Structural Change in Phosphorus-Bearing Dicalcium Silicates

Abstract

Structures of (Ca_2-x/2_??__x/2)(Si_1-xP_x)O₄ crystals were examined as a function of x ranging from 0.03 to 0.40. All of the samples were heated at the stable temperature region of the α-phase and then quenched in water. The phase constitution at ambient temperature was classified into three categories according to the fraction of the α-to-α′_H transition. When the transition was completed as in the crystals with x≤0.100, the β- and α′_L-phases were obtained, the relative amounts of which were determined by the start and finish temperatures of the α′_L-to-β martensitic transformation. With 0.125≤x≤0.150, the α-to-α′_H transition was incomplete. During further cooling, the product α′_H-phase was inverted to the α′_L-phase, and the residual α-phase was inverted to the incommensurate phase successively. Because the start temperature of the α′_L-to-β transformation was lower than ambient temperature, the α′_L-phase was stabilized. With x≥0.175, all of the crystals were free from the α-to-α′_H transition. The crystals with 0.175≤x≤0.225 were made up exclusively of the incommensurate phase. A good correlation existed between the modulation wavelength (=N) and the P/(Si+P) ratio (=x) as N=4.134-1.56x (0.175≤x≤0.250). With 0.275≤x≤0.300, the crystals were isostructural with α-Ca₂SiO₄. The hexagonal phase with 0.350≤x≤0.400, probably a transition product from the α-phase, showed two-dimensional modulations along the α-axis with N=2 and along the c-axis with N=3.