Simulated annealing structure solution of a new phase of dicalcium silicate Ca₂SiO₄ and the mechanism of structural changes from α-dicalcium silicate hydrate to α_L′-dicalcium silicate via the new phase

A new phase of dicalcium silicate (Ca₂SiO₄) was formed by heating α-dicalcium silicate hydrate [α-Ca₂(SiO₄H)OH = α-C₂SH] at temperatures of ∼663–763 K, and it was transformed into -Ca₂SiO₄ (= -C₂S) above ∼1193 K. The crystal structure of the new phase (hereafter called x-C₂S) has been determined by simulated annealing and refined by the Rietveld method using synchrotron radiation powder diffraction data. The structure consists of isolated SiO₄ tetrahedra and a three-dimensional CaO_n polyhedral network, forming a new structural type of dicalcium silicate. A structural change from α-C₂SH to x-C₂S is compelled by large displacements of SiO₄ tetrahedra, accompanied by dehydration, in the direction perpendicular to the two-dimensional Ca(O,OH)_n polyhedral network in α-C₂SH. With increasing temperature, sizes of CaO_n polyhedra in x-C₂S become too large to confine Ca atoms at the sixfold to eightfold coordination sites. Then the structure of x-C₂S is transformed into -C₂S, having eightfold to tenfold coordination sites for the Ca atoms.

Computing details

Cell refinement: WPPF(ver.3.00); data reduction: DATAPRO(ver.2.00); program(s) used to solve structure: DSS(ver.1.00); program(s) used to refine structure: PFLS(ver.5.00); molecular graphics: Ball&Stick.

Figures

dicalcium silicate

Crystal data

Ca2SiO4	Z = 8
Mr = 172.24	F(000) = 688
Monoclinic, P21/c	Dx = 2.91 Mg m^−3
a = 8.2127 (5) Å	Synchrotron radiation, λ = 1.598 Å
b = 9.7930 (4) Å	T = 298 K
c = 9.7954 (5) Å	Particle morphology: plate-like
β = 94.848 (5)°	white
V = 785.00 (7) Å^3	flat sheet, 29 × 29 mm

Data collection

MDS, 2-axis, multiple-detector system diffractometer	Data collection mode: reflection
Radiation source: synchrotron radiation	Scan method: step
Double-crystal Si(111) monochromator	2θmin = 10°, 2θmax = 155°, 2θstep = 0.005°
Specimen mounting: Sample was mounted on a glass specimen holder.

Refinement

Rp = 0.084	Excluded region(s): none
Rwp = 0.116	Profile function: pseudo-Voigt
Rexp = 0.051	1/(Yi)
RBragg = 0.070
χ^2 = 5.198	Background function: polynomial
28961 data points	Preferred orientation correction: symmetrized harmonic expansion (Jarvinen, 1993)

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ca1	0.8258 (3)	0.4210 (2)	0.0729 (2)	0.0160
Ca2	0.6621 (3)	0.4181 (2)	0.4121 (2)	0.0160
Ca3	0.3760 (3)	0.2889 (2)	0.0816 (2)	0.0160
Ca4	0.9081 (3)	0.0681 (2)	0.2068 (2)	0.0160
Si1	0.0221 (4)	0.2959 (3)	0.4326 (3)	0.0137
Si2	0.4618 (4)	0.5757 (3)	0.2026 (3)	0.0137
O1	0.8768 (7)	0.2993 (6)	0.5287 (7)	0.0175
O2	0.9625 (8)	0.3647 (5)	0.2856 (7)	0.0175
O3	0.0641 (8)	0.1350 (5)	0.4077 (7)	0.0175
O4	0.1831 (7)	0.3873 (5)	0.5038 (6)	0.0175
O5	0.5894 (9)	0.4557 (5)	0.1682 (6)	0.0175
O6	0.2903 (9)	0.5091 (5)	0.1502 (6)	0.0175
O7	0.4982 (7)	0.7130 (5)	0.1283 (7)	0.0175
O8	0.4557 (7)	0.5955 (5)	0.3702 (7)	0.0175