Phase transitions in La0.5Sr0.5FeO3−δ investigated by mechanical spectrum

Solid State Communications 152 (2012) 1252–1255

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Phase transitions in La0.5Sr0.5FeO3  d investigated by mechanical spectrum X.N. Ying n, L. Zhang National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 February 2012 Received in revised form 5 April 2012 Accepted 11 April 2012 by R. Merlin Available online 19 April 2012

The mechanical spectrum measurement was performed in ceramic La0.5Sr0.5FeO3  d from liquid nitrogen temperature to room temperature at kilohertz frequencies. From temperature dependent reduced modulus, a kink (corresponding temperature labeled as TM) was observed which evidenced a phase transition by the mechanical spectrum at two flexural resonance frequencies. This elastic manifested phase transition is a charge disproportionation transition. Around 170 K, an internal friction peak (labeled as P1) was observed accompanied with a large modulus hardening with the decrease in temperature. Two mechanisms are proposed for P1 peak, one is elastic manifestation of magnetic freezing, and the other is the ordering or freezing of oxygen vacancies. & 2012 Elsevier Ltd. All rights reserved.

Keywords: A. La1  xSrxFeO3  d D. Phase transitions E. Ultrasonics

1. Introduction In recent years, perovskite La1 xSrxFeO3 d (x¼0–1) series of samples have been extensively investigated due to the fundamental metal–insulator transition [1–7] and the application of oxygen separation membranes [8–10]. Metal–insulator transitions in oxides have been the matter of continuous studies, particularly in relation to charge ordering (CO) since the discovery of the Verwey transition in magnetite [11]. In La1 xSrxFeO3 d, a charge disproportionation transition was often observed in which a charge state is separated into two different charge states as Fe3 þ and Fe5 þ [1]. Furthermore, the metal–insulator transition in La1/3Sr2/3FeO3 d was ascribed to an order–disorder transition of the charge below charge disproportionation transition [2–4] with the charge ordering sequence of Fe3 þ Fe3 þ Fe5 þ Fe3 þ Fe3 þ Fe5 þ along the body diagonal /111S direction. Further study shows that the occurrence of above charge ordering pattern in La1/3Sr2/3FeO3 d mainly results from the minimization of the magnetic exchange energy [5]. However, x-ray absorption spectra did not agree with the presence of Fe5 þ below the metal–insulator transition [6]. And recent resonant x-ray scattering experiments showed the evidence for charge-density-wave nature of the charge-ordered phase in La1/3Sr2/3FeO3 d and charge disproportionation was not significant [7]. Therefore, the behavior of the charge and spin in La1 xSrxFeO3 d needs more systematic investigations. Usually, charge lattice coupling exists in transition metal oxides. Actually, a structural modulation measured by transmission electron microscopy (TEM) was used to infer the charge ordering pattern in

n

Corresponding author. Tel.: þ86 25 83593940; fax: þ86 25 83595535. E-mail addresses: [email protected], [email protected] (X.N. Ying).

0038-1098/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2012.04.017

La1/3Sr2/3FeO3 d [3]. Mechanical spectrum is a well-known probe for the study of phase transitions and relaxations of micro-units such as point defects, dislocations, grain boundaries, and domain walls in solids [12–22]. An elastic indication of the charge ordering in La1/3Sr2/3FeO3 d was observed in the ultrasound measurement [19,20]. Recently, mechanical spectrum was used to study the nature of the metal–insulator transition in Nd1 xEuxNiO3 [21]. In this work, mechanical spectrum was measured with a focus on the charge disproportionation transition in La0.5Sr0.5FeO3 d. In a similar La0.5Ca0.5FeO3 d sample, a cluster-glass-like state resulting from competition between antiferromagnetic interaction of Fe3 þ –Fe3 þ and ferromagnetic interaction of Fe3 þ –Fe5 þ was detected below the charge disproportionation transition [23,24]. The elastic manifestation of this magnetic glass transition in La0.5Sr0.5FeO3 d was also addressed in this work.

2. Experimental La0.5Sr0.5FeO3 d polycrystalline sample was prepared using the standard solid-state reaction techniques. The starting materials, high pure La2O3, Sr2CO3 and Fe2O3 powders, were weighed out at the nominal composition and thoroughly mixed. The powder mixtures were calcined at 1100 1C for 24 h in air. The product was ground and pressed into rectangular bars. The bars were calcined at 1100 1C for another 24 h in air. The samples were examined to be single phased by powder x-ray diffraction using Cu Ka radiation. Resistance measurement below room temperature was performed on the specimen of a rectangular shape by the conventional four-probe technique using heat-treatment-type silver paint as an electrode. The experiments were conducted on cooling at a rate of about 2.0 K/min and on heating at about 1.5 K/min.

X.N. Ying, L. Zhang / Solid State Communications 152 (2012) 1252–1255

The mechanical spectrum was measured for a rectangular bar (40  4  0.3 mm3) in the free-free flexural vibration mode from liquid nitrogen temperature to room temperature at the resonance frequency about several kilohertz. A combination of electrostatic drive and capacitor microphone detection was used with the details described in Ref. [18]. The mechanical spectrum was measured by the frequency bandwidth method in two-node and three-node modes on heating. During experiments, the resonance frequency and internal friction in the first two flexural vibration modes were measured alternatively in the same heating route. This two-vibration-mode measurement is attractive for the study of the frequency dependence of the internal friction. The experiments were conducted on cooling at a rate of about 2.0 K/min and on heating at about 1.0 K/min.

3. Results and discussions Fig. 1 shows the room temperature powder x-ray diffraction pattern of our La0.5Sr0.5FeO3  d sample. All diffraction peaks can be indexed with a perovskite-typed rhombohedral structure (the space group R3c). The best fitting lattice parameters are a ¼5.49 ˚ and c ¼13.43 A. Fig. 2 shows the resistance as a function of temperature below room temperature on cooling and subsequent heating of La0.5Sr0.5

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FeO3 d sample. On cooling, the resistance shows gradual semiconductorlike increase down to TC, and then undergoes a change of the slope of the resistance versus temperature, followed by a rapid increase below TC. For a better indication, the variation of d(lg R)/ d(1/T) versus temperature is plotted in the inset of Fig. 2. The characteristic transition temperature shows a clear hysteresis on cooling and heating, which indicates a first order phase transition. This transition corresponds to the charge disproportionation transition with Tc E198 K on heating in our La0.5Sr0.5FeO3 d sample. Fig. 3 represents the temperature dependent reduced Young’s modulus (Fig. 3a,

DY Y0

¼

YY 0 Y0

pf

2

f 0 2 f0

2

, f and f0 are the resonance

frequency and the one at a fixed temperature) and internal friction Q  1 (Fig. 3b) measured by the frequency bandwidth method in two-node and three-node modes in the La0.5Sr0.5FeO3  d sample on heating. The resonance frequencies at 133 K are 1474.6 and 3810.9 Hz in two-node and three-node modes, respectively. A kink in the reduced modulus is observed around 210 K (corresponding temperature labeled as TM). It does not show a clear dependence on the measuring frequency, which evidences phase transition nature. The transition at TM is thought to be an elastic indication of the charge disproportionation transition. In our experiments, no clear anomaly is observed in the temperature dependent internal friction at TM. In the temperature dependent internal friction in Fig. 3 measured by the frequency bandwidth method, two peaks can be well observed around 170 and 250 K (labeled as P1 and P2, respectively). With increasing temperature, P1 peak is accompanied with a large decrease of the reduced modulus. P1 peak shows a slight dependence on the measured frequency. The peak temperatures are 167.2 K in two-node mode and 169.3 K in three-node mode. As mentioned in the experimental section, internal friction was measured alternatively in the first two flexural vibration modes in the same heating route. It is expected that the shift of only 2.1 K of P1 peak in Fig. 3 shows an intrinsic feature of the frequency dependence and is not due to the experimental error. At P1 peak temperature, relaxation time t is satisfied with the following equation:

ot ¼ 1

ð1Þ

Fig. 1. Room temperature powder x-ray diffraction pattern of La0.5Sr0.5FeO3  d.

Fig. 2. (Color online) The resistance versus temperature on cooling and subsequent heating of the La0.5Sr0.5FeO3  d sample. In the inset, d(lg R)/d(1/T) versus temperature.

Fig. 3. (Color online) (a) Temperature dependent reduced Young’s modulus 1 YY 133 K DY measured by the frequency bandwidth Y 133 K ¼ Y 133 K and (b) internal friction Q method in the heating process in La0.5Sr0.5FeO3  d.

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where o ¼2pf is the resonance circle frequency. Usually, relaxation time is assumed to follow an Arrhenius law

t ¼ t0 expðE=kTÞ

ð2Þ

where E is the activation energy, t0 the relaxation time constant, k Boltzmann constant. The activation energy can be calculated from the shift of peak temperature with the variation of the resonance frequency E lnðf 2 =f 1 Þ ¼ k 1=T 1 1=T 2

ð3Þ

where f1 and T1, f2 and T2 are the resonance frequency and peak temperature in the first and second flexural vibration modes, respectively. A very large activated energy, 1.12 eV and an unreasonable small relaxation time constant t0 ¼2.00  10  38 s are obtained. Based on above discussions, the Arrhenius law can be safely excluded for P1 peak. At this moment, two possible mechanisms are proposed for P1 peak. Based on the frequency dependence, P1 peak might be ascribed to a freezing transition. The relaxation time of glass transition like freezing usually takes a Vogel–Fulcher relationship on temperature. Three parameters in Vogel–Fulcher law cannot be estimated here due to the fact that the mechanical spectrum was measured only at two frequencies. In La0.5Ca0.5FeO3  d sample, a glassy magnetic behavior was observed by the dc magnetization and ac susceptibility measurements [23,24]. The origin of the glassy magnetic behavior was argued to be from the competition between Fe3 þ –Fe3 þ antiferromagnetic interaction and Fe3 þ –Fe5 þ ferromagnetic interaction below the charge disproportionation transition where Fe4 þ ions were separated into to Fe3 þ and Fe5 þ ions. The freezing transition revealed by P1 peak may be an elastic manifestation of this magnetic glass transition. In the past, an internal friction peak correlated with the spin-glass transition was reported in Fe59Ni21Cr20 [25]. Below the charge disproportionation transition, a charge freezing transition seems also possible to take place and leads to P1 peak. However, following discussions are not in favor of regarding P1 peak as an elastic manifestation of the charge freezing transition. In the previous ultrasonic study of charge glass Sm3Se4, ultrasonic dispersion related to the charge fluctuation was observed and the activation energy of the charge fluctuation is calculated to be 0.14 eV by assuming an Arrhenius law [26]. At the same time, dielectric dispersion was observed in charge glass Sm3Se4 [27] while it is absent in our sample (dielectric measurement in our sample can be performed in the insulate state below the charge disproportionation transition and the data is not shown here). In addition, no Vogel–Fulcher relationship of the relaxation time on temperature was reported in the charge glass [28]. Another possible mechanism of P1 peak is some kind transition of the oxygen vacancies. Usually, oxygen vacancies exist in the perovskite oxides [29]. From this aspect, P1 peak might be due to the ordering of the oxygen vacancies which seems consistent with the observed large modulus change around P1 peak. Considering the frequency dependence of P1 peak, P1 peak might also relate to a freezing transition of the oxygen vacancies. In YBa2Cu3O7  d superconductor, a Vogel–Fulcher relationship of the relaxation time was observed in an internal friction peak around 220 K and it was suggested to be the freezing of the oxygen vacancies [30]. Further works are needed to judge above two proposals. Above the charge disproportionation transition, P2 internal friction peak is observed around 250 K as shown in Fig. 3. P2 peak is a low intensity and very broad peak. No clear frequency dependence of peak temperature can be discerned. In the following discussions, two approaches are applied to explain P2 peak. Firstly, P2 peak is suggested to be an indication of the charge

disproportionation fluctuation. Towards TM, the reduced modulus shows a continuous softening from the high temperature side. It can be ascribed to the elastic coupling with the order parameter of the charge disproportionation state. Based upon the charge lattice coupling, P2 peak might be an indication of slow dynamic of the charge disproportionation fluctuation. Secondly, P2 peak possibly relates to the Jahn–Teller distortion. In the ultrasonic study of La1  xSrxFeO3  d, the modulus softening was observed which was attributed to the Jahn–Teller distortion of Fe4 þ ions especially with Sr content around 2/3 [20,31]. However, the static Jahn–Teller distortion of Fe4 þ (d4) ion is expected to be small above the charge disproportionation transition otherwise the sample would become insulate [32]. Below the charge disproportionation transition, no Fe4 þ ion exists. The only left is the dynamic Jahn–Teller distortion of Fe4 þ ions above the charge disproportionation transition. If P2 peak in the mechanical spectrum at kilohertz frequency is caused by the dynamic Jahn–Teller distortion, the corresponded relaxation time t ¼ 1=2pf can be estimated to be about 10  4 s around 250 K. Considering no clear frequency dependence of the peak temperature was observed in our experiments, P2 peak could be related to a fluctuation of a phase transition or freezing transition. A lower temperature phase transition or freezing related to the Jahn–Teller distortion would be expected. This transition was not observed. Certainly, we can exclude the possibility that this transition is hindered by the charge disproportionation transition at 198 K. Much more works are needed to judge the possible Jahn–Teller distortion in La1  xSrxFeO3  d.

4. Summary The charge disproportionation transition is indicated by a kink in the temperature dependent reduced modulus. Around 170 K, an internal friction peak was observed at kilohertz frequencies. This peak is discussed by two proposals, an elastic manifestation of magnetic freezing and the ordering/freezing of the oxygen vacancies.

Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities, 973 Project of MOST (Grant no. 2009CB929501), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] [2] [3] [4]

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