  We have performed a systematic investigation of the variations of the lattice
constants with substituent rare-earth element concentration x in the nominally
undoped superconductors T'-(La3+)2-x(RE3+)xCuO4 (RE = Sm, Eu, Gd, Tb, Lu, and
Y), which we have recently discovered using MBE. The results show both the
in-plane and out-of-plane lattice constants (a0 and c0) linearly decrease with
x, whose extrapolation to x = 2 agrees well with the reported a0 and c0 values
for each T'-RE2CuO4. This behavior is what one would expect simply from the
ionic size difference between La3+ and RE3+. The absence of the Cu-O bond
stretching due to electron-doping, which is commonly observed in electron-doped
T' and infinite-layer superconductors, implies that electron doping via oxygen
deficiencies is, at least, not a main source of charge carriers.
