Charge Ordering Temperatures of Bi1−xSrxMnO3 (0.45 ≤ x ≤0.8)

B. H. Kima, J. S. Kima, M. S. Kima, C. J. Zhanga, K. H. Kimb, B. G. Kimb, H. C. Kimc, andY. W. Parka,

aSchool of Physics and Nano Systems Institute-National Core Research Center, Seoul NationalUniversity, Seoul 151-747, Korea

bDepartment of Physics, Pusan National University, Pusan 609-735, KoreacNational Research Laboratory for Material Science, Korea Basic Science Institute, Daejeon

305-333, Korea

ABSTRACT

We have investigated the charge ordering phenomenon from the temperature dependence of inverse suscepti-bility, resistivity, and thermoelectric power (TEP) for Bi1−xSrxMnO3 (BSMO) from 300 K to 700 K. At hightemperatures, susceptibility follows Curie-Weiss law. The resistivity data indicate insulating behavior of BSMO.TEP (S (T)) value is negative and weakly temperature-dependent in the high temperature regime. The slopeof TEP changes dramatically near the charge ordering temperature (TCO), indicating an increase of energy gapdue to the charge ordering. In the vicinity of TCO, thermal hysteresis is observed in TEP data as well as inthe resistivity data, which is consistent with the nature of the martensitic transition of the charge ordering phe-nomena. From this hysteretic behavior, we estimated TCO. As Sr concentration increases, TCO shifts to lowertemperature from TCO ∼ 490 K for x = 0.45 to TCO ∼ 435 K for x = 0.8, and the thermal hysteretic behaviorbecomes less pronounced. The electrical transport properties have been discussed in terms of carrier localizationdue to charge ordering transition accompanied by the local lattice distortions.

Keywords: Charge ordering temperature, BSMO

1. INTRODUCTION

Rare-earth mixed-valence manganites (Ln1−xAxMnO3) with perovskite structure have been focused on duringthe last years due to their colossal magnetoresistance (CMR) and charge ordering (CO) phenomena.1 In thecase of LaMnO3, it exhibits antiferromagnetism with Jahn-Teller distortion.2 The mixed valence states of Mnare induced by the partial substitution of Sr2+ for La3+, which results in the creation of hole carrier. Thismobile carrier causes ferromagnetic (FM) state. The FM and metallic transport is explained by the schemeof double exchange interaction.3 On the other hand, BiMnO3 shows a large ferromagnetic moment (3.5 µB)without hole-doping and it has triclinically distorted perovskite lattice at room temperature,4, 5 although theionic radius of La3+ (1.22 A) is similar to that of Bi3+ (1.24 A). The structure of BiMnO3 is distorted because ofthe shift of bismuth cations from the center of the hexagon of the oxide anion due to the lone pair of electrons onthe bismuth cation.6 As Sr concentration increases, the structure of Bi1−xSrxMnO3 (BSMO) transforms fromtriclinic to monoclinic and to tetragonal (x ≥ 0.36) lattices at room temperature.7 Also, magnetic property ofthis sample changes; FM state weakens rapidly with increasing of Sr substitution.4, 5, 7

CO state of mixed-valence manganites originates from the interaction between charge carriers and phonons.It is usually understood as a spatially ordered distribution of Mn3+/Mn4+ ions in the lattice in a purely ionicpicture accompanied by orbital ordering.8 In comparison with the ionic picture, Zener polaron ordering (ZPO)formation is considered as a more realistic scenario to explain the CO state.9 In this picture, eg electron istrapped into and shared with Mn - O - Mn trio instead of a single Mn cation. Two Mn ions in a trio maintain themixed valence state and they are coupled ferromagnetically by double exchange interaction. The transition from

Further author information: (Send correspondence to Y. W. Park)Y. W. Park: E-mail: ywpark@phya.snu.ac.kr, Telephone: 82 2 880 6607

Strongly Correlated Electron Materials: Physics and Nanoengineering, edited by Ivan Bozovic, Davor Pavuna,Proceedings of SPIE Vol. 5932 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.624690

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the normal state to the CO state is the first order phase transition. The main characteristic of this transition intransport property is a hysteretic behavior. In Ln1−xAxMnO3, charge ordering temperature, TCO, is known to besensitive to the size of A-site cation. When the ionic size of A-site cation is relatively small compared to the Mn-Obond length, the mobility of the itinerant eg electrons is significantly suppressed due to the lattice distortion andthe CO is reinforced with the increase of TCO.10 Recently, it was found that TCO of Bi0.5Sr0.5MnO3 is around520 K, which is the highest TCO among manganese compounds even though the ionic radius of Bi3+ is similar tothat of La3+.11–14 It has been proposed that the anomalously high TCO is related to 6s2 characteristics of Bi3+

which is highly polarized to certain Bi-O bond direction.6, 11 Although most published works reported only onetransition temperature (TCO), J. Hejtmanek et al.15 reported two transition temperatures of BSMO (x = 0.5) athigh temperatures. One is the TCO, the other is the Tcrit where the structural transition occurs. In this paper,we report TCO of Bi1−xSrxMnO3 (0.45 ≤ x ≤ 0.8) from the hysteretic behavior of the first order phase transitionin high temperature thermoelectric power (TEP) and resistivity, along with magnetic susceptibility. Especially,both the systematic change and the enhancement of TCO comparing to those of other manganites are explainedby the effect of Bi3+ lone pairs.

2. EXPERIMENTAL DETAILS

The polycrystalline samples of Bi1−xSrxMnO3 (x= 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8) were prepared bya conventional solid-state reaction method. The stoichiometric composition of high purity (≥ 99.99 %) Bi2O3,SrCO3, and Mn2O3 powders were mixed, pelletized and calcined at 850 0C in air for 10 h. These mixtureswere thoroughly ground again, pelletized, and reheated at 1050 0C in air for 10 h. Then the final sinteringof the samples was done at 1250 0C for 24 h followed by slow cooling. Each sample was characterized by x-ray diffraction (Rigaku D-max)(Cu Ka). DC magnetic susceptibility measurements were performed in a DCfield of 0.2 T from 300 K to 600 K using SQUID magnetometer with high temperature oven (QUANTUMDESIGN MPMS). Temperature-dependent resistivity was measured using conventional 4-probe method. TEPwas measured using a low frequency AC steady-state method.16 The TEP holder was placed in a home-madefurnace and the sample was mounted on top of two alumina blocks using silver paint. Temperature gradientacross the sample was typically 1 ∼ 2 K monitored by Chromel-Constantan thermocouple. The contribution ofthe Au wires for the voltage drop was subtracted using the published data.17

3. RESULTS AND DISCUSSION

Figure 1 shows typical x-ray diffraction (XRD) patterns for x = 0.5 sample at room temperature. Latticeconstants are obtained by least square fitting. The XRD data show that the samples have a single phase withtetragonal lattice structure, which is consistent with the previous report.7 As Sr concentration increases, latticeconstants along a and b axis decrease linearly from 3.906 A for x = 0.5 to 3.854 A for x = 0.8 (see the inset ofFig. 1). In contrast, the lattice parameter c is almost constant for the doping range 0.5 ≤ x ≤ 0.8. This variationof lattice parameters of BSMO is different from that of La1−xSrxMnO3 (LSMO). In case of LSMO, parameters aand b decrease but parameter c increases with the increase of Sr concentration in the range between x = 0 and x= 0.175 where the material has orthorhombic structure. For x ≥ 0.175 it has rhombohedral structure and latticeparameter decreases.18 The difference from LSMO is due to additional distortion of BSMO. J. L. Garcia-Munozet al. have suggested that the Bi 6s2 lone pair can be an origin of the additional lattice distortion.11 T. Atouet al. projected a part of the structure along [301] and drew a scheme of Bi 6s2 lone pair which is polarizedto some Bi-O bond direction.6 In the series of BSMO the amount of Bi3+ decreases with the increase of Sr2+

contents, so that the local distortion due to Bi3+ lone pairs is expected to weaken. This result indicates thatBi3+ lone pairs are polarized along the diagonal direction of a-b plane, which causes the lattice parameter a andb to be smaller with the decrease of the number of lone pairs. Therefore, the Sr doping strongly affects not onlythe hole carrier density but also the strength of local lattice distortion.

The temperature dependence of an inverse magnetic susceptibility, 1/χ(T) of BSMO on heating process isshown in Fig. 2. In the case of x = 0.5, it has been discussed that 1/χ(T) at high temperatures follows Curie-Weiss law, which can be represented as 1/χ = (T - θ)/C where θ is Curie-Weiss temperature and C is Curieconstant.11, 15 With variation of Sr content, all samples show the same Curie-Weiss behavior, but with differentθ as shown in the inset of Fig. 2. This result shows that FM correlation exists in the paramagnetic (PM) state

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Figure 1. Typical powder x-ray diffraction patterns for polycrystalline samples Bi1−xSrxMnO3 (x = 0.5). Latticeparameter a (=b) and c as a function of Sr concentration are shown in the inset.

Figure 2. The temperature dependence of the inverse susceptibility with applied magnetic field of 2 kOe forBi1−xSrxMnO3 (0.5 ≤ x ≤ 0.8) above 300 K. Arrows indicate the TCO. Inset: Curie-Weiss temperature as a function ofSr concentration.

in the high temperature regime, which is reduced upon Sr doping. As temperature decreases, 1/χ(T) curvesdeviate from Curie-Weiss behavior and exhibit relative minimun at certain temperature. Such a behavior hasbeen found in Bi0.18Ca0.82MnO3. Based on the neutron diffraction Bao et al.19 suggested that the minima of1/χ(T) corresponds to the charge ordering temperature. For Bi0.5Sr0.5MnO3 this temperature is also quite closeto the reported TCO ∼ 475 K estimated from synchrotron x-ray powder diffraction.11 As indicated with arrows,TCO estimated from the relative minimum of 1/χ(T) is shifted to lower temperature with Sr contents.

Figure 3 (a) shows the temperature dependence of resistivity for x = 0.45, 0.5, 0.55, 0.6, 0.7, and 0.8.Resistivity of BSMO shows an insulating behavior in the whole temperature range. For x ≥ 0.5, resistivity at300 K becomes larger by 2 orders of magnitude with Sr doping (from ∼ 0.3 Ω cm for x = 0.5 to ∼ 16 Ω cm

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Figure 3. (a) The temperature dependence of resistivity for Bi1−xSrxMnO3. The hysteresis is shown in all samples butthis behavior is weakened with the increase of Sr concentration. (b) The temperature dependence of TEP. There areoffsets for distinguishing the data: 0 µV/K for x = 0.45, -5 µV/K for x = 0.5, -10 µV/K for x = 0.55, -15 µV/K for x= 0.6, -25 µV/K for x = 0.7, and -35 µV/K for x = 0.80. The arrows in Fig.(a) and (b) represent cooling and heatingprocess. Inset of Fig. (b): TEP value at 650 K.

for x = 0.8) in contrast to the case of x < 0.5.7 There is a change in the slope of log ρ near TCO, which isattributed to the suppression of carriers hopping due to charge ordering. Figure. 3 (b) shows the temperaturedependence of TEP (S(T)). At high temperatures, S(T) is almost constant or weakly dependent on temperature.As Sr concentration increases, the absolute value of S(T) increases as shown in the inset of Fig. 2 (b), whichis consistent with the decrease of hole carrier density with Sr doping. The slope of S(T ) changes near the TCO

and the absolute magnitude of S(T) is rapidly increased with the decrease of temperature. This result suggeststhat the eg electrons are trapped for a longer time and as a result, the effective number of carriers participatingin transport is significantly decreased as CO is developed.

The thermal hysteresis is shown in both TEP and resistivity measurement (see Fig. 3 (a) and 3 (b)). Belowthe CO transition, the phase is separated, which results in the irreversibility of domain wall motion on heatingand cooling. As Sr concentration decreases, the hysteretic behavior decreases. This result shows that the chargeordering transition of x = 0.45 sample is a first-order phase transition and this first-order transition changesto a second-order phase transition with the increase of Sr concentration. TCO is defined as the temperaturecorresponding to the intersection point between S/Smean = 0 ( or ρ/ρmean = 0)line and the line along theright side of bell-shape feature. Fig. 4 (a) shows that the hysteresis for x = 0.5 sample develops at the sametemperature, TCO (∼ 480 K) in both TEP (filled circle) and resistivity data (opened circle). It is noticeable

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Figure 4. (a) shows the TCO for x = 0.5 from hysteretic behavior in TEP and resistivity, where S (or ρ) is Scooling -Sheating (or ρheating - ρcooling)and Smean (or ρmean) is (Scooling + Sheating)/2 (or (ρheating + ρcooling)/2). Filled circle andopened circle are obtained from TEP and resistivity, respectively. (b) TCO of BSMO based on susceptibility, resistivity,and TEP results.

that the temperature region of hysteresis for BSMO is about 200 K for x = 0.45 and 0.5 samples. Martensitictransformation (MT) is one of the possible explanation for this extraordinary behavior. MT is the cooperativemotion of atoms resulting in a formation of different crystal structure within a parent crystal. Generally, whenMT occurs, large hysteresis is shown.20 V. Podzorov et al.21 reported that the transport properties of COmanganites are determined by martensitic nature of CO phase which is formed by nucleation and growth of COdomains. Magnetic susceptibility, resistivity and TEP data indicate that CO transition does not occur abruptly,but slowly and CO state is maintained in a wide temperature region. It implies that CO domains are createdand CO phase is formed slowly by growth of them, which corresponds to the process of MT.

Figure 4 (b) presents the Sr doping dependence of TCO from the inverse susceptibility, resistivity, and TEPresults. With developing CO state, eg electrons are localized, which causes resistivity and the absolute value ofTEP to increase. Note that CO phase persists over the wide doping range and temperature and TCO decreaseswith the increase of Sr concentration. It shows that polarized Bi3+ lone pairs play an important role in theformation of CO state. Mn - O - Mn bond in Bi-based compound is more bent than that in La-based compound.11

It would be due to the polarized lone pairs which can produce a hybridization between Bi-6s orbitals and O-2porbitals in Mn - O - Mn bond. It suppresses eg electron density. As a result, CO state is maintained up to hightemperature.

The inset of Fig. 5 shows the additional measurement of 1/χ(T ) to the low temperature region. Note that

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Figure 5. Sr concentration dependence of CL/CH . Opened circle and filled circles represent theoretical and experimentalresult, respectively. The inset shows the two PM regions for x = 0.7 sample. Grey lines show the linear behavior of1/χ(T).

there are two paramagnetic regions which follow Curie-Weiss law indicated by two grey lines. Daoud-Aladineet al. suggested the ZPO model which predicts the formation and ordering of localized ZP below TCO. Also,It has been proposed that there is another PM phase whose effective magnetic moment (µeff ) is larger thanthat in the high temperature region (T > TCO).9 In order to confirm this behavior, we plotted CL/CH (filledcircle) as shown in Fig. 5, where CL and CH represent Curie constant in low temperature region (LT) and hightemperature region (HT), respectively. It indicates the enhancement of µeff in LT comparing to that in HT,which is consistent with ZPO model. And we calculated µeff/Mn of two temperature regions using the previousreport22 and obtained CL/CH for each sample (opened circle) with well-known equation as

C =Nµ2

eff

3kB, (1)

where N is the number of Mn atoms and kB is Boltzmann constant. Our result is similar to the theoreticalstudy except x = 0.6 sample. Therefore, we suggest that CO behavior of BSMO series can be described by theZPO model. The behavior for x = 0.6 sample is under investigation.

4. SUMMARY

In summary, we have reported the study of the CO state of the series of BSMO samples through TEP, resistivity,and magnetic susceptibility measurement up to 700 K. Resistivity shows an insulating behavior and the slopeof resistivity data is changed at TCO. TEP is almost independent of temperature at high temperatures (T >TCO) and starts to decrease near TCO. TCO was estimated by the minimum value in inverse susceptibility andby the onset of hysteresis shown in both TEP and resistivity. TCO decreases with Sr concentration. Such abehavior and the transport characteristics of BSMO are explained by the polarized Bi3+ lone pair effect. Theinverse susceptibility shows two PM states. Especially, the effective magnetic moment of PM phase in the lowtemperature region is larger than that in the high temperature region. It indicates CO transition of BSMO canbe explained by ZPO model.

ACKNOWLEDGMENTS

This work is supported by the National Research Laboratory program under Contact No. M1-0104-00-0023,Ministry of Science and Technology ( MOST ), Korea. The work at PNU is supported by University IT Research

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Center Project. The work at KBSI is supported by NRL fund of KBSI.

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