOP Publishing Ltd Printed in the UK

LDA + DMFT DOS (U,J)=(7.5eV,0.98eV) for Cu 3d and (6.80eV, 0.80eV) for Fe 3d electrons

Figure 18.

Figure 16.

■ Breathing distortion is always unstable

PM solution

3+- Band cross @ R-point in FM is below EF
No energy gain by the Peierls mechanism

Magnetic order in Ln Cu Fe O

3+

■ AF solution shows a gap @ R-point

B

B

3+

for CCFO. with almost equal intensities; one with the3 +isomerthat Cu3+ 3+ions at the site
produce no magnetic
ata
at Im−3
90 K arelistedintables 3temperatureand 4. Thistolow-temperaturecomponents3+(3−δ)+(3+δ)+ moment. The
−1
4+
La
Cu
Fe
O
to
the
transition
fromthehigh-temperature
3+ 4
8
from
above
the
transition
Pn−3
below
the
12
FeTiO
(Fe
+ Ti
→ Fe
andThethemagnetic
perand4c) forsites.
3
ferromagnetic
LCFO,
Cu
with
the
3d
electronconfigurawithahyperfine
fieldofordering
about
400
kOe
isTiobserved
in
shift
of
0.39
mm
s
and
the
hyperfine
field
of
417
kOe
and
the
those
Fe
in
CaFeO
and
SrFeO
[51,
52].
with the first-order nature of the transition. This intermetallic
magnetic
in
LCFO
is+
thus
ascribed )to [56]the theB-site3structure analysis gives an antiferromagnetic−1
3 ordered two
3
ructuregives
rock–salt-type
B
(4b
and
4c)
sites.
shift of 0.39
mmlow-temperatures and the hyperfine
field
417
kOe
and
the
3of
3+
+ tion3+
inO
the square-planar
oxygen coordination
is nonmagnetic
33+
5 which+ K,+ 2 +is below−1
Fe3+–O–Fe
antiferromagnetic
superexchange
coupling, superstructure3+
3+temperature,asindicated
by
very
weak
(1
1
1)
diffraction
intento
the
La
Cu
Fe
.
The
temperature
figure14.
The
temperaturedependenceofisomer
shift
shown
in
Mössbauer
spectrumat300
thetransitionhichisnearly
model
with
a
(1/2
1/2
1/2)
propagation
vector
other
with
the
isomer
shift
of
0.05
mm
s
and
the
hyperfine12
CCFO
showssuma transitionat 210 KCu
in is
the2.28,
crystalstructure,
Bi contrast
Ni toOthe3 →Bi Ni O ) [57] are two
CT contrasts sharply with the CD from Fe4+ to Fe3+ and Fe5+
43 (Bibecauseofhyperfine
the considerable energy splitting ovskite
between d3zBiNiO
which
is in0.5
sharp0.5
B-site Fe3+–O–Fe5+ 3ferrohebondvalencefor the A′-sitewhichisabruptly
nearly other with the isomer shift of 0.05 mm s−13 and−r
the
ASSOCIATED
CONTENT
sity
in
the
S-XRD
patterns,
and
the
unit
cell
volumetemperature,suggesting
that
at
low
temperaturesamagnetic
3+
the
same
as
that
at
300
K,
and
those
for
the
two
B-site
Fe
are
of
the
cubic
cell.
This
corresponds
toaG-typeantiferromagfigure
13 also supports the change in the Fe valence state. A simul- in the isostructural CCFO described in a previous section. In
Temperatureof
dependencecell
the transport and magnetic properties as shown in figure 5. With Figure 5.dependence
resistivityofinlattice
figureconstant
13and
shows
metal-to-insulatord(unitcoupling
in thethecharge-disproportionatedCCFO.
as shown in ACu O2rare(A =compounds
alkaline magnetic
thatexhibitintermetallic CT,
but onlyunder
x − y aorbitals
ordered state of Fe3+ spins is stabilized by the intermetallic netic
for4.04,
rightwhichaxis), resistivity,
magnetization,and isomer shift
decreases
thetransition
Thesymmetryrefined structuralordering(figure17) onwhere
B site moment43.60 andsuggeststheCDtransition.
taneousvalencechangebotheachthe A′-site
Cu and couples
B-site Fe ions LCFO both the S-XRD and Mössbauer experiments confirm
decreasingattemperature,thetemperature.
crystal structurechanges 4 volumetransition at the intermetallic CT transition temperature. TheCT.highpressure.linear
LCFO
is thefirst example
to show temperaturefor The
CCFO.
Supporting
Information
2+
−
3+ S
3.75+ neighbor−3+The
observed
field
dependenceofmagnetization
antiferromagnetically
with
the
six
nearestmoments.
parameters,thecation–oxygenbonddistances,
and
the
bond
Mössbauer
spectrumatalow
temperature
confirms
the
(3Cu
−
3e
→
3Cu
and
4Fe
+
3e
→
4Fe
) thus leads the absence of CD at temperatures down to 4.2 K. The ilmenite
9from Im−3 above the transition temperature to Pn−3 below thethermal hysteresis and the jump in the resistivity are consistentshown
inducedintermetallic
CTantransition.
infigure15
indicates
antiferromagnetic
ordering
The
refinedmagnetic
momentat
50
K
is4.03(2)µ,which
2 + is3.75 +
3+
B
valencesumvaluesobtainedfromthestructure
analysisfortheCDtransition(figure4).
The
spectrumat
4.2
K
consists
of
two
3+
3+
(3−δ)+
temperature, as indicated by very weak (1 1 1) diffraction intenLaSXRD
Cu3 FeO12 EDto the transition from theXRD
high-temperaturepatterns,
patterns,and ) [56] and the perFeTiOpatterns,+ TiMCD→ Fespectra,+ Ti(3+δ)+
4
3 (Fe
of
the
magnetic
moments
below
theintermetallicCTtransislightly
reduced
from
the
ideal
value
of
5
µ
for
a
high
spin
B
3
3
+
+
3+
data
are listed
in tables
andunit
4. Thiscomponents
withatalmost
intensities;
the Feisomer3
5++
2+
sity atin 90
theKS-XRD
patterns,
and3 thecell low-temperaturevolume abruptly thesame as that
300 K,equaland those
for theone
twowith
B-siteareto 3+thelow-temperature
La effects.
Cu3 Fe
The temperature
8 temperature. A decrease in susceptibility at 393 K shown Fe
ovskite
BiNiO
(Bi
Bi
O3 → Bi3+is
Ni3+O3) [57] are two
4 O12. contribution
tion
crystallographic
data
(CIF)
for
LnCu
Fe
.
This
3O
(S =
5/2) due to covalency
Magnetic
0.5
0.5 Ni material
−1
3
4
12
structure
gives
rock–salt-typeordered
two
B
(4b
and
4c)
sites.
shift
of
0.39
mm
s
and
the
hyperfine
field
of
417
kOe
and
the
decreases at the transition temperature. The refined structural 3.60 and 4.04, which suggests the CD transition.
dependenceof resistivity
in figureshows a metal-to-insulator
in figure 13 also agrees with the first-order change from the fromcompounds
that exhibit the intermetallic CT, but only underthe A′ site
is not detectedin the13refinement,
confirmingvia the rare
available
freeof charge
Internet
at http://pubs.acs.org.
Thebond valencesum for the A′-site
is 2.28, which
is nearly
the isomer
shift ofs−1 and thehyperfineparameters,
the cation–oxygen
bondCudistances,and thebond other
3+
ThewithMössbauer
spectrumat 0.05a lowmmtemperature
confirms
theFigure6.isTemperaturedependenceof the electrical resistivity forparamagnetic state to the antiferromagnetic state. In the anti- that
Cu ions
at thesite produceCT
no magnetic
The The high pressure.
transitionat theintermetallic
transitionmoment.
temperature.
LCFO
the first example
to show temperature3+
8
valence sum values obtained from the structure analysis for the CD transition (figure 4). The spectrum at 4.2 K consists of two
O12transition.
upon cooling. The resistivity was normalized to the
LnCu3Fe4CT
ferromagnetic LCFO, Cu with the 3d electron configura- magnetic
ordering in
LCFO
is thusascribed
to theare
B-sitethermal hysteresisandthe jump
in theresistivity
consistent induced intermetallic
data at 90 K are listed in tables 3 and 4. This low-temperature 4 components with almost equal intensities; one with the isomertion in the square-planar oxygen coordination is nonmagnetic Fe3+–O–Fe3+ antiferromagnetic superexchange coupling,values at 400 K (La), 350 K (Pr, Nd), and 300 K (others). The insets
AUTHORINFORMATION
5+
structure gives rock–salt-type ordered two B (4b and 4c) sites. shift of 0.39 mm s−1 and the hyperfine field of 417 kOe and the
because of the considerable energy splitting between d3z 2 − r 2 which is in sharp contrast to theB-site Fe3+–O–Fe
ferroshow the thermal hysteresis for Ln = (a) La, Nd, Gd and (b) Dy, Lu.
8
3+−1
The bond valence sum for the A′-site Cu is 2.28, which is nearly other with the isomer shift of 0.05 mm s and the hyperfine
and d x 2 − y2 orbitals as shown in ACu O2 (A = alkaline magnetic coupling in the charge-disproportionated CCFO.

8Fe3.75+ → 5Fe3+ + 3Fe5+

2

2

■

2

2

LDA+DMFT spin correlation function*

La, Pr-Nd, Sm-Tb: Antiferromagnetic (AF)3Cu2+ + 4Fe3.75+ → 3Cu3+ + 4Fe3+

Cu-O Zhang-Rice singlet
state on CuO4 unit
O■

Questionsd 9L

Zhang-RicesingletFigure 5. (a) TemperatureCorresponding
dependence of the magnetic
Authorsusceptibility

Figure 9. Electronic phase diagram for LnCu3Fe4Ofor
Red 3squares
12. LnCu
Fe4O12 on ZFC mode. Applied external ﬁeld was 1 kOe (Ln4*E-mail: i-yamada@21c.osakafu-u.ac.jp the bond strains represented as overbonding (underbonding)(blue circles) represent the CT (CD) transition temperatures,
which
9
= Sm,
Eu, Gd,Tb, Dy, Ho, Er, Tm, Yb, Lu) and 10 kOe (Ln = La, Pr,
Cu moment
Fe momentin the La−O (Fe−O) bond, while the CD transition occurs inNd).Eu,(b) Gd,Isothermal
LnCu3Fe4O12 (Ln = La−Lu)
were determined by XRD data for Ln = La, Pr, Nd, Sm,
Tb, magnetization
Presentfor
Address
screened!
3
4 and12
12 Lu.
measuredat 5TheK.
YCu3Fe4O12 because the opposite kind of bond strains, whichnot screened†
ferromagnetic transitions for Ln = Dy, 3
Ho, Er, 4Tm,
Yb,
Deutsches Elektronen Synchrotron (DESY),
22607
Hamburg,
are dominant in YCu3Fe4O12, are not necessarily relaxed by the
PM-M, FerriM-S, and AFM-I regions represent the paramagneticGermany
CT transition. Here, we attempt to make further reﬁnements to
Figure 6 shows the temperature dependence of the electricalmetal, ferrimagnetic-semiconductor, and antiferromagnetic-insulator
3
4
12
3
4
12
the above conclusions based on the structural data ofresistivity for LnCu3Fe4Notes
O12. Metal-to-insulator transitions arephases, respectively.
LnCu3Fe4O12. Figure 7a shows selected bond lengths andobserved at temperatures between 220 and 360 K for
The authors declare no competing ﬁnancial
interest.
LnCu3Fe4O12 (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb), in whichbond angle versus ionic radius of Ln ions for the
Ln3+Cu2+3Fe3.75+
very small temperaturedependences of dx.doi.org/10.1021/ic402344m
high-temperature
O12 phase in the vicinity of room temperature.
13760| Inorg. Chem. 2013,
52, 413751−13761
metallic phases change into insulator-like large dependences
The Ln−O bond shrinks almost monotonically as the rLn
below the CT transition temperature, as reported for
decreases. The Fe−O bond shrinks correspondingly, while26
LaCu3Fe4O12. In contrast, metal-to-semiconductor transitions
the Cu−O bond length is almost constant. This is because thelarger Ln ions expand 3
the unit4cell 12
volumes and the Fe−O
are observed for LnCu3Fe4O12 (Ln = Dy, Ho, Er, Tm, Yb, Lu)bonds are accordingly stretched in the relationship a = 4lFe−O ×
below ∼220 K (Figure 6b), which are similar to that of
YCu3Fe4O12 accompanying CD transition. The temperaturesin(ψ/2), where a is the lattice constant, lFe−O is the Fe−O
dependences of the resistivity in the charge-disproportionatedbond length, and ψ is the Fe−O−Fe bond angle. The Fe−O−
phases are much smaller than those in the charge-transferred
Fe bond angle slightly increases with decreasing rLn, which isphases. The inset of Figure 6a shows that a large thermal
attributed to the decrement in the diﬀerence in ionic radii
hysteresis exists in the CT transitions due to their ﬁrst-order
between A-site (rA) and A′-site (rA′) ions, ΔrA (ΔrA = rA − rA′),
characteristics,
while
the
thermalhysteresis
in
the
CD
as
demonstrated in our previous paper.38 When ΔrA decreases
Amplitude=0.20Å
Amplitude=3.22Å
Amplitude=0.33Å
CaCu3Fe4O12(b) Breathing mode(a) CaCu3Fe4O12
Cu
3d, Fe3d andOthe2ptransitions
is negligibly
small (seeinsetbandsof Figure 6b).
from 0.59 Å (La) to 0.407 Å (Lu), the Fe−O−Fe bond angle is
3
4 12
g
Fe3+
The above analysis of crystal structure, valence states, andseen to increase from 139.2 to 140.3°.
Ca
(LB)
Cu
physical propertiesof bands)
LnCu3Fe4O12 divides these compounds
The BVS analysis unveils the delicate relationship between
included(71
Fe (LB)
Ln−O and Fe−O bonds. Figure 7b shows the BVS, bondinto two classes, in which the ﬁrst class arouses a CT transition
Fe (SB)O
discrepancy, and GII in the valence unit (v.u.) as a function of
like that of LaCu3Fe4O12, and the other class generates a CD
Fe5+
25
rLn for the Ln3+Cu2+3Fe3.75+4O12 phase. The BVS of Cu is closetransition
like
that
of
YCu
Fe
O
.
In
a
previous
report,we
Calculate physicalquantities
3 4 12(SB)
investigated the origin of the alternative electronic phaseto the expected value (+2) for all of the compounds, indicating3+
in A Cu3Fe4O12 perovskites, concluding that thethat the Cu−O bond remains in a proper state. On the other++(c) M 2 mode(d) M 1 mode mode : Q- transitions
Densityofstates(DOS)
Breathing
FeO6 Rotational mode : QR
B
hand, the BVS of Ln drastically decreases from +3.60 (La) toCT transition predominantly occurs in LaCu3Fe4O12 to relieve

Cu■ The origin of MIT in CaCu Fe O with CD / LaCu Fe O with CT■ Different magnetic order in LaCu Fe O

(AF) and LuCu Fe O (FM)LDA + Dynamical mean-field theory (DMFT)+

Messagescba■ The origins of MIT:
CaCu Fe O : Peierls-type instability / LaCu3Fe4O12 : AF-ordering instability
Important !! : two instabilities switched by 1 electron doping (i.e. Ca2+ → La3+)■ The spin properties:
CaCu Fe O : Fe e -O spin singlet / LaCu3Fe4O12 : Cu-O Zhang-Rice singlet

Method: LDA + U, LDA + DMFT

Mode decomposition

S = 1/2s+

Result: LnCu3Fe4O12 (Ln3+=La, Lu)- Spin correlation function

Local density approximation (LDA) + U

χSpin(τ) = < Sz(τ)Sz(0) >2. Evolution of theoptimization
LDA+U total energy of the ferriBand/energy calculation,FIG.
structuremagnetic state with the breathing distortion Q for fixed Q
B

dx.doi.org/10.1021/ic402344m | Inorg. Chem. 2013, 52, 13751−13761

Band gap on EF at Γ-point

LDA + U: Volume optimizationR

rotation mode. Note that 100% corresponds to the experimental amplitude.

LaCFO(AF)

Include Coulomb interaction as static mean-field

FIG. 1. (a) Low-temperature structure of CaCu3 Fe4 O12(space group P n3). (b) breathing distortion of oxygen atoms.
(c) M2+ mode. (d) M1+ mode. The crystal structure is visualized using VESTA3 [48]. The mode analysis is performed
using AMPLIMODE [49]. Cu (Ca) atoms sitting on 3/4 (1/4)
of the A sites in the ABO3 perovskite lattice show no displacements from the high-symmetry positions in all temperatures.
Thus, Co and Ca atoms are not shown in panels (b)–(d).

13758

LuCFO(FM)LDA+U shows difference in electronic
structure between two compounds

tight-binding model spanning Cu 3d, Fe 3d and O 2p
Using WIEN2k implementation
bands, constructed using the wien2wannier [54] and wan-and thus rather di↵erent magnetic behavior is to be expected. Indeed, along with at the MIT, a ferrimagnetic(FM) order with a large magnetization of 9.7µB /f.u. isobserved [43, 47]. The large magnetization indicates aferromagnetic alignment of Fe moments antiferromagnetically coupled to the Cu2+ (S = 1/2) sublattice in CCFO.
We use the local-density approximation (LDA) + U
and LDA + dynamical mean-filed theory (DMFT) approaches to investigate the electronic response to the QR
and QB distortion. We characterize the local state of
Fe ions in the uniform and disproportionated phases anddetermine the preferred magnetic order. We discuss thesimilarities and di↵erences of CCFO to RNiO3 in thecontext of the Peierls and site-selective Mott scenarios.

nier90 [55] codes, is augmented with the local Coulombinteraction on the Fe and Cu sites. We use the interaction
parameters (in eV) (U, J) = (7.50, 0.98) and (6.80, 0.80)
for the Cu and Fe 3d shells, respectively [56, 57]. To correct for the double counting of the Coulomb interaction,
we shift the site energies of the Fe and Cu orbitals by therespective shell-averaged high-frequency limit (Hartreepart) of the self-energy [57]. We show that our conclusions hold for a wide range of the double-counting corrections (treated as adjustable parameters), see SM [44].
In the DMFT self-consistent calculation, the continuoustime quantum Monte Carlo solver with density-density
approximation to the on-site interaction was used to solve
the auxiliary Anderson impurity model [58–61].

LDA + U bands: FM solution

Result: CaCu3Fe4O12
Gap

II.

COMPUTATIONAL METHODEnergy(eV)The LDA+U calculations are performed using the augmented plane wave and local orbital (APW+lo) method
implemented in the Wien2k package [50]. Following previous GGA+U studies of CCFO [45, 46], the e↵ective U
values (Ue↵ = U J) are chosen as Ue↵ = 4.0 eV and
7.0 eV for Fe and Cu ions, respectively. We have checked
the robustness of the present result with respect to the
choice of the exchange-correlation potential and Ue↵ parameters, see SM [44]. The reciprocal space cut-o↵ Kmax
for the basis functions was chosen such that RMT Kmax =7.0. The Brillouin zone was sampled with an 8 ⇥ 8 ⇥ 8mesh. To perform LDA+DMFT calculations [51–53], aPeierls-type instability to MIT

LuCFOStructural distortion (FeO6 rotation QR) softens

Since the LDA+U approach cannot describe fluctuating local moments, we restrict ourselves to magneticallyordered phases (at T = 0). Overall, the FM solution has
lower energy than antiferromagnetic or ferromagnetic soR except for QR = 0 % where ferromagnetic alignlutions,ment of all Fe and Cu moments is favored. Fig. 2 showsB
the LDA+U total energies for various amplitudes of thebreathing QB and rotation QR distortions. For small QR
the breathing distortion is unstable, while for larger QR
a new local minimum appears at a finite QB amplitude.
We find
R the global energy minimum close to the experimental QR and QB amplitudes (i.e. QR = QB = 100 %)B
irrespectiveof the interaction parameters within a realistic range [44].

Q =100%Q =0%LDA+U energy: QR vs QB distortion■ Energy minimum at

PLu=0 GPa

(experimental structure)

LDA + U DOS: FM solution (QR=100%)■ QB opens the charge gap■ QR reduces the band width,■ Optimized energy reproduces correct

locating the band crossing

magnetic ordering for two materials@ R point to Fermi level EF

QR=0%
QB=0%

LaCFO

LuCFO

■ The difference of internal pressure is 6.7GPa■ LaCFO’s E-V curve mapped to LuCFO’s one

LDA+DMFT spin correlation function @ Fe site

Fe moment
reducedPLa=6.7 GPa

QR =QB = 100%

Q =50%
Q =100%

QR=0%
QB=100%Large
Fe moment

LaCFO

QR=100%III. RESULTS
A. LDA+U and Peierls mechanism
QB=100%the Fe-O bond breathing distortion QB

Energy (log scale)

LDA + U bands

∂E
p=−
∂V

Fe (SB)
Nd=5.2O

Fe-site and -orbital
selective spin screening
Fe eg electrons form a spin singlet
with two ligand holesby adding PLaV term

Messagescba■ LaCu3Fe4O12 can be seen as LuCu3Fe4O12 under the specific pressure (6.7GPa)■ Closing gap in FM solution may be responsible for the transition

5 2

d L
egσ

Fe (LB)
Nd=5.1a1g
egπ

Two ligand
holes
Many-bodyspin singlet

Acknowledgement
This work was supported by JSPS KAKENHI (Grant Number 21K13884), and the European ResearchCouncil (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant
Agreement No. 646807-EXMAG). The computational calculations were performed at the Vienna
Scientific Cluster (VSC).