[en] A detailed analysis of the vibrational spectra of 1,3-disilacyclobutane and 1,3-disilacyclobutane-1,1,3,3-d₄ in vapor, liquid, and solid phases shows that the molecules have puckered ring conformations in the former two phases but become planar in the solid state. This is reflected by the frequency shifts of the ring deformation and SiH₂ (or SiD₂) rocking modes which are uncoupled for the planar conformation but interact strongly for the puckered structure. Frequency differences between gas and solid phases for these two modes range from 27 to 42 cm^-1. Force constant calculations confirm that these shifts are due to the different amounts of vibrational mixing for different conformations in the two phases. 6 figures, 5 tables