  The influence of pressure on the hydrogen bond ordering in the perovskite
metal-organic framework [(CH3)2NH2]Mn(HCOO)3 has been investigated by
dielectric, pyroelectric adn magnetic measurements in a piston-cylinder cell.
Under ambient pressure the ordering of hydrogen bonds takes place at TC = 188 K
and induces a first-order ferroelectric phase transition. With increasing
pressure to p = 3.92 kbar, the order-disorder transition shifts to a lower
temperature and retains the first-order ferroelectric nature. However, under
higher pressures, the ordering process of hydrogen bonds is split into two
transitions: a broad antiferroelectric transition at high temperature and a
first-order ferroelectric transition at low temperature. With increasing
pressure, the antiferroelectric phase is enhanced whereas the ferroelectric
phase is greatly suppressed, which implies that compression of the perovskite
framework favors antiparallel arrangement of the hydrogen bonds. The canted
anti-ferromagnetic transition was almost unchanged when pressure up to 10.85
kbar. Our study demonstrated that the perovskite metal-organic frameworks are
more sensitive to external pressure than conventional perovskite oxides so that
their electric properties can be easily tuned by pressure.
