研究目的
To determine the valence states of elements in octahedral and tetrahedral sites of new anion-substituted garnets Ca3Mn2 – x(Ni, Co)xVyGe3 – yO12 (x = 0, 1; y = 0, 1, 2) and to study their electrical conductivity.
研究成果
The characterization of the prepared garnets Ca3Mn2 – x(Ni, Co)xVyGe3 – yO12 (x = 0, 1; y = 0, 1, 2) has shown a systematic increase in unit cell parameter in the order of samples as a function of the crystallographic radii of ions in the octahedral and tetrahedral sublattices. The X-ray photoelectron spectroscopic study has shown that all vanadium ions in these compounds are in the pentavalent state. The electrical conductivity values classify the prepared garnets with medium-gap semiconductors. For ceramic samples of garnets, the low-temperature total resistance is dominated by the bulk term. Above 500°С, the electronic conductivity term becomes dominant.
研究不足
The ionic conductivity term is insignificant (less than 3 × 10–6 S/cm) as judged from previous studies. The identification of the major charge carriers and conductivity type depending on temperature will be the subject of future investigation.
1：Experimental Design and Method Selection:
The citrate process was used to prepare samples. Vanadium oxide and germanium dioxide were dissolved in aqueous citric acid under heating. After the oxides dissolved, salt weights in the nitrate form were inserted into the solution. The resultant solutions were concentrated to a light brown xerogel followed by the heat treatment of the xerogel at 300°C. Afterwards, samples were annealed at temperatures in the range from 550 to 900–1100°C with intermittent grindings.
2：Sample Selection and Data Sources:
The initial reagents used were СаCO3, Mn(NO3)2, Co(NO3)2 ? 6H2O, Ni(NO3)2 ? 6H2O (all of pure grade), specialty grade V2O5,
3：94% pure GeO2, chemically pure grade nitric acid, and pure grade citric acid monohydrate. List of Experimental Equipment and Materials:
Shumadzu XRD-700 X-ray setup for phase composition monitoring, VGES-CALABMKII electron spectrometer for X-ray photoelectron spectroscopy (XPS), GM instek LCR-819 immitance meter for electrical conductivity measurements.
4：Experimental Procedures and Operational Workflow:
The annealing time at the highest temperature was 10–15 h. Electrical conductivity measurements were carried out on cylinder-shaped ceramic samples of prepared garnets, which were 10–15 mm in diameter and
5：5–2 mm thick, with silver electrodes by the electrochemical impedance method in the temperature range 90–900°C in 10°C increments. Data Analysis Methods:
The electrical conductivity σ for each temperature was calculated from the sample thickness, diameter, and ohmic resistance.
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