HAL Id: jpa-00228908
https://hal.archives-ouvertes.fr/jpa-00228908
Submitted on 1 Jan 1988
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of
sci-entific research documents, whether they are
pub-lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
MAGNETIC SUSCEPTIBILITY OF
(DIMETHYLPYRIDINIUM)2Cu6Cl14, CHAIN OF
LINEAR HEXAMERS
P. Zhou, John Drumheller, Gerarld Rubenacker, M. Bond, R. Willett
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplement au no 12, Tome 49, decembre 1988
MAGNETIC SUSCEPTIBILITY
O F (DIMETHYLPYRIDINIUM)2C~6C114,
CHAIN
O F
LINEAR HEXAMERS
P. Zhou (I), John E. Drumheller (I), Gerarld V. Rubenacker (I), M. Bond (2) and R. D. Willett (2) (I) Dept. of Physics, Montana State University, Bozeman, M T 59717, U.S.A.
(2) Chemical Physics Program, Washington State University Pullman, W A 99164, U.S.A.
Abstract. - The powder magnetic susceptibility of (1,2-dimethy~pyridinium)2Cu6C~14 consisting of Cu6Cl14 linear hexamers was measured in the temperature range from 5 K to 60 K in an applied field of 5 000 Oe and found to fit a mirror symmetric linear Heisenberg hexamer model with the end exchange coupling constant J1/ kg = -23 K , next to edge J 2
/
kg = -30 K and a central J3/
kg = -51 K.We have measured the magnetic susceptibility of powder (1,2-dirnethylpyridini~m)~Cu~Cl~~ (DMPCC) from 5 K to 60 K. DMPCC is an example of an oligomer of six cu2+ ions bibridged with chlorine to form a linear hexamer. These hexamers are stacked one above the other to form ladder-like chains [I]. Each hexamer orders antiferromagneticdy into a non- magnetic S = 0 ground state so that even with an assumed interaction between the hexamers, no long range ordering occurs. The ordering within the hex- amers occurs at about 26 K and the behavior is similar to that of the chlorine tetramer [ ( c H ~ ) ~ NH] Cu4Cll0 [21.
The l72-dimethylpyridinium chloride was prepared by condensing 25 ml of methyl chloride in a dry- ice-ethanol bath, sealing the condensed liquid with 50 ml of chilled, freshly-distilled 2-picoline in a chilled, 130 ml Parr bomb, and allowing the bomb to warm to room temperature. After sitting at room tem- perature for two days, the bomb was opened and a white solid mixed with a large amount of unreacted 2-picoline was found. The white, solid product was fil- tered, washed with acetone, and dried in vacuo. Cop- per(I1) chloride dihydrate was used without further purification. Crystals of 1,2-dimethylpyridinium te- tradecylchlorohexacuprate(II) were grown over mag- nesium perchlorate by slow evaporation of a concen- trated HC1 solution containing equimolar amounts of 1,2-dimethylpyridinium chloride and copper(I1) chlo- ride dihydrate. The composition of the crystals was determined by single crystal X-ray diffraction [I].
The powder magnetic susceptibility data were taken on a EG-G Model 155 Vibrating Sample Magnetome- ter in applied fields to 5 000 Oe. Paramagnetic im- purities were found to range from 1 % to 10 %
.
For the data presented below, a 5.1 % paramagnetic tail was subtracted. The data are shown in figure 1 and show the general behavior of antiferromagnetic linear system, which has a broad maximum at about 26 K.To fit the data, we have assumed a mirror symmetry linear hexamer model a s shown in figure 2. In addition
Fig. 1.
-
Powder magnetic susceptibility versw tempera- ture for DMPCC. Squares are experimental values and the solid line is the best fit to a mirror symmetic linear Heisen- berg Hexamer with Jl/
bB = -23 K, J 2/
kg = -30 Kand J3
/
kg = -51 K. ,/chain Direction / / / / /Fig. 2. - The structure of linear hexamer unit. J1, J 2 and
J 3 denoted the exchange constants between the coppers.
we have assumed a Heisenberg exchange so that the Hamiltonian may be written
C8
-
1472 JOURNAL DE PHYSIQUEwhere J1is the exchange constant between an edge copper and next neighbour, J 2 is the coupling be- tween that copper and next-central one, and the J 3 is the central coupling constant. The susceptibility was calculated by an exact numerical calculation of eigenvalves and eigenvectors. By minimizing the mean square difference between the calculated and exper- imental susceptibility, the Heisenberg antiferromag- netic exchange coupling constants of J1/ kg = -23 K, Jz
/ k g
= -30 K and J 3 / k g = -51 K were found and this best fit t o the data is shown as a solid line in figure 1. The susceptibility maximum occurs at ~ B T , ,/
1
JN1
= 0.828, where the JN is average of the coupling constants (251+
252+
53)/
5.Fig. 3.
- I
JNIx
/
( N ~versus ~ ~kgT ~/
1
JNl ~ for the dif- )ferent values of a with /3 = 0.4, where the normalization
JN = (251 + 2 J 2 + J3)/ 5 = J ( 2 - a + P ) / 5.
We believe this t o be the first study of any kind of hexamer, but because in this case the hexamers them- selves are linear and there is little interaction between the hexamers, this system might oKer many interest- ing possibilities of studying models and sign combi- nations of coupling constants in short chains. In ad- dition, more general models could be assumed. For example, other important models cam be obtained by letting JI = J (1
-
a ) , J 2 = J/3 and J3 = J a , and al- lowing a and/3
t o va'ry. If a = 0, two isolated trimers obtain and if a = 1, the model becomes a tetramer with two isolated spins. For a = 0.5, the Hamiltonian represents the alternating linear chitin model with/3
the so-called alternating ratio [3]. When a = 0.5 and/3
= 0.5, the uniform linear chain is presented. Finally, a three isolated dimers model is produced with /3 = 0. The plots ofx
I
J N ~/
( ~ ~ ~ ~ , u ~ ) versus k k T/
1
J N ~ for varied a and for /3 = 0.4 are shown in figure 3 for antiferromagnetic coupling. It is clear from figure 3 that the shape and peak of the curve are sensitively dependent on the value of a. Using this more sophisti- cated model, we attempted a fit t o the DMPCC pow- der susceptibility data and found the best fit t o be a tetramerized system with a = 0.69 vvith /3 = 0.41 and J / k ~ = -74 K .Acknowledgment
This
work is supported by NSF DMR 87-02933.[I] Unpublished results by Bond, M. and Willett, R. D., Chemical Physics Program, Washington State University, Pullman, WA '99164, U.S.A. [2] Rubenacker, G. V., Drumheller, J. E., Emerson,
K. and Willett, R. D., J . Magn. Magn. Muter. 54 (1986) 1485.
[3] D d y , W., Jr. and Barr, K. P., Phys. Rev. 165
