Determination of T N for Ba 6 Mn 4 MO 15 (M = Cu, Zn) by Neutron Diffraction
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Determination of TN for Ba6Mn4MO15 (M = Cu, Zn) by Neutron Diffraction P. D. Battle, E. J. Cussen Inorganic Chemistry Laboratory, Oxford University, South Parks Road, Oxford, OX1 3QR, U.K.
ABSTRACT
The Néel temperatures of Ba6Mn4MO15 (M = Cu, Zn) have been determined by neutron powder diffraction to be ~ 6 K. In both cases, short-range magnetic coupling within the individual chains of this pseudo-1D crystal structure produces a susceptibility maximum at a temperature somewhat higher than that at which the frustrated interchain interactions establish long-range magnetic order.
INTRODUCTION
There is currently a great deal of interest in the structural chemistry [1] and electronic properties [2, 3] of oxides having the general formula A3n+3A'nBn+3O6n+9. Their trigonal or rhombohedral crystal structures contain [001] chains consisting of a periodic sequence of A'O6 trigonal prisms and BO6 octahedra, the ratio of octahedra to prisms (o:p) being determined by the value of n; the A cations occupy sites in the interchain spaces. Much of the interest stems from the variation in the electronic properties that occurs as the value of n is changed, and recent work has focussed on the extent to which the electronic properties can be described as 1D [4], the degree of magnetic frustration present in the structure (which relates to the dimensionality of the structure) and the electronic consequences of the modulated structures that are observed in some compositions [5]. We have previously shown [6] that the n = 1, o:p = 4 structure (Figure 1) of Ba6Mn4MO15 (M = Cu, Zn) is magnetically frustrated as a consequence of interchain interactions, although a long-range ordered, antiferromagnetic 120 ° spin structure was observed for both compositions in powder neutron diffraction experiments carried out at 1.7 K. The molar magnetic susceptibility of the Zn compound (Figure 2) was relatively low ( ~ 11 x 10
–3
cm3 mol-1) throughout the temperature range 5
GG2.4.1
7. ZLWK D
FIGURE 1 Crystal structure of n = 1 A3n+3A’nBn+3O6n+9. BO6 octahedra are hatched, A’O6 prisms are shaded, shaded circles represent A cations
60
16
50
χ / 10-3 cm3mol-1
χ / 10-3 cm3mol-1
15
12
Ba6Mn4CuO15
Ba6Mn4ZnO15
14 13 12 11
10
40
ZFC FC χspin only S=1/2 FCobs- χspin only
30
100
200
20 10
10 0
0
50
100
150
200
250
300
0
T/K
50
100
150
200
250
300
T/K
FIGURE 2 Temperature dependence of the molar magnetic susceptibility of Ba6Mn4A’O15 (A’ = Zn or Cu) [6]
broad maximum at 40 K; a small increase was observed below 16 K. Our original interpretation of these data was that the maximum at 40 K is a consequence of short-range spin coupling within the tetramers, and that the 3D Néel temperature lies at a lower temperature and is invisible in the susceptibility data. We argued that the change in the bulk magnetic moment of the sample on passing from a short-range ordered state to a long-range ordered state would be
GG2.4.2
small and possibly undetectable in our experiment. The increase at low temperatures was assume
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