Direct observation of the near-surface layer in $\mathrm{Pb}\left({\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}\right){\mathrm{O}}_3$ using neutron diffraction

Spatially resolved neutron diffraction as a function of crystal depth in $\mathrm{Pb}\left({\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}\right){\mathrm{O}}_3$ reveals the presence of a distinct near-surface region where a strong distortion in the lattice exists. A dramatic change in both the lattice constant and the Bragg peak intensity as a function of crystal depth is observed to occur in this region over a length scale $\sim 100\mu \mathrm{m}$. This confirms a previous assertion, based on a comparison between high-energy x rays and neutrons, that such a near surface region exists in the relaxors. Consequences to both single crystal and powder diffraction measurements and previous bulk neutron diffraction measurements on large single crystals are discussed.

Figure 1

Schematic of the experiment used to measure spatially resolved strain with neutrons. The mosaic of the germanium monochromator was $\sim {15}^{\prime }$, and a single channel from the monochromator to incident slit defined a collimation of $\sim {30}^{\prime }$. A single slit was used on the scattered side. The slits on both the incident and scattered sides were $300\mu \mathrm{m}$ wide and $5\mathrm{mm}$ high.

Figure 2

The upper panel plots the (2, 0, 0) Bragg peak intensity as a function of translation. The lower panel displays the lattice constant (and hence the strain) as a function of distance into the sample. A large $\sim 100\mu \mathrm{m}$ surface layer is observed based on a large variation of the lattice constant. The vertical dashed line indicates the position of the sample surface.

Figure 3

Radial $\theta -2\theta$ scans through the (2, 0, 0) Bragg peak for various translation values. The decrease in intensity is due to extinction and a noticeable shift in the $2\theta$ position is clearly seen. The dashed lines are fits to a Gaussian lineshape.

Figure 4

The upper panel plots the germanium (2, 2, 0) Bragg peak intensity as a function of translation. The lower panel displays the lattice constant (and hence the strain) as a function of distance into the sample. Unlike PMN, no significant strain is measured in the near-surface region. The vertical dashed line indicates the position of the sample surface.