Structural, spectroscopic, and thermodynamic characterization of ammonium oxalate monohydrate mineral using theoretical solid-state methods

Abstract

In this study, the structural properties, main characteristics of the Raman spectrum, and the thermodynamic properties of the ammonium oxalate monohydrate mineral oxammite, (NH4)2(C2O4)·H2O, were investigated in theoretical solid-state calculations based on the periodic density functional theory using plane waves and pseudopotentials. The optimized structure of oxammite agreed very well with that obtained from low temperature X-ray diffraction data by structure refinement (orthorhombic symmetry, space group P21212; lattice parameters: a=8.017Å; b=10.309Å; c=3.735Å). The calculated structural properties, including the lattice parameters, bond lengths and angles, and X-ray powder pattern, accurately reproduced the experimental information. The Raman spectrum determined theoretically agreed with that obtained experimentally. The assignment of the Raman spectral bands significantly improved their previous empirical assignment. Thus, the assignment of a large series of bands was modified and the origins of several previously unassigned bands were found. Five bands in the experimental spectrum at 2344,2161,1933,1902, and 815cm−1, were absent from the computed spectrum and they were identified as combination bands. The band located at 2879cm−1 was confirmed as an overtone. Furthermore, the theoretical calculations clearly showed that some features described as single peaks in previous experimental studies were due to the contributions of several bands. The thermodynamic properties of the oxammite mineral, including the specific heats, entropies, enthalpies, and Gibbs free energies, were determined as functions of temperature. The specific heat calculated at 323 K, Cp=202.3J·K−1·mol−1, was in good agreement with the corresponding experimental heat capacity, Cp=211.7J·K−1·mol−1, where the values only differed by about 4%. Finally, using the computed thermodynamic data, the thermodynamic properties of the formation of oxammite as well as the free energies and reaction constants of the reaction for its thermal decomposition were determined.