Abstract (eng)
The accurate description of ionic contributions to the frequency-dependent polarisability of a solid
is essential for the understanding of optical phenomena in a wide range of materials. While the
calculation of the ionic dielectric function at the DFT groundstate has become a routine task
in electron structure theory, there is no standard method to describe the ionic polarisability at
higher temperatures. This is particularly relevant for systems such as perovskite crystals, whose
structures are notoriously unstable (dynamically stable), and exhibit a series of phase transitions
between the groundstate and room temperature.
In this thesis, we investigate a statistical-mechanical approach to simulate the ionic dielectric
properties of two perovskite systems, barium titanate and strontium titanate, at zero and non-
zero temperatures. By means of the fluctuation-dissipation theorem, we derive an expression for
the Kubo-Green relation of the ionic polarisability. According to this Kubo-Green relation, the
polarisability can be expressed in terms of a time-correlation function of the total dipole fluctua-
tions in a canonical ensemble at a set temperature T>0K.
Furthermore, we present a special case of a Kubo-Green relation which leads to significant reduc-
tion in the computational demands for low but non-zero temperatures, and we show that we can
recover the standard formula for the groud-state polarisability as a limiting case of Kubo-Green.
All three presented methods are then applied to a series of test systems. We give a proof of
principle with the example of the very stable α-SiO 2 . In addition, we show the effectiveness of
our approach by computing the finite-temperature polarisability of barium titanate and strontium
titanate.