Far infrared properties of the incommensurate ferroelectric Rb2ZnCl 4 J. Petzelt, A.A. Volkov, G.V. Kozlov, S.P. Lebedev et J.P. Chapelle J. Phys. France, 43 9 (1982) 1359-1363 DOI: 10.1051/jphys:019820043090135900