  X-ray diffraction studies on a Pb(Zn$_{1/3}$Nb$_{2/3}$)O$_3$ (PZN) single
crystal sample show the presence of two different structures. An outer-layer
exists in the outer most $\sim$ 10 to 50~$\mu$m of the crystal, and undergoes a
structural phase transition at the Curie temperature $T_C\approx410$ K. The
inside phase is however, very different. The lattice inside the crystal
maintains a cubic unit cell, while ferroelectric polarization develops below
$T_C$. The lattice parameter of the cubic unit cell remains virtually a
constant, i.e., much less variations compared to that of a typical relaxor
ferroelectric, in a wide temperature range of 15 K to 750 K. On the other hand,
broadening of Bragg peaks and change of Bragg profile line-shapes in both
longitudinal and transverse directions at $T_C$ clearly indicate a structural
phase transition occurring.
