A neutron diffraction study of crystal and low-temperature magnetic structures within the (Na,Li)FeGe2O6 pyroxene-type solid solution series

Abstract

an antiferromagnetic (AFM) coupling within the Fe 3+ O 6 octahedra zig-zag chains and an alternating AFM and ferromagnetic (FM) interaction between the chains, depending on the nature of the tetrahedral GeO 4 chains. The magnetic structure can be described in magnetic space group P a 2 1 /c. Close to the structural phase transition for sample with x(Na) = 0.75, magnetic ordering is observed below 15 K; however, it becomes incommensurately modulated with k = (0.344, 0, 0.063). At 4 K, the magnetic spin structure best can be described by a cycloidal arrangement within the M1 chains, the spins are within the a-c plane. Around 12 K the cycloidal structure transforms to a spin density wave (SDW) structure. For the C2/c structures, a coexistence of a simple collinear and an incommensurately modulated structure is observed down to lowest temperatures. For 0.78 ≤ x(Na) ≤ 0.82, a collinear magnetic structure with k = (0 1 0), space group P C 2 1 /c and an AFM spin structure within the M1 chains and an FM one between the spins is dominating, while the incommensurately modulated structure becomes dominating the collinear one in the samples with x(Na) = 0.88. Here the magnetic propagation vector is k = (0.28, 1, 0.07) and the spin structure corresponds again to a cycloidal structure within the M1 chains. As for the other samples, a transition from the cycloidal to a SDW structure is observed. Based on the neutron diffraction data, the appearance of two peaks in the heat capacity of Na-rich samples can now be interpreted as a transition from a cycloidal magnetic structure to a spin density wave structure of the magnetically ordered phase for the Na-rich part of the solid solution series.Keywords Clinopyroxenes · LiFeGe 2 O 6 · NaFeGe 2 O 6 · Neutron diffraction · Magnetic ordering · Cycloidal incommensurate structure · Spin density wave Abstract Solid solution compounds along the Li 1-x Na x FeGe 2 O 6 clinopyroxene series have been prepared by solid state ceramic sintering and investigated by bulk magnetic and calorimetric methods; the Na-rich samples with x(Na) > 0.7 were also investigated by lowtemperature neutron diffraction experiments in a temperature range of 4-20 K. For samples with x(Na) > 0.76 the crystal structure adopts the C2/c symmetry at all measuring temperatures, while the samples display P2 1 /c symmetry for smaller Na contents. Magnetic ordering is observed for all samples below 20 K with a slight decrease of T N with increasing Na content. The magnetic spin structures change distinctly as a function of chemical composition: up to x(Na) = 0.72 the magnetic structure can be described by a commensurate arrangement of magnetic spins with propagation vector k = (½, 0 0), Electronic supplementary material The online version of this article

Thermal expansion of minerals in the pyroxene system and examination of various thermal expansion models

“…Only Li can fully exchange for Na as a monovalent cation in the M2 site. Several studies collected diffraction data at temperatures higher or lower than 298 K on synthetic Li-Fe, Li-Cr or LiNi pyroxenes with silicon or germanium in the T site (Behruzi et al 1984, Redhammer et al 2001, 2016, 2017, Camara et al 2009, Tribaudino et al 2009, aiming to understand phase transitions and the magnetic behavior of these phases. Such studies, however, excluded the only naturally occurring Li-pyroxene end member, spodumene (LiAlSi2O6).…”

Low Temperature Synthesis of Aegirine NaFeSi2O6: Spectroscopy (57Fe Mössbauer, Raman) and Size/Strain Analysis from X-ray Powder Diffraction

“…Finally, Raman spectroscopy showed that some weakly-developed pre-ordering effects were present in the samples, which appeared to be amorphous in PXRD, while well-resolved spectra appeared as soon as the long-range ordered crystalline state could be found with X-ray diffraction.CaFeSi 2 O 6 -aegirine series [4][5][6], diopside CaMgSi 2 O 6 -aegirine series [7][8][9]; the thermal and high pressure behavior [10,11] and the implications of the presence of the aegirine molecule for the behaviour of naturally occurring augitic pyroxenes.In recent years, aegirine has also attracted the attention of numerous physicists due to the observation of multiferroic properties [12], resulting in several studies on its low temperature magnetic properties. In addition, the phase relations and magnetic properties within the (Na,Li)Fe(Si,Ge) 2 O 6 system were investigated with special interest [13][14][15][16][17][18][19][20][21][22][23][24][25]. Aegirine itself shows complex magnetic ordering at low temperatures and discrepancies have been found in the magnetic ground states of naturally-occurring single crystals (with diopsidic and hedenbergitic component dilutions) and synthetic end-member compositions.It was already noted by Ballet and co-workers [26] that phase-pure synthetic NaFeSi 2 O 6 charges are hard to obtain, and that material, produced in ceramic sintering experiments at high temperatures around 900 • C, always contains up to 3-10 wt.…”

“…In recent years, aegirine has also attracted the attention of numerous physicists due to the observation of multiferroic properties [12], resulting in several studies on its low temperature magnetic properties. In addition, the phase relations and magnetic properties within the (Na,Li)Fe(Si,Ge) 2 O 6 system were investigated with special interest [13][14][15][16][17][18][19][20][21][22][23][24][25]. Aegirine itself shows complex magnetic ordering at low temperatures and discrepancies have been found in the magnetic ground states of naturally-occurring single crystals (with diopsidic and hedenbergitic component dilutions) and synthetic end-member compositions.…”

A comprehensive study of the magnetic properties of the pyroxenes series CaMgSi2O6–Co2Si2O6 as a function of Co content

“…The differences originate from different pathways for magnetic super-exchange, in turn related to the interaction between structure and magnetic cations. As an example, in (Na,Li)FeGe 2 O 6 pyroxenes [2], commensurate and incommensurate orderings together with spin wave modulations and cycloidal magnetic structure are observed. However, a model relating the crystal structure/composition to magnetic behavior is to our knowledge not yet available.…”

“…The abundant literature on the magnetic properties of pyroxenes was mostly devoted to the end members; some studies were performed on the effect of cation exchange of non-magnetic atoms, while keeping fixed the magnetic cation [2,[25][26][27] or between magnetic cations [28,29]. The onset of magnetic behavior introducing a metal in a diamagnetic structure was studied only in (Fe,Mg) 2 Si 2 O 6 clino-and orthopyroxenes, due to their interest as major earth forming minerals [30].…”