Fluorite-Structured Nd2-2xLa2xCe2O7 Synthesis

Fluorite structure can be synthesized using various chemical routes, for instance, the hydrothermal method, solid-state method, and sol–gel auto-combustion method. However, here we used sol–gel auto-combustion approach to synthesize a series of Nd_2−2xLa_2xCe₂O₇ with (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) because it is simple to execute, inexpensive, and ensures crystalline phase in a quick time. To proceed, La(NO₃)₃·6H₂O [purity > 99%], Nd(NO₃)₃·6H₂O [purity > 99%], and Ce(NO₃)₃·6H₂O [purity > 99%] were utilized as precursors along with fuel agents such as urea (CH₄N₂O) and glycine (C₂H₅NO₂). All the chemicals were purchased from Sigma Aldrich. The metal nitrates to fuel agent's ratio was maintained as 1 : 2. The stoichiometric amounts of all the precursors were weighed using a precise digital balance and dissolved separately into deionized water. The individual transparent solutions were combined into a beaker. The beaker was then placed on the hot plate with a magnetic stirrer inside it. The hotplate temperature was set at 95 °C, and stirred magnetically at 320 rpm. The solution thickened over time as a result of the continuous elimination of fumes. The solution was kept on the hot plate till stirring became difficult. Then, the stirring was stopped, magnetic stirrer was taken out, and in a few moments, frothing started that converted viscous liquid into gel. To eliminate the nitrogenous gases from the gel and to trigger the auto-combustion process, the hot plate's temperature was gradually increased up to 310 °C.^{23 (link)} At this temperature urea and glycine catch fire and causes auto-combustion. This combustion yields the CO₂, NO_2, and water vapors according to the following balanced chemical equations:
The hitting of this temperature gave birth to the flame inside the beaker, which instantly burnt all the gel and converted it into ash. As a result of this combustion, the temperature inside the beaker promptly increased which lead to the following reaction for parent composition to occur:
The molecular oxygen (O₂) and nitrogen dioxide (NO₂) were evolved as a byproduct in this reaction. For subsequent samples, the stoichiometric amount of La(NO₃)₃·6H₂O was substituted at Nd(NO₃)₃·6H₂O site. After that, the ash was put into an Agate mortar and pestle for grinding and converted into fine powder. The synthesized powder was then placed into ceramic cups and calcined at 800 °C for 3 h in a Nabertherm furnace to develop a pure phase.^{24 (link)} This calcined powder was pressed using an Apex hydraulic press to make cylindrical pellets of 7 mm diameter and ∼1 mm thickness by applying a force of 30 kN. All the pellets were then sintered at 350 °C for 1 h to make them hard.^{25 (link)} The pictorial representation of this whole synthesis process is shown in Fig. 1.
An advanced Bruker D8 X-ray diffractometer (XRD) was used to analyze the crystalline phase of the synthesized series. A Nova NanoSEM-450 field emission scanning electron microscope (FESEM) was utilized to investigate the morphology and elemental composition. A Radiant's Technologies Inc., USA precision multiferroic tester was used to probe ferroelectric properties. Magnetic properties of synthesized series have been carried out by Cryogenic vibrating samples magnetometer (VSM).