Controlling the synthesis conditions for the fabrication of Pb (Mg1/3Nb2/3) O3 nano powders to diminish undesirable pyrochlore structure

The main problem in fabrication of relaxor lead magnesium niobate (PMN; Pb(Mg_1/3Nb_2/3) O₃) material is the formation of an undesirable pyrochlore phase (Pb_1.86Mg_0.24Nb_1.67O_6.5). This phase significantly impacts the electrical properties of the compound. Therefore, this article presents various experimental trials performed to minimize the formation of the undesirable pyrochlore phase during the preparation of relaxor lead magnesium niobate material. In this regard, a sol-gel pathway was utilized. The synthesis was conducted as follows; niobium ethoxide, metal nitrate and metal acetate were combined with 2-methoxyethanol to form transparent solution. The gel was then formed by heating the solution at 80 °C, to form the precursor powder. Subsequently, thermal treatment at elevated temperatures was performed to form the perovskite structure. The effects of several synthesis conditions such as the presence of surplus Mg²⁺ ions, excess of Pb²⁺ ion content, annealing and pre-annealing temperature, type of fuel and the annealing time, on the physicochemical properties and the structure of PMN phase were investigated. Thermal gravimetric analysis (TGA) results manifested that crystallization of PMN was occurred at a temperature of 580 °C. X-ray diffraction (XRD) profiles revealed that the ratio of the pyrochlore (Pb_1.86Mg_0.24Nb_1.67O_6.5) phase was significantly reduced by increasing the Mg²⁺ ion content up to 30%. The lowest concentration of the pyrochlore phase was predestined during a single annealing step at 900 °C for a period of 3 h, using an excess of 30% Mg²⁺ ion concentration. Rietveld refinement of the XRD data was conducted using Material Analysis Using Diffraction (MAUD) software. The refinement was performed to determine the crystal structure and lattice constants. From the refinement, the cubic structure was evinced with a lattice constant (a) of 4.048Å. Fourier transform infrared (FT-IR) spectroscopy of PMN nano powders annealed at 900 °C for 3 h demonstrated a spectrum characteristic of the most perovskite compounds with a common BO₆ oxygen-octahedral structure. Additionally, X-ray photoelectron spectrum (XPS) provided the composition and the chemical states of the prepared PMN compound. Brunauer–Emmett–Teller (BET) surface area analysis revealed that the PMN material exhibited a large specific surface area (155.8 m²/g) with a small average pore radius (2.8 nm). High-resolution transmission electron microscopy (HR-TEM) images indicated that the PMN particles manifested a regular cubic structure with an average particle size of 43 ± 0.1 nm under the optimum conditions. The selected area electron diffraction (SAED) technique proved the polycrystalline nature of the formed ceramic material. From characterization results obtained in this work, this material shows potential candidate for various metrological applications, including piezoceramic for the ultrasonic transducer cores.