Spectroscopic properties for high-pressure polymorphs of calcium and strontium metasilicates.

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The IR spectra for the high-pressure polymorphs of alkaline-earth metasilicates, viz. α-CaSiO3, δ-CaSiO3, α-SrSiO3, δ-SrSiO3, and δ′-SrSiO3, which consist of cyclic anions, (Si3O9)6− or (Si4O12)8− rings, were studied over a range of 1200–400 cm−1. These silicates gave absorption bands in wavenumbers of about 1050–820 cm−1, 750–620 cm−1, and below about 580 cm−1, based on symmetrical and anti-symmetrical stretches of Si–O bonds and Si–O bending modes for tetrahedral SiO4 units, and Ca–O or Sr–O stretches. Among them, the bands characteristic of the silicate anions with ring structure in the SrSiO3 polymorphs were found to shift to lower wavenumber with transformation into the δ′ form. This suggests that the (Si4O12)8− ring in δ′-SrSiO3 is less flexible than the (Si3O9)6− ring in the α or δ form. The experimental result that the quenching temperature of δ′-SrSiO3:Eu2+ phosphor is higher than that of the δ form supports this consideration.
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