Mechanochemical synthesis of Pb2MgWO6 piezoceramics with alloying additives

Pb2MgWO6 was prepared using mechanochemical activation and sintering in a temperature range of 600-1000oÑ in three ways: 1) from oxides of the corresponding metals, 2) using MgWO4 precursor; and 3) in the presence of over-stoichiometric amounts (1wt.% and 2wt.% ) of Li2CO3 alloying additive.


Introduction
Pb2MgWO6 piezoceramics has drawn particular attention because it can be used for the development and production of functional materials, such as Pb2MgWO6-based solid-state drives and solid solutions with the degree of order in the composite controlled by mechanical activation (m/a) [1,2]. Pb2MgWO6 is supposed to have a perovskite structure, while the presence of impurities worsens electro physical properties of this ceramics.

Experimental part
Pure WO3, MgO, PbO oxides, and Li2CO3 were used as the initial reagents for Pb2MgWO6 synthesis. In each case, the amount of lead oxide was 3 wt.% over stoichiometry [3]. Mechanical activation of the mixture was carried out in a planetary ball mill, AGO-2, equipped with steel jars and balls (d = 8 mm, m = 200g, ball acceleration 40 g). To prevent iron milling, pre-lining of the balls and jars was carried out [4]. The samples were formed as pellets with h = 2 mm and d = 10 mm under a pressure of 10 t/cm 2 in the absence of a plasticizing agent. Sintering of the samples was carried out in a furnace PVK-1.4-8 using a heating rate of 10 ºС/min. The samples were cooled down in the furnace after it was turned off. Weighting was carried out using UW220H SHIMATZU scales with 0.001g accuracy; geometrical size was measured with a micrometer with 0.01 mm accuracy. XRD patterns of the samples obtained were recorded on DRON-3 and Bruker D8 Advance powder diffractometers.
Synthesis of the precursor: WO3 and MgO samples were activated in the mill for 10 min, and then sintered at 900ºС for 4 hours. Later on, the as-prepared MgWO4 was milled and used in the synthesis of the samples.
According to the XRD data, immediately after m/a of the oxides, a two-phase system, consisting of cubic Pb2MgWO6 S.G. Fm-3m (no.225) and tetragonal PbWO4 S.G. I41/a (no.88) modifications, is formed. PbWO4 is well crystallized and its structure remains unchanged up to 800 ºС ( Fig. 1). After polishing the samples, this phase does not disappear, therefore, the structure is uniform throughout the sample thickness. After sintering at 950ºС, the amount of PbWO4 phase decreases drastically, and monoclinic modification of the Pb2WO5 phase S.G. C2/m (no.12) appears in the sample, which remains unchanged throughout all ranges of the samples (Fig. 2). MgO is a part of the forming solid solutions and its reflections do not appear in the XRD patterns.

Method 2: Synthesis of Pb2MgWO6 using MgWO4 precursor
In this case, immediately after mechanochemical activation, Pb2MgWO6 is formed. An increase in the sintering temperature results in a decrease in the amount of this phase and the appearance of the Pb2WO5 phase (Fig. 4). This mechanism is reported in studies [5,6]. Figures 4 and 5 show reflections of the Pb2MgWO6 (asterisks) and Pb2WO5 phases. As seen, at 600°C, a two-phase system consisting of Pb2MgWO6 and Pb2WO5 (Fig. 4) is formed, while at 800°C and 900°C, Pb2WO5 and Pb2MgWO6 respectively are formed (Fig.  5). As seen from Fig. 6, the density of the sample obtained after m/a for 15 min and sintering at 700°C is 8.73 g/cm 3 . For comparison, when synthesizing from oxides, after m/a for 15 min, the densest sample (8.24 g/cm 3 ) is achieved at 900°C.

Method 3: Study of the influence of alloying with lithium on sintering
A) 1 and 2 wt.% Li2CO3 were added to Pb2MgWO6 synthesized from the oxides. After m/a, a two-phase system consisting of Pb2MgWO6 and PbWO4 is formed. After sintering the samples in the presence of 1% Li2CO3 at 600 ºС, the Pb2WO5 phase appears, while in the presence of 2 wt.% Li2CO3, the PbWO4 phase remains in the sample (Fig. 7). After sintering at 800°C, the phase ratio is the same, while sintering at 1000°C results in the formation of almost Pb2MgWO6 phase alone (Fig. 8). In the presence of 1 wt.% Li2CO3, the densest samples (8.63 g/cm 3 ) were obtained at 700°C, while in the presence of 2 wt.% Li2CO3, the maximum density, 8.48 g/cm 3 , is achieved at 1000°C (Fig. 9). B) 1 and 2 wt.% Li2CO3 were added to Pb2MgWO6 synthesized from precursor. After m/a, at the initial sintering stages, the samples almost did not contain any impurity phases. At higher sintering temperatures, both in the presence of 1% and 2% Li2CO3, Pb2WO5 appeared as the second phase. After sintering at 1000°C in the presence of 2 wt.% Li2CO3, only Pb2MgWO6 perovskite phase was obtained (Fig. 10).   11 shows the dependence of the density of the samples on sintering temperature in the range from 600 to 1000ºС. In both cases, the highest density is achieved at 700°C. Then it decreases as the temperature increases from 700 to 900°C and it again increases as the temperature increases from 900 to 1000°C. The densities of the samples obtained at 700°C in the presence of 1 wt.% and 2 wt.% Li2CO3 were 8.33 and 8.63 g/cm 3 respectively.

Conclusion
Pb2MgWO6 was prepared from oxides of the corresponding metals (method 1) and from MgWO4 precursor (method 2) using mechanochemical activation and sintering in a temperature range of 600-1000ºС. In the first case, the densest samples (8.64 g/cm 3 ) were obtained after m/a for 30 min and sintering at 900°C, while in the second case, the highest density (8.73 g/cm 3 ) was achieved at 700°C and at activation for 15 min. The addition of lithium alloying improves the quality of the samples. In both cases, the densest samples were obtained in the presence of 1 wt.% or 2 wt.% Li2CO3 at 700°C and 1000°C respectively. The decrease in the density in the temperature range 700-900°C is due to (1) the formation of three-phase system with different lattice parameters (PbWO4, Pb2WO5 and Pb2MgWO6), (2) decomposition of lithium carbonate, and (3) an increase in the lead oxide vapor pressure. Pure perovskite phase was obtained during sintering at 1000°C in the presence of 2 wt.% Li2CO3 and in the presence of over-stoichiometric amounts (1wt.% and 2wt.% ) of Li2CO3 alloying additive.
The research was funded within the state assignment to ISSCM SB RAS (project No. 121032500062-4).