Mössbauer Spectral Study of the Low-Temperature Electronic and Magnetic Properties of α-FePO₄ and the Mixed Valence Iron(II/III) Phosphate SrFe₃(PO₄)₃

Abstract

The Mössbauer spectra of trigonal α-FePO4, measured between 4.2 and 300 K, exhibit hyperfine parameters characteristic of high-spin iron­(III) in a pseudotetrahedral oxygen environment. Between 24.5 and 300 K, the spectra show a paramagnetic quadrupole doublet and at 24.0 K the spectrum reveals the onset of antiferromagnetic exchange. At 4.2 and 16 K, a single magnetic sextet is observed with hyperfine fields of 51.36(1) and 42.74(1) T, respectively, with an angle, θ, of 90° between the principal axis of the electric field gradient tensor in the basal plane of the trigonal unit cell and the hyperfine field along the c axis. The spectra obtained between 21 and 18 K have been fitted with two equal-area magnetic sextets with θ angles of 25 and 85°, angles which indicate that the iron­(III) magnetic moments are canted away from the c axis. The reduced hyperfine field versus reduced temperature plot indicates a departure from a Brillouin S = 5/2 behavior, as a result of some magnetostriction at the Néel temperature. The Mössbauer spectra of class 1 mixed-valence SrFe3(PO4)3, measured between 4.2 and 300 K, exhibit hyperfine parameters characteristic of two high-spin iron­(II) ions and one high-spin iron­(III) ion in a pseudooctahedral oxygen environment. At and above 40 K, the spectra show two paramagnetic quadrupole doublets, whereas at 39.0 K the spectrum reveals the onset of ferrimagnetic exchange. Between 4.2 and 30 K, the spectra have been fitted with two magnetic sextets with θ angles of 85 and 10° for the iron­(II) and iron­(III) sites, respectively. The reduced hyperfine field versus reduced temperature plots for the iron­(II) and iron­(III) sites show a distinct departure from Brillouin S = 2 and S = 5/2 behavior, respectively, a departure that suggests a first-order magnetic transition at 39.5(5) K with differing magnetostrictions at the iron­(II) and iron­(III) sites.

A new iron-phosphate compound (Fe7P11O38) obtained by pyrophosphate stoichiometric glass devitrification

“…For the phase, IS values are relatively high like for a tetrahedrally coordinated Fe 3+ and are about 0.3 mm/s, and e 2 Q/2 is c.a. 0.62 mm/s 37 , 38 . Therefore, the last component (No.…”

The distortion of two FePO4polymorphs with high pressure

“…No anomalies are observed in the susceptibility of phase-I at ∼17 K as initially reported by Battle et al, 9 and confirmed more recently by Grandjean et al with Mössbauer, attributed to an antiferromagnetic spin reorientation, which may be due to differences in measuring field (see SI). 49 Phase-Ir, shows very different magnetisation behaviour from both phase-I and phase-II. While the Néel temperature for phase-II is greater than phase-I, it is greater still for phase-Ir, 66(2) K. Additionally there is a clear split in the FC and ZFC datasets, which is not observed to the same level in either phase-I or phase-II.…”

Low-Frequency Vibrational Spectroscopy: A Powerful Tool for Revealing Crystalline Magnetic Structures

“…In this work, Iron (III) phosphate (FePO 4 ) was chosen as a model system to explore the ability of terahertz timedomain spectroscopty (THz-TDS) to determine the magnetic configuration of crystals, owing to the detailed characterization data on this system available in the literature, including cryogenic Mössbauer spectroscopy and X-ray diffraction measurements. [14][15][16][17][18] While not a new material, in recent years FePO 4 has shown great promise as a cathode material in lithium ion batteries, [19][20][21] and understanding its magnetic and electrical properties is central to further innovation. FePO 4 is reported to crystallize in an α-quartz type structure, and is known to undergo a ferromagnetic (FM) to antiferromagnetic (AFM) transition at a Néel temperature of around 25 K. 14,16 While the iron atoms in FePO 4 are tetrahedrally coordinated, indicating that the iron is in the high-spin (HS) state, through the use of DFT simulations it is possible to explore all possible electronic configurations, and thus the low-spin (LS) state was also investigated here.…”

“…[14][15][16][17][18] While not a new material, in recent years FePO 4 has shown great promise as a cathode material in lithium ion batteries, [19][20][21] and understanding its magnetic and electrical properties is central to further innovation. FePO 4 is reported to crystallize in an α-quartz type structure, and is known to undergo a ferromagnetic (FM) to antiferromagnetic (AFM) transition at a Néel temperature of around 25 K. 14,16 While the iron atoms in FePO 4 are tetrahedrally coordinated, indicating that the iron is in the high-spin (HS) state, through the use of DFT simulations it is possible to explore all possible electronic configurations, and thus the low-spin (LS) state was also investigated here.…”

“…While the room-temperature spin configuration is believed to be ferromagnetic, 14 there is a reported FM to AFM transition at low-temperatures. 14,15 Thus, both the low-and high-spin AFM configurations were studied in order to explore the influence of the spin configuration on the lowfrequency dynamics.…”