Structural and magnetic properties of the ...

Zeitschrift für Kristallographie 183, 273-284(1988) C by R. Oldenbourg Verlag, München 1988 - 0044-2968/88 $3.00 + 0.00

Structural and magnetic properties of the clinopyroxenes NaFeSi 2 0 6 and LiFeSi 2 0 6 E. Baum, W. Treutmann Institut für Mineralogie der Universität Marburg, Hans-Meerwein-Straße, D-3550 Marburg, Federal Republic of Germany

Μ . Behruzi Institut für Kristallographie der RWTH Aachen, Templergraben 55, D-5100 Aachen, Federal Republic of Germany

W. Lottermoser and G. Amthauer Institut für Geowissenschaften der Universität Salzburg, Heilbrunner Straße 34, A-5020 Salzburg, Austria Received: December 7,1987

Synthetic clinopyroxenes / Crystal structure / Magnetic susceptibility / Mössbauer spectroscopy Abstract. Single crystals of the clinopyroxenes NaFeSi 2 0 6 , known as the mineral acmite, and LiFeSi 2 0 6 were synthesized by applying flux methods. They crystallize in the monoclinic space group C2/c, LiFeSi 2 0 6 undergoes a phase transition to Fl\\c at — 45°C. The Fe 3+ -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 (LiFeSi 2 0 6 ) = 19.5(5) K) with the paramagnetic Curie temperature also being negative. Angular dependent susceptibility measurements were carried out with a single crystal of LiFeSi 2 0 6 . 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 NaFeSi 2 0 6 (Na-acmite) is known as the mineral acmite, whereas LiFeSi 2 0 6 (Li-acmite) is not found in nature. Both compounds belong to the chain

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Ε. Baum. W. Treutmann, Μ. Behruzi, W. Lottermoser and G. Amthauer

Fig. 1. Projection of the tetrahedra and octahedra chains of the Pljc onto the (100) plane. The M2 polyhedra are not shown in this figure.

low-LiFeSi206

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 Si0 4 tetrahedra. (ii) Chains of edge-sharing Ml octahedra. (iii) Chains of edge-sharing M2 polyhedra having either 6- or 8-fold coordination. The crystal structure is shown in Figure 1. In both compounds the Ml octahedra are occupied exclusively by Fe 3 + . 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 LiFeSi 2 0 6 to 5.43 A in NaFeSi 2 0 6 . LiFeSi 2 0 6 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 3+ -environment such as the


Magnetic properties of NaFeSi 2 0 6 and LiFeSi 2 0 6

275

Fig. 2. Mössbauer spectra of LiFeSi 2 0 6 (Li-acmite) and NaFeSi20 6 (Na-acmite) taken at room temperature.

Fe —Fe intra- and 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 Fe 2 0 3 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


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Ε. Baum, W. Treutmann, Μ. Behruzi, W. Lottermoser and G. Amthauer

Li-acmite large enough (ca. 2 x 3 x 0 . 5 mm 3 ) 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 57 Co/Rh) was kept at room temperature whereas the absorber (5 mg Fe/cm 2 ) 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 3 + 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 NaFeSi 2 0 6 , whereas a deviation from linearity above 25 kOe is observed for LiFeSi 2 0 6 . 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


111

Magnetic properties of NaFeSi 2 O e and LiFeSi 2 0 5

Table 1. Lattice parameters and interatomic distances of NaFeSi 2 0 6 and LiFeSi 2 0 6 .

Γ[Κ] Λ-value [%] Space group

a [A] b[ A] C [A] β η Atomar distances Fe—Fe intrachain [A] Fe—Fe interchain [A] Bridging angle Fe—Ο —Fe [°] Tetrahedra chain angle 0 3 - 0 3 - 0 3 [°]

NaFeSi 2 0 6 *

LiFeSi 2 0 6 »*

LiFeSi 2 0 6 *»

295 3.5 C2/c 9.658 8.795 5.294 107.42

295 2.8 C2/c 9.675 8.668 5.297 110.22

213 4.0

3.19 5.43

3.18 5.31

P2i/c 9.642 8.694 5.281 110.03 3.18 5.28

100.7

99.5

99.6/99.3 (A-/B-chain)

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).

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10

20

30 40 Η [kOe]

50

60

Fig. 3. Magnetization Μ as a function of the external magnetic field Η for LiFeSi 2 0 6 and NaFeSi 2 0 6 at 4.2 K.

magnetic susceptibility follows the Curie Weiss law and reveals a linear dependence on temperature (Fig. 5). From this a negative paramagnetic Curie temperature θρ can be evaluated, which is —39 Κ for NaFeSi 2 0 6 and — 33 K. for LiFeSi 2 0 6 (Table 2). These negative paramagnetic Curie temperatures also indicate antiferromagnetic coupling in both compounds.


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r τ> ι ι τ 7~τ γ r Τf r1 r r r τ > τΓ f > > > > r ι » ι ι t « r » » > 10 20 30 U0 Τ IK]

Fig. 4. Magnetic susceptibility Χ of LiFeSi20 6 and NaFeSi 2 0 6 as a function of the temperature Τ (measured at Η = 10 kOe).

Table 2. Results of the magnetic susceptibility measurements.

C [Kcm J /mol] MCRR [ μ β ]

θρ [K] Tn[K] TN/0p Deviation from Curie-Weiss-Law

NaFeSi 2 0 6

LiFeSi 2 0 6

4.36 5.9 -39 5 0.128

4.60 6.1 -33 19.5 0.591

T> 60 Κ

Γ > 40K

From the Curie constant C an effective magnetic moment of 5.9 μΒ was calculated for the Na-acmite and 6.1 μΒ for the Li-acmite (Table 2). Both values are typical experimental values for Fe 3 + in the high spin state (HS). As mentioned above, a single crystal of LiFeSi 2 0 6 was large enough for magnetic measurements. The angular dependence of the magnetic susceptibility of that single crystal at 4.2 Κ is shown in Figure 6. With the vibration axis parallel to the c-axis as rotation axis isotropic behaviour of the susceptibility is observed whereas with the rotation axis parallel to a* or b a distinctly anisotropic behaviour is found. The easy direction of the magnetization is parallel to the c-axis, because always a minimum of Μ is measured parallel to c. A plot of the magnetic susceptibility versus the temperature Τ (external field Η = 10 kOe) parallel (X||) and perpendicular (Zx) to the c-axis is


279


Fig. 5. Inverse magnetic susceptibility X 1 of LiFeSi 2 S 6 and NaFeSi 2 0 6 as a function of temperature T. From the Curie-Weiss law the paramagnetic Curie temperatures 0 P = - 3 3 ( 2 ) Κ and 0P = - 3 9 ( 2 ) Κ are derived.

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60

120

180

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240

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1

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300

360

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Fig. 6. Angular dependence of the magnetic susceptibility X of a LiFeSi 2 0 6 single crystal at an external magnetic field Η = 10 kOe and at a temperature Τ = 4.2 Κ.


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Ε. Baum. W. Treutmann, Μ. Behruzi, W. Lottermoser and G. Amthauer

Τ [Kl Fig. 7. Temperature-dependence of the magnetic susceptibility X parallel (X||) and perpendicular (Xj.) to the c-axis for a LiFeSi 2 0 6 single crystal (H = 10 kOe). For comparison, the susceptibility of the powdered sample is plotted as a dashed line.

shown in Figure 7. The sharp kink very clearly marks the antiferromagnetic ordering temperature T N = 19.5(5) K. The difference of the magnetic behaviour of the two directions is quite obvious: below T N with Η parallel to the easy axis c X n drops to zero for T-»0 K, whereas with Η perpendicular to the easy axis XL remains nearly constant. For comparison the result of the powder measurement is also plotted as a dashed line in Figure 7. The well known prediction of the Neel theory X powder (0 K) = 2/3 X± = 2/3 * powder (TN) is quite well satisfied. Furthermore the magnetizations of the two directions behave differently when they are subjected to higher external fields below 20 K. Applying a strong magnetic field of about 50 — 60 kOe parallel to the c-axis a beginning spin-flop transition is indicated. With increasing temperature up to T N this effect disappears. Perpendicular to the c-axis the behaviour is always linear even below T N , as expected (Fig. 8). The low temperature Mössbauer spectra of both compounds exhibit relaxation features indicating fluctuations of the local magnetic fields. For NaFeSi 2 0 6 this is shown in Figure 9. The asymmetric quadrupole doublet at 15 Κ is replaced by a broad six-line pattern combined with a paramagnetic peak a 12.5 K. With decreasing temperature the six-line spectrum gains intensity with narrowing line widths (represented in Fig. 9 by the 10 Κ pattern) over the residual paramagnetic peak, which has


281


Table 3. Mössbauer parameters of NaFeSi 2 0 6 (Na-acmite) and LiFeSi 2 0 6 (Li-acmite). NaFeSi 2 0 6 T*[K] IS« [mm/s] QS* [mm/s] B* [mm/s] H„* [kOe] Tm*[K]

295 0.39(1) 0.33(1) 0.30(1) -

5 0.50(1) 0.01(1) 0.92(1) 461(5) 20

LiFeSi 2 0 6 295 0.39(1) 0.31(1) 0.28(1) —

5 0.49(1) 0.01(1) 0.40(1) 540(5) 12.5

* Τ = temperature at which spectrum was taken; IS = isomer shift relative to α-Fe at 295 K; QS = quadrupole splitting; Β = full width at half peak hight; H„ = local magnetic field; TM = 'magnetic ordering temperature' as observed by Mössbauer spectroscopy. Numbers in parentheses refer to the experimental error in the last digit.

Η tkOe] Fig. 8. Magnetization Μ of a LiFeSi 2 0 6 single crystal as a function of the external field Η. 1) c||H, Τ = 4.2 Κ; 2) c||H, Τ = 10 Κ; 3) c||H, Τ = 20 Κ and α*||Η, Τ = 4.2 Κ.

completely disappeared at 6.5 Κ. The comparably broad line width of the hyperfine spectrum at 5 Κ (IS = 0.92 mm/s, H c = 461 kOe) indicates relaxation effects of the magnetic fields with a fluctuation rate between about 109 s" 1 (12.5 K) and 107 s" 1 (5 K) (Barb, 1980). In the Mössbauer spectrum of LiFeSi 2 0 6 the transition from the quadrupole doublet to the magnetically ordered state is observed at 20(1) K. Relaxation effects as displayed by broadened line widths diminish with decreasing temperature within 2—3 Κ (see Fig. 10), until a quite normal hyperfine spectrum is established at 5 Κ (IS = 0.40 mm/s, H 0 = 540(5) kOe).


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Ε. Baum, W. Treutmann. Μ. Behruzi, W. Lottermoser and G. Amthauer

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