Performing polarized reflectivity and absorption measurements in the experimental range 151500 cm-1, we have investigated the infrared activity of phonons in aluminum phosphate. We report both 2 (Ec) and 3 (Ec) polarized components and, in order to identify all modes predicted by group theory arguments, we compare with similar measurements performed on -quartz. We have found the following. In polarization Ec, we resolve from reflectivity spectra all but one infrared-active 2 components. Since they are exclusively infrared active, they could not be found from any other technique. To investigate the missing mode, which corresponds to the folded acoustic branch, we performed a series of transmission measurements. In this way we could find a weak absorption structure, which appears at room temperature at 48 cm-1. In polarization Ec, we find 15 infrared-active 3 components which compare satisfactorily with previously published Raman data. Two modes, at 112 and 126 cm-1, form a close doublet which resolves only when running absorption measurements at liquid-helium temperature. The high-energy component (126 cm-1) comes from the folded acoustic branch and the second is the infrared counterpart in aluminum phosphate of the 128-cm-1 phonon of -quartz. This is clearly established from a comparison of both reflection and transmission experiments performed on the same sample. Finally, performing a series of oscillator fits, we get quantitative values for (i) the LO-TO splitting of the phonon modes under consideration and (ii) the corresponding oscillator strengths. The series of parameters obtained in this way compare satisfactorily with a simple model of infrared exclusive modes in -quartz admixed twice: first, with silent or Raman exclusive components (this is the case in the variety) and, second, with zone-boundary modes (this appears in berlinite when doubling the length of the unit cell). © 1988 The American Physical Society.
|Journal||Physical Review B|
|Publication status||Published - 1 Jan 1988|