Off-center Li+ ions: solid state rotators described by Mathieu's nonlinear-oscillator equation M. Georgiev, M. Mladenova, V. Krastev and A. Andreev Eur. Phys. J. B, 29 2 (2002) 273-277 Published online: 02 October 2002 DOI: 10.1140/epjb/e2002-00300-1