Structure Change of Ca 2 SiO 4 Solid Solutions with Ba Concentration

A series of Ba‐bearing Ca 2 SiO 4 solid solutions (C 2 S( ss )), (Ba x Ca 1− x ) 2 SiO 4 with 0.075 x 0.30, were prepared and examined by X‐ray and electron beam diffraction. They are all made up of orthorhombic domains 120° different in orientation around the common c axis of the former α phase. The C 2 S solid solution with x = 0.075 shows a superstructure incommensurate along the a axis with λ (modulation wavelength) = 3.5 and commensurate along the c axis with Δ= 3. With x = 0.15, modulation is observed only along the a axis and Δ= 3.4. No evidence of superstructure is found with x = 0.24; the space group and cell dimensions are comparable with those of pure α ’ H ‐C 2 S. The C 2 S( ss ) with x = 0.30 gives a superlattice with the cell‐edge length of 3 b . All the C 2 S( ss ), when reheated at 1000°C for 24 h, produced lamellae of the trigonal phase T nearly in parallel with (001) of the host α ’ L phase. The crystallographic orientation between the two phases is image This indicates that the above Ba‐bearing C 2 S( ss ) phases occur as precursors to the thermodynamically more stable two‐phase mixtures.