Structural and magnetic properties of the clinopyroxenes NaFeSi2O6 and LiFeSi2O6

Single crystals of the clinopyroxenes NaFeSi206, known as the mineral acmite, and LiFeSi206 were synthesized by applying flux methods. They crystallize in the monoclinic space group C2/c, LiFeSi206 undergoes a phase transition to Fl\\c at — 45°C. The Fe-cations occupy the centers of edge-sharing octahedra, connected in zig-zag-chains along c. Antiferromagnetic ordering occurs at low temperatures (TN(NaFeSi206) = 5.0(3) K, TN (LiFeSi206) = 19.5(5) K) with the paramagnetic Curie temperature also being negative. Angular dependent susceptibility measurements were carried out with a single crystal of LiFeSi206. The easy direction of the magnetization and also a spin-flop-transition were observed parallel to c. The Mössbauer spectra of both compounds reveal magnetic hyperflne interaction below the temperature of magnetic coupling. Relaxation features demonstrate the transition from short range to long range ordering. Introduction NaFeSi206 (Na-acmite) is known as the mineral acmite, whereas LiFeSi206 (Li-acmite) is not found in nature. Both compounds belong to the chain 274 Ε. Baum. W. Treutmann, Μ. Behruzi, W. Lottermoser and G. Amthauer Fig. 1. Projection of the tetrahedra and octahedra chains of the Pljc low-LiFeSi206 onto the (100) plane. The M2 polyhedra are not shown in this figure. silicates. At room temperature they are monoclinic and have the space group C2/c (Table 1). The structural details were determined by Clark et al. (1969) and Behruzi et al. (1984). The crystal structure of the clinopyroxenes consists of three kinds of chains, all of which are parallel to the c-axis. (i) Chains of corner-sharing Si04 tetrahedra. (ii) Chains of edge-sharing Ml octahedra. (iii) Chains of edge-sharing M2 polyhedra having either 6or 8-fold coordination. The crystal structure is shown in Figure 1. In both compounds the Ml octahedra are occupied exclusively by Fe . The intrachain Fe-Fe-distances are 3.19 Ä for the Na-acmite and 3.18 A for the Li-acmite. The substitution of Li by Na increases the coordination number of the M2 site from 6 to 8. This causes a structural expansion, especially an increase of the interchain distance from 5.31 A in LiFeSi206 to 5.43 A in NaFeSi206. LiFeSi206 undergoes a phase transition to the space group Flijc at 228 Κ (Behruzi et al., 1984), where two kinds of silicate chains exist, which are kinked by angles of 169.9° and 166.2°, respectively (Fig. l). In the C2/c structure, however, the chains are equal and fully extended to about 180°. Moreover, the Li-coordination changes from 6 to 5 in the low-temperaturestructure. But the structural details of the Fe-environment such as the Magnetic properties of NaFeSi206 and LiFeSi206 275 Fig. 2. Mössbauer spectra of LiFeSi206 (Li-acmite) and NaFeSi206 (Na-acmite) taken at room temperature. Fe —Fe intraand interchain distances or the Fe—Ο — Fe bridging angle remain more or less the same in both structures (Table 1). Because of their topological properties the clinopyroxenes may be considered as approximately one-dimensional systems with interesting magnetic properties. These were studied by means of magnetic susceptibility measurements and Mössbauer spectroscopy in the present work. Experimental Both compounds were synthesized from Fe203 and alkaline-silicate glasses at temperatures of about 1000°C by applying flux methods. The obtained single crystals (Behruzi, Baneijea-Appel et al., 1984) were in the case of 276 Ε. Baum, W. Treutmann, Μ. Behruzi, W. Lottermoser and G. Amthauer Li-acmite large enough (ca. 2 x 3 x 0 . 5 mm) for magnetic susceptibility measurements. The magnetic susceptibility measurements were carried out with a vibrating sample magnetometer (PAR 159) with automatic data recording. The maximum magnetic field generated by a superconducting split coil is 55 kOe. The sample can be cooled down to 2.6 K. It is also possible to rotate the sample on the vibration axis within the magnetic field in order to study anisotropic behaviour of single crystals. The adjustment of a sample in the sample holder is possible within + 2°. In our case, the vibration axis was always parallel to one of the crystallographic directions a*, b or c. Mössbauer spectra of the samples were taken with a multichannel analyzer (1024 channels) operated in conjunction with an electromechanical drive system with symmetric triangular velocity shape. The two simultaneously obtained spectra (512 channels each) were folded and evaluated assuming Lorentzian line shape. The velocity scale was calibrated with α-iron. Isomer shifts are reported relative to α-iron at room temperature. During the experiments the source ( ~ 25 mCi Co/Rh) was kept at room temperature whereas the absorber (5 mg Fe/cm) was cooled down to several temperatures below 4.2 Κ with a gas flow cryostate. The temperature was measured and controlled by a calibrated carbon resistor within ± 0.5 K. Results The quality of our samples was tested by X-ray diffraction and Mössbauer spectroscopy at room temperature. The lattice parameters are in good agreement with those reported by Clark et at. (1969) and Behruzi et al. (1984) (Table 1). The Mössbauer spectra of both compounds taken at room temperature reveal a doublet with isomer shifts IS = 0.39 mm/s, which are typical for Fe in sixfold oxygen coordination i.e. the Ml positions (Table 3). The small quadrupole splitting QS of both doublets indicates a slight deviation of the Ml sites from cubic symmetry. The comparatively small half widths Β of the lines of both compounds demonstrate the homogeneity and good crystallinity of the samples. The field dependence of the magnetization Μ of both powdered samples measured at 4.2 Κ is shown in Figure 3. A linear increase of the magnetization with the external magnetic field Η is observed for NaFeSi206, whereas a deviation from linearity above 25 kOe is observed for LiFeSi206. This deviation indicates a beginning spin-flop transition. Because of the linearity of the magnetization Μ in the lower field range its temperature dependence was measured at an external field Η = 10 kOe. Both compounds order antiferromagnetically, the Na-acmite at 5.0(3) Κ and the Li-acmite at 19.5(5) Κ (Fig. 4, Table 2). Above 50 Κ the inverse Magnetic properties of NaFeSi2Oe and LiFeSi205 111 Table 1. Lattice parameters and interatomic distances of NaFeSi206 and LiFeSi206. NaFeSi206* LiFeSi206»* LiFeSi206*» Γ[Κ] 295 295 213 Λ-value [%] 3.5 2.8 4.0 Space group C2/c C2/c P2i/c a [A] 9.658 9.675 9.642 b[ A] 8.795 8.668 8.694 C[A] 5.294 5.297 5.281 β η 107.42 110.22 110.03 Atomar distances Fe—Fe intrachain [A] 3.19 3.18 3.18 Fe—Fe interchain [A] 5.43 5.31 5.28 Bridging angle Fe—Ο —Fe [°] 100.7 99.5 99.6/99.3 (A-/B-chain) Tetrahedra chain angle 0 3 0 3 0 3 [°] 174.0 179.0 169.9/166.1 (A-/B-chain) • Structure refined by Clark et al. (1969). ** Structure refined by Behruzi et al. (1984).