Anomalous dielectric response of short hydrogen bonds under pressure: the case of (Mn,Fe) 2+ AlPO 4 (OH) 2 H 2 O.

An anomalous increase in the real part of dielectric response is observed in Mn0.5 Fe0.5 AlPO4 (OH)2 H2 O while cooling to ~70 K. This is addressed to field-induced proton dynamics in a short hydrogen bond of 2.480(3) Å. The absence of discontinuities in heat capacity curves above the Néel temperature (T N   ≈  7 K) excludes a paraelectric to antiferroelectric phase transition. Upon the application of mild hydrostatic pressures below 1.6 GPa, the maximum in the dielectric response is shifted from 70 K to lower temperatures near 2 K. This explains a narrow correlation between proton transfer and the compression of the short hydrogen bond length.