Synthesis and study of structural, thermodynamic, and magnetic properties of Na$_{x}$Li$_{1-x}$FeGe$_{2}$O$_{6}$ ($x$ = 0.1–0.9)

The properties of Na$_{x}$Li$_{1-x}$FeGe$_{2}$O$_{6}$ ($x$ = 0.1–0.9) solid solutions obtained via a solid-phase synthesis have been measured by X-ray diffraction, calorimetry, and magnetic method. The order–disorder transformations in low-dimensional Na$_{x}$Li$_{1-x}$FeGe$_{2}$O$_{6}$ ($x$ = 0.1–0.9) spin systems with predominately antiferromagnetic exchange interaction have been revealed in the low-temperature susceptibility dependences. The study of thermal and physical properties has confirmed that substituting the sodium ions with the lithium ones induces the first-order structural phase transitions of the displacement type which are characterized by a symmetry change in monoclinic crystals from high-temperature $C2/c$ space group to low-temperature $P21/c$ space group.