Crystal and magnetic structures, phase transitions in quasi-one-dimensional pyroxenes Na0.5Li0.5FeGe2O6

The possibility of cation substitution in pyroxenes allows the investigation the gradual change of structural and magnetic properties under doping in these compounds. Here, Na0.5Li0.5FeGe2O6 was prepared by the standard solid-phase reaction method and has been investigated using X-ray and neutron powder diffraction, and further characterized by magnetic and calorimetric measurements. The crystal structure of Na0.5Li0.5FeGe2O6 at 300 K is monoclinic C2/c (a=10.0333(1), b=8.8136(1), c=5.5295(9) Å, β=108.921(1)°). Calorimetric investigations indicate a displacive first order phase transition at T=271±1 K which is accompanied by the appearance of superstructure reflections in the X-ray patterns. At this transition Na0.5Li0.5FeGe2O6 undergoes a space group change from C2/c to P21/c (a=9.9692(3), b=8.8545(3), c=5.4752(2) Å, β=108.494(1)°). Magnetic order has been found below the Néel temperature TN≈18 K and has been refined from neutron diffraction. The quasi-low-dimensional magnetic spin system Na0.5Li0.5FeGe2O6 exhibits a collinear antiferromagnetic structure with the space group Pa21/c and the doubling of the unit cell along the crystallographic a-axis of the pyroxene crystal (propagation vector k=(1/2, 0, 0)). The critical modes which are responsible for the phase transition from C2/c to P21/c symmetry and for the magnetic transition from paramagnetic to antiferromagnetic state have been determined and their role in the transition is described. They are also the relevant mechanism driving the magnetic order in LiFeGe2O6.

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