Polarized infrared spectra of α-AlPO4

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Abstract
We report on polarized infrared reflectivity spectra of α-AlPO4. Since the unit cell of berlinite can be viewed as being twice as long as the unit cell of α-SiO2 along the C direction, one expects to find 8(4) strong Γ3 (Γ2) replica at energies similar to the optical modes of α-quartz. Working at room temperature in the range 100-1500 cm−1, we resolve 9 reflectivity structures in polarization E || C and 13 in polarization E ⊥ C. Concerning the Γ2 modes (E || C), we have found all quartz-like replica except for the low-energy folded acoustic branch. We find the mode at 370 cm−1 in berlinite to give rise to two modes at 280 cm−1 and 450 cm−1 of roughly the same intensity. This comes from a strong coupling between the corresponding zone-center and folded modes. Concerning the Γ3 modes (E ⊥ C) we find less complete data and identify only 6Γ3 replica.

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