Theoretical study of structural, mechanical and spectroscopic properties of boehmite (γ-AlOOH)

Abstract

The structural, mechanical and spectroscopic properties of boehmite (AlOOH polymorph)were investigated by means of first-principle density functional theory (DFT) andsemiempirical density functional based tight binding (DFTB) methods. Apart from amarginal underestimation of interlayer hydrogen bond distances the DFT method wellreproduces the experimental equilibrium low-pressure structure. For the DFTB methodsimilar good agreement was obtained for lattice parameters, however bond lengths andangles showed a larger deviation from experiment in comparison to DFT results. Theexperimental spectrum of the OH stretching region was interpreted by means of thecalculated frequencies within the frame of the harmonic approximation and by calculatingthe power spectra of the hydroxyl groups obtained from molecular dynamicssimulations. Using the latter approach, the strong coupling between the individualOH modes was demonstrated. Isostatic structural compression of the boehmitestructure was performed in order to obtain the bulk modulus and the dependenceof the vibrational spectrum on the pressure. The DFT method gives a value of97 GPa in the athermal limit. Comparison with available bulk moduli for otherAlOOH polymorphs reveals that boehmite shows the highest compression, forwhich mainly a strong shortening mechanism of interlayer hydrogen bonds isresponsible. The DFT method also described correctly the dependence of the OHstretch frequencies upon compression resulting in a strong red shift. Althoughgood performance is observed for the low-pressure region, the DFTB methodis not found to be suitable for high-pressure studies in cases such as boehmite.