X-ray structural study of calcium manganese oxide by rietveld analysis at high temperatures [Ca2MnO4.00]

Mat. Res. B u l l . , Vol. 28, p p . 565-573, 1993. P r i n t e d in t h e USA. 0025-5408/93 $6.00 + .00 C o p y r i g h t (c) 1993 P e r g a m o n P r e s s Ltd.

X-RAY STRUCTURAL STUDY OF CALCIUM MANGANESE OXIDE BY RIETVELD ANALYSIS AT HIGH TEMPERATURES [ Ca2MnO4.00 ]

Junichi Takahashi and Naoki Kamegashira Department of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441, Japan ( R e c e i v e d F e b r u a r y 8, 1993; Communicated b y M. Koizumi)

ABSTRACT The structure of the polycrystalline Ca2MnO4.oo has been investigated by high-temperature X-ray diffractometry and Rietveld analysis. The polycrystalline Ca2MnO4.0o has the KzNiF4-type tetragonal (14~/acd) super structure (a=V2a', c=2c') of tetragonal (I4/mmm) primitive structure in the temperature range 298977K in air. The composition remains unchanged during further conditions. This I4~/acd super structure contains the rotated MnO 6 octahedra about their c axes, and magnitude of the rotation decreases with increasing temperature. The temperature dependence of the a parameter is governed by the rotation of the MnO 6 polyhedra, and that of the c parameter is dominated by the tetragonal distortion of the CaO arrangements in the structure. MATERIALS INDEX: calcium, manganese, oxide

Introduction The polycrystalline Ca2MnO 4 was first prepared by Ruddlesden and Popper [1], and they reported that the crystal has the K2NiF4 structure; the space group is described by I4/mmm (D~, No. 139); lattice parameters are a'=3.67A and c'=12.08/~ at room temperature. However, Moreau and OUivier have found [2] that a single crystal (CaL72Pbo.28XMno.77Pbo.23)O4 shows the extra reflections corresponding to a tetragonal space group I41/acd(D~g,No. 142). Further565

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more, a single crystal Ca2MnO4.0 has also possessed the tetragnnal super structure as same as the Pb-substituted Ca2MnO4, the space group I41/acd, with lattice parameters a=5.183(1)/~ and c=24.117(4)/~ at room temperature [3]. This I41/acd super structure contains the tilted MnO 6 octahedra about their c axes, and the octahedra are contra-rotated along c axes. The cell parameters of this tetragonal I41/acd super structure are related to those of the tetragonal I4/mmm primitive structure as a=V'2a' and c=2c', and the volume of the I41/acd unit cell is quadruple larger than that of the 14/mmm unit cell. The investigation reported here was motivated to confirm whether the polycrystalline Ca2MnO 4 might possess the super structure, I41/acd, or the primitive structure, I4/mmm. Further, we become interested in the structural change of the Ca2MnO 4 with temperature, because we have recently studied phase transitions in Caz_xSmxMnO4 solid solutions [4,5]. In order to study these points, the polycrystalline Ca2MnO 4 was characterized by high-temperature powder X-ray diffractometry and Rietveld analysis.

Experimental Polycrystalline Ca2MnO 4 was prepared by solid state reaction from powders of CaCO 3 (4N-purity) and Mn203 (3N) with required atomic ratio. The mixed powders were fired at 1523K for 72h in air, and then slowly cooled. Unreacted raw materials could not be detected in the powder X-ray diffraction (XRD) patterns after calcination. Oxygen contents in the synthesized Ca2MnO 4 phase was determined by hydrogen reduction. After annealing in H2-gas for 10h at 773K the sample was reduced to the cubic rock salt structure of Ca2t3Mnlr30 with lattice parameter a=0.4697(1)nm. The oxygen contents of the sample was calculated from the weight loss by this hydrogen reduction, and the composition was determined to Ca2MnO4.oo within 1 at.% of amount of oxygen. The dependence of oxygen nonstoichiometry upon temperature was examined by thermogravimetry (TG) with TG/DTA 300 (Seiko Instruments Inc.), by heating and cooling in the temperature range 2981573K in dry air atmosphere (Po2~-0.2atm). The heating and cooling rates were 5K'min-L X-ray data of Ca2MnO4.00 were collected with CuKa radiation using a MAC MXP ~8 powder X-ray diffractometer equipped with a single crystal graphite monochromator. Hightemperature X-ray measurement was carried out with an oven mounted on the X-ray diffractometer over the temperature range from 298K to 977K in air. The temperature of the sample was measured with a Pt/Rh thermocouple. Diffraction data were collected by the conditions as follows: scan type; step scan method, 20 range; 20 ° < 20 < 109 °, step width (20); 0.04 °, counting time (sec); 4.5 - 6.0 (variable), number of data points; 1800 - 2650 (variable), number of variable parameters; 23, number of reflections; 70 - 144 (variable). The structural refinement of the powder X-ray diffraction data was performed using the Rietveld analysis computer program RIETAN [6,7].

Results and Discussions Figure 1 shows the TG curve of the synthesized sample Ca2MnO4 ill air. According to the proposed phase diagram for the C a - M n - O system [8], the Ca2MnO 4 phase is stable in solid state up to about 1800K in air. Therefore, the weight change in the TG curve corresponds to oxygen nonstoichiometry, just for Ca2MnO4_~, though the 6 values reached only about 0.06 at 1573K. This oxygen nonstoichiometry significantly appeared above about 1200K, and was reversible up to 1600K.

Vol. 28, No. 6

Ca2MnO4

Oxygen contents

I

i

I

567

I

I

I

t

I

i

TG (%)

I

~ , 10{}.0 ] --~ 99.9 I 99.8

4.00 3.99 3.98 3.97 3.96 3.95

Fig. 1 Temperature depende n c e of w e i g h t of

i

3.94 3.93

C a 2 M n O 4 ill a i r .

3.92 3.91

i

i

600

i

i

1000

I

I

1400

1400

i

Heating

I

~

1000

I

99.7

--

99.6

--

99.5

-1

99.4 99.3

600

Cooling

Temperature ( K )

According to the literature described for the polycrystalline Ca2MnO 4 [1] and single crystal Ca2MnO 4 system [2,3], the atom positions (Pos.) and coordinates in the I4/mmm and I4/acd structures are given as follows:

14/mmm Atom Ca Mn O1 02

Pos. 4e 2a 4e 4c

Symmetry 4mm 4/mmm 4ram mmm.

I

Coordinates +_(0,0,z); + (0,0,0); + _+(0,0,z); + (0,1/2,0); (1/2,0,0); +

I

I

I

Sample CazMnO4 Temperature 298 K

l 5K

t Fig. 2

4K 8

Powder X - r a y d i f f r a c t i o n p a t t e r n of CaaMnO4.oo at room temperature measured w i t h C u K a radiation. Indices are based on the I4/acd unit cell.

3K

LII

2K 1K OK

A

I 30

40 2 0 ( degree )

50

60

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I41/acd (2nd setting) Atom Ca

Pos. 16d

Symmetry 2..

Mn O1 02

8a 16d 16]"

~,.. 2.. .. 2

Coordinates

±(0,1/4,z); (0,1/4,1/2+z); (0,1/4,1/4-z); (0,1/4,3/4-0; + ±(0,1/4,3/8); (0,1/4,5/8); + ±(0,1/43); (0,1/4,1/2+0; (0,1/4,1/4-0; (0,1/4,3/4-z); +

±(x,1/4+x,1/8); (x,1/4-x,5/8); (x,3/4+x,7/8); (x,3/4-x,3/8); +

where + means body centering. The difference between these two structures is that the coordinate x for O2 atom is equal to zero (I4/mmm) or not zero (I4/acd). Then, for example in an XRD pattern, special reflections due to the 0 2 atom appear only for the I4/acd at 20 = 39.0, 40.4, 43.2 and 47.2 ° , which can be assigned to 2 1 1 , 2 1 3 , 215 and 217 respectively, using C u K a radiation. Figure 2 shows the XRD pattern of the polycrystalline Ca2MnO4.0o prepared in this study at room temperature. The super structure lines indicated with arrows in the figure

5th layer z = 7/8

4th layer z = 5/8 Ca (16d) 3rd layer z = 3/8

O1 (16d)

02 (160 Mn (8a)

2nd layer z = 1/8

z

Origin at

y

(o,-1/4,t/8) from ;~

x

1st layer z = -1/8 = 7/8

Fig. 3 The conventional unit cell for CaEMnO4.0o and rotation scheme on each layer containing MnO 2 plane.

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Ca2MnO 4

569

were completely agreed with the above diffraction angles 20. Hence, it has become apparent that the polycrystalline Ca2MnO4.oo has the I41/acd structure at room temperature. Those special reflections are so weak, because only the 0 2 atom contributes to its intensities. The schematic drawing of the structure of Ca2MnO4.oo is shown in Fig. 3. The cell parameters of a and c for this structure are related to those of a' and c' for I4/mmm structure as a=v'2a' and c=2c'. From the symmetry of I41/acd, the origin of the unit cell is set at (0, -1/4, 1/8) from 8a (Mn) site. However, in this figure, the origin is picked at the 8a site to make easy comparison with I4/mmm structure. The displaced 0 2 atoms at 16f site are attributed to the rotation of the MnO 6 octahedra in the xy plane, and each octahedron along the c axis, that is in a first layer and the third layer, is contra-rotated (the same relation between the second and fourth layers pair). There is naturally no rotation in the unit cell of the space group I4/mmm. Then, the Rietveld analysis were carried out on the conditions based on the above structural descriptions. Initial parameters for refinement were used from the resulted data of the single crystal Ca2MnO4.o [3]. The refined ionic coordinates and isotropic thermal parameters are given in Table 1. Table 1 Fractional coordinates and isotropic thermal parameters for Ca2MnO4.oo. T (K)

298

Ca x y z

Mn x y z B (nm2) O1 x y z B (nm2) 02 x y z B(nm2)

B (nm~)

Rwpb

Rp RB RF

365

548

700

843

977

0 0 1/4 1/4 0.5513(5)i 0.5510(6)

0 1/4 0.5508(6) 0.011(5)

0 1/4 0.5513(6) 0.014(5)

0 1/4 0.5511(6) 0.020(6)

0 1/4 0.5511(7)

0 1/4 3/8 0.017(3) 0 1/4 0.4555(12) 0.021(6) 0.215(4) 1/4+x 1/8 0.029(8)

0 1/4 3/8 0.004(4) 0 1/4 0.4546(14) 0.013(8) 0.217(6) 1/4+x 1/8 0.009(10)

o.020(63

0 1/4 0.5520(6) 0.024(6/

1/4 3/8 0.004(4) 0 1/4 0.4547(14) 0.007(8) 0.216(6) 1/4+x 1/8 0.003(9)

1/4 3/8 0.006(5) 0 1/4 0.4557(15) 0.009(8) 0.220(7) 1/4+x 1/8 0.010(11)

1/4 3/8 0.008(5) 0 1/4 0.4562(16) 0.017(9) 0.222(8) 1/4+x 1/8 0.015(12)

1/4 3/8 0.004(6) 0 1/4 0.4561(19) 0.025(10) 0.231(12) 1/4+x 1/8 0.003(12)

1/4 3/8 0.012(6) 0 1/4 0.4565(18) 0.025(10) 0.231(13) 1/4+x 1/8 0.019(13)

0.2111 0.1473 0.0501 0.0649

0.2127 0.1456 0.0515 0.0699

0.2182 0.1542 0.0553 0.0733

0.2214 0.1532 0.0574 0.0752

0.2180 0.1593 0.0787 0.0931

0.2278 0.1667 0.0651 0.0830

0.021(3)

0.2194 0.1680 0.0704 0.0644

314

0.012(5)

The numbers in parentheses following refined values of atomic coordinates and thermal parameters represent the estimated standard deviations in the last significant digit(s). b : The R-factors quoted in profile analysis are def'med as follows [5] : R. = {[Z.wi(yo~'s,y~)~y[zO,~*)~]} ,,~; R~ = [X, I Y~'~-Y~ I Y[XY°bq; RB = [Xk I l~-l~Z~ ]/[Z/~['~]; RF = a :

The temperature dependence of the lattice parameters of Ca2MnO4.00 are plotted in Fig. 4. The a parameter and the volume of the unit cell increased with increasing temperature up to about 1000K, and the c parameter decreased with increasing temperature. Moreover, the composition maintains Ca2MnO4.00 in the whole temperature range as revealed by the foregoing T G work.

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J. TAKAHASHI et al.

Vol. 28, No. 6

I

,-~

I

I

o

E =

0.518 2.414

,_,

2.410 2.406

o (bo I

o

I' I' Oo

I

,

II o

Fig. 4 4.62

"-

Temperature dependence of lattice parameters of

4.58 ~"

~o

I

o o

I

4.66

o

o

0.522

~I

Ca2MnO4.00.

I

0.526 F '

o

o

I

I

I

I

I

I t

0 o I I

I

o

I I

I I /

0.665 ~ i

o o o

E =

0.655

>

o %0

0.645

I

r ,

I

I

200 400 600 800 1000 T(K)

17411

I

I

I

I

I

I

I/

170

162 1 200

Fig. 5

I

I 400

I

I 600

I

I 800

Temperature ( K )

I

I 1000

Temperature dependence of Mn-O2~-Mn h bond angle a. The estimated errors are indicated with bars.

Vol. 28, No.

6

Ca2MnO 4

r

571

i

i

i

i

i

i

i

i

0.190 0.185 E "~=

_ _ % . o ~ o ~ O ~ O

~ ° Mn-02

0.180

h

I I I I I I I I 0.265 o

t.

Fig. 6

0.260

Temperature dependence of Mn-O2 h and Ca-O1 h bond distances.

0.255

0......-.~ 0 /

_Q).O/0"-/ Ca-Ol~

l

200

I

I

400

I

600

[

I

800

I

I 1000

Temperature (K)

In order to explain the temperature dependence of the lattice parameters, the major interatomic bond angle and lengths were calculated from the refinement results. The temperature dependence of M n - O 2 h - M n h bond angle (a) and that of M n - O 2 h and Ca-O1 h bond lengths are shown in Figs. 5 and 6, respectively: the atom with subscript h is horizontally located against the neighboring former atom. It can be seen that the angle a and bond lengths increase with increasing temperature. The relations between the Mn-O2h-Mn h angle a and the magnitude of the rotation of MnO 6 octahedra 0 is expressed by 0 = (1/2X180°-a), therefore, increasing a refers to the decreasing of the rotation of MnO 6 octahedra. It seems that all the results about the angle and bond lengths suggest the increasing a parameter with i n creasing temperature. However, it should be noted that (1) a Ca ion and the corresponding O1 h ion do not exist in the co-plane parallel to the xy plane; for example, at 298K, the coordinate z of a Ca is equal to 0.5513 and that of the corresponding O1 h is equal to 0.5445, and (2) since a Ca ion is coordinated by nine oxide ions (five O1 and four 02) consist of MnO 6 octahedra [3], the position of the calcium ion is relatively influenced by the neighboring oxide ions. These relations lead to the fact that the bond length between Ca and O1 h is accompanied by the rotation of MnO 6 octahedra. Therefore, it could be concluded that the increasing a parameter with increasing temperature is caused by the reducing the magnitude of the rotation of MnO 6 octahedra along c axes. When the value of a reaches 180 ° (0 = 0°), the structure becomes I4/mmm. From the tendency of increasing a, I4/mmm structure would appear at about 1500K. However, at that temperature, the oxygen nonstoichiometry emerges as a formula Ca2MnO3.95. Therefore, at least in air, the I4/mmm structure would not appear in stoichiometric form. It is interesting that the c parameter decreases with increasing temperature, because many oxides with K2NiF4 structure show that the c parameter increases with increasing temperature without structural phase transitions [9-13]. The c parameter of I4/acd cell consists of

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Vol. 28, No. 6

four Mn-Ol~, four Ca-O1 v and two Ca-Car: the atom with subscript v is vertically located against the former atom. Figure 7 shows the temperature dependence of those interatomic lengths. Taking into account the proportions of each length for c parameter, decreasing c parameter is most likely influenced by the decreasing Ca-O1~ bond length. In view of the structure of Ca2MnO4.00, we can see the double layers of CaO which consist of Ca and O1 ions between two MnO 2 plane. Those Ca and O1 ions are oriented in NaCI type, but no O1 ions take the form of a completely closest-packing arrangement like face-centered cubic array of chloride ions in NaCI. This CaO block has a tetragonal distortion shortened along z axes: compare with Ca-O1 h length (Fig. 6, bottom) and Ca-O1 v length (Fig. 7, middle). This tetragonal distortion is probably caused by that a thermal vibration with increasing temperature in CaO layer is relaxed into the direction within the xy plane by increasing a parameter.

I

I

I

I

I

I

I

I

0.200 0.195 0.190

- =

M.u-O1,

-

I

I

I

~,

I ,I

I

I

I

t-, O

I

I

I

I

I

0.235

'-]:~ ~a

I

-

I

0.230 0.225

"I

Ca-Ol.

--

I I I I II

I

0.360

-0 -

Fig. 7 0.355 Temperature dependence of M n - O l v , C a - O l v and C a Ca~ distances.

~

-

Ca-Ca,

0

0

0.350 i

200

J

I

400

I

600

I

I

800

I

I

1000

Temperature (K)

Conclusion Herein, we have investigated the structure of the polycrystalline C a 2 M n O 4 . 0 o o v e r the temperature range from 298K to 977K in air. Stoichiometric form, Ca2MnO4.00, is maintained in such conditions. The structure of Ca2MnO4.0o is ascertained to be K2NiF4-type tetragonal super structure which belongs to the space group 14z/acd with ¢2aXq2aX2c where a and c are proto-

Vol. 28, No. 6

Ca2MnO4

573

type I4/mmm tetragonal cell parameters. This super structure contains tilted MnO6 octahedra about c axes. Increasing a parameter of the unit cell of CazMnO 4 with increasing temperature is caused by the decreasing of the magnitude of the rotation of MnO 6 octahedra. Further, this is because the modification of the CaO layer become contracted along c axes, therefore c parameter of the unit cell decreases with increasing temperature.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

S.N. Ruddlesden and P. Popper, Acta Cryst., 10, 538 (1957). J.M. Moreau and G. Ollivier, J. Solid State Chem., 10, 51 (1974). M.E. Leonowicz, K. R. Poeppelmeier and J. M. Longo, J. Solid State Chem., 59, 71 (1985). J. Takahashi, T. Kikuchi, H. Satoh and N. Kamegashira, J. Alloys and Compds., in press. J. Takahashi and N. Kamegashira, Mat. Res. Bull., in press. F. Izumi, J. Crystallogr. Soc. Jpn., 27, 23 (1985). (Japanese) F. Izumi, J. Mineralogical Soc. Jpn., 17, 37 (1985). (Japanese) H.S. Horowitz and J. M. Longo, Mat. Res. Bull., 13, 1359 (1978). R. Berjoan, J. P. Coutures, G. Le Flem and M. Saux, J. Solid State Chem., 42, 75 (1982). K.K. Singh, P. Ganguly and J. B. Goodenough, J. Solid State Chem., 52, 254 (1984). T. Suzuki and T. Fujita, Phisica, C159, 111 (1989). D.J. Buttrey, J. D. Sullivan, G. Shirane and K. Yamada, Phys. Rev., B42, 3944 (1990). P. Zolliker, D. E. Cox. J. B. Parise, E. M. McCarron III and W. E. Farneth, Phys. Rev., B42, 6332 (1990).