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Regent theoretical interest in Kagomé antiferromagnets arose from the prediction that they possess a new type of magnetic ground state in which there is no magnetic long-range order even at <I>T =</I> 0. Prior to this work no detailed investigations had bee made of the magnetic properties of real materials that provide models for this class of magnet. Members of the series AB<SUB>3</SUB>(SO<SUB>4</SUB>)<SUB>2</SUB>(OH)<SUB>6</SUB> (A<SUP>+</SUP> = Na<SUP>+</SUP>, K<SUP>+</SUP>, Rb<SUP>+</SUP>, Ag<SUP>+</SUP>, NH<SUB>4</SUB><SUP>+</SUP>, and H<SUB>3</SUB>O<SUP>+</SUP>; B<SUP>3+</SUP> = V<SUP>3+</SUP>, Cr<SUP>3+</SUP>, and Fe<SUP>3+</SUP>) have been studied using a combination of experimental techniques- ac and dc susceptibility, specific heat, neutron diffraction, and MuSR. The hydronium compounds (A<SUP>+</SUP> = H<SUB>3</SUB>O<SUP>+</SUP>; B<SUP>3+</SUP> = V<SUP>3+</SUP>, Cr<SUP>3+</SUP>, and Fe<SUP>3+</SUP>) have been shown to provide the best model Kagomé antiferromagnets with <I>S </I>= 1, 3/2 and 5/2 respectively. (H<SUB>3</SUB>O)Fe<SUB>3</SUB>(SO<SUB>4</SUB>)<SUB>2</SUB>(OH)<SUB>6</SUB> has been shown to possess only short-range magnetic order with a correlation length of 19 ± 2Å, below a spin glass transition temperature, <I>T<SUB>f</SUB></I> ˜17K. Unlike normal spin glasses, muon spin relaxation measurements show that this magnetic phase is dynamic below <I>T<SUB>f</SUB></I> (H<SUB>3</SUB>O)Fe<SUB>3</SUB>(SO<SUB>4</SUB>)<SUB>2</SUB>(OH)<SUB>6</SUB> is a spin fluid: its dynamic and thermodynamic behaviour are completely unlike those seen in normal spin glass or long-range ordered ground states; the specific heat varies as <I>T<SUP>2</SUP></I> below <I>T<SUB>f</SUB></I>, rather than the linear <I>T</I> dependence observed in canonical spin glasses. Deliberate reduction of the concentration of magnetic atoms destabilises the spin fluid ground state and induces the formation of long-range order of the type seen in the other less magnetically concentrated members of the series. This is the first example of a system in which diamagnetic dilution causes it to order classically. (H<SUB>3</SUB>O)Cr<SUB>3</SUB>(SO<SUB>4</SUB>)<SUB>2</SUB>(OH)<SUB>6</SUB> has a broad transition to a dynamic short-range ordered state at ˜ 30K, similar to that seen in (H<SUB>3</SUB>O)Fe<SUB>3</SUB>(SO<SUB>4</SUB>)<SUB>2</SUB>(OH)<SUB>6</SUB>, but with no observed spin glass behaviour. A weak ferromagnetic transition at 2.2K is ascribed to a canting of antiferromagnetically coupled sublattices. Specific heat and dc susceptibility measurements show that (H<SUB>3</SUB>O)V<SUB>3</SUB>(SO<SUB>4</SUB>)<SUB>2</SUB>(OH)<SUB>6</SUB> undergoes a sharp antiferromagnetic transition at 21K to an ordered ground state. However, neutron diffraction data taken at 4.2K reveals no evidence for long-range magnetic order. It is suggested that the magnetic ground state could be a highly correlated spin fluid.

~rift mobili ty techni:ucs hnvc been \ \sec. to maD ,"urc the trDDP. 'oct of generated electrons in single crystals of orthorhombjc sulphur grown from solution. The drift mobility was found to be the SaT:1e for more than 20 specimens grown from starting materials of different degrees of purity, despite large differences in hole mobility measured on the same specimens. At 21 °c , JA .. = 6.2 x 10- 4 cm 2- V- 1 sec- 1 and was found to increase with temperature as exp( -E.ykl')) where Eo. = 0.167 eVe ~,. exhi bi terJ no measur~able Dnisotropyo The transformation to the monoclinic modification is accompanied by a fall in mobility by a factor of about ten and an increased density of deep traps. The electron mobility results are shovm to be consj stent wjth IOn intermolecular hopping mechanism jn vrhj ch the clcctT'll:n j l;(~Ci.'i'r:ts with one or more molecular vibrational modes. It is believed that this is the first time a strong case for a true hopping current has been presented. The polaron binding energy is shown to be 0.48 eV and the dimensionless electron-lattice interaction parameter, Y~18. The isolated 88 molecule and its ions are eXaIDjnea theoretically in terms of molecular orbital theory and the study is extended in an estimate of the electron and hole bandwidths in the solj(J by Em approximate method. A number of intraTllolecular ana intermolecul[cr orbital overlaps are calculated and the results are used in a discussion of the electron mobilityo The electron free lifetime at the surface and in the bulk was studied by a novel method involving interrupted electron transjts. The trapping spectrum was investigated by thermally stimulateo conductivity and by an electron beam probing technique which was used to study the polarization decay. The results lend support to the hopping interpretation cf electron transport. A subsisiary study of space-charge perturbed electron currents provides experimental confirmation of a recent theoretical treatment by Papadakis.

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Measurements are presented of the structural and magnetic properties of several series of pseudo-binary compounds formed between the (heavy) Rare Earth metals, or Yttrium, and the 3d transitional metals, in the Laves phase structure. This work has been undertaken in an attempt to obtain a better understanding of the nature of the moments and coupling mechanisms in these compounds, RX(_2).The magnetic results are discussed on the basis of a rigid band model for the 3d electrons in these compounds and this model is shown to provide a unified account of the properties of all the RX(_2) compounds. An elementary density of states histogram for the 3d band associated with the X component is presented. The C 36 phase is found to coexist with the C 15 terminal phase at intermediate compositions in some of the series. For the C 15 phases the lattice parameters show a positive deviation from Vegard's Law and these results are discussed in relation to the magnetic properties.

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