Phase Formation on the Surface of Lead Ferroniobate Depending on the Conditions of Mechanochemical Synthesis and Sintering

The perovskite structure is formed at lower temperatures with an increase in the time of preliminary activation. It is demonstrated that thermal treatment of mechanically activated for 25 minutes mixture composed of niobium oxide, lithium carbonate, lead oxide and iron oxide at a temperature of 650 – 700 °C results in the formation of perovskite structure. Further increase in the temperature of thermal treatment depending on experimental conditions can cause partial conversion of perovskite structure into the structure of pyrochlore and the formation of lithium ferrites. Pyrochlore structures and lithium ferrites are formed on the sample surface. For compacted materials, the thickness of the surface zone containing pyrochlore phase and lithium ferrite is 0.1-0.12 mm.


Introduction
Lead ferroniobate Pb(Fe 0.5 Nb 0.5 )O 3 (PFN) relates to multiferroics, materials combining electric and magnetic subsystems.They attract attention due to the possibility to make memory storage devices in which recording is performed by the electric field and reading by the magnetic field [1].PFN is also a promising basis for ceramic capacitor, piezoelectric, pyroelectric and electrostriction materials [2][3][4].
The synthesis of lead ferroniobate involves the formation of Pb(Fe 0.5 Nb 0.5 )O 3 phase with perovskite-type structure and also the formation of Pb 2 Fe 4 Nb 4 O 21 phase with pyrochloretype structure.This is characteristic not only for the synthesis of PFN [5,6], but also for a number of other compounds with perovskite structure, for example lead zirconate-titanate [7] and for the mechanochemical synthesis of lithium niobate doped with copper [8].
The formation of these phases at different ratios occurs both at the initial stages of synthesis and during subsequent high-temperature sintering.Chemical processes on the surface of PFN doped with Li during its synthesis are usually not considered in the literature.For example, the grain structure of Li-doped PFN is studied in [9], but the focus is on the strength properties of the intergranular boundaries and intercrystallite interlayers as well as on the electrophysical properties of the obtained material.The polarization properties of piezoceramics depend considerably on the state of the surface of the obtained samples.Therefore, it is a topical issue to consider chemical processes on the surface layers of ceramics during sintering.
Within temperature range 850-1100 °C, when the major stages of sintering process take place, the pressure of the vapor of lead and its oxides increases almost by three orders of magnitude [10][11].Because of this, sintering of lead-containing ceramics can be accompanied by lead evaporation from sample surface and cause changes of its phase composition.
It has been shown in the works [12][13][14][15] that lithium additives in lead ferroniobate affect dielectric, pyroelectric, piezoelectric, properties of resulting ceramics, and also affect the changes of the degree of B-cation ordering, temperature of magnetic and segneto-phase transitions.However, these works lack the data on the features of phase formation on sample surface due to the loss of lead, during the synthesis of PFN, while the surface properties of the ceramics affect also the above-mentioned characteristics.
The goal of the paper is investigation of the phase composition of surface layers in lithium-doped lead ferroniobate ceramics formed during thermal treatment of compacted mechanically activated mixture of reagents (Nb 2 O 5 , Fe 2 O 3 , PbO and Li 2 CO 3 ) above 650 °C, depending on the conditions of mechanical activation, filling and subsequent thermal treatment.Conditions for sample sintering were chosen by analogy with those used in [5][6][7][8].The structures of annealed samples were considered in the same works.The reference data on perovskite structures were taken from ASTM (the American Society for Testing Materials).

Experimental
Niobium oxide Nb 2 O 5 (Solikamsk Magnesium Works) of "pure" reagent grade, with the basic substance content not less than 98 % was used in the work.The reagent is a mixture of two phases: the basic orthorhombic phase and monoclinic admixture.In addition, the following reagents were used: rhombohedral iron oxide Fe 2 O 3 (Ural Plant of Chemical Reagents) in the form of hematite, "pure for analysis" reagent grade, orthorhombic lead oxide PbO -massicot (Ural Plant of Chemical Reagents), "pure for analysis" grade, and monoclinic lithium carbonate Li 2 CO 3 (Novosibirsk Rare Metals Plant) of "chemically pure" grade.The amounts of components were calculated on the basis of the following reaction describing the interactions between the components: 0.25Fe 2 O 3 + 0.25Nb 2 O 5 + (1-2Х)PbO + ХLi 2 CO 3 = Pb 1-2X Li 2X (Fe 0.5 Nb 0.5 )O 3 + XCO Here X is equal to 0.05.Taking into account the results obtained in [5], we added 3 % by weight (4.4 mol.%) lead oxide in excess over the stoichiometric composition into the initial mixture to prevent distortion of stoichiometry as a result of lead evaporation during sintering.
The synthesis of lithium-doped lead ferroniobate was carried out as follows.A mixture of the powders of three oxides and lithium carbonate was averaged in a mortar, and then activated in a ball planetary centrifugal mill AGO-2 in air, in steel cylinders 200 ml in volume, with the acceleration of 40 g.Steel balls 8 mm in diameter were used for activation; total mass of balls was 200 g; the ratio of the substance under treatment to ball mass was 1:20.Three powdered samples mechanically activated for 5, 15 and 25 minutes were prepared.Sample weighing was carried out with UW 220H scales, SHIMATZU, with the accuracy of 0.001 g.The powders were pressed into tablets 10 mm in diameter and 2 mm thick using a hydraulic press with the compression force of 10 t/cm 2 .Geometric size was determined with the help of micrometer with the accuracy of 0.001 mm.
After pressing, the samples were placed into a closable corundum crucible.To test the effect of synthesis conditions on lead loss from sample surface, some samples were sintered in aluminum oxide filling ("chemically pure" grade, for spectral analysis).A layer of aluminum oxide about 2 mm thick was placed on the bottom of the crucible; the samples were placed on this layer and covered with Al 2 O 3 layer about 3 mm thick.For sintering the second group of samples, lead oxide was placed on the bottom of the crucible.A ceramic substrate was placed onto PbO layer, and the samples were placed on this substrate.The samples of the third group were sintered on a substrate in a closed crucible but without filling and without lead oxide added.
Sample sintering was carried out in a PVK-1.4-8furnace with the working temperature range up to 1400 °C and with controlled heating rate.Samples were heated within temperature range 400 to 1050 °C.The rate of sample heating to the temperature of 600 °C was 20 degrees per minute, while heating above 600 °C was carried out at a rate of 10 degrees per minute.At any given temperature, samples were annealed for 2 hours.Samples were cooled together with the furnace after it was switched off.
The phase composition of the surface of compacted powders after thermal treatment was studied by means of powder diffraction using a DRON-3 diffractometer (CuKα radiation, focusing according to Bragg-Brentano scheme), and D8 ADVANCE diffractometer (Bruker, Germany).
Photographs were taken and the elemental analysis was carried out using a TM-1000 scanning electron microscope (HITACHI) with an energy dispersing spectrometer.

Results and discussion
As shown in Fig. 1a, activation of initial reagents leads to the formation of PFN with perovskite structure, though rather faulted.Then, at the initial stages of sintering (500-800 °C), pyrochlore -Pb 2  It follows from the data of X-ray phase analysis that the conditions of sample filling have no effect on the formation of the structure of resulting product up to the temperature of 800 °C (Tab.I).Filling starts to affect the structure only at higher annealing temperature.In the samples prepared in aluminum oxide filling, perovskite structure formed at a temperature of 650 -700 °C is conserved up to 1050 °C.This rule is not fulfilled when lead oxide is placed in the crucible, and also when sintering is carried out without any filling.During sample sintering in Al 2 O 3 filling, the surface layer of the filling is agglomerated into a dense rim, which is likely to prevent evaporation of lead and its compounds through the rim.As a result, the material with more perfect perovskite structure is formed; its surface contains a small amount of admixture phases.
Tab.I Density, lattice parameters, coherent length and micro-deformation, the ratio of perovskite to pyrochlore phases on sample surface depending on time of mechanical activation, filling, polishing and temperature of the sample annealing.According to X-ray data, after annealing at 650 °C the Pb 2 Fe 4 Nb 4 O 21 phase that was formed at the early stages of heating is still conserved in the samples activated for 5 and 15 minutes.The samples activated for 25 minutes contain only one phase: Pb(Fe 0.5 Nb 0.5 )O 3 .The samples annealed at 700 °C have perovskite structure (Pb(Fe 0.5 Nb 0.5 )O 3 phase for any activation time (Tab.I).The correlation of perovskite and pyrochlore phases has been calculated from the data of X-ray phase analysis with the use of PowderCell software.The fact that perovskite structure is formed at lower temperatures with an increase in the time of preliminary activation can be explained as follows: with an increase in activation time perovskite is formed already in larger amount and with more perfect structure.As mentioned above, perovskite-type structure is conserved in the samples prepared in aluminum oxide filling after subsequent annealing of the samples at a temperature up to 1050°C (Fig. 2).The phase Pb 2 Fe 4 Nb 4 O 21 starts to form on the surface of samples annealed with the addition of lead oxide into the crucible or without any filling; the amount of this phase makes it prevailing.In addition, also LiFeO 2 and LiFe 5 O 8 phases are formed; the LiFeO 2 phase dominates in the samples annealed without filling, while the LiFe 5 O 8 phase dominates after annealing with the addition of lead oxide in the crucible.

Sample characterization
Independently of filling kind and activation time, Pb(Fe 0.5 Nb 0.5 )O 3 phase with perovskite structure (Fig. 2a) is formed during thermal treatment at 800°C.An increase in annealing temperature to 900 °C causes the formation of pyrochlore phase Pb 2 Fe 4 Nb 4 O 21 and LiFeO 2 on the surface of samples (Fig. 2b).The formation of Pb 2 Fe 4 Nb 4 O 21 and LiFe 5 O 8 is also observed when lead oxide is added into the crucible at a higher temperature (Fig. 2c).Surprisingly, a pure Pb(Fe 0.5 Nb 0.5 )O 3 phase was obtained after annealing at 1050°C in aluminum oxide filling (Fig. 2d).These samples have been pressed from a powder mechanically activated within 25 minutes.The sintered layer of filling, which is rather dense, is likely to prevent evaporation of lead vapor from the crucible.
The photographs of the surface of samples obtained under different annealing conditions are presented in Fig. 3. Grained structure starts to appear on the surface of the sample treated at 800 °C, there are many pores yet (Fig. 3a).The diffraction pattern of this sample is shown in Fig. 2a.Three kinds of crystals are formed on the surface of the sample annealed at 900 °C (Fig. 3b, Fig. 3c), which agrees with the data of X-ray phase analysis.
The surface of the sample annealed at 1050 °C in aluminum oxide filling is shown in Fig. 3d and Fig. 3е.This is rather coarse-grained structure, according to the X-ray data (Fig. 2d), this is admixture-free Pb(Fe 0.5 Nb 0.5 )O 3 phase.However, dark zones of another phase are present on sample surface, though in small amounts (Fig. 3e).One can see in the photograph taken with low magnification (Fig. 4a) that a part of sample surface contains the zones composed of coarse white grains; another phase with dark grains is formed between the white grains, at their junctions and partially on their surface as clearly seen under high magnification (Fig. 4b).The feature of SEM images is that parts of the sample surface, containing the heavier elements look lighter, than the parts of the sample containing the easier elements.The elemental analysis of the light and dark crystallites of the sample (Fig. 4c) was carried out with the use an energy dispersing spectrometer.
The elemental analysis of the light region shows that this phase contains larger amount of heavy elements than the dark one, that is, it contains the larger amounts of lead and niobium.This is confirmed by the data shown in Fig. 5a  The elemental analysis of the dark grain in the center of Fig. 4c shows that this region contains much iron but substantially smaller amount of niobium and lead (Fig. 5b).It may be assumed that the dark grains are LiFe 5 O 8 phase, which was discovered with the help of X-ray data (Fig. 2c).
The diffraction pattern of the sample activated for 25 min and sintered at a temperature of 1000 o C without filling is shown in Fig. 6a.According to X-ray data, this is a two-phase system composed of Pb 2 Fe 4 Nb 4 O 21 and LiFe 5 O 8 .To what depth are these phases formed?To answer this question, the samples were polished.After a surface layer 0.06 mm thick was removed by polishing, the reflections of lead ferroniobate appeared on the diffraction pattern (Fig. 6b).After a surface layer 0.12 mm thick was removed by polishing, the diffraction pattern (Fig. 6c) contains only the reflections of Pb(Fe 0.5 Nb 0.5 )O 3 .Results of the calculation of density, lattice parameters, coherent lengths and their micro-deformations, the ratio of perovskite and pyrochlore phases on sample surface depending on obtaining conditions are presented in Tab.I.
It was demonstrated by us in [5][6][7][8] that admixture phases are formed in the surface layer of sintered samples; these phases are removed after polishing off a layer about 100 μm thick.However, chemical processes that take place on sample surface during sintering are not considered in those works.Determination of the crystal structure is considered in works [14][15], where the subject is the closest to that of our study.It is shown that the shape of grains is connected with the highest strength of the grain boundaries as well as the insides.There are no publications focused on the study of chemical processes that occur on the surface of samples PFN obtained with the use of mechanical activation and the subsequent sintering.
In As the electrophysical properties of ceramics depend on its polarization, which, in turn, depends on the surface properties, the knowledge of the conditions of heating, filling and the processes taking place both within and on the surface of samples can facilitate the technique for obtaining PFN to a considerable extent.The results presented in the paper can be used both in research and in technological processes.

Conclusions
The changes of the phase composition of samples during thermal treatment were followed.It was demonstrated that lithium-doped lead ferroniobate is formed at a temperature of 650 -700 °C, depending on activation conditions.
After activation of initial powders for 25 minutes, the synthesis of PFN is finished at 650 o C. For samples activated for 5 and 15 minutes, heating at a temperature not lower than 700 o C is necessary to complete the synthesis of PFN.Sample filling conditions do not affect the formation of the structure of resulting product up to the temperature of 800 °C inclusive.
For sintering with lead oxide added into the crucible, and for sintering without filling, temperature rise above 800 °C causes partial evaporation of lead from the surface ceramic layer.Lead losses are likely to be connected with the fact that lead vapor is free to evolve from the crucible, which is closed not tightly.As a result, the layer composed of Pb 2 Fe 4 Nb 4 O 21 , LiFeO 2 and LiFe 5 O 8 phases is formed on the surface of ceramics; these phases can have a negative effect on the electrophysical characteristics of ceramics.
The thickness of this layer is about 0.1 mm.After removing a surface layer 0.06 mm thick by polishing, the reflections of lead ferroniobate appear in diffraction patterns.After removing a 0.12 mm thick layer, only the reflections of Pb(Fe 0.5 Nb 0.5 )O 3 are present in the diffraction patterns.
Results of the calculations of lattice parameters, size and micro-deformations of coherent blocks, the ratio of perovskite to pyrochlore phases on sample surface depending on preparation conditions are presented.
The results obtained in this work can be used in the processes of ceramics production.
Fe 4 Nb 4 O 21 structure is also formed along with perovskite -Pb(Fe 0.5 Nb 0.5 )O 3 structure, Fig. 1b and Fig. c, and with temperature rise it is transformed into perovskite, Fig. 1d.

3 . 4 .
Photographs of the surface of samples after annealing: а) -annealing at 800 °C, without filling; diffraction pattern of this sample is shown in Fig. 2a; b, c) -annealing at 900 °C, without filling; diffraction pattern of this sample is shown in Fig.2b; d, e) -annealing at 1050 °C in aluminum oxide filling; diffraction pattern of this sample is shown in Fig. 2d; f)elemental analysis of the sample.The photographs of samples annealed at a temperature of 1000 °C with lead oxide added into the crucible with the samples are shown in Fig. 4. According to X-ray data (Fig. 2c) two phases Pb 2 Fe 4 Nb 4 O 21 and LiFe 5 O 8 are present on the surface.Photographs (at different magnification) of the samples annealed at a temperature of 1000 °C with lead oxide added into the crucible with samples: light grainspresumably Pb 2 Fe 4 Nb 4 O 21 , dark grains -LiFe 5 O 8 .

5 .
(acquisition of the light grain at the center of the photograph).It may be assumed that light grains are the Pb 2 Fe 4 Nb 4 O 21 phase.Elemental analysis: a) -of the central white crystallite and b) -elemental analysis of the central black crystallite (Fig. 4c).

Fig. 6 .
Fig. 6.Diffraction patterns of the surface of sample annealed without filling at 1000 °C, illustrating the effect of polishing on phase composition: a) -sample after sintering; b) -after polishing off the layer 0.06 mm thick; c) -after polishing off a layer 0.12 mm thick.

Fig. 7 .
Fig. 7. Sample density obtained depending on the time of sample activation and temperature of annealing in aluminum oxide filling.
this work, chemical processes that take place on sample surface during sintering are studied in detail.It is demonstrated what admixture phases are formed, in what form and to what depth.