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УДК 538.955
Crystal Structure and Magnetic Properties in Pyroxenes Li0.7Na03FeGe2O6
Tamara V. Drokina* German A. Petrakovskii^
Kirensky Institute of Physics SB RAS Akademgorodok, 50/38, Krasnoyarsk, 660036
Russia
Dmitrii A. Velikanov*
Kirensky Institute of Physics SB RAS Akademgorodok, 50/38, Krasnoyarsk, 660036 Siberian Federal University Svobodny, 79, Krasnoyarsk, 660041
Russia
Maxim S. Molokeev§
Kirensky Institute of Physics SB RAS Akademgorodok 50/38, Krasnoyarsk, 660036 Far Eastern State Transport University Serysheva, 47, Khabarovsk, 680021
Russia
Elena G. Rezina^
Siberian Federal University Svobodny, 79, Krasnoyarsk, 660041
Russia
Received 05.05.2015, received in revised form 08.06.2015, accepted 02.07.2015 The Lio,rNao,3FeGe2Oe solid solution has been synthesized by the solid-phase reaction. The clinopy-roxene compound has been investigated by X-ray diffraction and by bulk magnetic measurements. The structural properties of the cation substitution compound are presented. Using SQUID techniques the temperature dependence of the magnetic susceptibility was measured. It exhibits a sharp maximum, which is suggested the phase transition from the paramagnetic state to a magnetically ordered state below 15.5 K in the sample of pyroxene solid solution.
Keywords: solid solution, pyroxene, crystal structure, magnetic properties. DOI: 10.17516/1997-1397-2015-8-3-273-280
* tvd@iph.krasn.ru
t gap@ iph. krasn. ru
idpona@rambler.ru
§ msmolokeev@mail.ru ^prim67@mail.ru © Siberian Federal University. All rights reserved
Introduction
Pyroxene inorganic compounds with the general chemical formula ABX2O6 (A = Na, Li and Ca; B = Mg, Cr, Cu, Ni, Fe, etc.; X = Ge, Si) have significant interest in solid state physics due to their physical properties. Magnetic studies have been carried out for the family pyroxenes compound: there are various types of magnetic state (antiferromagnetic [1-4], ferromagnetic [3-5], modulated magnetic structure [6,7], spin gap state [8]) connected to the features of the pyroxene crystal structure which allows the existence of competing magnetic exchange interactions.
Characteristic feature of the iron pyroxene AFe3+X2O6 crystal structure is the isolated chains FeO6 octahedra running along the c axis (Fig. 1) [9]. Magnetic interaction between the chains (Fe-O-Ge(Si)-O-Fe) is weakened due to the presence of non-magnetic Ge(Si)O4 tetrahedra. Type of magnetic structure depends critically on the geometric topologies of the atomic structure which is determined the ratio of the intrachain (Fe-O-Fe) and interchain (Fe-O-X-O-Fe) exchange interactions parameters.
Fig. 1. Crystal structure of AFeX2O6 compounds: two chains of FeO6 octahedra and its connections via Ge(Si)O4 tetrahedra [9]. Balls represent Na(Li) ions
The NaFeGe2O6 and LiFeGe2O6 clinopyroxene-type germinates compounds exhibit different crystal and magnetic structures. The room temperature crystal structure of LiFeGe2O6 is mon-oclinic with space group P21/c [10]. The room temperature crystal structure of NaFeGe2O6 is monoclinic with space group C2/c [11].
Structures of magnetic subsystem are quasi-one-dimensional in AFeGe2O6 compounds [9]. The magnetic structure of LiFeGe2O6 pyroxene is described to have an antiferromagnetic arrangement of spins within and between the octahedral B chains ( TN = 20.5 K) [2]. NaFeGe2O6 characterized by Neel temperature TN = 13 K, below T = 11.5 K the incommensurate magnetic structure with a helical spin modulation is realized [6,12].
In this way, the possibility to synthesize the LiFeGe2O6 and NaFeGe2O6 solid solution is discovered the interesting physical properties. We have recently studied the crystal and magnetic structures of the Na0.5Li0.5FeGe2O6 compound [13]. The room crystal structure of Nao.5Li0.5FeGe2O6 is monoclinic C2/c (high temperature parameters of the cell: a = 10.0333(1), b = 8.8136(1), c = 5.5295(9)A, £ = 108.921(1)°). Calorimetric investigations indicate a displacive first order phase transition at T = 276 K, and at temperature decreasing Na0.5Li0.5FeGe2O6 undergoes a space group change from C2/c to P21 /c (low temperature parameters of the cell: a = 9.9692(3), b = 8.8545(3), c = 5.4752(2)À, £ = 108.494(1)°). Magnetic order has been found below the Neel 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 Pa2i/c and the doubling of the unit cell along the crystallographic a -axis of the pyroxene crystal (propagation vector k = (1/2, 0, 0)).
No magnetic properties and a detailed crystal structure data are available for germinate clinopyroxene-type solid solution compound Lio.7Nao.3FeGe2O6. In the present contribution in pyroxenes research we report the experimental data on the room crystal structure and magnetic characteristics of the synthesized solid solution Li0.7Na0.3FeGe2O6.
1. Sample preparation and experimental procedure
Polycrystalline sample of solid solution Li0.7Na0.3FeGe2O6 was synthesized by a solid-phase reaction method from the stoichiometric mixture of oxides GeO2, Fe2O3, Na2CO3, Li2CO3
2.00GeO2 + 0.50Fe2O3 + 0.15Na2CO3 + 0.35Li2CO3 ^ Li0.7Na0.3FeGe2O6 + CO2.
The mixture of the oxides was ground under ethanol, pressed into pellets and fired under ambient pressure at temperatures of 800-1000° C in air at three stages each with duration of 24 h with intermediate regrinding. At each heating stage the samples were slowly cooled to room temperature, ground and pressed into pellets for next heat treatment. The prepared pellets were pale brown. The chemical and phase composition of the pellet milled into the powder was examined by X-ray diffraction and shows a single phase.
The X-ray powder diffraction data of Li0.7Na0.3FeGe2O6 for Rietveld analysis was collected at room temperature with a Bruker D8 ADVANCE powder diffractometer (Cu-Ka radiation) and linear VANTEC detector. Refinement of the powder pattern was performed with TOPAS 4.2 (Bruker) [14].
A SQUID magnetometer operating from 300 K to 4 K at magnetic field 500 Oe was used to perform the dc magnetic measurements.
2. Experimental results and discussion
The X-ray powder diffraction pattern at 300 K reveals that Li0.7Na0.3FeGe2O6 has the cen-trosymmetric C2/c space group with 4 formula units per the unit cell and the lattice parameters are close to those of NaFeGe2O6 [7,11]. Therefore the crystal structure of NaFeGe2O6 was used as the starting structural model for the Rietveld refinement of the room temperature crystal structure of Li0.7Na0.3FeGe2O6. The site of Na ion was occupied by Li and Na ions with fixed occupation pli = 0.5 and pNa = 0.5 respectively. The refinement was stable and gave low R-factors (Tab. 1, Fig. 2). Main parameters of refinement are shown in Tab. 1.
To obtain information on the magnetic properties of polycrystalline sample material of Li0.7Na0.3FeGe2O6 compound the dc magnetic measurements were perform. Magnetic properties are determined by Fe3+ ions as a 3d5. The measured temperature dependence of the magnetic susceptibility in an external magnetic field of H = 500 Oe of monophased powder Li0.7Na0.3FeGe2O6 is shown in Fig. 3a. The magnetic susceptibility x(T) of Li0.7Na0.3FeGe2O6 exhibits a sharp maximum at 24.8 K in its temperature dependence with the point of inflection being located at 15.5 K. This behavior is usually related to a phase transition from the paramagnetic to the magnetically ordered state. Note, the pyroxene reported here the diamagnetic contribution in the magnetic susceptibility is negligible (Fig. 3 b).
Table 1. Main parameters of crystal structure refinement in Li0.7Nao.3FeGe2O6 at 300 K
Space group C/2c
ai, A 10.02346 (11)
bi, A 8.80980 (9)
a, A 5.52550 (6)
108.9286 (6)
V, A3 461.54 (1)
Dx, g/cm3 4.445
Z 4
Angle range 26, deg. 5-140
Number of reflections 4391
Number of refined parameters 71
Rwp, % 2.155
Rp, % 1.643
x2 1.450
Rb , % 1.333
Designations: a, b, c, and ¡3 are the unit cell parameters; V is the unit cell volume; Rb, RWp, Rp are the integrated (Bragg), weighted profile and profile factors, respectively; and x2 is the goodness-of-fit.
Fig. 2. Experimental (symbols), theoretical (line), and difference (lower line) X-ray powder diffraction pattern resulting from the Rietveld refinement of Li0.7Na0.3FeGe2O6 at 300 K
Fig. 3. Temperature dependence of the magnetic susceptibility, obtained in Li0.7Nao.3FeGe2O6 sample: the total magnetic susceptibility (a) and the diamagnetic contribution in the magnetic susceptibility (b)
In order to compare some details between NaFeGe2O6, LiFeGe2O6, Na0.5Li0.5FeGe2O6 and Li0 7Na0.3FeGe2O6 samples we summaries the room crystal structure and magnetic parameters in the Tabs. 2, 3.
Note, all materials of pyroxenes compound family NaxLi1-xFeGe2O6 (x = 1, 0.5, 0.3 and 0) are monoclinic. End members (x = 0, 1) exhibit different space group of crystal structure (P21/c, C2/c). Intermediate compositions (x = 0.5, 0.3) exhibit C2/c space group at T = 300 K. It is interesting to examine the A cation substitution influences on magnetic properties too. In this solid solution the substitution Li+ ions by Na+ ions led to an increase in the lattice parameters and change the picture of the competing magnetic exchange interactions in the pyroxenes NaxLi1-xFeGe2O6 compound.
Table 2. Summary of the monoclinic crystal structure parameters for different pyroxene-type compounds at room temperature
Sample at, A bt, A et, A ß0 Space group Reference
NaFeGe2Ü6 10.0100 8.9400 5.5200 108.0000 C/2c [11]
Nao.5Lio.5 FeGe2Ü6 10.0333(1) 8.8136(1) 5.5295(9) 108.921(1) C/2c [13]
Lio.7 Nao.3 FeGe2Ü6 10.02346(11) 8.80980(9) 5.52550(6) 108.9286(6) C/2c this study
LiFeGe2Ü6 9.8792(7) 8.8095(5) 5.3754(3) 108.844(6) P/2ic [10]
Table 3. Summary of the magnetic parameters for different pyroxene-type compounds
Sample Tn, K T K Reference
NaFeGe2O6 13 25 [12,15]
LiFeGe2O6 20.2 24.4 [2]
Nao.5Lio.5FeGe2O6 18 27.5 [13]
Lio.7Nao.3FeGe2O6 15.5 24.8 this study
Conclusion
The solid solution clinopyroxene compound Lio.7Nao.3FeGe2O6 was synthesized by solid-phase reaction. The chemical and phase composition of clinopyroxenes compound was examined by X-ray diffraction and shows a single phase. The room crystal structure and magnetic properties of Li0.7Na0.3FeGe2O6 were investigated as function of temperature in range 4 K - 300 K.
The room temperature crystal structure of Li0.7Na0.3FeGe2O6 is characterized by monoclinic symmetry and space group C2/c with parameters a = 10.02346(11), b = 8.80980(9), c = 5.52550(6)A, p = 108.9286(6)°. Li0.7Na0.3FeGe2O6 is isostructural to NaFeGe2O6 at T = 300 K.
The transition from the paramagnetic state to the magnetic long-range ordered state occurring at Tn = 15.5 K.
Note, the clinopyroxene compounds show a variety of phase crystal structure transitions as a function of temperature. In the case of LiFeGe2O6 and Na0.5Li0.5FeGe2O6 compounds the changes in symmetry from a low temperature P21/c to a high temperature C2/c structure were observed at a transition temperature Ttr = 789 K and 276 K correspondingly [10,13]. NaFeGe2O6 compound exhibits C2/c symmetry between 1.6 and 1000 K [10]. This result indicates that a change in composition of pyroxene-type compounds can have effect on the P21 /c ^ C2/c dis-
placive phase transition. It is not clear the crystal structure of Lio.7Nao.3FeGe2O6 with temperature changes in symmetry or not. It is expected that this pyroxene compound transforms to P2i/c structure at some low temperatures. The possible magnetic structure depends on crystal structure at low temperatures. In order to further characterize the magnetic state of Li0.7Na0.3FeGe2O6 compound it is necessary the neutron diffraction investigation to determine the magnetic structure.
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Структурные и магнитные свойства пироксена Lio.7Nao.3FeGe2Oe
Тамара В. Дрокина Герман А. Петраковский Дмитрий А. Великанов Максим С. Молокеев Елена Г. Резина
Поликристаллические образцы Lio,rNao,3FeGe2Oe были получены твердофазным синтезом и исследованы методами рентгеновской дифракции и СКВИД-метрии. Представлены характеристики структурных свойств. Показано, что температурная зависимость магнитной восприимчивости при температуре 15,5 К имеет излом, характерный для переходов в магнитоупорядоченное состояние.
Ключевые слова: твердый раствор, пироксен, кристаллическая структура, магнитные свойства.
