Investigation of the mechanism of sol-gel formation in the Sr(NO₃)₂/citric acid/ethylene glycol system by solution state ⁸⁷ Sr nuclear magnetic resonance spectroscopy

Abstract

The polymerization mechanism of a modified Pechini process was investigated by 87 Sr nuclear magnetic resonance spectroscopy for the mixed solution of strontium nitrate, citric acid, and ethylene glycol. The C-ratio (the ratio of citric acid to metal ions in the polymer complex) is suggested to have a strong influence on the quality of the derived film. An analysis of the chemical shift variation, as a function of C-ratio, indicates the presence in the solution of two species: solvated strontium ions and strontium ions bound to the polymer complex, with a stoichiometry of 1:7. A polymeric precursor model based on this stoichiometry is proposed. Through a relaxation rate study of the strontium sites, it was found that the polymerization mechanism is predominantly bimolecular within the concentration region being studied. The equilibrium rate constant for the polymerization was calculated to be 10 4 s −1 . A kinetic study of the fast cation exchange between the two identified strontium sites indicates that the inhomogeneity of the polymeric network leads to film cracking during pyrolysis.

Effect of strontium ion doping on structural, thermal, morphological and electrical properties of a co-doped lanthanum manganite system

“…The intensity of peaks due to the impurity phase was very weak to observe for the sample with x = 0.3 mole. The impurity is formed probably due to the strontium ion's properties relative to other cations, such as a large ionic radius, a high polarizability, a low acidity and a tendency to be free in the metal citrate environment [16][17][18][19]. It was shown that the less acidic nature of Sr 2+ relative to La 3+ slows down the protonation process of chelating agent and allows alternative side reactions for forming secondary phase [18].…”

Effects of anion and synthesis route on the structure of (La0.9 Sr0.1) (Cr0.85 Fe0.05 Co0.05 Ni0.05)O3−δ perovskite and removal of impurity phases

“…Substitution of a lower-valence ion on La-and/or Cr-sites of LaCrO 3 results in a charge-compensating transition of Cr 3+ to Table 2 Wyckoff notations for (a) LaCrO 3 [5], (b) SrCrO 4 [17], and (c) CoCr 2 O 4 [29] Atom [34], and (iv) binding environment present around M z+ [35]. The (i)-(iv) strongly affect the homogeneity of metal citrate complex and hence the structure of the perovskite after the pyrolysis.…”

“…Polymerization of metal citrates using ethylene glycol takes place via the following four steps [19,20,33,35,36].…”

“…Thus, the metal complex formation is found better in AY than NY. The SrCrO 4 phase is formed, in general, due to the following distinct properties of Sr 2+ relative to other cations, (i) ionic radius is large (1.44 Å), (ii) it is highly polarizable, (iii) hydrolysis rate is low (3.5 × 10 8 s − 1 ), (iv) it is less acidic, and (v) it has a tendency to be free in a metal citrate environment as observed by NMR study [23][24][25]35]. Furthermore, extremely high hydrolysis rate of Cr 3+ (2.7 × 10 − 6 s − 1 ) supports impurity phase formation by disobeying the rule that the reaction rate must be same for obtaining a single-phase material [24,25].…”

Dielectric properties of hexagonal perovskite ceramics prepared by different routes

“…Two materials were prepared with two different ratios of EG/CA = 1:1 and EG/CA = 1:2, respectively. It can be noted that a specific molar ratio of (AC + EG) to total metal ions (4:1) was achieved to have a homogeneous solution [18,19]. Heating and stirring were continued until the solution started solidifying, forming a gel-like porous mass.…”