Compressional behaviors of ammonium phosphomolybdate hydrate (APMH) with different pressure media.

This study investigates the pressure-dependent structural response of ammonium phosphomolybdate hydrate (APMH) under four distinct pressure-transmitting media (PTMs): distilled water, methanol, ethanol, and silicone oil. Synchrotron X-ray diffraction combined with Rietveld refinement confirmed that APMH maintains the archetypal Keggin-type framework while incorporating approximately ten crystallographic water molecules per unit cell, distributed over two distinct coordination sites (OW1 and OW2). High-pressure diffraction experiments revealed pronounced PTM-dependent compressional behaviors. In water, APMH undergoes an abrupt 2.6% volume collapse near 2 GPa followed by framework stiffening, while silicone oil induces significant densification above ~4 GPa. By contrast, methanol and ethanol promote smooth, elastic contraction without discontinuities. Bulk moduli derived from equation-of-state fitting span a wide range, from ~28 GPa under low-pressure silicone oil to 321 GPa at high pressures, highlighting the critical role of PTM chemistry and penetrability. Microstrain analysis further identified anisotropic deformation, with the (222) planes particularly sensitive to stress accumulation under both water and silicone oil. These results demonstrate that APMH compressibility is not an intrinsic constant, but a variable property governed by external medium.