1 Synthesis of (nRExi)2Si2O7 samples
1.1 Target rare-earth oxides and SiO2 powders were weighted according to the stoichiometry of (REI0.25REII0.25REIII0.25REIV0.25)2Si2O7. For example, 0.05 mol Dy2O3 (18.65 g), 0.05 mol Ho2O3 (18.89 g), 0.05 mol Yb2O3 (19.70 g), 0.05 mol Lu2O3 (19.90 g) and 0.8 mol SiO2 (48.07 g) powders were weighted for the synthesis of (Dy0.25Ho0.25Yb0.25Lu0.25)2Si2O7 material.
1.2 Weighted powders were ball milled and mixed for 24 h in a Si3N4 jar with Si3N4 balls and ethanol solution.
1.3. The obtained slip was dried in an air over at 60°C for 24 h and then passed through a 120-mesh sieve.
1.4 Target (nRExi)2Si2O7 powders were synthesis at 1550°C for 6 h in high temperature muffle furnace or at 1800°C for 1 h in high temperature hot-pressing sintering furnace.
2 Phase identification using X-ray diffraction method
2.1 Target (nRExi)2Si2O7 powders were ground using an agate mortar.
2.2 Load the powders into the XRD holder, press and flatten the surface of the powders.
2.3 Data collection. X-ray scanning range: 10-70 o, X-ray scanning rate: 10-20 o/min and 0.02o/step.
2.4 Date analysis using Jade or GSAS program.
3 Composition characterization using SEM-EDS method
3.1 Small bulk samples with dimensions about 4 mm×4mm×3mm were cut from hot-pressed samples using diamond blades.
3.2 The testing surfaces were polished to a diamond paste of 1.5 mm.
3.3 Fix the samples onto the SEM holder with conductive tapa.
3.4 SEM-EDS observation. For each concerned area, repeat the point analysis for at least 7 times.
4 Crystal structure and composition characterization using TEM-EDS method
4.1 Slice samples with dimensions about 3 mm×3mm×1mm were cut from hot-pressed samples using diamond blades.
4.2 The upper and lower surfaces of the slice samples were polished to a diamond paste of 1.5 mm.
4.3 The slice samples were further polished to a foil sample with thickness about 50-100 nm.
4.4 The thickness of the foil samples was further reduced using an ion beam thinner.
4.5 Load the grid into the TEM holder and insert the holder into the TEM.
4.6 TEM-EDS observation. First of all, we located different phases (b-RE2Si2O7, g-RE2Si2O7 or G-RE2Si2O7 grains) in the samples according to the EDS-point analysis results or selected area electron diffraction results. Then micro-structure and chemical composition analyzation of concerned aeras were conducted utilizing high angle annular dark field (HAADF) detector and energy dispersive X-ray spectroscopy (EDS) systems.