To achieve the true diffraction-limited emittance of a storage ring light
source, such as ~10 pm.rad for medium-energy electron beams, within a limited
circumference, it is generally necessary to increase the number of bending
magnets in a multi-bend achromat (MBA) lattice, as in the future upgrade plan
of MAX IV with a 19BA replacing the current 7BA. However, this comes with
extremely strong quadrupole and sextupole magnets and very limited space. The
former can result in very small vacuum chambers, increasing the coupling
impedance and thus enhancing the beam instabilities, and the latter can pose
significant challenges in accommodating the necessary diagnostics and vacuum
components. Inspired by the hybrid MBA lattice concept, in this paper we
propose a new unit lattice concept called the complex unit lattice cell, which
can reduce the magnet strengths and also save space. The complex unit cell is
numerically studied using a simplified model. Then as an example, a 17BA
lattice based on the complex unit cell concept is designed for a 3 GeV storage
ring light source with a circumference of 537.6 m, which has a natural
emittance of 19.3 pm.rad. This 17BA lattice is also compared with the 17BA
lattice designed with conventional unit cells to showcase the benefits of the
complex unit cell concept. This 17BA lattice also suggests a new type of MBA
lattice, which we call the MBA lattice with semi-distributed chromatic
correction