  Current nonlinear optical materials face a conventional limitation on the
tradeoff between band gap and birefringence, especially in the deep UV spectral
region. To circumvent such a dilemma, we propose a general principle, a
{\pi}-conjugated confinement, to partially decouple the inter group
{\pi}-conjugated interactions with the separation of a non-{\pi}-conjugated
group so as to maximize the band gap in comparison with those of simple
{\pi}-conjugated salts, such as borates, carbonates. Meanwhile, to maintain a
large optical anisotropy. We uncover that the {\pi}-conjugated confinement is a
shared structural feature for all the known DUV NLO materials with favorable
properties (45 compounds), and thus, it provides an essential design criterion.
Guided by this principle, the carbonophosphate is predicted theoretically for
the first time as a promising DUV candidate system, Sr3Y[PO4][CO3]3 and
Na3X[PO4][CO3] (X = Ba, Sr, Ca, Mg) exhibit an enhanced birefringence that is
3-24 times larger than that of the simple phosphate, as well as an increased
band gap that is 0.2-1.7 eV wider than that of the simple carbonate.
Especially, the shortest SHG output of Sr3Y[PO4][CO3]3 is at {\lambda}PM = 181
nm, being the shortest one among phosphates to date.
