Tripotassium citrate → Citric acid, lanthanum(3+) salt(1:1)

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In one embodiment for the synthesis of (La,K,□)2Nb2(O,□)7 (OH)2; (□=vacancies) (1), lanthanum acetate (0.42 g, 1.35 mmol) is dissolved in 4 grams of DI water by stirring in a 23 ml Teflon cup for a Parr reactor, which is a closed-cell reactor suitable for use in hydrothermal synthesis. Other reaction vessels or reactors that are suitable for use for reactions conducted under pressure and at elevated temperatures may be used. Potassium citrate, tribasic (0.44 g, 1.35 mmol) is dissolved in 3 grams of DI water and added dropwise to the lanthanum acetate solution. A thick white precipitate of lanthanum citrate is formed immediately. The pH at this point is 8.4. Thirty drops of potassium hydroxide solution (4 molar) is added, and the precipitate redissolves after about 20 drops. K8[Nb6O19].16H2O (0.33 g, 1.35 mmol Nb) is dissolved in 2 grams of water and added to the solution in the Parr reactor. Alternatively, the K8[Nb6O19].16H2O can be dissolved directly in the lanthanum citrate solution. The final pH is recorded at 12.7. The reactor is closed in its steel casing and heated in a 225° C. box furnace for 3-4 days. A white crystalline powder is collected (approximately 0.8 grams; around 100% yield) by vacuum filtration and washing with DI water and then methanol. ˜2.5% weight loss by TGA analysis (calculated=3%) Compositional analysis for (La1.7K0.1□0.2)Nb2(O6.6□0.4)(OH)2 (FW=565.8). Calculated (from formula derived from X-ray data): 41.8% La, 32.9% Nb, 0.69% K. Experimental: 40.2% La, 30.0% Nb, 0.82% K.

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