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The contribution of mixed-valence electron delocalisation to the ferromagnetic exchange between the iron (111) ions in Prussian Blue {Fe1"4[Fe11(CN)6],*1 4H,O) has been estimated theoretically. Agreement between the calculated and observed values of the Curie temperature is quite good. Oxford OX1 3QRPRUSSIAN BLUE, Fe111,[Fe11(CN),],*14H,0, is an interesting compound not only because it is one of the classical examples of a Class I1 2 mixed-valence compound whose physical properties have been very comprehensively investigated but also because at low temperatures the iron(rI1) spins order ferroniagnetically. Ionic transitionmetal compounds, especially co-ordination complexes, which order as three-dimensional ferromagnets, are extremely rare,3 so any insight which can be gained into the mechanism of the exchange interaction in Prussian Blue would be valuable.

From the pressure dependence of the lattice parameters measured by single-crystal X-ray diffraction at room temperature, the amplitudes and directions of the principal components of the isothermal compressibility tensor of κ-(BEDT-TTF)2Cu(NCS)2, bis(ethylenedithio)tetrathiafulvalene copper dithiocyanate, have been determined from 1 × 105 Pa to 12.9 × 105 kPa. The crystal structure has been determined at 1 × 105 Pa and 7.5 × 105 kPa. From the latter, the pressure dependence of the intermolecular electron-transfer integrals and the band structure have been calculated and compared with those at 1 × 105 Pa and 15 K.

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The bandshape of the intervalence absorption of SbC12-and SbC1, has been measured from 300 to 4 K in a crystal of (CH3NH3)2SbxSnl-xC16 in order to study the variation of the zeroth, first and second moments. At all temperatures the bandshape is Gaussian, as required by the vibronic model of Piepho, Krausz and Schatz (P.K.S.) in the weak-interaction limit. From the temperature dependence of the second moment, we estimate the electronphonon coupling constant (and hence the displacement in vibrational coordinate from the ground state to the intervalence charge-transfer state), together with the effective phonon frequency coupled to the transition. The latter is ver close to the mean of the ground-state totally symmetric Sb-C1 stretching modes of SbC16-and SbCl,, and the displacement in the vibrational coordinate is also about half the difference between the Sb-CI crystallographic bond lengths in the two complex ions. To explain the temperature dependence of the zeroth and first moments anharmonicity must be involved. A simple model for the variation of intervalence excitation energy with interionic distance, combined with an isotropic model of the thermal lattice expansion, gives a quantitative account of the change in first moment with temperature and a qualitative description of the change in zeroth moment.