  The thermal conductivity of glasses is well-known to be significantly harder
to theoretically describe compared to crystalline materials. Because of this
fact, the fundamental understanding of thermal conductivity in glasses remain
extremely poor when moving beyond the case of simple glasses, e.g., glassy
SiO$_2$, and into so-called 'modified' oxide glasses, that is, glasses where
other oxides (e.g. alkali oxides) have been added to break up the network and
alter e.g. elastic and thermal properties. This lack of knowledge is apparent
despite how modified glasses comprise the far majority of known glasses. In the
present work we study an archetypical series of sodium silicate
($x\text{Na}_2\text{O}\text{-}(100\text{-}x)\text{SiO}_2$) glasses. Analyses of
modal contributions reveal how increasing Na$_2$O content induces increasing
vibrational localization with a change of vibrations to be less ordered, and a
related general decrease in modal contributions to thermal conductivity. We
find the vibrational phases (acoustic vs. optical) of sodium vibrations to be
relatively disordered compared to the network-forming silicon and oxygen
species, explaining how increasing Na$_2$O content decreases thermal
conductivity. Our work sheds new light on the fundamentals of glassy heat
transfer as well as the interplay between thermal conduction and modal
characteristics in glasses.
