Crystals of n-butylammonium dihydrogenphosphate, C4H9NH3+. H2PO4-, reveal f
erroelasticity at room temperature and a number of phase transitions when h
eated up to approximately 373 K. Some of these phase transitions show hyste
resis effects. All atoms except two H atoms exist in pairs linked by the lo
st symmetry operations derived from the prototypic space group P2/b2(1)/n2(
1)/a. Each of these two different H atoms is involved in an asymmetric hydr
ogen bond between an oxygen pair. Ferroelastic switching is concomitant wit
h jumps of these hydrogen species from donor to acceptor O atoms. The studi
ed structure belongs to the structural family of n-hexyl- to n-decylammoniu
m dihydrogenphosphates and differs by localization of alternating layers fr
om n-propyl- and n-pentylammonium dihydrogenphosphates. The studied crystal
was slightly twinned; the minor domain constituted approximately 2 degrees
.