A sudden and reversible increase of about 5% in the unit cell volume of ammonium tungstoniobate at pressures around 2.5 GPa was observed with pressure transmitting media containing water. This behavior was attributed to high pressure water insertion from the pressure media into the host structure of ${\mathrm{NH}}_4{\mathrm{NbWO}}_6$. The effects of different water contents of the pressure transmitting medium on the high pressure behavior of ${\mathrm{NH}}_4{\mathrm{NbWO}}_6$ were studied by high pressure energy dispersive x-ray diffraction and by Raman spectroscopy of the ammonium ${\nu }_2$ bending mode. The water insertion occurs with no apparent change in the pyrochlore framework, and a preliminary analysis indicates a partial occupation of the site $16e$ ( $\mathrm{F}\overline{4}3m$) by the oxygen of the water molecules.

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