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An experiment was carried out to pinpoint the temperature of formation of the solid solution on heating LiFeP[O.sub.4]/FeP[O.sub.4] above 500 K to show that small polaron transport is responsible for electron transport. The onset of rapid small polaron hopping on the time scale is precisely correlated with the temperature that the lithium ions begins to disorder in the lattice, showing that the two events are correlated.

Transition metal phosphates such as LiFePO4 have been recognized as very promising electrodes for lithium-ion batteries because of their energy storage capacity combined with electrochemical and thermal stability. A key issue in these materials is to unravel the factors governing electron and ion transport within the lattice. Lithium extraction from LiFePO4 results in a two-phase mixture with FePO4 that limits the power characteristics owing to the low mobility of the phase boundary. This boundary is a consequence of low solubility of the parent phases, and its mobility is impeded by slow migration of the charge carriers. In principle, these limitations could be diminished in a solid solution, LixFePO4. Here, we show that electron delocalization in the solid solution phases formed at elevated temperature is due to rapid small polaron hopping and is unrelated to consideration of the band gap. We give the first experimental evidence for a strong correlation between electron and lithium delocalization events that suggests they are coupled. Furthermore, the exquisite frequency sensitivity of Mössbauer measurements provides direct insight into the electron hopping rate. [ABSTRACT FROM AUTHOR]

Electrons in solids often adopt complex patterns of chemical bonding driven by the competition between energy gains from covalency and delocalization, and energy costs of double occupation to satisfy Pauli exclusion, with multiple intermediate states in the transition between highly localized, and magnetic, and delocalized, and nonmagnetic limits. Herein, we report a chemical pressure-driven transition from a proper Mn magnetic ordering phase transition to a Mn magnetic phase crossover in EuMn 2 P 2 the limiting end member of the EuMn 2 X 2 (X = Sb, As, P) family of layered materials. This loss of a magnetic ordering occurs despite EuMn 2 P 2 remaining an insulator at all temperatures, and with a phase transition to long-range Eu antiferromagnetic order at T N ≈ 17 K. The absence of a Mn magnetic phase transition contrasts with the formation of long-range Mn order at T ≈ 130 K in isoelectronic EuMn 2 Sb 2 and EuMn 2 As 2 . Temperature-dependent specific heat and 31 P NMR measurements provide evidence for the development of short-range Mn magnetic correlations from T ≈ 250-100 K, interpreted as a precursor to covalent bond formation. Density functional theory calculations demonstrate an unusual sensitivity of the band structure to the details of the imposed Mn and Eu magnetic order, with an antiferromagnetic Mn arrangement required to recapitulate an insulating state. Our results imply a picture in which long-range Mn magnetic order is suppressed by chemical pressure, but that antiferromagnetic correlations persist, narrowing bands and producing an insulating state.

The series BaIn 1- x Fe x O 2.5+δ , x = 0.25, 0.50, and 0.75, has been prepared under air-fired and argon-fired conditions and studied using X-ray diffraction, d.c. and a.c. susceptibility, Mössbauer spectroscopy, neutron diffraction, X-ray near edge absorption spectroscopy (XANES), and X-ray pair distribution (PDF) methods. While Ba 2 In 2 O 5 (BaInO 2.5 ) crystallizes in an ordered brownmillerite structure, Ibm 2, and Ba 2 Fe 2 O 5 (BaFeO 2.5 ) crystallizes in a complex monoclinic structure, P 2 1 / c , showing seven Fe 3+ sites with tetrahedral, square planar, and octahedral environments, all phases studied here crystallize in the cubic perovskite structure, Pm 3̅ m , with long-range disorder on the small cation and oxygen sites. 57 Fe Mössbauer studies indicate a mixed valency, Fe 4+ /Fe 3+ , for both the air-fired and argon-fired samples. The increased Fe 3+ content for the argon-fired samples is reflected in increased cubic cell constants and in the increased Mössbauer fraction. It appears that the Pm 3̅ m phases are only metastable when fired in argon. From a slightly modified percolation theory for a primitive cubic lattice (taking into account the presence of random O atom vacancies), long-range spin order is permitted for the x = 0.50 and 0.75 phases. Instead, the d.c. susceptibility shows only zero-field-cooled (ZFC) and field-cooled (FC) divergences at ∼6 K [5 K] for x = 0.50 and at ∼22 K [21 K] for x = 0.75, with values for the argon-fired samples in [ ]. Neutron diffraction data for the air-fired samples confirm the absence of long-range magnetic order at any studied temperature. For the air-fired x = 0.50, a.c. susceptibility data show a frequency-dependent χ'(max) and spin glass behavior, while for x = 0.75, χ'(max) is invariant with frequency, ruling out either a spin glass or a superparamagnetic ground state. These behaviors are discussed in terms of competing Fe 3+ -Fe 3+ antiferromagnetic exchange and ferromagnetic Fe 3+ -Fe 4+ exchange. The PDF and 57 Fe Mössbauer data indicate a local structure at short interatomic distances, which deviates strongly from the average Pm 3̅ m model. Fe Mössbauer, PDF, and XANES data show a systematic dependence on x and indicate that the Fe 3+ sites are largely fourfold-coordinated and Fe 4+ sites are fivefold- or sixfold-coordinated., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

Sr(2)FeMnO(5+y) was synthesized under two different conditions, in air and in argon, both of which resulted in a cubic, Pm ̅3m, structure with no long-range ordering of oxygen vacancies. The unit cell constants were found to be a(0) = 3.89328(1) Å for argon (y = 0.0) and a(0) = 3.83075(3) Å for air (y = 0.5). In contrast, Ca(2)FeMnO(5) retains long-range brownmillerite oxygen vacancy ordering for either air or argon synthesis. Remarkably, Sr(2)FeMnO(5.0) oxidizes spontaneously in air at room temperature. A neutron pair distribution function (NPDF) study of Sr(2)FeMnO(5.0)(Ar) showed evidence for local, brownmillerite-like ordering of oxygen vacancies for short distances up to 5 Å. Mössbauer spectroscopy results indicate more than one Fe site for Sr(2)FeMnO(5+y)(Ar and air), consistent with the noncubic local structure found by NPDF analysis. The isomer shifts and quadrupole splittings in both air- and argon-synthesized materials are consistent with the 3+ oxidation state for Fe in sites with coordination number four or five. This is confirmed by an L-edge XANES study. Mn is almost entirely in the 3+ state for Sr(2)FeMnO(5.0)(Ar), whereas Mn(4+) is predominantly present for Sr(2)FeMnO(5.5)(air). Magnetic susceptibility data show zero-field-cooled/field-cooled (ZFC/FC) divergences near 50 K for the Ar sample and 25 K for the air sample, whereas Ca(2)FeMnO(5) is long-range G-type antiferromagnetically ordered at 407(2) K. Hyperfine magnetic splitting, observed in temperature-dependent Mössbauer measurements, indicates short-range magnetic correlations that persist up to 150 K for Sr(2)FeMnO(5.0)(Ar) and 100 K for Sr(2)FeMnO(5.5)(air), well above the ZFC/FC divergence temperatures. Neutron diffraction data confirm the absence of long-range magnetic ordering at room temperature and 4 K for Sr(2)FeMnO(5.0)(Ar) but indicate the presence of domains with short-range G-type order at 4 K with an average dimension of ∼50 Å (y = 0); thus, this material is actually a superparamagnet rather than a true spin glass. In sharp contrast, corresponding data for Sr(2)FeMnO(5.5)(air) show mainly a very weak magnetic Bragg peak, indicating that ∼4% of the sample has G-type antiferromagnetic ordering at 4 K., (© 2011 American Chemical Society)