  Ultramassive white dwarfs are extreme endpoints of stellar evolution. Recent
findings, such as a missing multi-Gyr cooling delay for a number of
ultramassive white dwarfs and a white dwarf with a quasi-Chandrasekhar mass,
motivate a better understanding of their evolution. A key process still subject
to important uncertainties is the crystallization of their dense cores, which
are generally assumed to be constituted of $^{16}$O, $^{20}$Ne, and a mixture
of several trace elements (most notably $^{23}$Na and $^{24}$Mg). In this work,
we use our recently developed Clapeyron integration technique to compute
accurate phase diagrams of three-component mixtures relevant to the modeling of
O/Ne ultramassive white dwarfs. We show that, unlike the phase separation of
$^{22}$Ne impurities in C/O cores, the phase separation of $^{23}$Na impurities
in O/Ne white dwarfs cannot lead to the enrichment of their cores in $^{23}$Na
via a distillation process. This severely limits the prospect of transporting
large quantities of $^{23}$Na toward the center of the star, as needed in the
white dwarf core collapse mechanism recently proposed by Caiazzo et al. We also
show that despite representing $\approx 10\%$ of the ionic mixture, $^{23}$Na
and $^{24}$Mg impurities only have a negligible impact on the O/Ne phase
diagram, and the two-component O/Ne phase diagram can be safely used in white
dwarf evolution codes. We provide analytic fits to our high-accuracy O/Ne phase
diagram for implementation in white dwarf models.
