X-ray- and neutron-diffraction measurements on BaMnF4

X-ray- and neutron-diffraction measurements on powder and single-crystal samples of BaMn${\mathrm{F}}_4$ have failed to show any evidence for the distortion with wave vector, in reciprocal-lattice units, $\overrightarrow{\mathrm{q}}=(0.4,0,0.5)$ recently reported by Scott, Habbal, and Hidaka. The distorted phase which is observed below 247 K is described by a wave vector $\overrightarrow{\mathrm{q}}=(\zeta ,0.5 ,0.5)$ with $\zeta$ incommensurate. There is a small (∼1%) increase in the magnitude of $\zeta$ as the temperature is lowered to 100 K, and the detailed temperature dependence of $\zeta$ is found to be different on heating and on cooling. The low-temperature distortion is characterized by a single wave vector $\overrightarrow{\mathrm{q}}=(\zeta ,0.5,0.5)$ rather than by a pair of wave vectors ($\zeta ,0.5,0.5$) and ($\zeta ,-0.5,0.5$).