Synthesis and crystal structure of the ordered perovskite Pb 2fesbo 6 Текст научной статьи по специальности «Химические науки»

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Synthesis and Crystal Structure of the Ordered Perovskite Pb2FeSbO6

Sergey V. Misjul*

Siberian Federal University, Svobodny, 79, Krasnoyarsk, 660041,

Russia

Maksim S. Molokeev^

Kirensky Institute of Physics, SB RAS, Akademgorodok, 50/38, Krasnoyarsk, 660036,

Russia

Nikolay M. Olekhnovich* Anatoliy V. Pushkarev§ Juriy V. Radyush^

Institute of Solid State and Semiconductor Physics, National Academy of Sciences of Belarus, P. Brovka, 19, 220072, Minsk, Belarus

Igor P. Raevski^

Research Institute of Physics, Southern Federal University, Stachki, Rostov-on-Don, 344090, Russia

Ivan N. Safonov**

Siberian Federal University, Svobodny, 79, Krasnoyarsk, 660041

Russia

Received 10.11.2012, received in revised form 10.12.2012, accepted 20.01.2013 This paper describes the synthesis of perovskite-like compound Pb2FeSbO6 which undergoes a phase transition between 210-220 K. According to X-ray studies of the powder sample the structure of the initial cubic and the low-temperature tetragonal phases of Pb2FeSbO6 were determined. The results obtained have been discussed using the group-theoretical analysis.

Keywords: phase transition, irreducible representations, X-ray experiment, perovskites, crystal, ceramics.

Introduction

Many oxides of A2BB'O6 type (where A — La, Ca, Sr, Ba, Pb; B, B' — Ni, Ru, Sb, Mo, Cd, Ta, Ho, La, Te, Er, Cu, W, Fe, Tm, Bi, Pr, Tb, Yb, Li) crystallize in an ordered perovskite

* [email protected] t [email protected] *[email protected] §[email protected] ^[email protected] H [email protected]

** [email protected] © Siberian Federal University. All rights reserved

structure (elpasolite), and may undergo a phase transition (PT) of different nature [1-3]. AX3 layers form elpasolite-like crystals A2BB/X6 (where X — Cl, F, O). All the octahedral voids of this package are filled by cations B and B/, alternating along the axes of the cube. Octahedra BX6 and B/X6 have different sizes due to the shift of the X anion towards B/ high-valency cation. The only arbitrary parameter of the elpasolite cubic structure is one of the coordinates of the ion X. Elpasolite cubic phase has FCC Bravais lattice, with the parameter of the unit cell twice of the perovskite’s one.

The ratio of the ionic radii and charges of cations B and B/ affect the nature of the ordering, and thus the structural characteristics and physical properties. These crystals belong to the space group G0 = O5 (Fm3m), even in cases when the ordering of B and B/ ion is only partial. Ordered compounds, which include oxygen elpasolites undergo «sharp» (nondiffused) transformations, the sequence of which depends strongly on the type of cations [1-3].

The recently synthesized new compound Pb2FeSbO6 is related to the family described above, and does not crystallize in the perovskite structure under normal conditions. Preliminary studies of Pb2FeSbO6 ceramics revealed the presence of a diffuse dielectric constant maximum at 210220 K, its temperature being independent on frequency of the measuring field. These results indicate the presence of ferroelectric or antiferroelectric phase transition in Pb2FeSbO6. This paper reports the synthesis of the perovskite modification of Pb2FeSbO6 under high pressure, and changes in the structure of Pb2FeSbO6 in the temperature range of 130-300 K, which includes the PT region.

1. Synthesis of the Pb2FeSbO6 compound

The starting reagents for the synthesis of Pb2FeSbO6 compound were PbO, Fe2O3, and Sb2O5 oxides (A.C.S. grade). The synthesis was performed in two stages. At the first stage by the means of pressing tablets were formed from the mixture of ball-milled powders of oxides corresponding to the stoichiometric composition of the target compound. After that, the tablets were subjected to heat treatment at 1020-1030 K for 4 hours in a closed corundum crucible filled by a mixture of lead and zirconium oxides. After the first stage of the synthesis, as shown by X-ray analysis, the resulting compound has the pyrochlore structure.

Product of the first stage of synthesis was ground, and then ball-milled. The tablets for the second stage of the synthesis were pressed from the resulting charge. The second stage of the synthesis was performed at the pressure of б GPa and the temperature of 1400-1500 K for 5 min. After the synthesis, the system was rapidly cooled to the room temperature under pressure. Then the high-pressure apparatus was unloaded. The product of the second high-pressure synthesis stage was dense ceramic of the perovskite phase of Pb 2FeSbO6 without any visible content of parasitic phases.

Annealing step method has shown that the resulting perovskite phase in the form of a monolithic tablets is stable when heated in air up to the temperature of 900-1000 K. The limit of the thermal stability of the perovskite phase in the powdered form is reduced to about 800-900 K. For removal of stresses the powder was annealed at 700 K for 2 hours before the X-ray diffraction studies.

2. The results of X-ray diffraction experiments

X-ray diffraction patterns of a powdered Pb2FeSbO6 samples were obtained using TTK450 Anton Paar temperature camera, mounted on an X-ray D8-ADVANCE diffractometer (Cu — Ka-radiation, 0-20 scanning linear detector VANTEC were used). Liquid nitrogen was used as a refrigerant.

To improve the data quality variable counting time (VCT) and variable step scaning (VSS) methods were used in the experiment [4-6]. The advantage of these methods as compared to the conventional one is that they equalize the weights of strong low-angled reflections and those weak ones at high angles, whereas in conventional X-ray experiment weights are non-equivalent, and the structure information contained in the X-ray diffraction pattern at high angles 26 is lost. In addition, the application of VCT and VSS methods significantly reduces the time of the experiment [7], without changing the quality of data.

Using the software, XRD Wizard [7] experimental X-ray diffraction patterns of the Pb2FeSbO6 sample were divided into two intervals of 26: 1) 26 from 5° to 60°, in the increments of 0.016° and exposure at each point for 4 seconds; and 2) 26 from 60° to 145°, in the increments of 0.032° and exposure for 16 seconds at each point. Structure refinement of the Pb2FeSbO6 crystal was carried out using software package [8].

Reflections positions on X-ray diffraction pattern of Pb2FeSbO6 (Fig. 1) obtained at room temperature of T = 300 K indicated FCC-cell of the Fm3m space group.

Fig. 1. X-ray diffraction pattern of the cubic Fm3m phase Pb2FeSbO6 at T = 300 K. Red dots — experimental intensity, solid black line — calculated intensity, the gray line shows the difference between them

In the cubic phase of the crystal of the ordered elpasolite Pb2FeSbO6 all atoms except oxygen are in special positions [3,9]. Thus from the structural parameters one coordinate of the oxygen atom, isotropic thermal parameters for all atoms, and occupancy positions for Fe and Sb were refined.

As a result of refinement it was found that the 4a position with (0, 0, 0) coordinates is occupied by 0.831(9) Fe ions and 0.169(9) Sb ions, and the 4b position (1/2,1/2,1/2) is occupied by 0.848(8) Sb ions and 0.153(8) Fe ions. That is, the total occupation for the two positions of the iron ions are 0.831 + 0.153 = 0.984, and Sb ions 0.848 + 0.169 = 1.017. In this case, the compound formula can be written as Pb2Feo.984Sbi.017O6. As the 4a (Fe) and 4b (Sb) sites population are equal to 1 within two standard deviations, the refinement was carried out with the restriction in the linear constraints form on the occupation of these positions, so that as a result we get the formula Pb2FeSbO6. At the end, the distribution of ions at the 4a and 4b positions in the structure of the cubic phase were defined in the Pb2 (Fe0.869(4) Sb0 131(4) ) (Sb0.869(4) Fe0.131(4) )O6 formula. The results of the structure refinement for the Pb2FeSbO6 cubic phase are given in Tab. 1 and 2. Table 3 presents the basic bond lengths. Fig. 7 a, shows the cubic Pb2FeSbO6 phase projection in the plane perpendicular to the axis of the fourth order. Refined profile

parameters of the cubic phase were further used without additional changes and refinements for the solving of low-symmetry phase structure.

Fig. 2. X-ray diffraction pattern of the tetragonal 14/m phase Pb2 FeSbO6 at T = 133 K. Red dots — experimental intensity, solid black line — calculated intensity, the gray line shows the difference between them

On the X-ray diffraction pattern of the low-temperature Pb2FeSbO6 phase (Fig. 2), obtained at T = 133 K, no superstructure reflections were observed. At low temperature, only subtle peaks of ice formed during the sample cooling could be seen.

To determine the point symmetry of the crystal low-temperature phase additional experiments in the temperature range from 143 K to 293 K with a step on a temperature of 10 K were carried out. Splitting of reflexes, by which the point of symmetry change can be determined, was not observed in X-ray diffraction patterns (Fig. 3). However, some information was obtained from the

(220)

20, deg

Fig. 3. Temperature evolution of the X-ray diffraction pattern of Pb2FeSbO6. Basic reflections of the prototype perovskite structure are shown

analysis of the full-width at half-maximum (FWHM) temperature dependence of several major

reflections (Figs. 4-6). It can be easily seen from the figures 4-6 that in the PT region reflexes broaden: (220) by 2.3% and (400) by 10.4%, while the FWHM of reflection (111) remains unchanged within the measurement error. Using the homology method [10], this broadening can be explained by the splitting of reflections, which most likely correspond to the tetragonal distortion of the face-centered cubic unit cell (Tab. 4).

Table 1. The main crystallographic data and experimental parameters for Pb2FeSbO6 at 300 K and 133 K

Experiment temperature 300 K 133 K

Space group Fm3m 14/m

A, A 7.98361(4) 5.64102(4)

C, A 7.9704(1)

V, A3 508.859(8) 253.627(5)

Z 4 2

20-range, ° 5-145 5-145

Reflections number 41 132

Refinement parameters number 24 18

Rwp, % 6.754 8.140

Rexp, % 3.410 3.386

Rp, % 5.054 5.930

Rb , % 3.42 3.97

GOF (x) 1.981 2.404

Table 2. Atomic coordinates, occupancy p, and isotropic thermal Biso parameters for Pb2FeSbO6 at 300 K and 133 K

T = 300 K

X y z P Biso, A2

Pb 1/4 1/4 1/4 1 2.2(4)

Fei 0 0 0 0.869(4) 0.7(4)

Sbi 0 0 0 0.131(4) 0.7(4)

Fe2 1/2 1/2 1/2 0.131(4) 0.7(4)

Sb2 1/2 1/2 1/2 0.869(4) 0.7(4)

O 0.2472(9) 0 0 1 1.1(1)

T =133 K

X y z P Biso, A2

Pb 0 1/2 1/4 1 1.43(2)

Fe1 0 0 0 0.867(7) 0.86(3)

Sbi 0 0 0 0.133(7) 0.86(3)

Fe2 0 0 1/2 0.133(7) 0.86(3)

Sb2 0 0 1/2 0.867(7) 0.86(3)

Oi 0.775(6) 0.733(8) 0 1 1.0(2)

O2 0 0 0.244(9) 1 1.0(2)

Further, the space group choice of low-temperature tetragonal phase was carried out using the results of Ref. [11] on the group-theoretical analysis of the structural phase transitions in

crystals with space group Fm3m and ISODISTORT program complex [12].

Table 3. Characteristic bond lengths for Pb2FeSbO6 at 300 K and 133 K

T = 300 K T = 133 K

bond d, A bond d, A

Fei (Sbi) - O 1.973(7) Fei (Sbi) - Oi 1.97(3)

Fei (Sbi) - O2 1.95(7)

Fe2 (Sb2) - O 2.019(7) Fe2 (Sb2) - Oi 2.03(4)

Fe 2 (Sb2) - O2 2.04(7)

O - b P 2.823(5) Oi - b P 2.70(3)

2 O b P 2.821(1)

Fig. 4. Temperature dependence of the (220) reflection FWHM

Fig. 5. Temperature dependence of the (222) reflection FWHM

Following [11,12], all the variants of tetragonal space groups, in which the PT from Fm3m without changing the volume of the unit cell is possible were obtained, as the superstructure

reflections in the low-temperature phase were not detected. There were six variants: 14/mmm, 142m, 14mm, 14/m, 14, and 14. All cells have a basis of atetr = (acub + bcub) /2; btetr = (acub - bcub) /2; ctetr = Ccub, where acub, bcUb, c^b — the face-centered cubic cell basis, atetr, btetr, ctetr — the tetragonal unit cell basis.

Since the experimental data that would help in the selection of the tetragonal phase space group were absent, the following considerations were taken into account.

Fig. 6. Temperature dependence of the (400) reflection FWHM

Refinement of the structure of tetragonal Pb2FeSbO6 phase in all of the groups gave identical values within the standard deviations of the uncertainty factors. However, the number of structural parameters refined in the polar groups is significantly larger than in centrosymmetric ones. In addition, numerous studies (see, e.g. [2,3,9]) over an ordered perovskite (elpasolite) indicate the phases in which the critical distortions are rotations of octahedral groups, leading to structures with the presence of the center of symmetry. For these reasons, subsequently polar groups 14mm and 14 were not considered. Furthermore, the transition to the space group 14/mmm is associated with the emergence of a critical order parameter (OP) (a, 0), which transforms to r+ of Fm3m representation leading to the compression/expansion of the octahedron Fe(Sb)O6. Hereafter in the article the notation of irreducible representations (IR) groups are given according to the handbook [13]. Transition to 142m is associated with the critical (a, 0,0) parameter r- representation, which leads to the Fe(Sb)O6 polyhedron deformation. Transition to 14 is associated with the appearance of two critical (a, 0, 0) OP r- representation and (b, 0,0) OP r+ representations, which lead to the rotation and distortion of the Fe(Sb)O6 polyhedron. More interesting is the transition to the group 14/m, in which there is a critical (a, 0, 0) OP r+ representation, resulting in a pure ^-type rotation of the Fe(Sb)O6 polyhedron. Rotations are designated according to [14].

Given the considerations above, the further consideration of the structure of the tetragonal phase was carried out in the 14/m space group. Coordinates of the two independent oxygen atoms, thermal parameters of all atoms and population of the sites occupied by Fe and Sb atoms were refined from the structural parameters. The sites populations were refined with the restrictions imposed earlier, so that the Pb2FeSbO6 formula was true. The result was the following ions distribution over the B- positions: Pb2 (Feo.867(6)Sbo.m(6) ) (Sbo.867(6)Feo.m(6) )Oa. Thus, the distribution of Fe and Sb over occupied positions in the tetragonal phase has not changed in comparison to the cubic phase. That is the charge ordering has not happened. The coordinates, thermal parameters, the atoms positions populations and the bond lengths of the tetragonal Pb2FeSbO6 phase are given in the Tab. 2 and 3. The structure projection of the tetragonal phase in the plane perpendicular to the cubic cell axis is presented in Fig. 7 b, c and d.

Table 4. Splitting of the main reflections on X-ray diffraction pattern during deformation of FCC-cell according to Mikheyev [10]

Distortion type № Syngony Split reflections number

(111) (200) (220) (311) (222) (400)

1 Tetragonal 1 2 2 2 1 2

2 Trigonal 2 1 2 3 2 1

3 Rhomboid1 2 2 3 4 2 2

4 Rhomboid2 1 3 3 3 1 3

5 Monoclinic3 2 3 4 6 2 3

6 Monoclinic4 3 2 4 7 3 2

7 Triclinic 4 3 6 12 4 3

Notes:

aorth

2

aorth

3 b

bmon

4 b bmon

(acub + bcub) /2, borth || (acub bcub) /2, corth || ccub

acub, borth 1 bcub, corth 1 ccub

acub, (bmon — monoclinic axis)

(acub + bcub) /2, (bmon — monoclinic axis)

Fe1(Sb1)06 Fe2(Sb2)Of

Fe1(Sb1)06 Fe2(Sb2)06 d

Fig. 7. The Pb2FeSbO6 structure: a — projection of cubic phase structure Fm3m along a fourfold axis, b, c, d — projection of tetragonal phase structure I4/m: b — along the fourfold axis (the ctetr axis), a — along acub of the initial phase (the axis of atetr — btetr), d — along the initial phase bcub (the axis of atetr + btetr). Distortion symbol is (0, 0, ^)

c

3. Discussion

The results are discussed using the results of the work [11] on the group-theoretical analysis of the structural phase transitions in crystals with the Fm3m space group and ISODISTORT [12] and ISOTROPY [15] program complexes. From Tab. 2,3 and figure 7 it is clear that the critical displacement is the displacement of the oxygen atoms, which together can be seen as ^-type octahedral groups rotations. Such displacements are associated with the critical OP r+ representation.

Here we note that the critical IR and OP determine the symmetry of the distorted phase. But in addition to the critical distortions of initial phase structure in the distorted (dissymmetric) phase displacement of atoms are compatible with the symmetry of this phase and are defined by non-critical (secondary) OP and IR. The entire set of OP, both critical and non-critical, arising during PT, forms a complete condensate of OP [16].

Works [11, 12] show that together with the critical distortions of ^-types there should be non-critical distortion of the octahedral groups associated with r+ IR of Fm3m space group. After decomposition of the atomic displacements in the tetragonal phase of the cubic phase basis functions of Fm3m IR we got the displacements of atoms in the tetragonal phase with respect to their positions in the cubic one. So, the critical displacement of the O atoms, associated with a r+ critical representation, is 0.17 A, while an offset of O atoms due to uncritical r+ representation is 0.02 A, that is almost an order in magnitude smaller. So, obviously just r+ makes the greatest contribution to the displacement of oxygen atoms. All these displacements can be represented as ^>-mne rotations [3,9,14].

Conclusion

Using the synthesis under high pressure, we managed to obtain the ordered Pb2FeSbG6 perovskite phase, which remains stable when heated in air up to temperature of 900-1000 K.

X-ray powder diffraction techniques, involving the complete symmetry analysis of condensate of order parameters defined structural changes in the ordered Pb2FeSbG6 perovskite. Schematically, the change of the spatial symmetry of the Pb2FeSbG6 crystal at the phase transition is

Г+

as follows: Fm3m ------ —> 14/m. The main critical changes at the phase transition are the

(vA°)

shifts of the oxygen atoms, which together can be represented as ^-type rotations of octahedral groups. Thermodynamic analysis carried out the basis of Ref. [15], shows that such a change of the structure can be realized at the second order phase transition.

This work was supported by RFBR (12-02-90019 Bel_a), the President of the Russian Federation grant for Support of Leading Scientific Schools (NS-4S2S.2012.2) and the Belarusian Republican Fundamental Research Foundation (T12R-077 grant).

References

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Синтез и кристаллическая структура упорядоченного перовскита Pb2FeSbO6

Сергей В. Мисюль Максим С. Молокеев Николай М. Олехнович Анатолий В. Пушкарев Юрий В. Радюш Игорь П. Раевский Иван Н. Сафонов

В статье описывается синтез перовскитоподобного кристалла Pb2FeSbO6, претерпевающего фазовый переход в области 21Q-22Q K. По данным рентгеновского эксперимента, от порошкового образца определены структуры исходной кубической и низкотемпературной тетрагональной фаз кристалла. Обсуждение результатов проводится с привлечением симметрийного анализа.

Ключевые слова: фазовый переход, неприводимые представления, рентгеновский эксперимент, пе-ровскиты, кристалл, керамика.