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High-pressure optical floating-zone growth of Li(Mn,Fe)PO$$_4$$ single crystals

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Abstract

Mm-sized LiMn$$_{1-x}$$Fe$$_x$$PO$$_4$$ single crystals with $$0\leq x \leq 1$$ were grown by means of the traveling floating-zone technique at elevated Argon pressure of 30~bar. For the various doping levels, the growth process was optimized with respect to the composition-dependant effective light absorption and transparency of the materials. A convex crystal/melt interface, determined by the angle of incident light, was identified to be particularly crucial for a successful growth. The resulting large single crystalline grains are stoichiometric. Structure refinement shows that lattice parameters as well as the atomic positions and bond lengths linearly depend on the Mn:Fe-ratio. Oriented cuboidal samples with several mm$$^3$$ of volume were used for magnetic studies which imply an antiferromagnetic ground state for all compositions. The N\'eel-temperature changes from $$T_N$$ = 32.5(5) K in LiMnPO$$_4$$ to 49.5(5) K in LiFePO$$_4$$ while the easy magnetic axis in the ordered phase flips from the crystallographic $$a$$- to the $$b$$-axis upon Fe-doping of $$x<0.2$$.

Most cited references7

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Observation of ferrotoroidic domains.

(2007)
Domains are of unparalleled technological importance as they are used for information storage and for electronic, magnetic and optical switches. They are an essential property of any ferroic material. Three forms of ferroic order are widely known: ferromagnetism, a spontaneous magnetization; ferroelectricity, a spontaneous polarization; and ferroelasticity, a spontaneous strain. It is currently debated whether to include an ordered arrangement of magnetic vortices as a fourth form of ferroic order, termed ferrotoroidicity. Although there are reasons to expect this form of order from the point of view of thermodynamics, a crucial hallmark of the ferroic state--that is, ferrotoroidic domains--has not hitherto been observed. Here ferrotoroidic domains are spatially resolved by optical second harmonic generation in LiCoPO4, where they coexist with independent antiferromagnetic domains. Their space- and time-asymmetric nature relates ferrotoroidics to multiferroics with magnetoelectric phase control and to other systems in which space and time asymmetry leads to possibilities for future applications.
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Comparison of small polaron migration and phase separation in olivine LiMnPO4and LiFePO4using hybrid density functional theory

(2011)
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Hole polaron formation and migration in olivine phosphate materials

(2011)
By combining first principles calculations and experimental XPS measurements, we investigate the electronic structure of potential Li-ion battery cathode materials LiMPO4 (M=Mn,Fe,Co,Ni) to uncover the underlying mechanisms that determine small hole polaron formation and migration. We show that small hole polaron formation depends on features in the electronic structure near the valence-band maximum and that, calculationally, these features depend on the methodology chosen for dealing with the correlated nature of the transition-metal d-derived states in these systems. Comparison with experiment reveals that a hybrid functional approach is superior to GGA+U in correctly reproducing the XPS spectra. Using this approach we find that LiNiPO4 cannot support small hole polarons, but that the other three compounds can. The migration barrier is determined mainly by the strong or weak bonding nature of the states at the top of the valence band, resulting in a substantially higher barrier for LiMnPO4 than for LiCoPO4 or LiFePO4.
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Author and article information

Journal
2017-02-03
1702.01138
10.1016/j.jcrysgro.2017.01.046