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|Title: ||Brillouin and neutron scattering study of hexagonal ABX3 ternary halides|
|Authors: ||Hashim, Dayang M.A.|
|Issue Date: ||1995|
|Publisher: ||© Dayang Maryani Awang Hashim|
|Abstract: ||The interest in one dimensional (1D) magnetism has been strongly renewed
with the synthesis of many magnetic compounds which exhibit a quasi one
dimensional magnetic behaviour. One of the peculiarities of this 1D system is
the absence of a long range magnetic ordered phase at any finite
temperature for the ideal 1D system with short range interaction.
Tetramethylammonium manganese chloride (CH3)4NMnCl3(TMMC) exhibits
the properties of an ideal one dimensional antiferromagnets for temperature
above 1 K, the transition to a three dimensional (3D) long range ordered state
only occurs at 0.84K. In addition to its magnetic transition, TMMC exhibits
structural phase transition due to the ordering of the tetramethylammonium
(TMA) ions which makes also this compound very attractive from a lattice
dynamical point of view.
Structural phase transitions of tetramethylammonium manganese chloride
(TMMC), tetramethylammonium manganese bromide (TMMB) and
tetramethylammonium manganese chloride doped with 8% Cu (TMMC:Cu) of
the hexagonal type compounds are investigated using the Brillouin scattering
method. These crystals show pronounced acoustic anomalies in the region of
the structural phase transition. The acoustic anomalies were observed by
measuring sound velocity and hence the elastic constant can be deduced.
The phase transition temperatures were observed at 129.6K and 388.6K
(TMMC), 114.6K and 377.6K (TMMB) and at 108.6K and 359.6K (TMMC:Cu).
The elastic constant at room temperature were C11 = 2.10 (TMMC) and C11 =
1.59 (TMMB) in units of 1010 Nm-2.
The phase transition of these compounds were further investigated
macroscopically using the Differential Scanning Calorimetry (DSC) method.
Activation energies of TMMC, TMMC:Cu, TMMB and deuterated TMMB at the
phase transition were determined using this method. The values are 70.612
kJ/mol (TMMC), 49.224 kJ/mol (TMMC:Cu), 51.747 kJ/mol (TMMB) and
69.909 kJ/mol (d12-TMMB).
The elastic constant of the linear chain antiferromagnet CsNiCl3 and RbNiCl3
was also determined using the Brillouin scattering method. The room temperature measurements give C11 = 3.77 (3.71) and C33 = 5.62 (5.42) in
units of 1010 Nm·2 for CsNiCl3 and RbNiCl3 respectively.
The phonon dispersion curves at room temperature in the hexagonal
CsFeBr3 have been studied using the inelastic neutron scattering technique.
From the initial slope of the dispersion curve, the sound velocity was deduced
which enable us to calculate the elastic constant of CsFeBr3 at room
temperature. The values obtained are C11 = 7.33, C66 = 1.01, C33 = 2.58 and
C44 = 0.56 in units of 1010 Nm·2.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Physics)|
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