Common Building Motifs in Ba2Fe3(PO4)4·2H2O, BaFe3(PO4)3, and Na3Fe3(PO4)4: Labile Fe2+/Fe3+ Ordering and Charge-Dependent Magnetism

Structural and Spectroscopic Studies of NaCuCr2(PO4)3: A Noncentrosymmetric Phosphate Belonging to the α-CrPO4-Type Compounds

An electron and joint neutron and X-ray diffraction study of the synthetic copper/chromium phosphate NaCuCr₂(PO₄)₃ (NaCuP) is reported. A noncentrosymmetric Imm2 space group belonging to the well-known α-CrPO₄ type is observed contrary to what is reported in NaMCr₂(PO₄)₃ (M = Co and Ni) phosphates. The structural model is validated by bond valence sum analysis and charge-distribution (CHARDI) calculations and supported by complementary infrared and Raman spectroscopy investigations. Both Raman spectroscopy and theoretical study by deformation density approach further suggest the presence of Cu²⁺ (3d⁹) and Cr²⁺ (3d⁴) Jahn-Teller polaron effects as a key factor to the centro Imma to noncentrosymmetric Imm2 phase change.

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

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.