Polymorphs and high-temperature phase transformations␣Ј L -Ca 2 SiO 4 (with ␣Ј L generally considered as a superstructure referring to the ␣Ј H lattice) 3,4,10,11 and ␣-Ca 2 SiO 4 , structurally of dicalcium silicate (Ca 2 SiO 4 ) are investigated on powdered samples, using Raman scattering techniques. Raman specstill uncertain with trigonal 12,13 or hexagonal symmetry, 14 are stable as pure Ca 2 SiO 4 only at elevated temperatures. The tra at room conditions of pure ␥-and ␤-Ca 2 SiO 4 and stabilized ␤-, ␣ L -, and ␣-Ca 2 SiO 4 are presented in the frequency sequence of phase transformations generally accepted is that in Fig. 1, 15 and the proposed transformation mechanisms are as range of 110-1100 cm ؊1 . Each polymorph can be identified by its Raman spectrum, even though strong similarities follows: a semireconstructive mechanism between ␣ and ␣Ј H ; 14 a second-order transformation 16 or a displacive transformation exist between the ␤, ␣ L , and ␣ phases. In-situ hightemperature Raman spectroscopic studies in the temperamechanism between ␣Ј H and ␣Ј L ; 14 a displacive, 14 possibly martensitic, mechanism 17 or a first-order transformation between ture range of 298-1433 K on heating and cooling also are ␣Ј L and ␤; 16 a reconstructive 14 or a semireconstructive ␥ → reported, starting from ␥-Ca 2 SiO 4 . Modifications occurring ␣Ј L transition; 5 and a reconstructive, 14 semireconstructive, 5 or in the Raman spectra recorded in the wavenumber range of displacive mechanism for the ␤ → ␥ phase transformation. 18,19 90-1000 cm ؊1 show up during heating, the irreversible Besides these five phases, an additional phase intermediate ␥ → ␣ L phase transformation. Coexistence of ␥ and ␣ L between ␥ and ␣Ј L has sometimes been observed. 20-22 grains is observed in the temperature range of 1091-1119 K In most cases, X-ray diffractometry (XRD) and differential at least. The differences observed between the ␥-Ca 2 SiO 4 thermal analysis (DTA) are the techniques used to attempt and ␣ L -Ca 2 SiO 4 Raman spectra suggest that this phase to clarify the polymorphism and phase transformations in transformation is a reconstructive first-order transition.Ca 2 SiO 4 . 4,14,16,21 In this study, application of Raman spectrosThe reversible ␣ L → ← ␤ phase transformation also is copy is described and viewed as a complementary technique to described. Raman spectra of pure ␤ and ␣ L phases resemble the previous ones. Indeed, vibrational spectroscopy is a useful each other. The transition occurs from 949 K, on cooling, to tool for sample characterization and analysis and for the calcu-960-988 K, on heating, with a hysteresis probably depenlation of thermodynamic and elastic properties. Vibrational dent on the thermal history of the starting product. From frequencies also are very sensitive to interatomic forces. Indithe Raman scattering observations, this phase transformavidual peaks may be associated with the presence of particular tion also is considered as a first-order transformation and is structural groups within the sample. Thus, Ram...

Energy-dispersive X-ray powder diffraction experimentstemperature; 1-3,11 and finally ␣, the highest-temperature polyhave been investigated at high temperature and room presmorph whose structure is still uncertain and has trigonal 12,13 or sure, and at high pressure and room temperature, starting hexagonal symmetry. 14 The structures of all of these phases are from either ␥or ␤-Ca 2 SiO 4 . High-temperature studies built from isolated SiO 4 tetrahedra. The differences come from were performed up to 1980 K, using a versatile heating cell.the orientation of the SiO 4 tetrahedra and the movements of The high-temperature phase transformations previously calcium ions. described were reexamined. Volume and linear thermalDespite the numerous X-ray diffraction studies at elevated expansions were measured for each Ca 2 SiO 4 polymorph, ␥, temperature, few papers include the investigation of all of the ␤, ␣ L , ␣ H , and ␣. Volume thermal expansion increases Ca 2 SiO 4 polymorphs. Indeed, in most cases the starting material with increasing temperature except for ␣ H , whose thermal is either the ␥ 5,15 or the ␤ form of Ca 2 SiO 4 , 3,4 and because of the expansion tends to decrease at elevated temperature. Highdifficulty in reaching the highest temperatures, the ␣ phase pressure investigations were performed in the 0-15 GPa has scarcely been studied. 2,16,17 It also appears that the thermal pressure range, using a diamond anvil cell, with silicon oil as expansion data are not well-defined, in particular for ␣Ј H -and the pressure-transmitting medium. The value of the room-␣-Ca 2 SiO 4 . Authors generally report the unit-cell parameters or pressure bulk modulus K 0 , assuming a second-order Birchvolume data only at selected temperatures. 2-4 Furthermore, a Murnaghan equation of state with K 0 ‫؍‬ 4, is 140(8) GPa for great confusion still persists in the literature about the exact ␥-Ca 2 SiO 4 . The ␥ olivine form exhibits anisotropic comsequence of the pure Ca 2 SiO 4 phase transformations with pression, with the c axis as the most compressible. From increasing temperature. The scheme usually proposed is that in such in situ high-pressure X-ray investigations, the ␥-→ Fig. 1 but other possibilities have been reported. For instance, ␤-Ca 2 SiO 4 phase transformation induced by cold compresanother monoclinic form called ␣Ј m has been observed in the sion is clearly evidenced and extends from 2 to about 5 GPa.range 984-1252 K. 19 Again, Sarkar suggested that a "transient phase" between ␥ and ␣Ј L existed over the temperature range I. Introduction 1089-1183 K, 5 and the existence of a cubic phase at temperatures higher than 1873 K was also mentioned. 16 The characteris-

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