Very interesting question! The wavelength of X-rays are very small  and is comparable with interatomic spacing. Therefore, X-ray  propagates through the opening between atoms and also exhibit diffraction. i believe the refractive index of  interatomic spacing is same as vaccum.   

At X-ray frequencies, the refractive index of materials is slightly less than 1. Refraction angles of X-rays at material-air or -vacuum interfaces are therefore very small, and the direction of the refraction angle is the opposite of what would be expected for visible light. In most cases, the refraction effect can be neglected except at very small incident angles (where total external reflection of X-rays occurs) or for high-resolution diffraction studies of nearly perfect crystals. However, total external reflection can be very useful for studying the surface of materials or thin films, since the penetration of X-rays into the material is minimized.

Lambda has n in it. The true characteristic of a wave

I like the question. It also applies to standard, i.e. non-Bragg, planar gratings. Think of a CD, where the structure is buried in plastics of n clearly away from unity. The crucial hint from Igor leads to the answer...

I agree, Lambda has the refractive index already inside. Sometimes (e.g. in reflection from multi-dielectric mirrors) the refractive index is considered in the  thickness of layer introducing an optical or effective thickness (which is simply the geometrical thickness multiplied the index) and everything else is expressed then in vacuum.

this may be useful:

http://scripts.iucr.org/cgi-bin/paper?pii=S0108767388012243&sentby=wiley