Section 3.1.5.2.7. Barium manganese tetrafluoride

BaMnF₄ is an unusual material whose room-temperature structure is illustrated in Fig. 3.1.5.11(a). It consists of MnF₆ octahedra, linked by two shared corners along the polar a axis, with ribbons of such octahedra rather widely separated by the large ionic radius barium ions in the b direction. The resulting structure is, both magnetically and mechanically, rather two-dimensional, with easy cleavage perpendicular to the b axis and highly anisotropic electrical (ionic) conduction.

Figure 3.1.5.11 | top | pdf | (a) Structure of barium metal fluoride BaMF4 (M = Co, Mn, Mg, Zn, Ni) at ambient temperature (300 K). (b) Raman spectroscopy of barium manganese fluoride above and below its structural phase transition temperature, ca. 251 K. (c) Temperature dependence of lower energy phonons in (b).

Most members of the BaMF₄ family (M = Mg, Zn, Mn, Co, Ni, Fe) have the same structure, which is that of orthorhombic () point-group symmetry. These materials are all ferroelectric (or at least pyroelectric; high conductivity of some makes switching difficult to demonstrate) at all temperatures, with an `incipient' ferroelectric Curie temperature extrapolated from various physical parameters (dielectric constant, spontaneous polarization etc.) to lie 100 K or more above the melting point (ca. 1050 K). The Mn compound is unique in having a low-temperature phase transition. The reason is that Mn⁺² represents (Shannon & Prewitt, 1969) an end point in ionic size (largest) for the divalent transition metal ions Mn, Zn, Mg, Fe, Ni, Co; hence, the Mn ion and the space for it in the lattice are not a good match. This size mismatch can be accommodated by the r.m.s. thermal motion above room temperature, but at lower temperatures a structural distortion must occur.

This phase transition was first detected (Spencer et al., 1970) via ultrasonic attenuation as an anomaly near 255 K. This experimental technique is without question one of the most sensitive in discovering phase transitions, but unfortunately it gives no direct information about structure and often it signals something that is not in fact a true phase transition (in BaMnF₄ Spencer et al. emphasized that they could find no other evidence that a phase transition occurred).

Raman spectroscopy was clearer (Fig. 3.1.5.11b), showing unambiguously additional vibrational spectra that arise from a doubling of the primitive unit cell. This was afterwards confirmed directly by X-ray crystallography at the Clarendon Laboratory, Oxford, by Wondre (1977), who observed superlattice lines indicative of cell doubling in the bc plane.

The real structural distortion near 250 K in this material is even more complicated, however. Inelastic neutron scattering at Brookhaven by Shapiro et al. (1976) demonstrated convincingly that the `soft' optical phonon lies not at () in the Brillouin zone, as would have been expected for the bc-plane cell doubling suggested on the basis of Raman studies, but at (). This implies that the actual structural distortion from the high-temperature () symmetry does indeed double the primitive cell along the bc diagonal but in addition modulates the lattice along the a axis with a resulting repeat length that is incommensurate with the original (high-temperature) lattice constant a. The structural distortion microscopically approximates a rigid fluorine octahedra rotation, as might be expected. Hence, the chronological history of developments for this material is that X-ray crystallography gave the correct lattice structure at room temperature; ultrasonic attenuation revealed a possible phase transition near 250 K; Raman spectroscopy confirmed the transition and implied that it involved primitive cell doubling; X-ray crystallography confirmed directly the cell doubling; and finally neutron scattering revealed an unexpected incommensurate modulation as well. This interplay of experimental techniques provides a rather good model as exemplary for the field. For most materials, EPR would also play an important role in the likely scenarios; however, the short relaxation times for Mn ions made magnetic resonance of relatively little utility in this example.