Ternary fluorides BaMF4 (M = Zn, Mg and Mn) at low temperatures.

Ternary fluorides BaMF4 (M = Zn, Mg, Mn) have been studied in the temperature range from 300 to 10 K using synchrotron and laboratory powder and single-crystal diffraction. The first two compounds are stable down to 10 K, while the third one undergoes a phase transition to an incommensurately modulated structure at approximately 250 K. The modulated phase is stable down to 10 K. The magnetic anomalies at 45 and 27 K observed previously in BaMnF4 are exclusively reflected in the behavior of the gamma component of the q vector, which assumes an irrational value of approximately 0.395 A(-1) at the temperature corresponding to the onset of the magnetic ordering and then stays constant down to 10 K. Mn-Mn distances do not indicate any structural response to the magnetic ordering. The formation of the modulated phase can be explained on the basis of simple geometrical criteria. The incorporation of the large Mn cation in the octahedral sheets implies an increase of the cavity in which the Ba ion is incorporated. This leads to the formation of the modulated structure to adapt the coordination sphere around Ba in such a way that the bond-valence sums can be kept close to the ideal value of two. With further lowering of the temperature, the charge balance around the Ba ion requires an increasingly anharmonic character of the modulation function of Ba, until finally at 10 K a crenel-like shape is assumed for the modulation of this atom.