  Realizing a quantum spin liquid (QSL) ground state in a real material is a
leading issue in condensed matter physics research. In this pursuit, it is
crucial to fully characterize the structure and influence of defects, as these
can significantly affect the fragile QSL physics. Here, we perform a variety of
cutting-edge synchrotron X-ray scattering and spectroscopy techniques, and we
advance new methodologies for site-specific diffraction and L-edge Zn
absorption spectroscopy. The experimental results along with our
first-principles calculations address outstanding questions about the local and
long-range structures of the two leading kagome QSL candidates, Zn-substituted
barlowite Cu$_3$Zn$_{x}$Cu$_{1-x}$(OH)$_6$FBr and herbertsmithite
Cu$_3$Zn(OH)$_6$Cl$_2$. On all length scales probed, there is no evidence that
Zn substitutes onto the kagome layers, thereby preserving the QSL physics of
the kagome lattice. Our calculations show that antisite disorder is not
energetically favorable and is even less favorable in Zn-barlowite compared to
herbertsmithite. Site-specific X-ray diffraction measurements of Zn-barlowite
reveal that Cu$^{2+}$ and Zn$^{2+}$ selectively occupy distinct interlayer
sites, in contrast to herbertsmithite. Using the first measured Zn L-edge
inelastic X-ray absorption spectra combined with calculations, we discover a
systematic correlation between the loss of inversion symmetry from
pseudo-octahedral (herbertsmithite) to trigonal prismatic coordination
(Zn-barlowite) with the emergence of a new peak. Overall, our measurements
suggest that Zn-barlowite has structural advantages over herbertsmithite that
make its magnetic properties closer to an ideal QSL candidate: its kagome
layers are highly resistant to nonmagnetic defects while the interlayers can
accommodate a higher amount of Zn substitution.
