Resolve the discrepancy in measured optical properties of ammonium dihydrogen phosphate

Determine whether the discrepancy between the ammonium dihydrogen phosphate refractive-index measurements of Zernike et al. and Querry et al. is caused by differences in experimental setup or by the distinct physical states of the material, namely solid crystal versus aqueous solution.

Background

Ammonium dihydrogen phosphate (ADP) is considered as a potentially important high-altitude cloud condensate in cool, metal-rich substellar atmospheres. Incorporating ADP into cloud models requires reliable real and imaginary components of its complex refractive index across the relevant wavelengths. The paper compares measurements from Zernike et al., which provide optical-to-near-infrared real refractive indices, with measurements from Querry et al. and Nikolic et al. at longer wavelengths. Around 2 μm, the Zernike extrapolation and Querry measurement differ by approximately 10%, and the overlapping 5–20 μm measurements from Querry et al. and Nikolic et al. also disagree substantially. The authors identify the differing experimental conditions and material phases as possible explanations, but do not resolve the cause.