Raman spectrum of ammonia IV

We present an extensive analysis of the Raman spectrum of ammonia IV, for both NH3 and ND3, between 6 and 11 GPa. Polarized Raman scattering experiments were performed at low temperature (20–80 K) on single-crystalline samples grown in a diamond anvil cell. This method enabled us to discern very weak features of the Raman spectrum, including previously unobserved bending modes nu2–nu4. In parallel, we used first-principles methods, based on density functional theory, to compute the theoretical Raman spectra at the conditions of our experiments. An overall good agreement is found between experiment and theory. The results of the measurements with different polarizations and a comparison with our calculations allowed an unambiguous determination of the Raman modes. The assignment was further confirmed by examination of the isotopic ratios and Grüneisen parameters. This study thus provides a much more complete picture of the Raman spectrum of phase IV than available before and constitutes a solid basis for the investigation of ammonia at higher pressures