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Hội nghị Vật lý chất rắn và Khoa học vật liệu toàn quốc lần thứ 6 (SPMS-2009)-Đà nẵng 8-10/11/2009

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THE MAGNETIC ANOMALY OFTHE DOUBLE RARE-EARTH ELEMENTS PEROVSKITE

COMPOUNDS(La1-xYx )FeO3 AND (La1-yNdy)FeO3

Dang Le Minh1, Nguyen Minh Tuan1 Nguyen Thi Thuy2 Nguyen Phu Thuy3, Nguyen Thanh Trung4

1 Hanoi University of Science,VNU, Hanoi2 Hue Pedagogical University-Hue University

3 College of Technology, VNU, Hanoi and ITIMS, HUT, Hanoi4 Delft University – Netherland

Abstract. The double rare-earth elements perovskite compounds of (La1-

xYx)FeO3 and (La1-yNdy)FeO3 (x, y = 0.00; 0.15; 0.35; 0.55; 1.00 ) were prepared by ceramic method. Magnetic measurements have shown that the samples are ferromagnetic. The magnetic hysteresis loops of two series of samples measured from 5K to 300K have the anomalous shapes. It can be suggested that the magnetic anisotropy and temperature influence on the magnetization process are the reasons of the observed phenomena in these samples.Keywords : Magnetic anomaly, Double rare-earth elements, Magnetization, Ferromagnetic, Hysteresis loop.

INTRODUCTION

Perovskite-type oxides with the general formula ABO3 , where A can be an alkali, alkaline earth, or a lanthanide metal and B can be a transition metal, play an important role in the preparation of catalysts for specific applications. Their composition can be varied over a wide range by partial substitution of cation in position A and B yielding compound of the formula (AxA’

1-x)(ByB’1-y)O3. Particularly, lanthanium transition metal perovskite

oxides, which are denoted as the chemical formula LaMBO3 (MB

= 3d transition metal), have been attracting the scientific interests due to substitution effects of various impurity ions into the perovskite A and B sites.


The LaFeO3-based oxides is characterized as an orthorhombic perovskite structure. They are important for cathode and interconnection materials for solid oxide fuel cell (SOFCs), gas sensors, and humidity sensors [1,2,3,4].

In our previous report [5], the study of the properties of LaFeO3 has shown that it is a semiconductor with weak ferromagnetic behavior. In this work, the double rare-earth elements perovskite compounds (La1-xYx)FeO3

and (La1-yNdy)FeO3 (x, y = 0, 0.15, 0.35, 0.55, 1.00) were prepared and the influence of second rare-earth elements in the A-site on the magnetic properties of the samples has been investigated.

EXPERIMENTAL

(La1-xYx)FeO3 and (La1-yNdy)FeO3 (x, y = 0, 0.15, 0.35, 0.55, 1.00 ) were prepared using the conventional solid-state synthesis technique. La2O3, Nd2O3 ,Y2O3 and Fe2O3 powders were used as starting materials. The mixed powder was presintered in air at 10000C for 10h. The presintered powder was pressed into pellets which were finally sintered at 12300C for 5h. Powder X-ray diffraction (XRD) with Cu-Kα radiation was used to measure these pellets and investigate their crystal structures. Magnetic measurements were carried out using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design) under the applied magnetic field of 5T in the temperature range between 5K and 300K.

RESULTS AND DISCUSSION

The X-ray diffraction analysis of (La1-xYx)FeO3 and (La1-yNdy)FeO3

indicates that all samples are single phase of the orthorhombic structure. The lattice parameters are listed in Table 1. The requirement for stability of this orthorferrite is that the Goldschmidt tolerance factor (t) should be of nearly unity, and (t) is defined via RA+ R0 = t

Where RA, RB and R0 are radii of ions at A , B sites and oxygen ion, respectively.

Table 1. The lattice parameters (a, b, c, V), tolerance factor (t) , and RA, RB

radii in (La1-xYx)FeO3


x Compound a, A0 b, A0 c, A0 V,(A0)3 t RA

A0

RB

A0

0 LaFeO3 5.56 5.55 7.86 242.24 0.921 1.36 0.6450.15 La0.85Y0.15FeO3 5.54 5.57 7.84 241.93 0.913 1.33 0.6450.35 La0.65Y0.35FeO3 5.55 5.52 7.84 240.19 0.903 1.3 0.6450.55 La0.45Y0.55FeO3 5.55 5.45 7.83 236.84 0.892 1.27 0.645

1 YFeO3 5.28 5.59 7.60 224.32 0.870 1.19 0.645

Table 2. The lattice parameters (a, b, c, V), tolerance factor (t) , and RA, RB

radii in (La1-yNdy)FeO3

y Compound a, A0 b, A0 c, A0 V,(A0)3 t RA

A0

RB

A0

0 LaFeO3 5.56 5.55 7.85 242.24 0.921 1.36 0.6450.15 La0.85Nd0.15FeO3 5.55 5.54 7.86 241.67 0.917 1.35 0.6450.35 La0.65Nd0.35FeO3 5.54 5.55 7.84 241.06 0.911 1.33 0.6450.55 La0.45Nd0.55FeO3 5.53 5.55 7.81 239.70 0.906 1.31 0.645

1 NdFeO3 5.58 5.45 7.76 235.99 0.894 1.27 0.645

It can be seen from Tab.1 that the volume of the unit cell decreases with increasing the Y and Ni content because the radius of Nd3+(0.109Ǻ) and Y3+

(0.097 Ǻ) are both smaller than that of La3+(0.114Ǻ).However the structure distorsion caused by Nd substitution is much less than that caused by Y substitution. This fact should strongly influence on the magnetic property of the samples.

The M(H) curves of two series of (La1-xYx)FeO3 and (La1-yNdy)FeO3

obtained from SQUID magnetometer have shown in Figure 1, 2 and 3. It can be noted that both of them are anomalous in different ways.

Concerning the hysteresis loops of the (La1-yNdy)FeO3 samples (Fig.1), it can be seen that while the magnetization curve measured at 5K raises strongly with increasing of y from y = 0.15 to y = 0.55 (Fig.1b,c,d) making the magnetization process much easier with larger Nd content, the curves measured in the (100K-300K) range (Fig.1b,c,d) and (5K-300K) range with y = 0 (Fig.1a) are nearly not changed for all compositions of the samples.

We explain this anomaly by supporting that the total anisotropy energy of this series composes of two compositions coming from substituted Nd and from Fe-sub-lattice and the Nd-anisotropy fast decreases with increasing of


temperature while the Fe-anisotropy is nearly constant in the whole range of (5K-300K). That makes the total anisotropy and consequently the magnetization processes drastically change at 5K from sample to sample while it is nearly the same in the (100K-300K) range.

As far as the (La1-xYx)FeO3 series is concerned (Figs.2 and 3) it can note firstly that they become very ”soft” in the small magnetic field for all samples which x≠0 while at higher field they are all hardly saturated. In comparison with the case of (La1-yNdy)FeO3 (Fig.1) , the hysteresis loops of the samples (La1-xYx)FeO3 are more rectangular (Fig.2,3). All of them have the magnetization transition at a certain magnetic field called the critical field (Hcrit). At 5K the virgin magnetization curve is nearly coincided with the decreasing branch of the hysteresis loop (Fig.1a).

These anomalous behaviors can be understood by suggesting that the Y substitution for La causes the distortion of the crystal field created by the ligands of the Fe ions making the low order anisotropy constant of this sub-lattice drastically reduces and the Fe-lattice anisotropy is mainly due to the higher order anisotropy constant. It makes the materials become “soft” and the magnetization transition at Hcrit was occurred. This transition can be due to the abrupt change of the easy axis which is determined by this anisotropy constant under the influence of the magnetic field.


Fig.1 The curves M(H) of the samples of (La1-yNdy)FeO3

y=0 (a); y=0.15 (b); y=0.35(c); y=0.55(d)

(La1-yNdy)FeO3

y=0.00 (a)

(La1-yNdy)FeO3

y=0.15 (b)

(La1-yNdy)FeO3

y=0.35 (c)

(La1-yNdy)FeO3

y=0.55 (d)

Fig. 2 The curves M(H) of the samples of (La1-xYx)FeO3

y=0.15 (a); y=0.35(b)

5K

100K-300K

5K

100K-300K

5K

100K-300K

(a) (b)

100K-300K

5K


Fig. 3 The curves M(H) of the samples of (La1-xYx)FeO3 , x =1measured at 5K(a); 100K(b); 200K(c); 300K(d)

CONCLUSION

The perovskite lattice of orthorhombic LaFeO3 was distorted by Nd and Y substitution and the tolerance factor varies far from unity causing the decrease of unit cell volume and the magnetic anomaly.

Interesting magnetic anomalies observed in both sample series have been qualitatively explained based on the differences between the anisotropy of the substituted samples and that of the hot sample LaFeO3. Of course, in order to confirm these explanation, the measurements on single crystals are strongly recommended.

(a)

(c)

(b)

(d)


AcknowledgmentsThe authors would like to thank “National Foundation for Science and

Technology (NAFOSTED)”, with the project code “103.03.69.09” for financial support.References 1. G. Shbbir, A.H Qureshi, K. Saeed Materials Letters 60 (2006) 3706-

37092. Ivar Warnhus et all Solid State Ionics 178 (2007) 907-9143. Define Bayraktar et all Journal of the European Ceramic Society 27

(2007) 2445-24614. Peter Dinka, Alexander S.Mukasyan Journal of Power Sources 67 (2007) 472-4815. Dang Le Minh, Nguyen Van Du and Nguyen Thi Thuy Proceeding of the Eleventh Vietnamese-German Seminar on Physics and

Engineering Nha Trang City 31 March – 05 April, 2008, p.267-270.


THE ELECTRIC PROPERTY OF THE DOUBLE RARE-EARTH ELEMENTS PEROVSKITE COMPOUND (La1-xYx )FeO3

Nguyen Thi Thuy 2 , Nguyen Minh Tuan 1 , Dang Le Minh 1, Nguyen Phu Thuy 3

1 Hanoi University of Science-VNU 2 Hue Pedagogical University-Hue University

3 College of Technology-VNU of Hanoi and ITIMS-HUT, Hanoi

Abstract : The double rare-earth elements perovskite compound of (La1-xYx)FeO3 (x = 0; 0,15, 0,25; 0,35; 0,45; 0,55; 1 ) were prepared by ceramic method. Measurement of the electric property has shown that the resistivity of the samples is very high, like a dielectric material, the transition of “metallic-semiconductor” conductivity behaviors is observed at high temperature and the conductivity mechanism obey the nearest-neighbour hoping (NNH) with the Arrhenius law and the variable-range hopping (VRH) conductivity law.Key words : Double rare-earth elements, resistivity, conductivity, Arrhenius, VRH.

INTRODUCTION

Technologies for recycling waste heat are important for the efficient use of energy. Thermoelectric materials are candidates for the recycling of waste heat since they can directly convert heat into electric energy. Oxide ceramics have received attention as thermoelectric materials since they are relatively stable compared to inter-metallic materials in high temperature atmospheres. Several perovskite-type oxides have been reported to show high thermoelectric properties, such as the oxides CaMnO3-based, LaFeO3-based [1,2]. Though the mentioned oxides have high Seebeck coefficient, the electrical conductivity is low, it is necessary to improve the conductivity for applications as thermoelectric materials. Generally, the thermoelectric perovskite oxides are magnetic and semiconductor (n-type or p-type) behaviors. The electrical conduction mechanism is hopping process [3] . In


the present report, the electric property of the double rare-earth elements perovskite compounds of (La1-xYx)FeO3 ( y = 0; 0,15, 0,25; 0,35; 0,45; 0,55; 1) have been investigated.

EXPERIMENTAL

(La1-xYx)FeO3 (x, y = 0; 0,15, 0,25; 0,35; 0,45; 0,55; 0; 1 ) were prepared using the conventional solid-state synthesis technique. La2O3 ,Y2O3, and Fe2O3

powders were used as starting materials. The mixed powder was pressed into the pellets and pre-sintered in air at 10000C for 10 h. The calcined pellets were ground again and pressed as tablets and finally sintered at 12300C for 5 hrs.

The samples we examined under X-ray diffraction using D5005-Bruker X-ray diffractometer and Cu-Kα radiation. The DC resistivity was measured using a Keitheley 197A electrometer at various temperatures.

RESULTS AND DISCUSSION

The X-ray diffraction analysis of (La1-xYx)FeO3 indicates that all samples possess an orthorhombic structure. The requirement for stability of this orthorferrite is that the Goldschmidt tolerance factor t should be of nearly unity, where t is defined via

RA+ R0 = tWhere RA, RB and R0 are radii of A , B sites and oxygen ion respectively.The lattice parameters (a, b, c), tolerance factor (t), and RA, RB radii of A

and B of the samles(La1-xYx)FeO3 as function of Y content are listed in the table 1.


Table 1. The lattice parameters, tolerance factor, and RA, RB radii as function of Y content

X Compound a, A0 b, A0 c, A0 V,(A0)3 t RA

A0

RB A0

0 LaFeO3 5.56 5.55 7.86 242.24 0.921 1.36 0.6450.15 La0.85Y0.15FeO3 5.54 5.57 7.84 241.93 0.913 1.33 0.6450.25 La0.75Y0.25FeO3 5.54 5.53 7.84 240.19 0.908 1.32 0.6450.35 La0.65Y0.35FeO3 5.55 5.52 7.84 240.19 0.903 1.3 0.6450.45 La0.55Y0.45FeO3 5.55 5.47 7.83 237.71 0.862 1.28 0.6450.55 La0.45Y0.55FeO3 5.55 5.45 7.83 236.84 0.892 1.27 0.645

1 YFeO3 5.28 5.59 7.60 224.32 0.87 1.19 0.645

The volume of the unit cell is decreased with increasing the Y content because the radius of Y3+ (0.097Ǻ) are smaller than La3+(0.114Ǻ).

In general, the samples of (La1-xYx)FeO3 are insulator. However, during sintering at high temperature, some of Fe3+ transfer to Fe2+. Depending on sintering temperature, cooling speed and atmosphere, the ratio of Fe3+//Fe2+ is varied. The samples of (La1-xYx)FeO3 become conductor but their resistivity is very high.

Fig.1 shows the ρ(T) dependences of (La1-xYx)FeO3 with x=0.15; 0.25(a); x=0.35; 0.45(b); x=0.55; 1(c) and x = 0(d). The small figures in the top right corner are the decreasing branch of the ρ(T) curves.


It can be seen from these curves, the resistivity (ρ) is increased from room temperature to around 350-400K (as metallic behavior) and then it is decreased with increasing temperature (as semiconductor behavior). Around between 300 and 450K, the resistivity of the samples with x=0.35; 0.45 is too high to measure.

It is clear that, here, in the samples, there are two conductivity behaviors of metal and semiconductor at different temperature ranges. In the work [1], the electrical conductivity of LaFeO3 was not measured below 600K because of the high electrical resistance of the sample. In our work, for the first time, the transition of “metallic-semiconductor” conductivity behavior was observed at the high temperature (300K-600K) in the very high resistivity perovskite compound. From the lnρ(T) (Fig.2) of the decreasing branch of ρ(T) curves (Fig.1), the electrical conduction mechanism in the samples can be suggested as following.

Fig.1 The ρ(T) dependence of (La1-xYx)FeO3 x = 0.15; 0.25 (a); x = 035; 0.45 (b); x = 0.55; 1.00(c); x = 0 (d)

(a) (b)

(c)(d)


Here, we can apply two conductivity mechanisms : the nearest-neighbor hopping (NNH) conductivity with the Arrhenius law

and the variable-range hoping (VRH) conductivity satisfying a general law

The VRH conductivity is analysed within the Mott( p = ¼) or the Shklovskii-Efros (SE, p = ½) concepts , depending on the importance of the Coulomb interaction between the localized electrons [4]. Moreover, in [4] , the authors also give the different values of p : 3/4 ; 1/4 ; 1/3 or 1/2.

Fig.2,3,4 show the lnρ(T) curves of the samples La1-xYxFeO3 , x = 0.15;

0.25; 0.55, respectively. In our cases, with x = 0.15 ( Fig.2), in the temperature range of (442K-

489K), the law lnρ ~ T-1 is satisfied (a) and in one of (533K-564K), the plot of lnρ vs. T-3/4 can be fitted approximately with a linear law (b).

With x = 0.25 (Fig.3), in the range of (437K-455K) and (467K-500K) the law lnρ ~T-1/4 (a) and lnρ ~T-3/4 (b) are fitted, respectively.

With x = 0.35; 0.45; 0.55 (Fig.4), in the temperature range of ( 437K-553K), the law lnρ ~T-1/2 (a); lnρ ~T-1/4 (b) and lnρ ~T-1 (c) are satisfied , respectively.

Fig.2 The lnρ vs. T curves of La1-xYxFeO3 , x = 0.15

(a) (b)


CONCLUSION

The double rare-earth elements perovskite compound of (La1-xYx)FeO3 (x = 0; 0,15, 0,25; 0,35; 0,45; 0,55; 0 ; 1 ) were prepared successfully by ceramic method. The samples are single phase of orthorhombic structure. The volume of unit cell is decreased with increasing of Y content. For the first time, the transition of “metallic-semiconductor” conductivity behaviors is observed in the doped perovskite having high resistivity like a dielectric material at high temperature ( >300K ). The samples are semiconductor at the temperature higher about 400K and the conductiviy mechanism obey the Arrhenius law ( the nearest-neighbor hopping-NNH) and the variable-range hopping (VRH) law.

Fig.3 The lnρ vs. T curves of La1-xYxFeO3 , x = 0.25

(a) (b)

Fig.4 The lnρ vs. T curve of La1-xYxFeO3

x = 0.55


AcknowledgmentThe authors would like to thank “National Foundation for Science and

Technology (NAFOSTED)”, with the project code “103.03.69.09”, for financial support.

References1 Kouta Iwasaki, Tsuyoshi Ito, Masahito Yoshino, Tsuneo Matsui,

Takanori Nagasaki, Yuji Arita Journal of Alloys and Compounds 430 (2007) 297-301.

2. S. Komine, E. Iguchi Journal of Physics and Chemistry of Solids 68 (2007) 1504-1507.

3. Ming-Hao Hung, M.V Madhava Rao, Dah-Shyang Tsai Materials Chemistry and Physics 101 (2007) 297-302.

4. K.G Lisunov, E.A Arushanov, B. Raquest, J M Broto, F C. Chou, N. Wizent and G. Behr J.Phys.

Condens. Matter 18 (2006) 8541-8549.

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