Synthesis And Characterization Of Apatite-type Lanthanum Silicates/Germanates Materials As Electrolyte

In recent years, the investigations of apatite-type lanthanumsilicates/germanates (ATLS/ATLG) as a new oxide ions conductor has gainedconsiderable interest owing to its clean energy conversion, high energy efficiencyand the potential for developing fuel cell devices [1-10]. In contrast to a vacancymechanism commonly observed for fuorite-and perovskite-type oxide ionconductors, ATLS/ATLG has a unique interstitial conduction mechanism. Therefore,they exhibit a higher ionic conductivity at intermediate temperature (500-800°C).However, the research of them at the early exploration stage, many problems stillneed to be solved. Firstly, the preparation of ATLS/ATLG is commonly used by theconventional solid state synthesis method, while there are many disadvantages in thismethod that limits their preparations, particularly the preparations of ATLG.Secondly, the doped investigations of ATLS/ATLG are not perfect until now, thecorresponding studies on the correlation of crystal structural changes and electricalproperties have to be further completed. Finally, up to now the structural alterationmechanism cannot be thoroughly built, owing to lack of quantitative data.1. Through the study of the crystal structure and ionic conductivity ofMg-doped apatite-type lanthanum silicates La10Si6-Mg O27-(=0í0.4), to optimizethe Mg doping on Si site for apatite-type lanthanum silicates. La10Si6-Mg O27-(=0í0.4) samples were prepared by solid state synthesis method, and their crystalstructure, morphology and electrical property were measured and analyzed. Theresults indicate that: Mg doping on Si site results in a significant lattice expansion,owing to the effective ionic radius of Mg2+for fourfold coordination (0.57) ismuch larger than that of Si4+(0.26); Mg doping on Si site leads to the significantenlargement of channels, favoring oxide-ion conduction; La10Si5.8Mg0.2O26.8exhibitsthe highest conductivity with a value of3.0×10-2Scm-1at700°C among the Mgdoped samples, which is related to its highest linear density of interstitial oxygen. 2. Through the high-pressure behaviors of apatite-type La10Si6O27, tounderstand the structural alteration mechanism and compressibility of lanthanumsilicates. La10Si6O27powders were prepared by by the molten-salt synthesis methodusing NaCl as eutectic salt, and in situ X-ray diffraction study of apatite-typeLa10Si6O27with P63/m space group was carried out using angle-dispersivesynchrotron radiation technique. The results indicate that: A reversible phasetransition from P63/m to P63symmetry was observed at~14.5GPa, which wascompanied by~2.0%c-axis shrinkage and~2.2%volume collapse; This structuralphase transition was attributed to the reduction of symmetry caused by the gradualtilting of the SiO4tetrahedra; the high-pressure phase exhibits an unusual bulkmodulusB0=117.2±6.6GPa in contrast with the low-pressure phase with B0=124.8±1.7GPa.3. Through the high-pressure behaviors of Mg doped apatite-type lanthanumsilicate La10Si5.8Mg0.2O26.8, to understand the influence of Mg doping effectmechanism on the structural alteration and compressibility of lanthanum silicates.La10Si5.8Mg0.2O26.8powders were prepared by by the molten-salt synthesis methodusing NaCl as eutectic salt, and in situ X-ray diffraction study of apatite-typeLa10Si5.8Mg0.2O26.8was carried out using angle-dispersive synchrotron radiationtechnique. The results indicate that: The sample underwent a reversible structuraltransformation with the reduction of symmetry from P63/m to P63at~16.1GPa, andthe phase transition should be attributed to the tilting of the Si/MgO4tetrahedra.La10Si5.8Mg0.2O26.8exhibited higher compressibility for the high-pressure phase thanthe initial phase; In contrast with the previous reported undoped apatite-typeLa10Si6O27sample, the transition pressure of the Mg doped sample was elevated by~1.6GPa, while the compressibility was lowered for the initial P63/m phase.4. Through the study on the preparation and electrical property of apatite-typeLa9.33Ge6O26, to optimize the preparations of apatite-type lanthanum germanates.La9.33Ge6O26powders have been successfully synthesized by facile molten-saltsynthesis method using NaCl as eutectic salt, and the morphology, structure andelectrical property of the powders and corresponding pellets have been measured. The results indicate that: Apatite-type La9.33Ge6O26powders have been successfullysynthesized at900°C by a facile molten-salt synthesis method, which is lower atleast200°C than that of solid state synthesis method, but its displays a bettercrystallinity; The as-prepared La9.33Ge6O26powders are homogeneous, nano-size,less agglomerated and well crystallized particles, which are favorable to obtain thehigh dense pellets; Compared with the conventional methods, dense pellets havebeen successfully sintered at a relatively low temperature of1100°C, which inhibitsthe vaporization of GeO2; The pellets exhibit a higher conductivity with a value of2.4×10-2Scm-1at850°C, due to the high density and the avoidance of impurityLa2GeO5; The main problems such as Ge loss, agglomeration and bad crystallinityencountered in the conventional preparations have been effectively solved via themolten-salt synthesis method.5. Through the high-pressure behaviors of apatite-type La9.33Ge6O26, tounderstand the structural alteration mechanism and compressibility of lanthanumgermanates. La9.33Ge6O26powders were prepared by by the molten-salt synthesismethod using NaCl as eutectic salt, and in situ X-ray diffraction study of apatite-typeLa9.33Ge6O26was carried out using angle-dispersive synchrotron radiation technique.The results indicate that: A reversible phase transition from P63/m to P63symmetrywas observed at~16.5GPa, which was companied by~1.9%c-axis shrinkage and~2.2%volume collapse; This structural phase transition was attributed to thereduction of symmetry caused by the gradual tilting of the GeO4tetrahedra; the bulkmodulus of high-pressure phase is B0=85.8±3.9GPa, which is lower than that ofthe low-pressure phase (B0=124.8±1.7GPa).