3FeO3

Physica B 434 (2014) 118–121

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Charge-ordering correlated elastic anomalies in Nd1/3Sr2/3FeO3 Kong Hui a,n, Tong Lianhai a, Zhu Changfei b a

School of Metallurgy and Resources, Anhui University of Technology, Maanshan, Anhui 243002, PR China Laboratory of Advanced Functional Materials and Devices, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China b

art ic l e i nf o

a b s t r a c t

Article history: Received 11 September 2013 Accepted 4 November 2013 Available online 12 November 2013

The resistivity and longitudinal ultrasonic velocity have been measured as a function of temperature from 50 K to 300 K in the single-phase polycrystalline Nd1/3Sr2/3FeO3. At about 167 K (TCO), the resistivity shows sharp increase, corresponding to the charge ordering transition (CO). The velocity softens conspicuously as the temperature decreasing from 210 K and then stiffens dramatically below TCO. This feature is similar in character to that of charge-ordered La1/3Sr2/3FeO3, and implies strong electron– phonon coupling. This unusual elastic stiffening can be fitted by the mean-field theory, which hints the presence of the Jahn–Teller effect originating from the Fe4 þ . Below TCO, the different ultrasonic behaviors between Nd1/3Sr2/3FeO3 and La1/3Sr2/3FeO3 have been observed. The expected ultrasonic anomalies corresponding to the breathing type distortion are absent in Nd1/3Sr2/3FeO3. This feature is attributed to the unique feature of charge ordering transition in perovskite-type iron oxides. & 2013 Elsevier B.V. All rights reserved.

Keywords: Jahn–Teller effect Transition metal oxides Ultrasonic

1. Introduction Transition metal oxides R1  xAxMO3 (R: trivalent ions, A: divalent alkaline-earth ions, M: Mn, Fe, Co) have become a focus of recent studies due to their special structural, magnetic, and electronic properties as well as the potential applications [1,2]. Among them, iron oxides have attracted much attention due to their interesting features originating from the interplay among different valence states of iron. In R1/3Sr2/3FeO3 (R¼ La, Pr, Nd, Gd) [3,4], charge ordering (CO) accompanies both antiferromagnetic (AFM) transition and charge disproportionation (CD) of 2Fe4 þ -Fe3 þ þ Fe5 þ . When the R-site ionic radius decreases (from La to Gd), the charge ordering transition temperature TCO shifts to lower temperature, and finally the CO transition disappears for R ¼Gd. From optical properties studies [4], Park et al. indicated that this evolution is due to the decrease of the electron bandwidth, which is caused by the increase of the rhombohedral lattice distortion with decreasing the R-site ionic radius. In R1/3Sr2/3FeO3 system (R¼ La, Pr, Nd, Gd), the most novel feature is the high valence state of Fe:Fe4 þ . According to the softX-ray absorption spectroscopy studies [5], the electronic structure of Fe4 þ might be a mixture of 3d4 (high spin) and 3d5L (L denotes a hole in the O 2p band). For 3d4 (high spin), it is isoelectronic with Mn3 þ . Since the Jahn–Teller effect of Mn3 þ in charge ordered

n

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0921-4526/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2013.11.002

manganites is widely accepted [6–8], similar Jahn–Teller effect is expected in R1/3Sr2/3FeO3 system. However, the neutron powder diffraction did not observe any structural changes accompanying the CO transition [9]. This point was also supported by the unrestricted Hartree–Fock band-structure calculations [10]. On the contrary, the superlattice spots related to CO state have been detected by the transmission electron microscopy [11], which is explained by the breathing-type distortion of Fe–O octahedron. In 2005, Ghosh et al. indicates that the lattice distortions are present only above TCO from the temperature-dependent micro-Raman study [12]. These contradictory results arise from the unstability of Fe4 þ . The coexistence of 3d4 (high spin) and 3d5L weakens the Jahn–Teller effect of Fe4 þ , which make its distinction and confirmation more difficult. As a sensitive tool, ultrasonic technique has proven to be successful for studying the Jahn–Teller effect in charge ordered manganites [7,8]. Thus, to explore the Jahn–Teller effect of Fe4 þ , ultrasonic characterization has been taken in R1/3Sr2/3FeO3 system [13,14]. For La1/3Sr2/3FeO3, two anomalies in velocity have been observed above and below TCO. The former is attributed to the electron–phonon coupling arising from the Jahn–Teller effect of Fe4 þ , and the latter is interpreted by the breathing-type distortion of Fe–O octahedron via the charge disproportionation. For Gd1/3 Sr2/3FeO3 with no CO transition, the anomaly in V is small, and may be due to the antiferromagnetic spin fluctuations. However, the study on ultrasonic property of Nd1/3Sr2/3FeO3 is still lacking though its feature is special. In this sample, the CO transition still exists, while the breathing type lattice distortion is weaker than that in La1/3Sr2/3FeO3. Thus, Nd1/3Sr2/3FeO3 is a good specimen to confirm the Jahn–Teller effect in R1/3Sr2/3FeO3 system. So in this

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paper, we present the ultrasonic velocity study on charge-ordered Nd1/3Sr2/3FeO3 in order to clarify the Jahn–Teller effect of Fe4 þ around CO transition.

2. Experimental procedure The polycrystalline sample of Nd1/3Sr2/3FeO3 was synthesized through a solid state reaction method. A stoichiometric mixture of high purity starting materials Fe2O3, SrCO3 and Nd2O3 powders were well mixed and calcinated at 1100 1C, 1150 1C, 1150 1C in air for 15 h. The final obtained powder was pressed into pellets at 300 MPa and then sintered at 1180 1C in air for 20 h, and cooled to room temperature at a rate of 1 1C min  1. The crystal structure was determined by powder X-ray diffraction on a Japan Rigaku MAX-RD powder X-ray diffractometer with Cu Kα radiation (λ ¼1.5418 Å) at room temperature. The resistivity was measured as a function of temperature by the standard fourprobe technique. The longitudinal ultrasonic velocity measurement was performed on the Matec-7700 oscillator/receiver series with a conventional pulsed echo technique. The experiment was taken in a closed-cycle refrigerator during the warm-up from 50 to 300 K at the rate of about 0.25 K/min. The sound velocity V was found through the following relationship: V ¼ 2L=t ¼ 2Lf

Fig. 2. Temperature dependence of resistivity for Nd1/3Sr2/3FeO3. The inset is the logarithmic derivative, d(lnρ)/d(T  1), of the resistivity with temperature.

ð1Þ

where L is the thickness of the specimen, t is the sound velocity transit time determined from the distances between corresponding cycles of two successive echoes, and f ¼ ð1=tÞ is the trigger frequency displayed on a Sabtronics model 8000C frequency counter. The relative change of the sound velocity ΔV/V in Fig. 3 was defined according to the following equation: ΔV V  V min ¼ V V min

ð2Þ

where Vmin (m s  1) is the minimum longitudinal ultrasonic velocity from 50 to 273 K.

3. Experimental results and discussion The X-ray diffraction pattern of Nd1/3Sr2/3FeO3 is shown in Fig. 1. It is of single phase without detectable secondary phase. The diffraction peaks are sharp and can be indexed in the pseudo-cubic structure. Fig. 2 shows the temperature dependence of resistivity for Nd1/3 Sr2/3FeO3. It can be seen that the resistivity shows semiconductorlike transport behavior, and increases steeply around 167 K

Fig. 1. XRD pattern of Nd1/3Sr2/3FeO3 at room temperature.

Fig. 3. Temperature dependence of the ultrasonic velocity for Nd1/3Sr2/3FeO3. The inset (a) is the temperature dependence of the ultrasonic velocity for La1/3Sr2/3FeO3. The inset (b) is the temperature dependence of the Cl(T) for Nd1/3Sr2/3FeO3 below TCO. (Open symbols are experimental data, solid line is the results calculated using Eq. (3) below TCO.)

(charge-ordering transition temperature TCO, defined as the corresponding peak temperature of d(lnρ)/d(T  1) vs. T curve) [8]. Fig. 3 displays the temperature dependence of the longitudinal ultrasonic velocity (V) for Nd1/3Sr2/3FeO3 at a frequency of 10 MHz. It can be seen that V smoothly softens as the temperature decreases from 210 K and substantially increases below TCO. It should be noticed that this kind of velocity anomaly is observed around TCO. The simultaneous occurrence implies strong electron– phonon coupling. In fact, similar phenomenon was observed in charge ordered manganites [7,8] and La1/3Sr2/3FeO3 [13,14]. For comparison, the temperature dependence of V for La1/3Sr2/3FeO3 at a frequency of 10 MHz is shown in the inset (a) of Fig. 3. It is known that in normal state, the ultrasonic velocity increases with decreasing temperature. The typical relative change in sound velocity caused by AFM spin fluctuations is of the order 0.1% [15]. In Nd1/3Sr2/3FeO3, the velocity increase is more than 4%, which cannot be explained only based on spin–phonon coupling near a conventional AFM phase transition. In La1/3Sr2/3FeO3, the ultrasonic anomalies around TCO have been well discussed and attributed to the Jahn–Teller effect of Fe4 þ [13,14]. For Fe4 þ , its electronic structure is unique due to the unconventionally high valence. Abbate indicated that Fe4 þ might be a mixture of 3d4 and 3d5L by soft-X-ray absorption spectroscopy, where L denotes a hole in the O 2p band [5]. Moreover, Bocquet et al. suggested that d4 is a high spin state with one electron filling the two-fold degenerate eg band [16]. Thus, when

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charge ordering transition occurs, the high spin Fe4 þ (3d4) may be the Jahn–Teller active in an octahedral crystal field. Since Fe4 þ also exists in Nd1/3Sr2/3FeO3, similar ultrasonic anomalies around TCO may also be related to the Jahn–Teller effect. To verify this conclusion, two theoretical results are applied to Nd1/3Sr2/3FeO3. First, based on the Hamiltonian of small Jahn–Teller polarons with strong electron–phonon coupling, Min et al. found that in manganites, the CO interaction induces the softening of V above TCO and the hardening below TCO [6]. This conclusion has been verified by the experimental results in charge-ordered manganites [7,8], and is qualitatively similar to our observation. Second, according to the Jahn–Teller theory, which describes the coupling of the Jahn–Teller ions to the lattice distortion under the mean field approximation, the relationship between the long2 itudinal modulus (ClðTÞ ¼ ρVl [17], ρ is the mass density) and temperature T (To TCO) can be written as [18]      CðTÞ λ þ μ λ þμ Δ λ λ Δ 2 2 ¼ 1 þ tanh 1 þ tanh C0 kB T kB T kB T kB T kB T kB T ð3Þ where λ is the phonon exchange constant, μ is a measure of the strength of the coupling of the ions to the uniform strain, and Δ represents the effect of the ion–strain coupling, which can be calculated according to the following equation:   Δ Δ ¼ kB T CO tanh ð4Þ kB T Eq. (3) can be used to fit the experimental data below TCO. In fact, Eq. (3) has proven to be successful for studying the Jahn– Teller effect in charge ordered manganites, and has been applied to quantitatively characterize the Jahn–Teller effect. The results of fitting are consistent with other features, and can explain the evolution of CO phenomenon. For example, Zheng et al. pointed out that in La0.25Ca0.75Mn1  xCrxO3 [19], the Jahn–Teller energy, which derives from Eq. (3), decreases with increasing Cr content. This leads to the decreasing of TCO. Moreover, this equation has been applied to confirm the presence of short-range charge ordering state in similar perovskite-type iron oxides La2/3Sr1/3 FeO3 [20] Thus, the validity of upper equations has gained acceptance. In the inset (b) of Fig. 3, the open symbols are the experimental data and the solid line is the theoretical result (the values of λ, μ, C0/Cmin are 5.9 meV, 1.05 meV, 1.08, respectively). The good agreement between experiment and theory indicates that the Jahn– Teller effect indeed exists in Nd1/3Sr2/3FeO3 below TCO. The different behaviors in V between La1/3Sr2/3FeO3 and Nd1/3 Sr2/3FeO3 are observed when the temperature decreases below TCO. V of R ¼La shows another unexpected softening below TCO. This softening is attributed to the breathing type distortion of Fe3 þ and Fe5 þ , and be related the charge disproportionation [13]. This kind of distortion arises from the size difference between Fe3 þ and Fe5 þ , and is proven by the measurements of transmission electron microscopy [11] and optical spectroscopy [4]. The charge disproportionation phenomenon also exists in Nd1/3Sr2/3FeO3. However, the expected ultrasonic anomalies corresponding to the breathing type distortion are absent, and the elastic stiffening below TCO can be well fitted by the Jahn–Teller theory. These behaviors are probably due to the unique feature of charge ordering transition in R1/3Sr2/3FeO3. When the R-site ionic radius decreases (from La to Nd), the bond angle (∠Fe–O–Fe) decreases from 1801, which weakens the p–d hybridization of the middle O 2p bands and the neighboring Fe5 þ and makes the Fe5 þ unstable [4]. This means that the CD transition is suppressed, and the resulting breathing type distortion is weaker in Nd sample than that in La sample, which is proven through measurements of optical spectroscopy [4]. Moreover, the

suppression of CD transition hints that more Fe4 þ ions exist below TCO, thus the Jahn–Teller effect is stronger in Nd sample than that in La sample. These effects lead to the fact that in Nd1/3Sr2/3FeO3, the Jahn–Teller theory is applicable to fit the elastic stiffening below TCO, and the expected ultrasonic anomalies corresponding to the breathing type distortion are absent. It should be mentioned that due to the speciality of Fe4 þ , the Jahn–Teller effect in R1/3Sr2/3FeO3 system (R¼La, Pr, Nd, Gd) is still in discussion. Different conclusions have been drawn through different experimental and theoretical tools [9–12]. In 2001, the superlattice reflections in Nd1/3Sr2/3FeO3 have been observed by neutron diffraction technique [21], and Kajimoto et.al pointed out that q1/3 reflections originate from the charge ordering of … Fe3 þ Fe3 þ Fe5 þ … and accompanying local lattice distortion. This conclusion is different from our ultrasonic results. To further clarify the nature of the CO transition in perovskite-type iron oxides, more experiments are needed, and the ultrasonic data from single crystal with different directions would be more valuable.

4. Conclusion In summary, we have studied the behaviors of longitudinal ultrasonic velocity and resistivity in polycrystalline Nd1/3Sr2/3FeO3 as a function of temperature. The resistivity shows semiconductorlike transport behavior, and exhibits an obvious slope change at about 167 K, corresponding to the charge ordering transition. Upon cooling down from 210 K, an obvious softening in V above TCO and dramatic stiffening below TCO are observed. This feature is similar in character to that of charge-ordered La1/3Sr2/3FeO3, and gives clear evidence for electron–phonon coupling. Moreover, this unusual elastic stiffening below TCO can be described well by the meanfield theory, which hints that this coupling originates from the Jahn– Teller effect of Fe4 þ . Below TCO, the expected ultrasonic anomalies corresponding to the breathing type distortion, which are found in La1/3Sr2/3FeO3, are absent. This behavior is due to the unique feature of charge ordering transition in perovskite-type iron oxides.

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