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HEAT CAPACITY OF FOUR MANGANESE DOUBLE CHLORIDES
H. Blöte, W. Huiskamp
To cite this version:
H. Blöte, W. Huiskamp. HEAT CAPACITY OF FOUR MANGANESE DOUBLE CHLORIDES.
Journal de Physique Colloques, 1971, 32 (C1), pp.C1-1005-C1-1007. �10.1051/jphyscol:19711358�.
�jpa-00214394�
CHANGEMENTS DE PHASE
HEAT CAPACITY OF FOUR MANGANESE DOUBLE CHLORIDES
H. W. J. BLOTE and W. J. HUISKAMP Kamerlingh Onnes Laboratorium, Leiden, The Netherlands
Rksumk. - Les chaleurs spkifiques de CssMnCls, CszMnC14.2 HzO, K4MnC16 et cl-CszMnC14 ont kt6 mesurkes aux tempkratures entre 0,05 et 3 OK. Pour chacun des quatre composks, la courbe de la chaleur spkcifique montre une ano- malie pointue. Des mesures des susceptibilit6s ont montrk que ces anomalies sont causkes par des transitions de phase antiferromagnktiques. En raison de l'etat S de l'ion MnZ+, les rksultats seront interprktks en terme du modkle Heisenberg.
Abstract. - The heat capacities of Cs3MnCls, Cs~MnC14.2 H20, K4MnC16 and a-CszMnC14 have been measured at temperatures between 0.05 and 3 OK. Each of the four salts shows a sharp anomaly in the heat capacity curve. Suscep- tibility measurements showed that these anomalies are due to antiferromagnetic phase transitions. In view of the S-state of the Mnzi ion, the results will be explained in terms of the Heisenberg model.
I. General part. For an analysis of the data of Heisenberg spin systems we shall utilize expressions relating the exchange constant J for q neighbours to the magnetic energy gain E, the Curie-Weiss constant 8 and the asymptotic high temperature heat capacity
Ch :
E/R = qs2 J/k (1)
Eq. (1) is a molecular field approximation for a simple two-sublattice antiferromagnet.
11. Cs3MnC1, was prepared by fusing together stoichiometric quantities of CsCl and MnCI,. Accor- ding to IJdo [l] it has the same tetragonal structure as Cs3CoC15 [2]. The unit cell contains 4 Mn ions.
For our purposes it is useful to divide this unit cell into four tetragonal cells which are magnetically equivalent. Therefore, as far as magnetic interactions are concerned, we may consider the Mn ions to be in a simple tetragonal Bravais lattice. EPR measure- ments of Henning [3] on Mn2+ in the isomorphous Cs,ZnCI5 lattice have shown that, after neglecting a few small terms, the Hamiltonian can be written as X = gpB H . s + As. I, in which g = 2.00 and
Alk = - 0.012 3 OK.
The heat capacity of Cs,MnCI5 is shown in figure 1.
A phase transition is seen at T = 0.59 OK. At low T, the heat capacity originates from the last term of the Hamiltonian (h. f. s. splitting). A Schottky curve for six equidistant levels can be fitted for
I A/k1 = 0.011OK.
Integration of the electronic c / R yields the magnetic energy gain E/R = - 0.89 OK. For simplicity, we shall now assume one average exchange constant J for coupling to nearest neighbours in the aa plane and along the c axis. Using formula (1) we derive
I J/k I = 0.024 OK .
From the measured Curie-Weiss constant 8 = - 0.9 OK
and eq. (2) we find J/k = - 0.0260K. Further, c, T 2 / R = 0.16 K2 and eq. (3) gives I J/k I = 0.023 OK.
Conversely, the approximate agreement between the
FIG. 1. - Heat capacity of Cs3MnC15.
three J/k values suggests that the Mn2+ ions are anti- ferromagnetically coupled to the 4 nn in the aa plane and the 2 nn along the c axis.
111. Cs2MnC14.2 H 2 0 crystallizes in a triclinic structure [4] in which the Mn ions are arranged in a simple Bravais lattice. NMR data [5] at low tempera- tures show that the Mn spins are ordered in two sublat- tices having antiparallel spin orientation. The heat capacity (Fig. 2) clearly exhibits a phase transition at TN = 1.80 OK. To the low temperature h. f. s. heat capacity a theoretical curve can be fitted for
I A/k 1 = 0.012 OK. Integration of the electronic heat capacity leads to E/R w - 3.5 OK, thus to q I J/k I = 0.56 OK in a simple approximation for a two sublattice antiferromagnet. From ch T2/R w 3 K2 we roughly estimate q~'/k' = 0.058 OK. For q = 6, we obtain I J / k I values of resp. 0.093 OK and 0.098 OK.
IV. K4MnC16 was prepared by fusing of the compo- nents, grinding and heating to temperatures just below the melting point of K4MnCl6, The structure is rhom-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711358
FIG. 2. - Heat capacity of Cs2MnC14.2 H20.
bohedral [6], nearly body centered cubic (a = 890 32').
Recent X-ray anvestigations of the isomorphous K4CdCI, have shown that the Ci octahedra around the Mn2+ at 000 are not the same as that of the ion
at +&+, EPR experiments [7] showed that the D para-
meter was small, and A/k = - 0.011 5 OK.
The heat capacity curve of K,MnCl, shows two maxima, at T = 0.33 OK and at T = 0.44 OK. From
FIG. 3. - Heat capacity of K4MnCl.5.
chfs, 1 A/k 1 = 0.01 1 OK was calculated. The electro- nic magnetic energy gain amounted to
E/R = - 0.78 OK.
The magnetic susceptibility data between 1 OK and 3.5 OK could be fitted by a Curie-Weiss relation for 6 = - 1.7 OK. The El8 value is too small to be explai-
ned by nn exchange only or nnn exchange only. It is necessary to assume negative values for both the nn as well as the nnn exchange parameters. Since then no magnetic structure will allow all interacting spins to be antiparallel, a low El8 value and a negative T-3 term in the high temperature heat capacity expansion are expected. A negative T-3 term causes the high T heat capacity to decrease less steeply than T-', as is observed.
V. a-Cs,MnCl, has the same structure as Cs,CoC14 [8] which is orthorhombic [9]. The sample was pre- pared by fusing of the components. The measured heat capacity (Fig. 4) cannot be ascribed to a-Cs2Mn Cl,,
FIG. 4. - Heat capacity of a-CszMnCL.
which orders at about 550K according to Ep- stein [lo]. Moreover, the electronic heat capacity of the sample had decreased drastically when after some days the measurements were repeated. Therefore we attri- bute the heat capacity to a-Cs2MnC14 which is unsta- ble below 297 OC, thereby explaining the decrease of heat capacity in time. Figure 4 shows a sharp maximum at 0.93 OK. For the hyperfine parameter we found I Alk I = 0.010 OK. Integration of the hfs corrected c/RT gave an electronic entropy gain ASIR = 1.50, which is significantly smaller than the expected value In 6 = 1.79. We ascribe the difference to the observed chemical instability, and a correction factor 1.79/1.50 is applied to E/R and ch T 2 / ~ , yielding E/R = 1.79 OK and ch T 2 / ~ z 1.1 K2. Further, sus- ceptibility measurements showed that
8 = - 4 + 1 0 K .
Also in this case the low El8 value can be explained by mutually opposing nn and nnn exchange interac- tions. From the low c,, T 2 / ~ 0 2 ratio we deduce that exchange interactions occur among a large number of neighbours, which is in accordance with the crystal structure. Twelve neighbours are found within a distance of 7.6 Pi (using the lattice parameters and constants of Cs2CoC14).
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