J. Phys. Chem. C 2009, 113, 12509–12516
12509
High Temperature Metal-Insulator Transition Induced by Rare-Earth Doping in
Perovskite CaMnO3
Yang Wang, Yu Sui,* Jinguang Cheng, Xianjie Wang, Zhe Lu, and Wenhui Su
Center for Condensed Matter Science and Technology (CCMST), Department of Physics, Harbin Institute of
Technology, Harbin 150001, People’s Republic of China, and International Center for Materials Physics,
Academia Sinica, Shenyang 110015, People’s Republic of China
ReceiVed: October 13, 2008; ReVised Manuscript ReceiVed: February 28, 2009
This paper reports the crystal structure and high-temperature transport properties of electron-doped perovskite
manganites Ca0.9R0.1MnO3 (R ) La, Pr, ..., Yb). All the samples crystallized into a orthorhombic distorted
perovskite structure (space group Pnma). The structural distortion increases as the average A-site cation size
〈rA〉 decreases; the average Mn-O-Mn bond angle θMn-O-Mn monotonously decreases, while the average
Mn-O bond length dMn-O exhibits a minimum at R ) Dy. The substitution of R3+ for Ca2+ markedly enhances
the electrical conductivity of parent CaMnO3 and induces a metal-insulator transition at TMI which varies
from 325 K to 420 K with R3+. Among all the samples, Ca0.9Dy0.1MnO3 shows the lowest resistivity and TMI,
mainly because of its shortest dMn-O and largest effective bandwidth W. Thermally activated conductivity is
valid below TMI, and the activation energy increases slightly with the structural distortion. All of the samples
exhibit metallic-like behavior above TMI. The measurements of magnetic susceptibility and the Seebeck
coefficient indicate that the metal-insulator transition with R3+ doping is induced by the spin-state transition
of Mn3+ ions, and the transition is strongly influenced by the crystal structure. Moreover, the observed TMI
can be well described as a function of the structural parameters θMn-O-Mn and the A-site cation size variance
σ2.
1. Introduction
Manganese perovskites R1 - xAxMnO3 (R ) rare-earth, A )
alkaline-earth) have been studied extensively in recent years
because of their unusual physical properties such as colossal
magnetoresistance (CMR), charge ordering (CO), orbital ordering (OO), phase separation, and spin-glass behavior.1-5 These
properties can be tuned by changing the nature and concentration
of trivalent rare-earth or divalent alkaline-earth cations, which
determine both the distortion of crystal structure and the
concentration of eg electrons at Mn sites.6 The average size of
cations at A-site 〈rA〉, Mn-O bond length, and Mn-O-Mn
bond angle also have significant effect on physical properties.
As 〈rA〉 reduces, the tilt of MnO6 octahedron is induced in favor
of the localization and ordering of Mn3+/Mn4+ cations. At the
same time, double exchange (DE) interactions can induce a
ferromagnetic (FM) metallic ground state; long-range Coulomb
repulsion and Jahn-Teller distortion can cause the localization
of the Mn3+ and Mn4+ species and their antiferromagnetic
(AFM) coupling.6,7 In a word, the interplay among spin, charge,
orbital, and lattice degrees of freedom provides a rich phase
diagram and an important challenge to existing theories of
electronic states in solids as well.
In comparison with hole-doped manganites, less attention has
been paid to the electron-doped counterparts, in which the CMR
effect is only observed in a narrow region in a phase diagram
and the magnetic phase diagrams heavily depend on 〈rA〉.8-11
In addition, most of the researches on electron-doped manganites
concentrate on their low-temperature properties only and neglect
their high-temperature behavior. However, it is also of significance to investigate their high-temperature properties, such as
* Corresponding author. Tel: +86-451-86418403; e-mail: suiyu@
hit.edu.cn.
their transport or thermoelectric behavior. Recently, the hightemperature thermoelectric properties of electron-doped manganites have attracted much attention. The studies on
(Ca0.9M0.1)MnO3 (M ) Y, La, Ce, Sm, In, Sn, Sb, Pb, and Bi)
and Ca1 - xRexMnO3 (Re ) rare-earth) indicate that the doping
at the Ca site causes a dramatic enhancement of electrical
conductivity and thus a good thermoelectric performance at high
temperature.12,13 Some investigations on electron-doped
Ln1 - xCaxMnO3 (Ln ) rare-earth and Th) show that all of these
manganites are n-type semiconductors at low temperatures and
exhibit a metal-insulator (M-I) transition at high temperatures
without any crystallographic variation, but the nature of this
M-I transition is still unclear.12,14-18 Taguchi et al. postulated
that there may be four t2g electrons in Mn3+ at a lower
temperature, but some t2g electrons transfer to the eg band at a
higher temperature, which induces this transition.14-16 However,
this point of view is quite problematic because it is well-known
that the ground state of Mn3+ is t2g3eg1. Melo et al. attributed
the M-I transition to the formation of CO at low temperature,17,18
but this conclusion is not convictive either. First, CO occurs
only in Ca1 - xRxMnO3 at a high doping (x > 0.15), but the M-I
transition takes place in Ca1 - xRxMnO3 at x ) 0.1 even if x ∼
0.02.17 Second, the CO transition temperature TCO is much lower
than TMI. The fact that TCO differs from TMI by more than 100
K suggests that these two phenomena are probably not related
to each other. Therefore, these viewpoints on this M-I transition
may be all controversial. Although electron-doped CaMnO3 also
have additional phenomena at an elevated temperature, for
example, endothermic/exothermic phenomena and variation of
thermogravity, enthalpy, and entropy, as reported by Rørmark
et al. and Bakken et al.,19-21 the temperatures at which these
phenomena take place are much higher than TMI, and these
phenomena do not appear in a system at the same time; thus,
10.1021/jp809049s CCC: $40.75  2009 American Chemical Society
Published on Web 06/12/2009
12510
J. Phys. Chem. C, Vol. 113, No. 28, 2009
Wang et al.
Figure 1. XRD patterns and Rietveld refinement results for (a) Ca0.9La0.1MnO3 and (b) Ca0.9Dy0.1MnO3 at room temperature. The experimental
data are shown as dots, the global fitting profile and the difference curve are shown as solid lines, and the calculated reflection positions are
indicated by stick marks.
the phenomena have nothing to do with the M-I transition. In
addition, the M-I transition seems to have an intimate correlation
with the structure and the 3d electrons of Mn ions because the
substitution of different elements or slight variation of doping
level in Ca1 - xRxMnO3 can have a strong effect on the M-I
transition.16-18 On the other hand, it has been reported that the
resistivity and Seebeck coefficient of the electron-doped CaMnO3 system increase simultaneously above the M-I transition
temperature TMI, and the thermoelectric power factor also
increases; thus this system can be considered as potential hightemperature thermoelectric material.12,13 Therefore, it is essential
to understand the nature of M-I transition at TMI, which appears
to play an important role in determining the high-temperature
transport properties of the electron-doped perovskite CaMnO3.
In this paper, the structural and transport properties of the
electron-doped perovskite manganites Ca0.9R0.1MnO3 (R ) La,
Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) are studied in detail
so that the nature of the high-temperature M-I transition and
the correlation between structural parameters and this M-I
transition can be well understood. It has been pointed out that
the concentration of the electron carrier is predominant
in transport properties of the Mn4+-rich manganites,22 so the
effect of structural factors on the M-I transition is studied with
the carrier density of the system fixed. Furthermore, we choose
the doping level of x ) 0.1 where Ca1 - xRxMnO3 families exhibit the lowest resistivity and a considerable Seebeck coefficient, that is, good thermoelectric performance. The study also
provides information for discussing the role of the 3d electrons
of Mn ions in the transport mechanism of the perovskite
manganite systems.
2. Experimental Section
Ca0.9R0.1MnO3 (R ) La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho,
Er, Yb) polycrystalline samples were prepared by the solidstate reaction method. Reagent grade CaCO3, MnO2, La2O3,
Pr6O11, Sm2O3, Eu2O3, Gd2O3, Tb4O7, Dy2O3, Ho2O3, Er2O3, and
Yb2O3 powders were mixed at stoichiometric ratios and calcined
in air at 1273 K for 12 h for decarbonation. Then the mixture
was ground, pressed into disk-shaped pellets, and sintered in
air at 1573 K for 24 h. Finally, the products were ground,
pressed into pellets again, and then sintered in air at 1623 K
for 36 h. The pellets were slowly cooled down to room
temperature in the furnace.
X-ray diffraction (XRD) data were collected at room temperature for all the samples, using a Bede D1 XRD diffractometer with Ni filtered Cu KR (λ ) 0.15406 nm) radiation and
scanning (0.02° step in 2θ) over the range 15° e 2θ e 120°.
Iodometic titration was employed (under the assumption that
the valences of Ca, R, and O are +2, +3, and -2 in acidic
solution, respectively) to determine the average Mn valence and
excess oxygen content. The results show that the oxygen
stoichiometry for all the samples is equal to 3.00 ( 0.01. The
oxygen stoichiometry was further checked through TGA, using
SDT 2960 of TA Instruments. The temperature dependences
of resistivity from 300 K to 1073 K were measured using the
standard four-probe method. Silver leads were attached to four
points on the top surface of the specimens with silver paste
and used as electrodes. The Seebeck coefficient was also
measured with a four-probe technique in the temperature range
of 300-423 K. By measuring thermoelectric voltage (∆V) with
a 2000 Multimeter (KEITHLEY) and temperature gradient (∆T)
across the samples, the Seebeck coefficient S was obtained
according to S ) ∆V/∆T. Both resistivity and the Seebeck
coefficient were measured in air. Magnetic susceptibility from
5 K to 350 K was measured using the commercial Quantum
Design physical property measurement system (PPMS-9T).
3. Results and Discussion
The XRD patterns for Ca0.9La0.1MnO3 and Ca0.9Dy0.1MnO3
at room temperature are shown in Figure 1. Other samples have
similar XRD patterns. All the samples are assigned to be singlephase of the orthorhombic perovskite structure described by the
symmetry of the Pnma space group. The structural parameters
were determined by Rietveld refinement method, using the
profile analysis program Fullprof. The Rietveld refinement of
all of the samples was carried out by considering an orthorhombic structure, the same as the other electron-doped
CaMnO3,17,18,21,23-25 with a ∼ c ∼ 2ap and b ∼ 2ap (where ap
is the lattice parameter of an ideal cubic perovskite structure).
The initial positional parameters were taken by referring to the
previously reported data of CaMnO3. As shown in Figure 1, all
reflections can be indexed as orthorhombic perovskite structures,
and a good agreement is obtained between the observed and
the calculated XRD patterns for all of the samples; the refined
quality factors (Rwp, RB, and RF) are satisfactory. From these
results, the structural parameters were obtained.
Table 1 lists the refined structural parameters and calculated
tolerance factor t, defined as t ) (rA + rO)/2(rB + rO),26 of all
of the samples. Here we use the ionic radius for nine-coordinated
Ca2+ (1.085 Å), since the coordination number in orthorhombic
distorted perovskites is usually nine.27 Figure 2 shows the
variations of lattice parameters as a function of 〈rA〉; b/2 has
also been shown. It can be seen that all of a, b, c, and unit cell
volume V decrease as 〈rA〉 decreases from La to Yb, which is
Metal-Insulator Transition in Ca0.9R0.1MnO3
J. Phys. Chem. C, Vol. 113, No. 28, 2009 12511
TABLE 1: Room-Temperature Cell Parameters, Atomic Position, Isotropic Thermal Factor Biso, Fitting Factors (Rwp, RB, and
RF), Mn-O Bond Length, Mn-O-Mn Bond Angles, A-Site Average Ionic Radii 〈rA〉, A-Site Cation Size Variance σ2, and
Tolerance Factor t for All of the Samples
R ion
a (Å)
b (Å)
c (Å)
V (Å3)
x (Ca/R)
z (Ca/R)
Biso (Ca/R) (Å2)
Biso (Mn) (Å2)
x (O1)
z (O1)
Biso (O1) (Å2)
x (O2)
y (O2)
z (O2)
Biso (O2) (Å2)
Rwp (%)
RB (%)
RF (%)
Mn-O(1) (Å)
Mn-O(2) (Å)
La
5.3164(5)
7.5201(9)
5.3034(7)
212.03(7)
0.0314(1)
-0.0070(2)
0.78(3)
0.32(2)
0.4876(9)
0.0666(5)
1.04(6)
0.2730(4)
0.0340(1)
-0.2750(9)
1.36(9)
12.8
4.36
3.86
1.9142
1.8963
1.9102
Mn-O(1)-Mn (deg) 158.325
Mn-O(2)-Mn (deg) 161.079
〈rA〉 (Å)
1.0981
σ2 (Å2)
1.54 × 10-3
t
0.90983
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Yb
5.3132(5)
7.5105(9)
5.2945(3)
211.28(2)
0.0322(6)
-0.0071(5)
0.75(4)
0.30(1)
0.4878(3)
0.0672(5)
1.08(5)
0.2822(7)
0.0285(1)
-0.2828(8)
1.41(5)
11.9
4.04
4.13
1.9122
1.9017
1.9044
158.196
160.370
1.0944
7.95 × 10-4
0.90849
5.3111(7)
7.5066(2)
5.2906(8)
210.93(4)
0.0326(3)
-0.0076(3)
0.80(3)
0.32(1)
0.4882(5)
0.0677(3)
1.10(7)
0.2830(1)
0.0288(6)
-0.2839(7)
1.39(6)
10.8
3.81
3.76
1.9116
1.9005
1.9060
158.068
159.946
1.0928
5.48 × 10-4
0.9079
5.3087(9)
7.4957(5)
5.2871(9)
210.39(5)
0.0330(4)
-0.0079(2)
0.73(3)
0.26(3)
0.4884(3)
0.0679(5)
1.07(6)
0.2859(6)
0.0290(4)
-0.2850(9)
1.35(7)
11.2
3.95
3.61
1.9090
1.8996
1.9091
158.001
159.216
1.0897
1.99 × 10-4
0.90677
5.3023(3)
7.4925(9)
5.2857(7)
209.99(4)
0.0332(5)
-0.0080(5)
0.78(4)
0.26(2)
0.4885(2)
0.0679(7)
1.12(8)
0.2859(9)
0.0299(3)
-0.2860(3)
1.44(6)
10.5
3.58
3.09
1.9083
1.9035
1.9052
157.973
158.760
1.0885
1.10 × 10-4
0.90634
5.2989(3)
7.4818(2)
5.2833(7)
209.46(3)
0.0335(1)
-0.0082(2)
0.82(3)
0.30(2)
0.4888(4)
0.0682(5)
1.06(4)
0.2889(7)
0.0300(2)
-0.2870(8)
1.38(8)
10.2
3.16
2.87
1.9058
1.8973
1.9136
157.906
158.080
1.0872
4.36 × 10-5
0.90586
5.2951(9)
7.4742(1)
5.2815(1)
209.02(8)
0.0336(5)
-0.0085(1)
0.80(2)
0.29(3)
0.4890(1)
0.0685(2)
1.13(5)
0.2896(3)
0.0303(2)
-0.2890(4)
1.40(7)
9.78
2.92
2.88
1.9041
1.9032
1.9091
157.809
157.551
1.086
9.02 × 10-6
0.90543
5.2900(6)
7.4709(8)
5.2779(6)
208.59(5)
0.0341(2)
-0.0088(6)
0.77(3)
0.33(2)
0.4902(7)
0.0688(2)
1.11(3)
0.2902(8)
0.0304(6)
-0.2903(4)
1.37(6)
8.86
2.56
2.71
1.9034
1.9055
1.9061
157.778
157.188
1.0848
3.60 × 10-7
0.90499
5.2896(7)
7.4683(4)
5.2765(1)
208.44(8)
0.0339(7)
-0.0090(4)
0.76(2)
0.31(2)
0.4903(5)
0.0696(1)
1.09(7)
0.2904(6)
0.0308(1)
-0.2909(1)
1.48(5)
9.31
2.78
2.96
1.9036
1.9052
1.9074
157.534
156.943
1.0837
1.52 × 10-5
0.90459
5.2866(7)
7.4618(7)
5.2748(4)
208.08(4)
0.0346(8)
-0.0092(9)
0.75(3)
0.32(3)
0.4910(3)
0.0712(6)
1.06(6)
0.2909(2)
0.0316(1)
-0.2912(3)
1.42(5)
11.7
3.34
3.57
1.9035
1.9062
1.9071
157.057
156.593
1.0827
4.76 × 10-5
0.90422
5.2815(6)
7.4532(5)
5.2719(6)
207.53(1)
0.0351(2)
-0.0096(2)
0.79(4)
0.28(2)
0.4872(6)
0.0738(3)
1.17(5)
0.2918(5)
0.0322(7)
-0.2920(3)
1.39(6)
13.2
4.55
4.21
1.9047
1.9066
1.9070
156.068
156.140
1.0807
1.66 × 10-4
0.9035
related to the reduction of R3+ ionic radii. Additionally, the
values of V for all of the samples are larger than that of undoped
CaMnO3 (∼207.47 Å3).17,18 This is because the substitution of
the trivalent cation R3+ for the Ca site produces the Mn3+ cation
in the Mn4+ matrix, where the ionic radius is larger than that of
Mn4+ (0.645 Å for Mn3+ and 0.530 Å for Mn4+, respectively).
Although the ionic radii of Dy3+, Ho3+, Er3+, and Yb3+ are
smaller than that of Ca2+, this change in ionic radii at the A
site is so small compared to the change in cationic size at the
Mn site that the unit cell volume is still increasing. Specifically,
the values of a, b, and c are all larger than that of CaMnO3,
except for Ca0.9Yb0.1MnO3. In Ca0.9Yb0.1MnO3, the values of a
and c are still larger than those of CaMnO3, but the b value is
smaller than that of CaMnO3, which indicates the stronger
structural distortion and the contraction of the b-axis in the
Ca0.9Yb0.1MnO3 sample. It can also be seen from Figure 2 that,
except for Ca0.9Yb0.1MnO3, all of the other samples follow the
relation c < b/2 < a, that is, an O-type orthorhombic structure.
However, the values of b/2 tend to c gradually with the
decrease of 〈rA〉, which means the increment on the structural
distortions. Ca0.9Yb0.1MnO3 has turned out to be an O*-type
orthorhombic structure with a large distortion, that is, b/2 <
c < a.
Figure 3 shows the variation of the average Mn-O bond
length dMn-O and Mn-O-Mn bond angle θMn-O-Mn as a
function of 〈rA〉. Within the limits of error, it is clear that
θMn-O-Mn decreases monotonously with 〈rA〉 decreasing, but
dMn-O exhibits a minimum for Ca0.9Dy0.1MnO3. The significant
lowering in θMn-O-Mn from 180° also indicates that there is a
strong structural distortion and the distortion increases gradually
from Ca0.9La0.1MnO3 to Ca0.9Yb0.1MnO3. The enhancement of
the tilt angle of MnO6 octahedra may be the dominant reason
for the contraction of the b-axis. Moreover, tolerance factor t
also deviates away from its ideal value 1 with the decrease of
〈rA〉. The reduction of dMn-O from Ca0.9La0.1MnO3 to
Ca0.9Dy0.1MnO3 should be attributed to the decrease of unit cell
volume. Although the decrease of the Mn-O-Mn bond angle
may lengthen the Mn-O bond, this effect is overcome by the
obvious constriction of the unit cell. As for the Ho3+-, Er3+-,
and Yb3+-doped samples, because of their similar ionic radii,
the constriction of the unit cell is slight; however, the decrease
of the Mn-O-Mn bond angle becomes noticeable, so dMn-O
begins to gradually increase.
Figure 4 shows the temperature dependence of resistivity (F)
of Ca0.9R0.1MnO3 samples from 300 K to 1173 K. All the
samples show similar transport behavior, namely, metallic-like
Figure 2. Structural parameters of Ca0.9R0.1MnO3 as a function of 〈rA〉: (a) lattice parameters and (b) unit cell volume V.
12512
J. Phys. Chem. C, Vol. 113, No. 28, 2009
Wang et al.
TABLE 2: Room-Temperature Resistivity GRT, M-I
Transition Temperature TMI, Effective Bandwidth W, and
Activation Energy Ea for Ca0.9R0.1MnO3
FRT
TMI
(mΩ cm) (K) W (Å-3.5)
Figure 3. Average Mn-O bond length dMn-O and average Mn-O-Mn
bond angle θMn-O-Mn as a function of 〈rA〉.
Ca0.9La0.1MnO3
Ca0.9Pr0.1MnO3
Ca0.9Nd0.1MnO3
Ca0.9Sm0.1MnO3
Ca0.9Eu0.1MnO3
Ca0.9Gd0.1MnO3
Ca0.9Tb0.1MnO3
Ca0.9Dy0.1MnO3
Ca0.9Ho0.1MnO3
Ca0.9Er0.1MnO3
Ca0.9Yb0.1MnO3
9.88(15)
9.12(18)
7.84(12)
7.12(21)
6.46(16)
5.89(11)
5.59(15)
4.69(16)
5.04(12)
6.23(16)
7.08(17)
420
399
388
365
357
339
333
325
331
376
383
0.102571(28)
0.102701(16)
0.102707(20)
0.102712(17)
0.102743(16)
0.102756(21)
0.102753(14)
0.102808(12)
0.102727(17)
0.102595(15)
0.102334(18)
temperature range
for obtaining
Ea
Ea (K)
(meV)
300-416
300-396
300-385
300-362
300-355
300-335
300-330
300-322
300-328
300-372
300-380
56.9
57.8
56.3
59.6
58.8
60.3
62.1
64.9
62.9
64.1
65.6
difference in F value for the samples comes from the structural
distortion induced by the substitution of different R3+ ions. It
can be seen from XRD refinement results (Figures 2 and 3)
that θMn-O-Mn decreases monotonously with the reduction of
〈rA〉 from Ca0.9La0.1MnO3 to Ca0.9Yb0.1MnO3, but dMn-O decreases first and then increases, with the shortest dMn-O for
Ca0.9Dy0.1MnO3. Then we consider the overlap integral between
Mn 3d and O 2p orbitals of the system, which is strongly
influenced by 〈rA〉 and structural distortions. For orthorhombic
perovskite manganites, the effective bandwidth W is represented
within the tight-binding approximation:
Figure 4. Temperature dependence of resistivity F for Ca0.9R0.1MnO3
(R ) La, Sm, Dy, Er, and Yb) samples.
at a higher temperature and semiconducting-like at a lower
temperature, while the M-I transition temperature (TMI) varies
from 325 K to 420 K. We define TMI as the temperature at which
the slope is zero in F-T curves. Undoped CaMnO3 exhibits
semiconductive behavior from 5 K to 1000 K, and G-type AFM
order with Néel temperature (TN) is ∼120 K.28-30 With minor
electron doping at the Ca site, resistivity can decrease significantly and exhibit metallic behavior.17,18,22,31 Figure 4 shows that
substitution of R3+ for Ca2+ with x ) 0.1 lowers resistivity by
several orders of magnitude, compared to the undoped CaMnO3
with a resistivity of about 2 × 104 mΩ cm at room temperature.17 The reduction of F should be attributed to the change in
the valence state of Mn caused by R doping because the electron
hopping between Mn3+ and Mn4+ is responsible for the electric
conductivity in perovskite manganites.32,33 On the basis of the
valence equilibrium, substitution of R3+ for Ca2+ will bring
Mn3+, thus the electron hopping between Mn3+ and Mn4+ is
facilitated by DE interactions, and then F will decrease as a
result. These results are similar to the results of previous studies
on electron-doped perovskite manganites.12,13,17,18,22 Furthermore,
the doping level is same for a different R, which introduces
equal electron carriers, so the values of F are at the same order
of magnitude for all of the samples.
It should be noted that both room-temperature resistivity (FRT)
and TMI vary with R3+ from La3+ to Yb3+ regularly. With the
decrease of 〈rA〉, FRT first decreases from 9.88 mΩ cm for
Ca0.9La0.1MnO3, reaches a minimum of 4.69 mΩ cm for Ca0.9Dy0.1MnO3, and then increases to 7.08 mΩ cm for
Ca0.9Yb0.1MnO3. Similarly, TMI first decreases from ∼420 K for
Ca0.9La0.1MnO3 down to ∼325 K for Ca0.9Dy0.1MnO3 and then
increases up to ∼383 K for Ca0.9Yb0.1MnO3. As shown in Table
2, Ca0.9Dy0.1MnO3 exhibits the lowest FRT and TMI. The small
W∝
cos ω
dMn-O7/2
(1)
where ω ) (π - θMn-O-Mn)/2.34 The calculated W values for
all the samples are shown in Table 2. With the variation of the
R ions, the change in W is clear; W exhibits an opposite
evolution behavior of dMn-O approximately. Ca0.9Dy0.1MnO3
exhibits the largest W, mainly because of its shortest Mn-O
bond length and moderate Mn-O-Mn bond angle. Since the
conduction is governed by an eg electron, the enhancement of
effective eg bandwidth will consequently decrease the resistivity
of the system. Therefore, the variation of FRT with R3+ doping
approximately corresponds to W. In addition, it is the minor
difference in effective bandwidth arising from structural distortion that results in the different resistivity of these samples, even
though they have equal electron carrier concentration. These
results also suggest that structure and electrical transport
properties are coupled closely to each other in perovskite
manganites.
The plots of ln(σT) versus 1/T for all of the samples lie on
straight lines below TMI, as shown in Figure 5. This means the
conduction mechanism is thermally activated small-polaron
hopping between Mn3+ and Mn4+ states. Activation energy (Ea)
can be obtained from the slopes of the plots, as listed in Table
2. Ea are almost the same for all of the samples, in the range of
56-66 meV, for their same transport mechanism and hopping
process. Ea increases a little for Ca0.9R0.1MnO3 (R ) Tb, Dy,
Ho, Er, and Yb), mainly because their notable crystal distortions
can heighten the energy barrier for polaron hopping. Nevertheless, although the system exhibits metallic-like transport behavior above TMI, polarons cannot disappear at the point of TMI
immediately. Polarons still exist in a certain temperature range,
and conduction electrons gradually develop into dominant
carriers of this system.12 That is why the end temperature of
Metal-Insulator Transition in Ca0.9R0.1MnO3
Figure 5. ln(σT) versus 1000/T for Ca0.9R0.1MnO3 (R ) La, Sm, Dy,
Er, and Yb) samples. The solid lines represent the linear fitting, and
the arrows point to the M-I transition temperature TMI of each specimen.
J. Phys. Chem. C, Vol. 113, No. 28, 2009 12513
splitting between the t2g and eg states and the Hund coupling
energy (exchange energy), the spin state configuration of Mn4+
is t2g3eg0 (yields a total spin S ) 3/2 and an effective magnetic
moment of 3.9 µB/Mn4+ ion), but that of Mn3+ can be the high
spin (HS) state t2g3eg1 (yields a total spin S ) 2 and an effective
magnetic moment of 4.9 µB/Mn3+ ion) or the low spin (LS)
state t2g4eg0 (yields a total spin S ) 1 and an effective magnetic
moment of 2.8 µB/Mn3+ ion). Because the crystal structural
distortion induced by MnO6 octahedron rotation can give rise
to the change in crystal-field splitting energy (∆o), exchange
splitting energy (Eex), and the overlap of t2g orbitals with β spin
and eg orbitals with R spin, thus the HS state and LS state of
the Mn3+ ions may be close in energy and transform to each
other. This has been shown in perovskite LaCoO3, which
exhibits a two spin-state transition as temperature increases, from
the LS state to the intermediate spin (IS) state and then to the
HS state.36 By assuming that only Mn4+, Mn3+, and R3+
moments are responsible for the paramagnetic behavior of χ at
a temperature above TN, the Curie-Weiss law can be written
as
χ)
NMnµeff,Mn2 + NRµeff,R2
3kB(T - Θp)
(2)
where NMn and NR are the number densities of Mn and R ions
per formula unit, µeff,Mn and µeff,R are the effective magnetic
moments of Mn and R ions, kB is Boltzmann’s constant, T is
the absolute temperature, and Θp is the paramagnetic Curie
temperature. Considering the ratio of Mn4+, Mn3+, and R3+ of
the system is 9:1:1, eq 2 can be expressed as
Figure 6. Temperature dependence of the inverse susceptibility 1/χ
and magnetization M (inset) of Ca0.9R0.1MnO3 (R ) La, Sm, Dy, Ho,
and Yb), measured in a field-cooled mode under a 2 T magnetic field.
the linear dependence for ln(σT) versus 1/T may be slightly
higher than TMI.
Next, we focus on the M-I transition induced by R3+ doping.
To identify the origin of this M-I transition, the temperature
dependence of field-cooled magnetization (M) for the samples
was measured, as shown in Figure 6. It can be seen that TN
values are in the range of 100-120 K for all of the samples,
where TN is defined as the temperature of the maximum slope
in M-T curves, and all of the samples are paramagnetic above
TN. Since the M-I transition occurs in the high-temperature
paramagnetic range, it cannot be correlated with magnetic order
or charge order. Melo et al. also reported that electron-doped
Ca1 - xCexMnO3 and Ca1 - xHoxMnO3 systems exhibit the M-I
transition at high temperatures but without crystallographic
variation at TMI,17,18 so there are other factors leading to the
M-I transition. The temperature dependence of inverse susceptibility (1/χ) for Gd3+-, Tb3+-, Dy3+-, and Ho3+-doped samples
from 300 K to 350 K is shown in Figure 7. The 1/χ-T curves
follow the Curie-Weiss law, and a clear variation in slope can
be observed at 339 K, 333 K, 325 K, and 331 K for the four
samples, respectively, which is consistent with the TMI obtained
from F-T curves. The abrupt rise of slope in the 1/χ-T curves
indicates the change of the effective magnetic moment in the
system, suggesting a change in the electron configuration (i.e.,
spin state) of Mn ions at TMI.
According to the energy band scheme for perovskite manganites,35 the electron configurations of Mn4+ and Mn3+ ions
are 3d3 and 3d4, respectively. With respect to the crystal-field
0.9µeff,Mn4+2 + 0.1µeff,Mn3+2 + 0.1µeff,R3+2
χ)
3kB(T - Θp)
(3)
We can use eq 3 to obtain the values of µeff,Mn3+ and Θp for the
samples below and above TMI from the 1/χ-T curves, as listed
in Table 3. Here we ignore the contribution from the Pauli
paramagnetism of conduction electrons to susceptibility above
TMI, because the electron density in the Ca0.9R0.1MnO3 system
is low, at most 10% of the density of magnetic Mn ions. When
applying the magnetic field, only partial electrons around the
Fermi level contribute to the Pauli paramagnetism. The estimated
Pauli paramagnetic susceptibility of conduction electrons is so
weak that it can hardly influence the total susceptibility.
Therefore, the Pauli paramagnetism here is much weaker than
the Langevin paramagnetism arising from magnetic Mn and R
ions and can thus be ignored. The observed value of µeff,Mn3+
below TMI is quite close to the ideal value 4.9 µB, which means
that almost all of the Mn3+ ions are in the HS state below TMI.
This result is consistent with many previous studies and a band
scheme.35 In the ground state of perovskite manganites, Eex is
larger than ∆o, so the electron configuration of Mn3+ should be
t2g3eg1, that is, the HS state. The value of µeff,Mn3+ above TMI is
less than that below TMI, suggesting that partial Mn3+ ions
change from the HS to the LS as temperature rises over TMI.
As the temperature increases, the energy of electrons increases,
so some eg electrons can be activated to the t2g orbital with β
spin in the case of overlapping between t2g orbitals with β spin
and eg orbitals with R spin, forming a t2g4eg0 configuration. In
this case, although the strong Hund’s exchange present in
manganites forces the spin of an electron in the eg state to align
with the t2g electrons on the same Mn3+ site, there is still a
12514
J. Phys. Chem. C, Vol. 113, No. 28, 2009
Wang et al.
Figure 7. 1/χ versus T (open circles) and d(1/χ)/dT versus T (solid squares) curves for (a) Ca0.9Gd0.1MnO3, (b) Ca0.9Tb0.1MnO3, (c) Ca0.9Dy0.1MnO3,
and (d) Ca0.9Ho0.1MnO3; the inflection in the 1/χ-T curves corresponds to TMI, as indicated with hatching.
TABLE 3: Observed Effective Magnetic Moment of Mn3+ Ions µeff,Mn3+, Fractional Concentration of Mn3+ in the LS State and
the HS State, and the Paramagnetic Curie Temperature Θp below and above TMI, Respectively
below TMI
Ca0.9Gd0.1MnO3
Ca0.9Tb0.1MnO3
Ca0.9Dy0.1MnO3
Ca0.9Ho0.1MnO3
above TMI
µeff,Mn3+a
NMn3+,LS
NMn3+,HS
Θp (K)
µeff,Mn3+a
NMn3+,LS
NMn3+,HS
Θp (K)
4.87
4.85
4.81
4.86
0.002
0.003
0.005
0.002
0.098
0.097
0.095
0.098
49
43
51
56
4.58
4.54
4.47
4.52
0.019
0.021
0.025
0.022
0.081
0.079
0.075
0.078
63
60
71
72
a
µeff,Mn3+(µB/Mn3+ ion) is calculated using eq 3 with µeff,Mn4+ ) 3.9 µB/f.u., µeff,Gd3+ ) 8.0 µB/f.u., µeff,Tb3+ ) 9.5 µB/f.u., µeff,Dy3+ ) 10.6 µB/
f.u., and µeff,Ho3+ ) 10.4 µB/f.u., while NMn4+ ) 0.9 and NR3+ ) 0.1. The effective magnetic moment values of R3+ ions used are all
experimental values that have included the contribution from Van Vleck paramagnetism of the excited states of the R3+ ions.
great probability that partial eg electrons can exist on t2g orbitals.
As a result, the susceptibility of the system will diminish, and
1/χ-T curves will display an inflection around TMI. We can
also estimate the number of Mn3+ ions in the LS and HS state
below and above TMI, as shown in Table 3, where around onefourth to one-fifth of the Mn3+ ions change their spin state above
TMI. Nearly all of the Mn3+ ions are in the HS state below TMI,
so the hopping of eg electrons between neighbor Mn sites favors
the thermally activated polaronic behavior. Above TMI, since
partial Mn3+ ions transfer from the HS state to the LS state,
some electrons occupy t2g orbitals rather than eg orbitals. More
electrons in the same t2g orbital will lead to a strong interaction
and then widen the t2g orbital. When the t2g orbital is widened
to approach the narrow-band limit for an itinerant electron
because of the electron correlation, the system will exhibit
metallic-like behavior, and the M-I transition will be induced
consequently.
To confirm this postulation, the temperature dependence of
the Seebeck coefficient (S) for Gd3+-, Tb3+-, Dy3+-, and Ho3+doped samples is measured in the range of 300-423 K, because
the Seebeck coefficient is a powerful tool in probing the change
Figure 8. Temperature dependence of the Seebeck coefficient S for
Ca0.9R0.1MnO3 (R ) Gd, Tb, Dy, and Ho); ∆Sexp represents the jump
in the S-T curves around TMI.
in electron states and configurational entropy.37-42 As shown
in Figure 8, the S-T curves for different samples are similar,
and the values of S are almost equal. The sign of S is negative,
making clear the nature of the electron carriers in this system
in the whole temperature range. The most notable feature of
Metal-Insulator Transition in Ca0.9R0.1MnO3
J. Phys. Chem. C, Vol. 113, No. 28, 2009 12515
the S-T curve is an obvious jump around TMI; the value of S
below TMI is lower than that above TMI. Although some models
were reported to calculate the temperature dependence of S for
electron-doped CaMnO3,43 the effect of electron configuration
on S should be considered since S is determined by not only
the carrier concentration but also the configurational entropy
of the system. Accordingly, the jump in the S-T curve would
be corresponding to the variation of electron configuration.
In a small polar system, the Seebeck coefficient S is given
by the sum of two terms, the charge carrier term (Sc) and the
configurational entropy term (Sg) in which both spin and orbital
configuration are involved, and S ) Sg + Sc. Sc is given by
Sc )
(
)
kB
ES
ln
+R
e
kBT
(4)
where ES is the activation energy determined by the measurement of the Seebeck coefficient and R is a sample-dependent
constant.44 At a high temperature (kBT . ES), Sc can reach the
value of a constant, Sc,∞, determined by the Heikes formula,
and Sc,∞ is only dependent on the number of electrons per active
transport site.45 Above room temperature, S can be considered
to obey S ) Sg + Sc,∞. The electron concentration is the same
at the same doping level, which gives the same Sc,∞ for all of
the samples. Since Sc,∞ does not vary as temperature traverses
TMI, the jump of S arises from the change in Sg. Namely,
∆S ) (Sg + Sc,∞)above TMI - (Sg + Sc,∞)below TMI
) Sg,above TMI - Sg,below TMI
(5)
In the paramagnetic state, the Sg term in mixed-valence
manganites can be expressed as
Sg )
()
kB
g4
ln
e
g3
(6)
where g3 and g4 represent the total degeneracy of Mn3+ and
Mn4+, respectively; g ) gσ × go in which gσ and go are spin
degeneracy and orbital degeneracy, respectively. For Mn4+, spin
σ is equal to 3/2, so gσ ) 4; all of the three t2g orbitals are
filled, so go ) 1. Thus, g4 ) 4. As for Mn3+, there are two
situations, the HS state and the LS state. Although Jahn-Teller
distortion exists in Mn3+ ions, the thermal energy is large
compared to Jahn-Teller splitting (namely, kBT > ∆JT) in
manganites at high temperatures, so all eg orbitals can be
considered degenerate.39 In this case, (i) for Mn3+ in the HS
state (below TMI), σ ) 2, gσ ) 5, and go ) 2, thus g3 ) 10, and
(ii) for Mn3+ in the LS state (above TMI), σ ) 1, gσ ) 3, and go
) 3, thus g3 ) 9. Therefore, we can calculate the jump value
of S around TMI according to eqs 5 and 6 by assuming all of
the Mn3+ ions are in the HS state below TMI and all of them
change to the LS state above TMI:
∆S ) Sg,above TMI - Sg,below TMI
)
kb 4
4
ln - ln
) 9.2 µV/K
e
9
10
(
)
The observable difference in S below and above TMI in the S-T
curves confirms the variation of the spin state of Mn3+ ions at
TMI unambiguously. It should be emphasized that the calculated
∆S is much larger than the observed value ∆Sexp (3-3.2 µV/
K) from S-T curves, indicating that only about one-third of
the Mn3+ ions change their spin state above TMI, which is
roughly consistent with the susceptibility results.
From the discussion above, we can conclude that the M-I
transition arises from the spin-state transition of Mn3+ ions with
the increase of temperature. However, the variation of TMI with
〈rA〉 (Table 2) suggests that structural distortion is also important
to determine TMI of the system for different R3+ doping. Since
Ca0.9Dy0.1MnO3 gives the shortest average Mn-O bond length
dMn-O, as confirmed by XRD refinement, Ca0.9Dy0.1MnO3 should
exhibit the largest crystal-field splitting energy ∆o according to
∆o ∝ 1/dMn-O5. Therefore, the crystal-field splitting energy ∆o
is the closest to the exchange splitting energy Eex in
Ca0.9Dy0.1MnO3, and thus the overlap of t2g orbitals with β spin
and eg orbitals with R spin is the largest. Therefore partial eg
electrons with R spin in Ca0.9Dy0.1MnO3 can be activated to t2g
orbitals easily at a lower temperature, and thus Ca0.9Dy0.1MnO3
exhibits the lowest TMI. As dMn-O increases with other R3+
doping, ∆o decreases, which will weaken the overlap of t2g
orbitals with β spin and eg orbitals with R spin; as a result a
higher energy is needed to activate the eg electron to the t2g
orbital, and TMI will increase.
We have seen that the crystal structure has a strong effect on
the M-I transition here. The different crystal distortions of this
system give rise to the different situations of the overlapping
between the eg and the t2g orbitals and then induce that the spin
state transition of Mn3+ together with the concomitant M-I
transition happens at different temperatures. It is understandable,
because structural, magnetic, and electrical transport properties
are strongly coupled to each other in perovskite manganites.
Since TMI depends on structural distortion, crystal structural
factors such as θMn-O-Mn or dMn-O can be used to describe TMI.
Glancing at θMn-O-Mn and dMn-O, it seems that the variations
of dMn-O are corresponding to TMI, but the change of dMn-O for
the system is too small to describe TMI; that is, the relationship
TMI ∝dMn-O is not valid. Actually, θMn-O-Mn has a more
important effect on the physical properties than dMn-O because
θMn-O-Mn is determined by the distortion of the MnO6 octahedron, which can affect the hybridization between Mn 3d and O
2p orbitals and connect to the band structure. Another structural
factor, A-site cation size variance σ2 (defined by ∑yiri2 - 〈rA〉2,
where ri is the ionic size and yi is the fractional occupancy of
the ith atoms of the A-site), is also crucial to determine the
transport properties of system. It has been reported that in holedoped manganese oxide perovskites the FM-metal to paramagnetic-insulator transition temperature is affected by σ2 because
the size mismatch of A-site cations can induce ordered or
disordered oxygen displacements and then lead to the change
of strain fields.46,47
Here we propose an empirical relation between TMI and
θMn-O-Mn and σ2, namely,
TMI ) B1θMn-O-Mn + B2σ2
(7)
where B1 and B2 are coefficients. In eq 7, θMn-O-Mn and σ2 are
known, so B1 and B2 can be determined, respectively. In
Ca0.9Dy0.1MnO3, σ2 ∼ 0, so B1 can be determined by dTMI/
dθMn-O-Mn as B1 ∼ 1.2 × 102 K. Using the values of θMn-O-Mn,
σ2, and B1, B2 ∼ 7 × 104 K/Å2 can be obtained. As shown in
Figure 9, although the variation of θMn-O-Mn is not consistent
with TMI, when the modulation term σ2 is added, the values of
12516
J. Phys. Chem. C, Vol. 113, No. 28, 2009
Figure 9. Observed (filled circles) and calculated (open squares) TMI
as a function of 〈rA〉.
TMI calculated using eq 7 have a good agreement with the
observed ones. Namely, the linear combination of θMn-O-Mn and
σ2 can well describe TMI over the whole range of 〈rA〉 for all
the samples. The two coefficients B1 and B2 connect TMI and
structural factors, clearly demonstrating that the structure and
transport properties are strongly coupled to each other in such
electron-doped perovskite manganite systems.
4. Conclusions
All of the Ca0.9R0.1MnO3 samples display orthorhombicperovskite structures, but the crystal structural distortion is found
to have a significant effect on the transport properties. From
La to Yb, the variations of structural distortions result in different
effective bandwidth and thus different resistivity values. At the
same time, all of the samples exhibit a M-I transition. At a lower
temperature, the transport mechanism is thermally activated
small-polaron conductivity, and the activation energy increases
slightly with the increase of structural distortion because of the
enhancement of the energy barrier for polaron hopping. At a
higher temperature, all of the samples exhibit metallic-like
behavior. In the vicinity of TMI, 1/χ-T curves show an inflection,
and S-T curves present a jump, which both indicate that the
M-I transition is induced by the spin-state transition of Mn3+
ions. The crystal structural distortion brought by doping tends
to induce the overlap of t2g orbitals with β spin and eg orbitals
with R spin in a Mn3+ ion. As a result, partial electrons in the
eg orbitals of the Mn3+ ions are activated to the t2g orbitals as
temperature increases, which widens the t2g orbital to an itinerant
band and then results in M-I transition consequently. This M-I
transition is influenced by structural distortions, and TMI of all
of the samples can be described satisfactorily by the linear
combination of θMn-O-Mn and σ2 over the whole range of 〈rA〉.
These results strongly suggest that the structural, magnetic, and
electrical transport properties are coupled to each other in
perovskite manganites.
Acknowledgment. This work is supported by the National
Natural Science Foundation of China (Grant No. 50672019).
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