  Deuterated molecules and their molecular D/H-ratios (RD(D)) are important
diagnostic tools to study the physical conditions of star-forming regions. The
degree of deuteration, RD(D), can be significantly enhanced over the elemental
D/H-ratio depending on physical parameters. Within the Cygnus Allscale Survey
of Chemistry and Dynamical Environments (CASCADE), we aim to explore the
large-scale distribution of deuterated molecules in the nearby Cygnus-X region.
We focus on the analysis of large-scale structures of deuterated molecules in
the filamentary region hosting the prominent Hii region DR21 and DR21(OH). Here
we discuss the HCO+, HNC and HCN molecules and their deuterated isotopologues
DCO+, DNC and DCN. The spatial distributions of integrated line emissions from
DCO+, DNC, and DCN reveal morphological differences. DCO+ displays the most
extended emission, characterized by several prominent peaks. Likewise, DNC
exhibits multiple peaks, although its emission appears less extended compared
to DCO+. In contrast to the extended emission of DCO+ and DNC, DCN appears the
least extended, with distinct peaks. Focusing only on the regions where all
three molecules are observed, the mean deuteration ratios for each species are
0.01 for both DNC and DCN, and = 0.005 for DCO+. Anti-correlations are found
with deuterated molecules and dust temperature or N(H2). The strongest
anti-correlation is found with RD(DCO+) and N(H2). The anti-correlation of
RD(DCO+) and N(H2) is suggested to be a result of a combination of an increased
photodissociation degree and shocks. A strong positive correlation between the
ratio of integrated intensities of DCN and DNC with their 13C-isotopologues,
are found in high column density regions. The positive relationship between the
ratios implies that the D-isotopologue of the isomers could potentially serve
as a tracer for the kinetic gas temperature.
