High-temperature susceptibility of copper compounds containing CuCl2-4 or Cu2Cl2-6 ions

691 HIGH-TEMPERATURE SUSCEPTIBILITY OF COPPER COMPOUNDS CONTAINING CuC!24OR Cu,CI6z- IONS M.F. MOSTAFA, M. SEMARY and M.A. AHMED Physics Department, Faculty of Science, Cairo University, Giza, Egypt

We have measured the magnetic susceptibility of [(C2H~)2NH:]2 CuCL, (iso-C3H,NH3)2CuCL, and (iso-C~HTNH3)CuCI3 in the temperature range 80 < T < 400 K. These compounds show thermochromic first order phase transitions at 315.5, 329.5 and 324.5 K, respectively, which is related to the change in geometry of the chlorine coordination of Cu 2÷. From the susceptibility data, estimates of the super-exchange interaction can be inferred. For [(C2H~)2NH2]2CuCL and (isoC~HTNH~)2CuCL our results evidenced the presence of ferromagnetic interactions between the Cu ions within the network formed by CuCI,~- ions. For the (iso-C,H,NH3)CuCI, the interaction between two Cu 2÷ ions forming the dimer can be estimated. The interaction between the dimers is found to be much weaker.

The compounds (iso-C3H7NH3)2CuC14, [(C2Hs)2NH2]2CuCI4 and (iso-C3HTNH3)CuCI3, henceforth, (IPA)2CuC14, (DEA)2CuC14 and (IPA)CuC13, respectively, have been investigated. The first two compounds contain CuCI~- ions while the third is made up of Cu2C12- ions. The compounds show thermochromic transitions at 315.5, 329.5 and 324.5K, respectively, caused by a change in the structural symmetry of the transition metal ion from tetrahedral into square planar [1,2]. These changes will be reflected in the magnetic behaviour of these compounds. The magnetic susceptibility as a function of temperature for the three compounds, in the range 300--400 K, is shown in fig. 1. The midpoints between susceptibility maxima and i

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Physica 86-88B (1977) 691-692 © North-Holland

minima occur at the corresponding thermochromic temperature (Tth©rmo). The thermal hysteresis observed in the neighbourhood of Tth.... is an indication that the transition is of first order. The near infra-red spectra obtained for (IPA)CuC13 below and above Ttherrao indicates that the Cu(II) ions have a square planar geometry and tetrahedral distortion respectively. This is similar to the case found for the other two compounds. Willett et al. [1] have reported that both (IP A)ECuC14 and (DEA)2CuCL consist of CuCI~- ions of tetrahedrally distorted geometry at T > T t h .... • However, the building units in (IPA)CuCI3 are Cu2Cl~- dimers as indicated by its pleochromic property and its similarity to the other compounds of known crystal structures. At temperatures above Tth . . . . . the molar susceptibility could be fitted to the Curie-Weiss law with X u = 0.55/(T + 18), Xu = 0.54/(T + 58), and Xu = 1.17/(T - 30) for (IPA)2CuCI4, (DEA)2CuCI4 and (IPA)2CuzC16, respectively. The corresponding /xen are 2.09, 2.09 and 3.06B.M., respectively. At temperatures below Tthermo, the data can be approximated by X u = 0.47/(T - 20), Xu = 0.46/(T- 9) and Xu = 0.96/(T + 4) for the three compounds, respectively. The corresponding values of /z,n are 1.95, 1.92 and 2.77 B.M. The results are summarized in table I. The values of #~n obtained above and below Tth. . . . are in agreement with what would be expected for CuCI 2- and Cu2CI~- ions of tetrahedral and square planar geometries. The plot of the reciprocal molar magnetic susceptibility vs. temperature showed a small

692 Table I Compound

(IPA)2CuCL (DEA)2CuCL (IPA)~CuC|~

~55 T > Tth.....

T < Tth.....

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~.onB.M.

0.55 0.54 1.17

-18 -58 30

2.09 2.09 3.06

20 9 -4

1.95 1.92 2.77

0.47 0.46 0.96

but significant deviation from the Curie-Weiss behaviour below 200 K for the first two compounds, and below 95 K for the (IP A)CuC13. For the former this is due to a ferromagnetic interaction in the system, as is evidenced when plotting XMT vs. T. As the temperature decreases below 200 K, the value of XMT, for the first two compounds, increases implying that the magnetic moment/particle is increasing. This proposed ferromagnetic interaction, is supported by the results of De Jongh et al. [3] on the magnetic interaction of Cu(II) complexes of the general formula (C, H2,÷INH3)2CuC14. These complexes where the CuC124- ions are almost of square planar geometry were reported to behave as two-dimensional Heisenberg ferromagnets at low temperatures. The results obtained for (IPA)2CuC14 and (DEA)2CuC14 were compared to the series expansion of XMT/C in powers of J/kT for both the linear chain and the quadratic Heisenberg ferromagnets [4, 5]. A good fit is obtained with the simple quadratic model giving [J/kI = 12.5 and 4.5, respectively (see fig. 2). The (IPA)2Cu2C16 compound shows a quite interesting behaviour. At T > Tth..... the obtained value of C corresponds to a g = 2.27 for a system of N spin 1 moments. It is thus possible, from structural information and magnetic behaviour of similar compounds, to predict that the compound consists of magnetically isolated Cu2CI~- dimers which are tetrahedrally distorted at T > Tth but are of square planar geometry at T < Tth The high temperature expansion of the Bower-Bleany equation [6] yields 0 = 2J/3k. This gives lJ/kl=45K in reasonable agreement with the value obtained for similar compounds [7]. At T < Tth The value of C corresponds to g = 2.26 for a system of 2N spin ions with I]/kl = 6 K. The deviation from the .....

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Curie-Weiss behaviour observed below 95K could be a result of a dimer-dimer interaction and the system becomes a linear chain (possibly antiferromagnet) at lower temperatures. This argument is supported by the EPR results obtained by Willett et al. [7]. At 78 K, they observed a single exchange narrowed anisotropic line, rather an S = 1 spectrum. Detailed magnetic behaviour of this compound has to await crystal structure analysis and low temperature magnetic work. The authors wish to thank Prof. R.D. Willett for the enlightening discussions. References [1] R.D. Witlett, J.A. Haugen, J. L e b s a k and J. Morry, Inorg. Chem. 13 (1974) 2670. [2] M.A. A h m e d , M.Sc. Thesis, University of Cairo, Egypt (1976). [3] L.J. De Jongh and W.D. van Amstel, J. de P h y s i q u e 32 CI (1971) 880. [4] G.S. R u s h b r o o k and P.J. Wood, Proc. P h y s . Soc. 68A (1955) 116. [5l G.A. Baker, H.E. Gilbert, J. Eve and G.S. Rushbrook, P h y s . Lett. 25A (1967) 207. 16l B.Bleany and K.D. Bowers, Proc. Roy. Soc. (London) A214 (1952) 451. [7] R.D. Willett, private communication, W a s h i n g t o n State University, Pullman, Washington.