As a part of the International Avogadro Project we have utilized a state-of-the-art lattice comparator at the National Institute of Standards and Technology (NIST) to measure relative lattice parameter differences that are of the order of 10E-8. Respective samples have been prepared from NRLM3, NRLM4, WASO04 and WASO17 material, which are of high importance in terms of a precise determination of the silicon d_220 lattice parameter. The experimental setup at NIST consists of a two-source, two-crystal Laue x-ray diffractometer. It applies a heterodyne interferometer to control the position of the first one with an uncertainty of 2?10E-9 rad. From a least-squares fit we estimate the measurement uncertainty for a relative lattice parameter comparison to about 3?10E-9. The measured lattice parameter variation across each individual sample indicates that lattice uniformity of a particular specimen has to be carefully considered in view of intercomparing similar, however physically different, samples made out of the same material.