Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.660698
Title: Structural and magnetic study of ammonium iron halides
Author: Porter, Deborah Aitken
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1998
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Abstract:
Some ABX3 materials, in which A is a monovalent cation, B a divalent cation and X a monovalent anion, are known to exhibit the hexagonal perovskite structure; these have a chain-like structure produced by face sharing BX6 octahedra that lie parallel to the crystal c axis. For materials in which B is a first row transition metal, this structural anisotropy leads to pseudo-one dimensional magnetic behaviour, the interchain exchange is several orders of magnitude larger than the intrachain exchange. This thesis discusses the investigation of the crystal structure and crystallographic and magnetic phase transitions in the hexagonal perovskites, ammonium iron chloride and bromide; specifically to consider the effect of the non-spherical ammonium ion on both the structure and the magnetic properties. Powder samples have been studied between 420 and 4.2 K using high resolution synchrotron X-ray diffraction, and between 470 and 1.5 K using neutron diffraction. The rotation of the ammonium ion within the structure was studied via quasi-elastic neutron diffraction. Single crystals of both materials were obtained and studied using SQUID magnetometry and X-ray diffraction (between room temperature and 10 K). Ammonium iron chloride is found to undergo a structural phase transition at 181.0(2) K where the symmetry is lowered as the FeCl3 chains twist slightly. A second structural phase transition is observed at 19.5(2) K; in this case the unit cell is found to double in the a direction with respect to the primitive cell. Quasi-elastic neutron scattering measurements show NH4FeCl3 exhibits quantum tunnelling in its low temperature phase, proving the NH4 motion is frozen at that temperature. Neutron diffraction reveals antiferromagnetic reflections corresponding to a helical magnetic array below 3.7(2) K; susceptibility data also show a transition at this temperature.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.660698  DOI: Not available
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