Crystal systems and dielectric constants of high-pressure phases of KH2PO4

High-pressure phases of a hydrogen-bonded ferroelectric, KH2PO4, were studied by angle-dispersive x-ray diffraction using a diamond-anvil cell and synchrotron radiation and by a dielectric constant measurement using a multianvil at room temperature. The analysis of the obtained x-ray-diffraction pattern clarified that the crystal system of phase V is orthorhombic, and the volume reduction for the IV-V phase transition was estimated to be 2.9% at 5.4 GPa. The decrease of the dielectric constant was observed at the TV-VI transition beginning at around 7 GPa for the single-crystal parallel to the a axis. As to the IV-V transition, the results were different according to the crystallographic axis; the decrease for the a axis and the increase for the c axis. All phases of IV, V, and VI are assumed to be paraelectric at room temperature, although the ferroelectric or antiferroelectric state may appear at low temperature.