Magnetic structure of binary fluorides containing Mn2

2014 Spin orderings of the binary fluorides with chemical formula XMnF3 (X = Na, Rb, Cs, and NH4) were investigated by neutron diffraction. The antiferromagnetic lattices of the compounds with X = Na, Rb, and NH4 are found to be of the same basic type as found in KMnF3 (G-type). In CsMnF3, which is hexagonal, the Mn2+ layers are arranged in an antiparallel sequence along the c-axis, with the spin direction normal to the axis. LE JOURNAL DE PHYSIQUE TOME 25, MAI 196fit, Introduction. Binary fluorides of the type XMnF (X = Na, Rb, Cs, and NH4), which were investigated previously by means of x-ray [1, 2], electron spin resonance [3, 4] specific heat [5], and magnetization [6, 7] measurements, have been reported to be antiferromagnetic. The present study reports powder neutron diffraction measurements on these compounds undertaken to confirm the antiferromagnetism and to study details of the magnetic ordering. Some information was also obtained concerning the nuclear structures. Experimental. The measurements were taken on powdered samples, which were kindly furnished by M. Shafer. Temperatures intermediate between 4.2 oK and 77 oK were attained by heating the sample holder, which was insulated from the liquid helium bath by thin teflon sheets, or by pumping on liquid nitrogen. In this way the sample could be held at any desired temperature, which was measured by a carbon resistance thermometer in contact with the sample holder. For small temperature changes, the sample came into equilibrium after about one-half hour. Structure information. All of the compounds can be considered as based on perovskite. RbMnF3 and NH4MnF3 are cubic (tolerance ratios 1.00 and 0.97, respectively), while NaMnF3 and CSNInF have tolerance ratios outside the cubic range. The Na compound is orthorhombic [1] and has a pseudomonoclinic cell derived from a (1) Work performed under the auspices of the U. S. Atomic Energy Commission. slight distortion of the cubic cell ; the Cs compound is hexagonal [2], with the ABCABC stacking of the cubic phase altered to ABCACB, in analogy with the hexagonal form of BaTio3’ In the case of RbMnF3 we observe only cubic perovskite reflections, as expected. However, if the tabulated nuclear scattering amplitudes ~ bRb = bF = 0.55) are used, there is a small discrepancy between weak reflections which can be removed only by making bRb > bF. Indications are that bxb should be increased to about 0. 63. The hydrogen positions in NH4IVInF3 should be obtainable from the neutron data. Good agreement ~~ == 0 .044) was obtained by distributing the hydrogen statistically in the twelvefold position 12(1) of Pm3m, i.e., along the direction toward the neighboring F ions. The N-H distance is 1.14 ~., which is slightly larger than that found [8] in NH4Cl and ND4Cl. The nuclear scattering from the CsMnF3 was examined to see if any change in the fluorine parameters might be required. Using the x-ray parameters determined by Zalkin et al. [2], we obtained an R factor of 0 . 053, which, since fluorine is the heaviest scatterer, indicates little necessity for change. Although the neutron data for NaMnF3 are still being analyzed, we note that none of the weak reflections that required an orthorhombic cell in the x-ray case were observed. However, contrary to the x-ray data, several of the reflections with mixed indices that require doubling the monoclinic cell are quite strong, indicating that the departures from the ideal perovskite structure may be Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01964002505056500