Two types of adjacent dimer layers in the low temperature phase of   BaCuSi2O6
  The low-temperature crystal structure of BaCuSi2O6 has been investigated with high-resolution synchrotron x-ray and neutron powder diffraction techniques and has been found to be on average (ignoring the incommensurate modulation) orthorhombic, with the most probable space group Ibam. The Cu-Cu dimers in this material are forming two types of layers with distinctly different interatomic distances. Subtle changes also modify the partially frustrated interlayer Cu-Cu exchange paths. The present results corroborate the interpretation of low-temperature nuclear magnetic resonance and inelastic neutron scattering data in terms of distinct dimer layers. 
D. V. Sheptyakov, V. Yu. Pomjakushin, R. Stern, I. Heinmaa, H.   Nakamura, T. Kimura
cond-mat.str-el