The crystal structure of manganese trifluoride, MnF₃

Abstract

MnF 3 is monoclinic with a=8.904±0.003, b--5-037±0.002, c---13-448+0.005 A; fl=92.74+0.04 °.The structure is pseudo-rhombohedral and has atoms in the following positions of space group C2 /c-C~h : 4Mn at (a): 8Mn at (f~), with x 1 --=~, Yl=½, z~----½; 4F at (e), with ye=0"617; 8Fat (f2), with x~ = 0.310, y~. ----0.714, z2 = 0"244; 8F at (f3), with x 3 ----0-167, Y3 = 0-117, z a = 0.583; 8F at (f4), with x 4 = 0.477, Ya = 0.214, z 4 ----0.577; 8F at (fh), with xh----0-143, Yh----0.214, z5----0.911.Since its fluorine-atom packing is mid-way between close-packed hexagonal and an ReO3-type cubic close-packing, MnF 3 is classified as a VF3-type transition-element trifluoride. The lower symmetry of the structure, in comparison with other trifluorides, results from three different Mn-F bond lengths (2.09, 1.91 and 1.79 A) in each MnF, octahedron. Reasons for this unsymmetrical bonding and for its unique occurrence in MnF 3 are offered in terms of crystal-field theory.

On the tetragonal distortion of octahedral systems in an Egelectronic state

“…the distortion is not purely tetragonal. In these compounds, the distortion of the octahedra is such that there are three different bond lengths [16,17]. The origin of the distortion, however, is essentially the same and an extension of the present model taking into account the effect of both Q3 and Q2 (D~h symmetry case) should lead to the desired result.…”

“…It would be useful to write (16) in a form which separates the terms arising due to covalency from those which follow from an electrostatic model. Making use of relations (12) to (15) we get:…”

Origin of the Antiferroorbital Ordering in LaMnO3and Other Related Compounds

“…This coexistence of q 2 and u 3 is consistent with the observed orbital orders. 2,5,8) According to experimental observations, the main order parameters of the AFO in LaMnO 3 , MnF 3 , and KCuF 3 are all q 2 , but their Q's are ð; ; 0Þ in LaMnO 3 , 1,2) ð; ; Þ in MnF 3 , 5) and both in KCuF 3 . 7) These facts do not contradict with our theoretical prediction of the properties of the AFO order on the basis of the joint mechanism.…”

“…Instead, the CJTE mechanism contributes not only to T c itself, but also to determination of the most stable order parameter among the degenerate AFO ones. For example, the observed wide temperature regions of phases with AFO order in the paramagnetic phase 1,5,7) can be naturally ascribed to the presence of the CJTE. Moreover, the coexistence of the AFD and FD is directly related to the coexistence of the AFO and FO, which is therefore attributed to the presence of the CJTE.…”

“…In all these compounds, the orbital degree of freedom is quenched to be subjected to the AFO orders, which are accompanied not only by an antiferrodistortion (AFD) but also by a ferrodistortion (FD). 1,5,7) In LaMnO 3 , the AFO order occurs at the transition temperature T c ¼ 750 K and continues also in the antiferromagnetic phase below the Néel temperature T N ¼ 141 K. 1,4) Similarly, T N 's in MnF 3 and KCuF 3 are 43 K and 38 K, respectively, 6,8) but T c 's are so high that they have not been detected.…”