P21/c structure there are two symmetrically independent chains
with different deformation angles and Si-O bond lengths (Fig. 2).
Because the 670 cm–1 peak is related to the bending motion of the
tetrahedral chain, a splitting of the 670 cm–1 peak was expected
from the presence of two independent tetrahedral chains. Raman
bands at lower frequencies, which are related to M2 and M1-O
motion, did not show peak spitting because of the presence of
uniquely symmetric and independent M2 and M1 sites in both
P21/c and in C2/c. The mode splitting at 670 cm–1 is therefore
evidence in the Raman data of the phase transition of Ca-rich
samples from the C2/c to P21/c space group, as previously observed using TEM and powder diffraction analysis (Tribaudino
2000; Tribaudino et al. 2003, 2005).