Synthesis and properties of complex oxides, Ln2Mn23Ta43O7 phases (Ln = rare earth or Y)

Journal of

ALLOYS

A~D CO~POU~DS ELSEVIER

J o u r n a l o f A l l o y s a n d C o m p o u n d s 238 (1996) 2 8 - 3 4

Synthesis and properties of complex oxides, Ln2Mn2/3Ta4/307 phases ( L n - rare earth or Y) G. Chen, H. Takenoshita, N. Kamegashira* Department of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan Received 4 October 1995; in final form 16 December 1995

Abstract

The complex oxides with a pyrochlore-related structure, L n e M n 2 / 3 T a 4 / 3 0 7 , have been synthesized by the usual ceramic method in a reducing atmosphere at 1523 K. The X-ray diffraction patterns of these compounds can be indexed by a hexagonal symmetry. The lattice parameters of the hexagonal crystal are linearly dependent on the ionic radii of Ln 3-. The chemical stability range of L n 2 M n z / 3 T a 4 / 3 0 7 phases was determined by the isothermal thermogravimetric analysis at 1273 K. The temperature dependence of the magnetic susceptibility of E u 2 M n 2 / 3 T a 4 / 3 0 7 obeys the Curie-Weiss law in the temperature range above 77 K, and the valence numbers of rare earth, manganese and tantalum are 3 +, 2 + and 5 + respectively. The results of electrical conductivity exhibit a semiconducting property in the measured temperature range. Keywords: Synthesis; Complex oxide; Phase stability; Magnetic susceptibility; Electrical conductivity; Pyrochlore

1. Introduction

The complex oxides with pyrochlore-related structure have been studied in various fields [1,2]. Among them, compounds containing rare earth and manganese, Ln2Mn2_xMxO 7, where Ln is rare earth or Y and M is W, Mo, Nb, Ta or Sb, have been synthesized by Bazuev and coworkers [3-6], Casado and Rasines [7] and Casado et al. [8]. As shown in Table 1, Ln2Mn4/3W2/307 and Ln2Mna/3Mo4/307 with the cationic combinations of (Mn3+W 6÷) and (Mn 7+, Mo 5÷) were obtained in air and in vacuum respectively [3,4,6]. Moreover, using niobium and tantalum of 5A main group, Y2MnMO 7 with the cationic combinations of (Mn 3+, Nb 5+ or Ta s+) and Y2Mna/3M4/307 with those of (Mn 2+, Nb s+ or Ta s+) were also ob-

tained [5]. In these cases YaMnMO 7 was synthesized in air and Y2Mn2/3M4/307 in vacuum, although they contained a certain number of the second phase, YTaO 4. According to Bazuev et al. [5], Y2Mna/3Ta4/307 compound was synthesized by the ceramic method in a vacuum at 1.33 × 10 -3 Pa. Its X-ray diffraction patterns could be indexed by hexagonal (or rhombohedral) symmetry. It is described by Bazuev et al. that the temperature dependence of the magnetic susceptibility of Y2Mn2/3Ta4/307 shows paramagnetic behavior in the temperature range 77300 K, indicating ferrimagnetism below 77 K. In our preceding study [9], complex oxide Dy/MnTal+xOT+ ~ has been obtained as a single phase with composition ratio of excess tantalum of Mn:Ta = 1:1 + x (x = 0.240.27) at 1253K in air. However, it has not been

Table 1 Composition and synthetic atmosphere of Ln2Mn2_,MxO 7 Ln

x

Mn ion

M ion

Synthetic atmosphere

Y, Sm to Lu Rare earth Y Y Gd

2/3 4/3 1 4/3 1

Mn 3+ Mn 2+ Mn 3+ Mn z+ Mn 3÷

W 6÷ Mo 5+ Nb s÷, Ta 5+ Nb 5+, Ta 5+ Sb s+

in in in in in

* Corresponding author. 0925-8388/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved PII S 0 9 2 5 - 8 3 8 8 ( 9 6 ) 0 2 2 2 7 - X

air [3] vacuum [4,6] air [5] vacuum [5] air [8]

G. Chen et al. / Journal of Alloys and Compounds 238 (1996) 28-34

investigated whether Ln2Mn2/3Ta4/307 compounds similar to Y2Mnz/3M4/307 are formed for other rare earths besides yttrium or not. Therefore, it is interesting to synthesize new Ln2Mnz/3Ta4/307 compounds with pyrochlore-related structure. The synthetic conditions for complex oxides of Ln2Mnz/3Ta4/307 phases are described in the present paper. Furthermore, the results of powder X-ray diffract:ton, electrical conductivity and magnetic susceptibility of these phases are also described. The phase s~ability of these new compounds was studied by the isothermal thermogravimetric method.

29

thermogravimetric analysis. The electrical conductivity measurement was carried out by the conventional d.c. four-probe method [16,17] in the temperature range 800-1300 K. The dependence of electrical conductivity on oxygen partial pressure was measured after equilibrating a sample pellet at a constant oxygen partial pressure at the measured temperature. The magnetic susceptibility was measured by the Faraday [18] method in the temperature range 77-600 K.

3. Results and discussion The results of synthesis of Ln2Mn2/3Ta4/307phases are given in Table 2. When synthesis was done with the stoichiometric composition ratio of Mn:Ta = 2/3:4/ 3 for Ln = Eu, Gd, Dy, Ho and Er, Ln2Mn2/3Ta4/307 single phases were obtained. However, for other rare earths the Ln2Mn2/3Ta4/307phases obtained included LnTaO 4 as a second phase with amounts of about 3-10%. Therefore, in order to obtain a single phase, the synthesis was tried by adjusting the composition ratio of manganese and tantalum under the same experimental conditions. The results showed that Ln2Mn(2+x)/3Ta(4_x)/30 7 x/2single phase could be formed for Ln = Sm and Y, when x = 0.1-0.2. When excess amount x of manganese was larger than 0.2, the X-ray diffraction peaks of the second phase, such as MnO, became stronger, while the LnTaO4 phase was contained in the products when x was smaller than 0.1. The synthesized Ln2Mn2/3Ta4/307 and Ln2Mn~2+x)/3Ta~4_x)/3Oy_x/2 phases are all yellow in color. The effort to synthesize single phases was unsuccessful for Ln = La to Nd, Tb and Yb, where the second phase was still contained with about 5-10%. Table 3 shows the powder X-ray diffraction data of Ln2Mnz/3Ta4/307 phases at room temperature. These data are similar to those of Y2Mn2/3Ta4/307 synthesized by Bazuev et al. [5] in vacuum. The X-ray diffraction patterns are indexable on the basis of hexagonal (or rhombohedral) symmetry. In the case of Ln = Eu (Mn:Ta = 2/3:4/3), the lattice parameters of Eu2Mn2/3Ta4/307 phase obtained in the hexagonal settings are a = 15.099(2) and c = 17.522(4)A, and

2. Experimental For tlae synthesis of Ln2Mn2/3Mn2/3Ta4/307phases, Ln20 3 (99.99% purity), MnO and Ta20 5 (99.9% purity) were used. Tb203 and Pr20 3 were made by the reduction of Tb407 and Pr60 u at 1273 K in a hydrogen stream for 24 h [10,11] respectively. Ln203 was prepared at 1273 K in Ar gas for 24 h, MnO at 1273 K in H 2 gas for 4 h and Ta20 5 at 1073 K in air for 12h be,fore mixing. These starting materials were mixed in an appropriate molar ratio corresponding to the following equation: (2 + x)MnO + 3Ln203 + (2 - x)/2Ta205 = 3Ln2Mn~2+x)/3Ta(4

x)/307-x/2

where C <~x ~<0.2. The mixtures of these powders were pressed into pellets and heated at 1523 K in I % H 2 - A r gas for 48 h. The formation of Ln2Mn2/3Ta4/307 phases was identified by powder X-ray diffractometry using Cu K a radiation with an Ni filter [9,12]. The isothermal thermogravimetric analysis was carried oul using the weight change of quenched samples after equilibrating at 1273 K under various oxygen partial pressures. Mixtures of Ar and 0 2 gases at 1 atm total pressure were used for controlling higher oxygen partial pressures (105-1 Pa), and H 2 and CO 2 gases for lower oxygen partial pressures (less than 1Pa) [13-15]. A linear flow rate of 0.3cms -1 was maintained for all gas mixtures throughout the

Table 2 Results of synthesis of Ln2Mnz/3Ta41307 phasds Rare earth and Y La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Y

A

--

A

A

©

@

©

A

©

©

©

--

A

--

O

@, (3, A, --,

Single phase synthesized by Mn:Ta = 2/3:4/3. Single phase synthesized at the composition Ln2Mnt2+x)/3Ta(4_x)/307 x/2 (x = 0.1-0.2). Second phase contained. Synthc,sis not carried out.

30

G. C h e n et al. / J o u r n a l o f A l l o y s a n d C o m p o u n d s 238 (1996) 2 8 - 3 4

Table 3 X-ray diffraction data of Ln2Mn2/3Ta4/307 phases at room temperature (hexagonal assignment) hkl

dobs (.~x)

dcalc (,4,)

I/I o (%)

dob s ( A )

d¢,,c ( A )

I/1 o ( % )

401 402 006 044 045 440 408 082 446 0404 804 0012 088 842 4010 484 4014 4412 1200 848 1206

3.229 3.076 2.930 2.632 2.398 1.897 1.826 1.614 1.591 1.548 1.539 1.463 1.314 1.229 -1.195 1.172 1.158 1.094 1.083 1.025

Ln = Sm 3.228 3.075 2.926 2.630 2.398 1.896 1.825 1.614 1.591 1.549 1.538 1.463 1.315 1.229 1.199 1.194 1.172 1.158 1.095 1.083 1.025

1 100 21 35 1 25 13 10 14 5 7 1 3 5 -4 1 2 2 2 1

-3.064 2.917 2.620 -1.887 1.817 1.608 1.585 1.542 1.532 1.461 1.310 1.224 1.195 1.189 1.169 1.155 1.089 1.076 1.021

Ln = Eu 3.216 3.063 2.920 2.620 2.389 1.888 1.819 1.607 1.585 1.544 1.531 1.460 1.310 1.224 1.195 1.189 1.169 1.155 1.089 1.076 1.021

-100 29 42 -32 22 14 22 9 9 2 6 9 3 7 2 5 3 5 2

Lattice parameters

a = c= c/a V=

401 402 006 044 440 408 082 446 0410 804 0012 088 842 484 4014 4412 1200 848 1206

15.166(2) ,~ 17.558(3) ,~ = 1.158 3497.4 A. Ln = Gd 3.202 3.052 2.913 2.611 1.880 1.814 1.601 1.580 1.540 1.526 1.456 1.306 1.219 1.185 1.166 1.151 1.086 1.072 1.017

3.207 3.056 2.914 2.612 1.881 1.814 1.602 1.580 1.541 1.526 1.457 1.305 1.220 1.184 1.165 1.151 1.085 1.072 1.018

a = c= c/a V=

2 100 26 37 26 14 10 14 6 5 1 3 5 3 1 2 2 2 1

3.175 3.035 2.901 2.596 1.868 1.804 1.590 1.570 1.532 1.516 1.450 1.298 1.211 1.177 -1.145 1.079 1.065 1.010

15.099(2) ,~. 17.522(4) = 1.161 3459.6 ~3 Ln = Dy 3.179 3.031 2.902 2.596 1.867 1.805 1.590 1.570 1.533 1.516 1.451 1.298 1.210 1.178 1.161 1.146 1.078 1.066 1.010

2 100 22 36 27 14 10 14 5 5 1 3 4 3 -2 1 2 2

Lattice parameters

a = c= c/a V=

401 402 006 044 440 408

3.175 3.031 2.897 2.593 1.865 1.802

15.041(2) ,~ 17.475(4) ,~, = 1.162 3423.9/~3 Ln = Ho 3.173 3.026 2.891 2.589 1.863 1.800

a = 14.936(5) ,~ c = 17.361(5) ,~ c / a - 162 V = 3363.4 ,~3

1 100 19 32 23 12

-3.016 2.886 2.582 1.856 1.795

Ln = Er 3.161 3.014 2.882 2.580 1.857 1.794

-100 17 27 18 10

G. Chen et al. / Journal o f Alloys and Compounds 238 (1996) 28-34

31

Table 3 (Continued)

hkl

d,,b~ ( A )

dcal~ (.~)

I/1 o (%)

dob~ (A)

d¢.,~ (A)

I / I o (%)

082 446 0410 804 0012 088 842 484 4412 1200 848 1206

1.587 1.568 1.529 1.513 -1.294 1.208 1.175 1.142 1.075 1.063 1.008

1.586 1.566 1.528 1.512 1.446 1.294 1.208 1.174 1.142 1.076 1.063 1.008

8 12 4 5 -3 4 3 1 1 2 1

1.579 1.561 1.523 1.507 1.442 1.289 1.203 1.170 1.138 1.072 1.060 --

1.581 1.561 1.523 1.507 1.441 1.290 1.204 1.170 1.138 1.072 1.059 1.004

8 11 4 6 1 2 3 3 1 1 2 --

Lattice parameters a = 14.905(3) .~ c = 17.346(9) .~ c/a = 1.164 V = 3337.3 .~3

a = 14.852(3) ,~ c = 17.293(6) .,~ c/a = 1.165 V = 3303.6 A~

Ln = Y 401 402 006 044 045 440 408 082 446 0410 804 088 842 484 4412 1200 848 1206

3.175 3.031 2.893 2.593 2.361 1.863 1.801 1.587 1.566 1.528 1.513 1.294 1.207 1.174 1.143 1.076 1.063 1.009

3.173 3.025 2.892 2.590 2.364 1.863 1.800 1.587 1.566 1.529 1.513 1.295 1.208 1.174 1.143 1.076 1.063 1.008

8

100 18 28 5 23 12 9 12 4 5 2 4 3 l 2 2 2

Lattice parameters a = 14.907(2) ,~ c = 17.355(7) A c/a = 1.164 V = 3339.7 .~3

those of the rhombohedral cell are a = 10.491(1),~ and a = 92.10(1) °. In the case of Ln = other rare earth ( M n : T a > 2 / 3 : 4 / 3 ) , the X-ray diffraction data of Ln2Mn(:+x)/3Ta(4_x~/307_x/2 phases are close to those of Eu2Mnz/3Ta4/307. Among these compounds, the lattice parameters of Y2Mnz.1/3Ta3.9/307 are a = 14.887(3) and c = 17.333(6) A, in good agreement with those of YzMn~/3Ta4/307 ( a = 1 4 . 8 7 ( 1 ) and c = 17.31(1),~) reported by Bazuev et al. [5]. The variation of the lattice parameters in LnzMnz,3Ta4/307 compounds is plotted against ionic radii of Ln 3+ in Fig. 1. From this figure it can be seen that the parameters a and c, the ratio c/a and the volume of the unit cell are almost linearly dependent on ionic radii of Ln 3+. The a-axis, c-axis and the

volume of the unit cell increase, while the ratio c/a decreases with increasing ionic radii of Ln 3+. Isothermal thermogravimetric analysis was carried out at 1273 K as a function of oxygen partial pressure, as shown in Figs. 2(a)-2(g). In these figures the standard point of weight change was depicted as 10 -15 atm. From the results of isothermal thermogravimetric analysis, it is shown that not all of the Ln2Mn2/3Ta4/30 7 phases have decomposed, and some exist stably over a wide range of oxygen partial pressure (1-10-2°atm), or even in hydrogen gas. When the oxygen partial pressure is lower than 10 -5 atm, the isothermal weight of specimens changes only slightly with decreasing oxygen partial pressure to 10 -2o atm. However, when the oxygen partial pressure

G. Chen et al. / Journal of Alloys and Compounds 238 (1996) 28-34

32

Ho Dy Gd

i

Sm

-0.5

.20

1.74

I

~I T

,

,

~l

,

I

0.5 .18

~

"A

\ 1.68 ~

>

"

~

.15

o

<3 1.5, ~

I

-A

1.80

E

i

0.5

3

.

8

0

co 3.60

1.47

-0.5 I

,

I

0.5 "A

-0.5

~-E

>

0.5 A

3.40 -0.5 1.0

0.12

0.128

0

Ln 3÷ radii / nm Fig. 1. Dependence of lattice parameters, c/a ratios and volumes of unit cell for Ln2Mn2/3Ta4~307 on ionic radii of Ln 3÷.

I

I

-10 Log(Po2/ atm) '

0

I

[

0.5 zx is higher than 10 .5 atm, the specimens are oxidized and the isothermal weight increases a little. This corresponds to the powder X-ray diffraction data. Fig. 3 shows the powder X-ray diffraction patterns of quenched samples of Eu2Mn2/3Ta4/307 after annealing under Po2 = 1, 0.21, 10 -5, 10 -15 atm and H 2 gas. All of the X-ray diffraction patterns show the same type, and new reflection peaks were not found. Moreover, when the oxygen partial pressure is lower than 10 -5 atm, the positions of the reflection peaks change only little, while when the oxygen partial pressure is higher than 10 5 atm, those of the reflection peaks shift to higher angle. The change in lattice parameters of Eu2Mn2/3Ta4/307 against oxygen partial pressure is shown in Fig. 4. As shown in the figure, the lattice parameters a and c in the hexagonal settings increase slightly in the range 1-10 -5 atm of oxygen partial pressure, and then remain almost constant with decreasing oxygen partial pressure. The ratio c/a and the volume of the unit cell also show similar behavior. These results reflect those of thermogravimetric analysis. From the results mentioned above, it is considered that synthesized Eu2Mna/3Ta4/307 is fundamentally a stoichiometric compound, and the valence state of manganese and tantalum ions in this compound remains unchanged over a considerably wide range (10-5-10 -20 atm) of oxygen partial pressure (or even hydrogen gas). In contrast, EuzMn2/3Ta4/30 7 phase has a non-stoichiometry of excess oxygen under o x y -

o " 2 - 0 . 5,~ ~

,

,

,

t

,

,

,

,

I

,

-0.5 <3

-0.5~ ,

-lO

I

o

Log(Po2/ atm) Fig. 2. Isothermal weight change in LnzMn2/3Ta4/30 7 phases as a function of oxygen partial pressure at 1273 K: ©, data under various oxygen partial pressures; A, data in hydrogen gas. (a) Ln = Sm; (b) Ln = Eu; (c) Ln = Gd; (d) Ln = Dy; (e) Ln = Ho; (f) Ln = Er; (g) Ln = Y.

gen partial pressures higher than 10 -5 atm, and then the present compound should be formally expressed as Eu2Mn2/3Ta41307+8, where 6 is an oxygen non-stoichiometry. The detailed defect structure of this phase should be clarified further. When the specimen annealed in hydrogen gas at 1273 K is taken as the stoichometric point (6 = 0 in Eu2Mnz/3Ta4/307+8),the largest deviation from stoichiometry is 6 = 0.32 (at 1 atm of 0 2, at 1273 K) which corresponds to 4% Mn 2+ and 96% Mn 3+. This result is consistent with a decrease in lattice parameters.

G. Chen et al. / Journal of Alloys and Compounds 238 (1996) 28-34 I

'

I

33

<~ P(Oa)=lm p{o'z)-:O.21

l

'

'

I

'

A:AE=0.53eV

-2.5

aim

I

E

c"

13.)

)~

^AP(°g='°-s'~" ,.

,

,

,

J

r

O9

o -3.0

20

30

40 20/deg

50

03

60

o _..I

-3.5

Fig. 3. Pcwder X-ray diffraction patterns of E u 2 M n 2 / 3 T a 4 / 3 0 7 annealed under various oxygen partial pressures at 1273 K.

I

0.8 I

i

I

1.77

,

I

,

1.1

Fig. 6. Electrical conductivity of E u 2 M n 2 / 3 T a 4 / 3 0 7 as a function of the reciprocal of temperature under the synthetic atmosphere.

v

<

I

1.0

103 K / T

1.18 v

,

0.9

c

1.17

1.71

O

1.16 c/a

E c" 1.65 A

O

c6

>

A

<

a

1.4(

,.60

O9

E t-"

1 .,20

"

~

3.40 "" > >

V 1.00

'

-20

'

'

'

-10

I 0

i3.20

Log(P02 / atm) Fig. 4. Change in lattice parameters, c/a ratios and volumes of unit cell for Eu,Mn2/~Ta4/307 as a function of oxygen partial pressure at 1273 K.

I

I

tJ o b s = 7 . 8 8

I

I

I

The results of magnetic susceptibility measurements for Eu2Mn2/3Ta4/3 are shown in Fig. 5. The plot of inverse magnetic susceptibility against temperature follows a Curie-Weiss law. It is considered from this result that Eu2Mn2/3Ta4/307 shows a paramagnetic property in the measured temperature range. From this slope of the susceptibility curve the effective magnetic moment can be calculated as tZobS -----7.88/-~B, while the theoretical value is /Xcalc=7.01/~ with assumptions of E u 3+, Mn 2+ and Ta 5+ in Eu2Mn2/3Ta4/307. The logarithm of electrical conductivity of E u 2 M n 2 / 3 T a 4 / 3 0 7 is shown in Fig. 6 as a function of the reciprocal of temperature. It is seen from this figure that Eu2Mn2/3Ta4/307 has semiconducting properties, two different values of activation energy

-2.75

I

i

i

I

I-t B

la cal(EU3+,Mn2+,Ta5+) ~;p "7

:3

6 'E -3.00

E~ (1) E~ C)

O9

4

O

O

o, -3.25

7 >"

O

2

I

0

I

200

I

I

400 T/K

I

I

600

Fig. 5. Variation of the inverse of the magnetic susceptibility of Eu2Mn213q-a4/307 against temperature.

-3.50

-5.0

-2.5

0

Log(Po2 / atm) Fig. 7. Isothermal electrical conductivity of E u 2 M n 2 / 3 T a 4 / 3 0 7 function of oxygen partial pressure at 1273 K.

as

a

34

G. Chen et al. / Journal of Alloys and Compounds 238 (1996) 28-34

for the electrical conductivity being found according to the temperature range of measurement. The two kinds of activation energy calculated from Fig. 6 are A E - 0.53 and 0.95 eV respectively, corresponding to the regions A and B. The isothermal electrical conductivity was measured as a function of oxygen partial pressure. Fig. 7 shows the dependence of electrical conductivity on oxygen partial pressure. It is considered that the electrical conductivity of Eu2Mn2/3Ta4/30 7 was p-type in the high oxygen partial pressure region, because the slope of the electrical conductivity curve is positive (about 1/9 slope) with increasing oxygen partial pressure.

4. Conclusions New complex oxide Ln2Mn2/3Ta4/30 7 single phases can be obtained from Ln203, M n O and T a 2 0 5 at 1523K in I % H 2 - A r for various rare earths. The stability ranges of these new compounds were determined by thermogravimetric analysis. U n d e r low oxygen partial pressure, below Po2 = 10-5 atm, Eu2Mn2/3Ta4/307+ 8 compound almost keeps a stoichiometry (6 < 0 . 0 7 ) and unchanged lattice parameters. In contrast, under higher oxygen partial pressures above Po2 = 10-5 arm, it has a certain oxygen nonstoichiometry and the lattice parameters decrease with increasing oxygen partial pressure. Eu2Mn2/3Ta4/307 phase shows paramagnetic behavior in the temperature range 7 7 - 6 6 0 K . The valence numbers of rare earth, manganese and tantalum ions in Eu2Mn2/3Ta4/307 are estimated at 3 +, 2 + and 5 + respectively from the results of magnetic susceptibility. Eu2Mn2/3Ta4/30 7 shows semiconducting properties in the temperature range 800-1300K. The electrical conductivity increases with increasing oxygen partial pressure in the measured range.

Acknowledgements The authors are grateful to the Ministry of Education, Science, Sports and Culture for financial support

through Priority Areas "New Development of Rare Earth Complexes" No. 07230242.

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