Phase gratings can be written in acentric ${\mathrm{Ba}[\mathrm{Fe}(\mathrm{CN})}_{5}{\mathrm{NO}]\mathrm{\ensuremath{\cdot}}3\mathrm{H}}_{2}\mathrm{O}$ single crystals on the basis of extremely long-living metastable electronic states at temperatures below T=213 K after preexposure with light in the range of 350--580 nm. The modulation of the refractive index reaches values up to $\ensuremath{\Delta}n=5.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ in the red and $\ensuremath{\Delta}n=1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ in the near-infrared spectral range. Beam-coupling experiments show that there is no phase shift between the light interference pattern and the resulting refractive index grating. The dynamical behavior of the diffraction efficiency during the writing process depends on the depopulation velocity of the metastable electronic states SI and SII and on the transfer velocity from SI into SII. The maximum efficiency of \ensuremath{\eta}=72% is reached at an exposure of $Q=27 {\mathrm{J}/\mathrm{c}\mathrm{m}}^{2}$ with \ensuremath{\lambda}=632.8 nm. The grating is stable against irradiation with light of wavelength \ensuremath{\lambda}=1047 nm and cannot be destroyed with an exposure of up to $Q=600 {\mathrm{J}/\mathrm{c}\mathrm{m}}^{2}$. In comparison to centrosymmetric ${\mathrm{Na}}_{2}{[\mathrm{Fe}(\mathrm{CN})}_{5}{\mathrm{NO}]\mathrm{\ensuremath{\cdot}}2\mathrm{H}}_{2}\mathrm{O}$, where the refractive index cannot be modulated perpendicularly to the mirror plane m, such a restriction does not exist in the acentric single crystal.