The (Na,Li)FeGe2O6 clinopyroxene-type series: a temperature-dependent single-crystal X-ray diffraction and 57Fe Mössbauer spectroscopic study

Pyroxene-type compounds along the solid solution series LiFeGe2O6 (P21/c space group symmetry) and NaFeGe2O6 (C2/c) have been synthesized at 1273–1373 K and investigated by single-crystal X-ray diffraction at 298 K and between 105 and 298 K for Li0.4Na0.6FeGe2O6 in order to study structural variations associated with the chemical- and temperature-driven P21/c ⇔ C2/c phase transition. 57Fe Mössbauer spectroscopy, differential thermal analysis, and determination of thermal expansion coefficients from high-temperature powder X-ray diffraction data complete the investigations. The C2/c phase is stabilized with increasing Na+ content and temperature, and the phase boundary is found at 789 K in pure LiFeGe2O6, decreasing to 109 K in the compound Li0.3Na0.7FeGe2O6. From the available data, a T–X structural phase diagram is constructed. The P21/c ⇔ C2/c phase transition is accompanied by distinct variations in lattice parameters, rearrangements in M2 site coordination as well as changes in tetrahedral site chain kinking. Also, chemical strain and the thermal expansion tensor change in size and orientation with Li+ by Na+ replacement. The study will provide a detailed description of the observed variations in bond lengths, bond angles, and distortion parameters.