Structural studies on the Fresnoite type compound Ba₂VSi ₂O₈

Single crystals of Ba₂VSi₂O₈ have successfully been grown from a high-temperature solution and single crystal X-ray diffraction used to refine its fresnoite type average crystal structure. In contrast to other recently investigated representatives of the fresnoite family of compounds, Ba₂VSi₂O₈ did not at first appear to be incommensurately modulated. Neither single crystal X-ray diffraction nor powder neutron diffraction gave any indication for the existence of satellite reflections in addition to the strong average structure Bragg reflections. A careful electron diffraction study, however, has revealed the existence of weak incommensurate satellite reflections characterized by the primitive primary modulation wave-vectors q₁ = 0.297 [1 1 0]* + 1/2c* and q₂ = 0.297 [1 1 0]* + 1/2c*. The reciprocal space positioning of these incommensurate primary modulation wave-vectors, the overall (3 + 2)-d superspace group symmetry, as well as the shapes of the refined displacement ellipsoids determined from the single crystal XRD refinement are all consistent with their arising from condensed Rigid Unit Mode (RUM) modes of distortion of the Ba₂VSi₂O₈ parent structure. The weakness of the satellite reflections in Ba ₂VSi₂O₈ relative to, for example, Ba ₂TiGe₂O₈ or Sr₂TiSi ₂O₈ arises from the significantly smaller amplitude of the condensed RUM mode as is clear from the magnitudes of the refined average structure anistropic displacement ellipsoids.