Optical absorption spectra of an aqueous frozen solution of neodymium chloride at various temperatures and pressures

A dilute aqueous solution of neodymium chloride NdCl₃ has been taken through two thermodynamic cycles along transformations with varying pressure (between 1 bar and 3.3 kbar) and temperature (between 300 K and 77 K). The characteristic optical line absorption spectrum which shows a Stark splitting of the Nd³⁺ electronic levels due to a quasi-crystalline surrounding of the ions appears only in that part of the cycles where pure ice I_h phase would be present. In any other part of the cycles where pure ice phases II, III, IX and I_C would be present such a phenomenon does not exist. It is shown that ice I_h may constitute a frame for neodymium aquo-ions.