High‐Temperature, High‐Pressure X‐ray Investigation of Dicalcium Silicate

Energy‐dispersive X‐ray powder diffraction experiments have been investigated at high temperature and room pressure, and at high pressure and room temperature, starting from either γ‐ or β‐Ca 2 SiO 4 . High‐temperature studies were performed up to 1980 K, using a versatile heating cell. The high‐temperature phase transformations previously described were reexamined. Volume and linear thermal expansions were measured for each Ca 2 SiO 4 polymorph, γ, β, α′; L ,α′; H , and α. Volume thermal expansion increases with increasing temperature except for α′; H , whose thermal expansion tends to decrease at elevated temperature. High‐pressure investigations were performed in the 0–15 GPa pressure range, using a diamond anvil cell, with silicon oil as the pressure‐transmitting medium. The value of the room‐pressure bulk modulus K 0 , assuming a second‐order BirchMurnaghan equation of state with K' 0 = 4, is 140(8) GPa for γ‐Ca 2 SiO 4 . The γ olivine form exhibits anisotropic compression, with the c axis as the most compressible. From such in situ high‐pressure X‐ray investigations, the γ‐→Ca 2 SiO 4 phase transformation induced by cold compression is clearly evidenced and extends from 2 to about 5 GPa.