Chromium-based clinopyroxene-type germanates NaCrGe(2)O(6) and LiCrGe(2)O(6) at 298 K

The structure analyses of sodium chromium digermanate, NaCrGe(2)O(6), (I), and lithium chromium digermanate, LiCrGe(2)O(6), (II), provide important structural information for the clinopyroxene family, and form part of our ongoing studies on the phase transitions and magnetic properties of clinopyroxenes. (I) shows C2/c symmetry at 298 K, contains one Na, one Cr (both site symmetry 2 on special position 4e), one Ge and three O-atom positions (on general positions 8f) and displays the well known clinopyroxene topology. The basic units of the structure of (I) are infinite zigzag chains of edge-sharing Cr(3+)O(6) octahedra (M1 site), infinite chains of corner-sharing GeO(4) tetrahedra, connected to the M1 chains by common corners, and Na sites occupying interstitial space. (II) was found to have P2(1)/c symmetry at 298 K. The structure contains one Na, one Cr, two distinct Ge and six O-atom positions, all on general positions 4e. The general topology of the structure of (II) is similar to that of (I); however, the loss of the twofold symmetry makes it possible for two distinct tetrahedral chains, having different conformation states, to exist. While sodium is (6+2)-fold coordinated, lithium displays a pure sixfold coordination. Structural details are given and chemical comparison is made between silicate and germanate chromium-based clinopyroxenes.