Perturbation of the 2201/cm absorption line of the Al-H complex, by uniaxial stressing, was shown to be consistent with the stretching mode of a center with an effectively trigonal symmetry. Quantitative analysis of the effects of stress and temperature revealed the presence of a very low frequency transverse mode which transformed as the E irreducible representation in the trigonal symmetry. Its frequency was 64/cm in the ground state of the stretching mode (A1). An A1 + E combination mode, which was observed only under stressing, lay at 24/cm above the excited state of the A1 mode. Upon substituting D for H, the frequencies of the stretching and transverse modes were reduced by a factor of about 1/2; as expected for vibrations which predominantly involved H (D) atoms. The symmetries of the center and of its modes were unchanged by such isotopic substitution. It was noted that the Al-H axis could be preferentially aligned along one of the <111> axes by applying a stress. The barrier to reorientation was about 0.37eV.

M.Stavola, Y.M.Cheng, G.Davies: Physical Review B, 1996, 54[16], 11322-30