Original Reaction Sequence of Pb(Yb1/2Nb1/2)O-3-PbTiO3: Consequences on Dielectric Properties and Chemical Order

The solid-solution [Pb(Yb 1 / 2 Nb 1 / 2 )O 3 ] 1 − x -[PbTiO 3 ] x was synthesized with x ≤ 60%, using several high-temperature techniques as well as room-temperature mechanosynthesis. The high-temperature synthesis reveal a reaction path involving the synthesis ﬁrst of the end-members before the solid solution. The density and dielectric constant measured on the ceramics prepared from these powders indicate the crucial role of the synthesis technique on the subsequent properties. Mechanosynthesis results in ceramics with higher density and dielectric constant. Identical optimized sintering conditions were then applied to all investigated compositions and the resulting dielectric properties and chemical orders compared. All ferroelectric orders were evidenced. The 1:1 chemical order on the B-site of Pb(Yb 1 / 2 Nb 1 / 2 )O 3 results in the formation of a double perovskite Pb 2 YbNbO 6 , and the superstructures in the X-ray diagrams signing the existence of this order persist up to 30% PbTiO 3 . The underlying mechanism for substitution of Yb or Nb by Ti is presented.

The synthesis of these A(BB')O 3 compounds is delicate as pyrochlore phases may easily be formed when all oxides are reacting and are detrimental to the properties. An alternative synthesis route has been designed for Pb(BB')O 3 perovskites, the so-called "B-site oxide mixing route" [7,8] involving the reaction of all oxides but PbO. Depending on the valence of the B-site elements, this leads to the synthesis of a columbite through B 2+ O 2− +B' 5+ 2 O 2− 5 →BB' 2 O 6 [9] or of a wolframite through B 3+ 2 O 2− 3 +B' 5+ 2 O 2− 5 →2BB'O 4 [10] or of a rutile through B 4+ O 2− 2 +B' 4+ O 2− 2 →BB'O 4 [11]. In a second step, this columbite, wolframite, or rutile is mixed with PbO to get Pb(B,B')O 3 .
In the case of Pb(B 1/2 B' 1/2 )O 3 ferroelectric perovskites, the order on the B site has a dramatic influence on the properties of these materials. Generally speaking, disorder on the B site of the perovskite (i.e. random occupancy by any of the two elements) leads to a relaxor behavior whereas an ordered alternating occupancy of the B site along the [111] direction by the two elements leads generally to antiferroelectric behavior [12]. In the later case, the structure may be described as a "double perovskite" of general formula Pb 2 BB'O 6 . This order doubles the crystallographic unit cell along the [111] direction, giving rise to super-structure X-ray reflections, and was shown to be driven more by the sum of the ionization energies of the two B-site elements rather than by size or charge difference [13]. In the case of Pb(Yb 1/2 Nb 1/2 )O 3 (PYN), the sum of the ionization energies is large (75 eV) leading to a 1:1 chemical order on the B site which could only be broken by adding Lithium to the compound [14].
In this work we investigate three techniques to synthesize [Pb(Yb 1/2 Nb 1/2 )O 3 ] 1−x -[PbTiO 3 ] x (PYN-PT) with x≤60% (Ti-richer compositions were considered analogous to PbTiO 3 ) and compare the properties of the ceramics made out of these powders. The investigated techniques are: conventional solid-state mixing, reactive sintering, and mechanosynthesis. An original intermediate reaction step involving the formation of the two end-members is evidenced. The chemical order on the B-site, reminiscent of the one of PYN, is shown to persist in the powders obtained from mechanosynthesis up to composition with 30% PbTiO 3 . For the solid-state technique, we investigated various calcination temperatures and times (800-1000 • C and 2-12 h) to get the pure perovskite phase. The reactive sintering technique literally enables to carry out both the synthesis and the sintering of the ceramics from a pellet composed of the reactants [11]. For all compositions investigated, the optimized conditions were 950 to 1050 • C for 4 h. For the mechanosynthesis, we used a Retsch mill PM 100, and the pure perovskite phase was obtained after 9 h at 450 rpm. This process takes place at room temperature as the thermal energy of the other techniques is replaced here by mechanical energy.

II. EXPERIMENTAL PROCEDURE
The X-ray diffraction was carried out on a Bruker D2 phaser to check the crystallinity and purity of the products and on an 18 kW high-precision diffractometer equipped with a rotating Cu anode for the super-structures detection. The X-ray diagrams were either compared with the corresponding JCPDS file (for PbTiO 3 ) or to the literature [15] (for Pb(Yb 1/2 Nb 1/2 )O 3 , as no JCPDS file is available) to confirm the perovskite phase and its purity. Scanning Electronic Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX) (Leo Gemini 5130) were performed on fractured samples to investigate the purity and microstructure of the samples. The average grain size were determined using the line interception method on SEM images [16]. The relative density was calculated using geometrical dimension or the Archimedes' method. Dielectric measurements were carried out using an Agilent 4294A impedance analyzer on ceramics with gold-sputtered electrodes.

III. RESULTS AND DISCUSSION
Starting with the conventional solid-state technique, after 7 h at 950 • C, PYN-PT is formed (see Fig.1(a)). However, for shorter calcination times (4 h), extra diffraction peaks related to Pb(Yb 1/2 Nb 1/2 )O 3 and PbTiO 3 also appear in Fig.1(b). This suggests that the reaction sequence is actually PbO+TiO 2 +YbNbO 4 →Pb(Yb 1/2 Nb 1/2 )O 3 +PbTiO 3 →PYN-PT. It is highly unusual that the reaction sequence of a solid solution involves forming first the end-members. In order to test whether it is actually possible to synthesize the solid solution from its end members, we synthesized Pb(Yb 1/2 Nb 1/2 )O 3 and PbTiO 3 separately, before mixing them and calcining them for 950 • C 4 h. We indeed obtained PYN-PT, as can be seen in Fig.1(c). The pyrochlore impurity phase can be eliminated and pure PYN-PT synthesized under optimized synthesis conditions (namely at 800 • C for 12 h), see Fig.1

(d).
Pyrochlore phases have been shown to form at moderate temperatures (600-700 • C) in Pb(BB')O 3 and Pb(BB')O 3 -PbTiO 3 solid solutions, even using the B-site mixing route [7,9,17,18] and may still be present in the final product. In PYN-PT, a pyrochlore phase may also be formed but then the end members, PYN and PT, are formed before reacting together to form the PYN-PT solid solution at higher temperature. With optimized synthesis parameters it is possible to obtain pyrochlore-free PYN-PT samples.
The powder of pure perovskite obtained from solid-state synthesis must be shaped and sintered in order to get the final ceramic. As the reaction temperature and the sintering temperature were the same, a more practical solution to prepare ceramics was investigated: reactive sintering. In this case, the raw reactants are pre-pressed into a so-called "green" pellet and both the calcination and sintering occur simultaneously. Following this route we observed the same reaction sequence, i.e. starting from PbO+YbNbO 4 +TiO 2 leads to the formation of PYN and PbTiO 3 and only afterwards of PYN-PT.
In order to prevent the formation of pyrochlore phases, alternative high-pressure and/or hightemperature techniques have been developed, e.g. hot isostatic press. They, however, require heavy equipment. The last technique used to synthesize PYN-PT, mechanosynthesis, was shown to enable the synthesis of pure perovskite phase without such heavy equipment. This room-temperature technique, also known as ball-milling, was first applied to metallic alloys in the late 70's and to ferroelectric oxides 20 years later [8,19,20]. The strain and the grain size reduction taking place during the mechanosynthesis lead to broad and asymmetric peaks in the diffraction patterns making the identification of minority phases in presence difficult. The pure perovskite phase was considered to be obtained when subsequent milling did not change the diffraction pattern. The corresponding X-ray diagram is reported in Fig.1(e) where the large width and asymmetric profile of the peaks associated with the mechanical strain developed in the nanograins can be clearly observed. The difference with the high-temperature synthesis techniques here is that the grain size of the reactants is much smaller (∼20 nm vs micro-sized grains) and therefore present a much higher reactivity.
Powders obtained by the solid-state or mechanosynthesis techniques have been sintered (at 950 • C for 4 h) and the absence of pyrochlore after sintering was confirmed by X-ray diffraction (not shown). The density and dielectric constant at room temperature of the resulting ceramics are compared with the ceramics obtained by reactive sintering in Table I  if the ceramics density increases with PbO excess compared to reactive sintering, adding 3 wt% excess PbO leads to a normalized dielectric constant lower than with a 1.3 wt% excess PbO. This is because the excess PbO over-compensates for Pb loss during sintering and remains as PbO in the ceramics. Its low dielectric constant (∼20) therefore contributes to lower the ceramic dielectric constant.
The influence of the synthesis technique on dielectric properties is presented in Fig.2.
Mechanosynthesis leads to the highest dielectric constant at the para-ferroelectric phase transition (T C ∼530 • C) with max >12 000. The ceramics obtained from solid-state technique exhibit slightly lower max , and, as mentioned earlier, excess of PbO decreases max . From SEM, we found that even after sintering, the grain size of PYN-PT obtained by mechanosynthesis remains smaller (∼ 1 µm) than with other techniques (≥5 µm). This grain size is close to the optimum grain size giving rise to the best properties. This smaller grain size, combined with strain effects [21], also contribute to lower the transition temperature by ∼10 • .
Sintering conditions for ceramics obtained by mechanosynthesis were then further optimized with the objective to determine sintering parameters valid over the entire compositional range investigated here (from 0 to 60% PbTiO 3 ). The SEM investigation (Fig.3) reveals that, as expected, the grain size increases with increasing sintering temperature (from 0.086 µm at 800 • C for 4 h to 1.27 µm at 1000 • C for 4 h) and/or time (from 0.25 µm at 900 • C for 2 h to 0.78 µm at 900 • C for 6 h). Together with the decrease in the number and size of porosities, this larger grain size lead to an increase of the dielectric constant [1,4] (Fig.4). At the smallest grain size (0.086 µm), the dielectric peak at the Curie temperature is broad, but still existent indicating the persistence of the ferroelectric properties down to this grain size.
Our study focuses on the synthesis of the ceramics, the complete description of their properties together with the type of ferroelectric order they derivate from shall be presented elsewhere. The details of the microstructural model explaining the origin of these super-structures will be pub-    Triangles ( ) mark super-structures due to antiparallel displacements.