Some studies of some rare earth halides and related compounds

The work described in this thesis falls into four major divisions. Firstly, it is concerned with the preparation of large, single crystals of rare-earth halides and mixed halides of rare-earth and alkali metals. Secondly, it describes the discovery of new magnetic materials, which incorporate rare earth ions, by means of thermal and X-ray analysis of a number of mixed halide systems. Thirdly, the results of the measurements of A.C. magnetic susceptibility and specific heat of some rare-earth trichlorides are presented. Finally, crystallographic studies of some rare-earth and Group III B compounds are described.
In the first part, the preparation and purification of rare-earth chloride and bromides, and the method used to grow both pure and doped single crystals for various physical measurements are discussed in detail. A very large number of crystals have been grown by the Bridgman-Stockbarger method, and have been used for physical measurements of various types, not only in the laboratory, but also in France and the U.S.A. It has been found that, to prepare optically clear crystals of mixed rare-earth chloride-alkali metal chloride compounds, the starting materials must be of very high purity. The methods of purification used were either distillation or zone refining.
An investigation of phase systems containing sodium fluoride and praseodymium or terbium tetrafluorides showed that compounds of the formula NaMF5, Na2MF6 and Na3MF7 were formed, but none of these were thermally stable; and therefore could not be grown as single crystals from the melt. The phase diagram of the system LiCl/YCl3 showed the presence of a congruently melting compound Li3YCl, which has since been grown as a single crystal, both pure and doped with 10% Er3+.
An extension of previous work on materials similar to elpasolite, K2NaAlF6, revealed the existence of isomorphous compounds CS2KMCl6, where M is any trivalent rare earth or yttrium. In these compounds the rare earth ion occupies a site of cubic symmetry. A single crystal of CS2KYCl6 has been grown from zone-refined materials.
Several phase diagrams of systems containing two rare-earth chlorides of different crystallographic structures have been constructed. The mutual solubilities of several rare-earth chlorides were determined.
The dimorphism of TbCl3, DyCl3 and HoCl3 was investigated by differential thermal analysis. The transition temperature of TbCL3 was found to be 535°C, and that of DyCl3, to be 350°C. The latter transition was never observed directly, but could be inferred from other data. DyCl3, in the purely low temperature form could not be made, but the transformation was found to be aided by high pressure. The transition temperature of HoCl3 was estimated to be about 160°C, but was never observed.
Techniques for handling the hygroscopic rare earth halides were evolved, and methods of shaping and orienting single crystal fragments for various types of physical experiments are described. In addition various forms of container used to transfer the sample from a drybox to a cryostat are shown.
Measurements of the A.C. susceptibility of DyCl3 and HoCl3 at low temperatures are described, but a full interpretation of the phenomena was not possible. The specific heats of DyCl3. HoCl3 and ErCl3 were measured at temperatures in the liquid He3 temperature range.
At the start of this work, the low temperature crystallographic form of TbCl3 had not been fully characterised. The crystal structure was published while work on the structure was still in progress, and the author's results agree with the published results within experimental error.
The positions of the rare-earth ions in Li3YCl6 and CeCl3.7H2O were determined from Patterson syntheses. The unit cell size and symmetry of GdBr3 was also determined.
Several Group III B. compounds were studied by X-ray powder diffraction, in order to determine their unit cell size and symmetry. The only compound of possible interest as a host lattice for rare-earths, appears to be In(SCN)3 which possesses cubic symmetry.