Abstract
Raman spectra are reported for crystalline akermanite (Ak, Ca2MgSi2O7), hardystonite (Har, Ca2ZnSi2O7), gehlenite [Geh, Ca2Al(AlSi)O7], sodium melilite (SM, CaNaAlSi2O7) and for glasses of corresponding compositions. The spectra of melilites are dominated by the vibrational modes of pyrosilicate (T2O7) units (T = Si or Al) and not by the sheet-like structure formed by interconnected TMO4 tetrahedra (M = Mg, Zn and Al). The frequency of vs(T-O-T), t symmetric stretching mode of the bridging oxygen in the pyrosilicate unit, is directly related to the angle of the T-O-T linkage. The symmetric stretching bands of nonbridging oxygens vs(T-O−) appear in the spectral range characteristic of T-O− stretching in pyrosilicate units. The intensity of the vs(T-O−) band is, however, affected by th presence of A13+ in tetrahedral sites adjacent to the pyrosilicate units. The lowering of the intensities of nonbridging oxygen stretching bands in the spectra of Geh and SM is attributed to a change in the degree of covalency of T-O− bonds in the pyrosilicate unit resulting from substitution of A13+ for Mg2+ in the adjacent tetrahedral sites. Comparison of the spectra of crystals with glasses indicates that most Al3+ ions act as network-forming cations, whereas Mg2+, Zn2+ and Ca2+ retain their role as network modifiers. Spectra of glasses of Ak and Har composition show SiO4−4 and Si2O6−7 bands, indicating a redistribution of silicate species among monomer, dimer, trimer and tetramer chains. Glasses of Geh and SM compositions are more highly polymerized than their respective crystalline counterparts because of the role of Al3+ as network-forming cations in the glass structures.
Published Version
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