High Temperature Metal-Insulator Transition Induced by Rare-Earth

(AFM) coupling.6,7 In a word, the interplay among spin, charge, orbital, and lattice ... transition takes place in Ca1 - xRxMnO3 at x ) 0.1 even if x ...
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J. Phys. Chem. C 2009, 113, 12509–12516

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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

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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

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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.

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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 Ne´el 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

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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

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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

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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). References and Notes (1) Liu, G. L.; Zhou, J. S.; Goodenough, J. B. Phys. ReV. B 2001, 64, 144414. (2) Coey, J. M. D.; Viret, M.; von Molnar, S. AdV. Phys. 1999, 48, 167.

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