Structure–Property Relationships in α‑, β′‑, and γ‑Modifications of Mn₃(PO₄)₂

The manganese orthophosphate, Mn₃(PO₄)₂, is characterized by the rich variety of polymorphous modifications, α-, β′-, and γ-phases, crystallized in monoclinic <i>P</i>2₁/<i>c</i> (<i>P</i>2₁/<i>n</i>) space group type with unit cell volume ratios of 2:6:1. The crystal structures of these phases are constituted by three-dimensional framework of corner- and edge-sharing [MnO₅] and [MnO₆] polyhedra strengthened by [PO₄] tetrahedra. All compounds experience long-range antiferromagnetic order at Neel temperature <i>T</i>_N = 21.9 K (α-phase), 12.3 K (β′-phase), and 13.3 K (γ-phase). Additionally, second magnetic phase transition takes place at <i>T</i>* = 10.3 K in β′-phase. The magnetization curves of α- and β′-modifications evidence spin-floplike features at <i>B</i> = 1.9 and 3.7 T, while the γ-Mn₃(PO₄)₂ stands out for an extended one-third magnetization plateau stabilized in the range of magnetic field <i>B</i> = 7.5–23.5 T. The first-principles calculations define the main paths of superexchange interaction between Mn spins in these polymorphs. The spin model for α-phase is found to be characterized by collection of uniform and alternating chains, which are coupled in all three directions. The strongest magnetic exchange interaction in γ-phase emphasizes the trimer units, which make chains that are in turn weakly coupled to each other. The spin model of β′-phase turns out to be more complex compared to α- or γ-phase. It shows complex chain structures involving exchange interactions between Mn2 (Mn2′, Mn2″) and Mn3 (Mn3′, Mn3″). These chains interact through exchanges involving Mn1 (Mn1′, Mn1″) spins

Crystal Structure, Physical Properties, and Electronic and Magnetic Structure of the Spin <i>S</i> = ⁵/₂ Zigzag Chain Compound Bi₂Fe(SeO₃)₂OCl₃

We report the synthesis and characterization of the new bismuth iron selenite oxochloride Bi₂Fe­(SeO₃)₂OCl₃. The main feature of its crystal structure is the presence of a reasonably isolated set of spin <i>S</i> = ⁵/₂ zigzag chains of corner-sharing FeO₆ octahedra decorated with BiO₄Cl₃, BiO₃Cl₃, and SeO₃ groups. When the temperature is lowered, the magnetization passes through a broad maximum at <i>T</i>_max ≈ 130 K, which indicates the formation of a magnetic short-range correlation regime. The same behavior is demonstrated by the integral electron spin resonance intensity. The absorption is characterized by the isotropic effective factor <i>g</i> ≈ 2 typical for high-spin Fe³⁺ ions. The broadening of ESR absorption lines at low temperatures with the critical exponent β = ⁷/₄ is consistent with the divergence of the temperature-dependent correlation length expected for the quasi-one-dimensional antiferromagnetic spin chain upon approaching the long-range ordering transition from above. At <i>T</i>_N = 13 K, Bi₂Fe­(SeO₃)₂OCl₃ exhibits a transition into an antiferromagnetically ordered state, evidenced in the magnetization, specific heat, and Mössbauer spectra. At <i>T</i> < <i>T</i>_N, the ⁵⁷Fe Mössbauer spectra reveal a low saturated value of the hyperfine field <i>H</i>_hf ≈ 44 T, which indicates a quantum spin reduction of spin-only magnetic moment Δ<i>S</i>/<i>S</i> ≈ 20%. The determination of exchange interaction parameters using first-principles calculations validates the quasi-one-dimensional nature of magnetism in this compound