title: Structure and physical properties of mixed-metal chalcogenides for energy applications
Creators: name: Aldowiesh, Alaa Soluman Mohammed
Publisher: name: University of Reading
Year: 2024

description: Thermoelectricity offers a promising solution to help address the ongoing energy crisis by 
utilizing waste heat and reducing the reliance on conventional energy sources thereby 
contributing to a more sustainable and environmentally friendly energy future. A thermoelectric 
device allows for the direct conversion of thermal energy to electrical energy or vice versa 
through several principles. The structural and thermoelectric properties have been investigated 
for three families of mixed-metal chalcogenides materials. Chemical substitution approaches 
have been used on all the prepared materials as a method to enhance the thermoelectric 
performance. 
Focusing on Cu2BGeSe4 (B = Fe, Mn and Co), the influence of magnetic ions on crystal 
structure and thermoelectric properties was explored. Different crystal structures are adopted
based on the magnetic ion present, with Cu2FeGeSe4 and Cu2MnGeSe4 exhibiting structural 
phase transitions at ~ 500 K affecting their electrical properties. The phase Cu2CoGeSe4, which 
was characterised by the increased tetragonal distortion parameter, achieved a maximum figure 
of merit ZT of 0.52 at 875 K. Magnetic studies uncovered antiferromagnetic order in 
Cu2MnGeSe4. Neutron diffraction data revealed magnetic scattering in Cu2MnGeSe4 phase,
while Cu2FeGeSe4 showed no evidence of long-range magnetic ordering which indicates a spin-glass transition. The magnetic unit-cell of Cu2MnGeSe4 is doubled in the a and c directions and 
is defined by the propagation vector k = [1/2, 0, 1/2] with an ordered magnetic moment of μ = 
3.950(2) μB at 5 K. The room-temperature analysis of neutron diffraction data for Cu2FeGeSe4 
shows an antisite cation disorder at the 4d and 2a sites. 
Investigations of the effect of magnetic ions on the thermoelectric properties were extended to 
the mixed derivatives, Cu2Fe1-xMxGeSe4 (M=Mn and Co) (0 ≤ x ≤ 1), in which improvements 
in figure of merit and the average figure of merit were observed, with Cu2Fe0.925Co0.075GeSe4 
achieving the maximum ZT = 0.53 at 800 K. Tunning the carrier concentration through electron 
doping was investigated in the series Cu2FeGeSe4-x (0 ≤ x ≤ 0.3). Reducing the Se content 
increases the figure of merit reaching ZT = 0.47 at 875 K for Cu2FeGeSe3.90 in comparison with 
the end-member phase. 
The new kiddcreekite-type materials have been investigated as a potential TE materials. The 
materials are of interest for their complex crystal structure, large unit-cell and the presence of 
heavy atoms. X-ray powder diffraction coupled with Rietveld refinements confirmed the cubic 
structure with the space group F4̅3m and lattice parameters a = 10.8328(1) Å and a = 11.2781(2) 
Å for Cu6SnWS8 and Cu6SnWSe8 respectively. Cu6SnWS8 outperformed Cu6SnWSe8 in the 
figure of merit due to its lower electrical resistivity and enhanced Seebeck coefficient, attributed 
Alaa Aldowiesh to point-defects and off-stoichiometry that altered the electronic band structure. The maximum 
figure of merit reached ZT = 2.3 × 10-4 at 575 K for Cu6SnWSe8 and ZT = 0.021 at 675 K for 
Cu6SnWS8. Substituting components at different sites within the kiddcreekite materials (24f , 
4c, 4a and 16e) through electron, hole, and isovalent substitution shows limited effectiveness in 
improving ZT, as the substitution negatively impacted the electrical transport properties.
Work on p-type two-dimensional materials demonstrated that Cr2-xInxGe2Te6 (x = 0, 1, 2) based 
compounds are good candidates for thermoelectric applications. The phases Cr2Ge2Te6 and 
In2Ge2Te6 crystallise in the trigonal space group R3̅, whereas the phase CrInGe2Te6 crystallises 
in the trigonal space group P3̅1c. Replacing one Cr3+ with In3+ increased the figure of merit 
reaching ZT⊥ = 0.18 at 730 K in CrInGe2Te6. However, the power factor in the undoped material, 
CrInGe2Te6, is lower than that of conventional thermoelectric materials. This indicates that 
further enhancement of the electrical properties can be achieved through chemical substitution. 
The thermoelectric performance is greatly enhanced in the two series Cr1-xMnxInGe2Te6 and 
CrIn1-xMnxGe2Te6 (0 ≤ x ≤ 0.1). The substitution reduced the electrical resistivity, ρ, and this 
reduction offset the insignificant increase in the thermal conductivity. Hole doping enhanced 
the thermoelectric performance by up to 67% and 60% in Cr0.92Mn0.08InGe2Te6 and 
CrIn0.92Mn0.08Ge2Te6 ,respectively, reaching ZT⊥ = 0.52 and ZT⊥ = 0.42 at 730 K.