Crystal Structure, Defects, Magnetic and Dielectric Properties of the Layered Bi_3<i>n</i>+1Ti₇Fe_{3<i>n</i>–3}O_9<i>n</i>+11 Perovskite-Anatase Intergrowths

The Bi_3<i>n</i>+1Ti₇Fe_{3<i>n</i>–3}O_9<i>n</i>+11 materials are built of (001)_p plane-parallel perovskite blocks with a thickness of <i>n</i> (Ti,Fe)­O₆ octahedra, separated by periodic translational interfaces. The interfaces are based on anatase-like chains of edge-sharing (Ti,Fe)­O₆ octahedra. Together with the octahedra of the perovskite blocks, they create S-shaped tunnels stabilized by lone pair Bi³⁺ cations. In this work, the structure of the <i>n</i> = 4–6 Bi_3<i>n</i>+1­Ti₇Fe_{3<i>n</i>–3}­O_9<i>n</i>+11 homologues is analyzed in detail using advanced transmission electron microscopy, powder X-ray diffraction, and Mössbauer spectroscopy. The connectivity of the anatase-like chains to the perovskite blocks results in a 3<i>a</i>_p periodicity along the interfaces, so that they can be located either on top of each other or with shifts of ±<i>a</i>_p along [100]_p. The ordered arrangement of the interfaces gives rise to orthorhombic <i>Immm</i> and monoclinic <i>A</i>2/<i>m</i> polymorphs with the unit cell parameters <b>a</b> = 3<b>a</b>_p, <b>b</b> = <b>b</b>_p, <b>c</b> = 2­(<i>n</i> + 1)<b>c</b>_p and <b>a</b> = 3<b>a</b>_p, <b>b</b> = <b>b</b>_p, <b>c</b> = 2­(<i>n</i> + 1)<b>c</b>_p – <b>a</b>_p, respectively. While the <i>n</i> = 3 compound is orthorhombic, the monoclinic modification is more favorable in higher homologues. The Bi_3<i>n</i>+1­Ti₇Fe_{3<i>n</i>–3}­O_9<i>n</i>+11 structures demonstrate intricate patterns of atomic displacements in the perovskite blocks, which are supported by the stereochemical activity of the Bi³⁺ cations. These patterns are coupled to the cationic coordination of the oxygen atoms in the (Ti,Fe)­O₂ layers at the border of the perovskite blocks. The coupling is strong in the <i>n</i> = 3, 4 homologues, but gradually reduces with the increasing thickness of the perovskite blocks, so that, in the <i>n</i> = 6 compound, the dominant mode of atomic displacements is aligned along the interface planes. The displacements in the adjacent perovskite blocks tend to order antiparallel, resulting in an overall antipolar structure. The Bi_3<i>n</i>+1­Ti₇Fe_{3<i>n</i>–3}­O_9<i>n</i>+11 materials demonstrate an unusual diversity of structure defects. The <i>n</i> = 4–6 homologues are robust antiferromagnets below <i>T</i>_N = 135, 220, and 295 K, respectively. They show a high dielectric constant that weakly increases with temperature and is relatively insensitive to the Ti/Fe ratio

Crystal Structure, Defects, Magnetic and Dielectric Properties of the Layered Bi_3<i>n</i>+1Ti₇Fe_{3<i>n</i>–3}O_9<i>n</i>+11 Perovskite-Anatase Intergrowths

The Bi_3<i>n</i>+1Ti₇Fe_{3<i>n</i>–3}O_9<i>n</i>+11 materials are built of (001)_p plane-parallel perovskite blocks with a thickness of <i>n</i> (Ti,Fe)­O₆ octahedra, separated by periodic translational interfaces. The interfaces are based on anatase-like chains of edge-sharing (Ti,Fe)­O₆ octahedra. Together with the octahedra of the perovskite blocks, they create S-shaped tunnels stabilized by lone pair Bi³⁺ cations. In this work, the structure of the <i>n</i> = 4–6 Bi_3<i>n</i>+1­Ti₇Fe_{3<i>n</i>–3}­O_9<i>n</i>+11 homologues is analyzed in detail using advanced transmission electron microscopy, powder X-ray diffraction, and Mössbauer spectroscopy. The connectivity of the anatase-like chains to the perovskite blocks results in a 3<i>a</i>_p periodicity along the interfaces, so that they can be located either on top of each other or with shifts of ±<i>a</i>_p along [100]_p. The ordered arrangement of the interfaces gives rise to orthorhombic <i>Immm</i> and monoclinic <i>A</i>2/<i>m</i> polymorphs with the unit cell parameters <b>a</b> = 3<b>a</b>_p, <b>b</b> = <b>b</b>_p, <b>c</b> = 2­(<i>n</i> + 1)<b>c</b>_p and <b>a</b> = 3<b>a</b>_p, <b>b</b> = <b>b</b>_p, <b>c</b> = 2­(<i>n</i> + 1)<b>c</b>_p – <b>a</b>_p, respectively. While the <i>n</i> = 3 compound is orthorhombic, the monoclinic modification is more favorable in higher homologues. The Bi_3<i>n</i>+1­Ti₇Fe_{3<i>n</i>–3}­O_9<i>n</i>+11 structures demonstrate intricate patterns of atomic displacements in the perovskite blocks, which are supported by the stereochemical activity of the Bi³⁺ cations. These patterns are coupled to the cationic coordination of the oxygen atoms in the (Ti,Fe)­O₂ layers at the border of the perovskite blocks. The coupling is strong in the <i>n</i> = 3, 4 homologues, but gradually reduces with the increasing thickness of the perovskite blocks, so that, in the <i>n</i> = 6 compound, the dominant mode of atomic displacements is aligned along the interface planes. The displacements in the adjacent perovskite blocks tend to order antiparallel, resulting in an overall antipolar structure. The Bi_3<i>n</i>+1­Ti₇Fe_{3<i>n</i>–3}­O_9<i>n</i>+11 materials demonstrate an unusual diversity of structure defects. The <i>n</i> = 4–6 homologues are robust antiferromagnets below <i>T</i>_N = 135, 220, and 295 K, respectively. They show a high dielectric constant that weakly increases with temperature and is relatively insensitive to the Ti/Fe ratio

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