Synthesis, Structure, and Magnetic Properties of (Tb1–xMny)MnO3−δ

Crystal and Magnetic Structures and Properties of (Lu1- xMn x)MnO3 Solid Solutions

(Lu_{1- x}Mn _x)MnO₃ solid solutions, having the perovskite-type structure and Pnma space group, with 0 ≤ x ≤ 0.4 were synthesized by a high-pressure, high-temperature method at 6 GPa and about 1670 K from Lu₂O₃ and Mn₂O₃. Their crystal and magnetic structures were studied by neutron powder diffraction. The degree of octahedral MnO₆ tilting decreases in (Lu_{1- x}Mn _x)MnO₃ with increasing x. Only the incommensurate (IC) spin structure with a propagation vector of k = ( k₀, 0, 0) and k₀ ≈ 0.44 remains in (Lu_0.9Mn_0.1)MnO₃ in the whole temperature range below the Neel temperature T_N = 36 K, and the commensurate noncollinear E-type structure that has been reported in the literature for undoped o-LuMnO₃ is not observed. (Lu_{1- x}Mn _x)MnO₃ samples with 0.2 ≤ x ≤ 0.4 have a ferrimagnetic structure with a propagation vector of k = (0, 0, 0) and ferromagnetic (FM) ordering of Mn³⁺ and Mn⁴⁺ cations at the B site, which are antiferromagnetically coupled to a noncollinear predominantly FM arrangement of Mn²⁺ at the A site. The ferrimagnetic Curie temperature, T_C, increases monotonically from 67 K for x = 0.2 to 118 K for x = 0.4. Magnetic and dielectric properties of (Lu_{1- x}Mn _x)MnO₃ and a composition-temperature phase diagram are also reported.

Mn Self-Doping of Orthorhombic RMnO3 Perovskites: (R0.667Mn0.333)MnO3 with R = Er-Lu

Orthorhombic rare-earth trivalent manganites RMnO₃ (R = Er-Lu) were self-doped with Mn to form (R_0.667Mn_0.333)MnO₃ compositions, which were synthesized by a high-pressure, high-temperature method at 6 GPa and about 1670 K from R₂O₃ and Mn₂O₃. The average oxidation state of Mn is 3+ in (R_0.667Mn_0.333)MnO₃. However, Mn enters the A site in the oxidation state of 2+, creating the average oxidation state of 3.333+ at the B site. The presence of Mn²⁺ was confirmed by hard X-ray photoelectron spectroscopy measurements. Crystal structures were studied by synchrotron powder X-ray diffraction. (R_0.667Mn_0.333)MnO₃ crystallizes in space group Pnma with a = 5.50348(2) Å, b = 7.37564(1) Å, and c = 5.18686(1) Å for (Lu_0.667Mn_0.333)MnO₃ at 293 K, and they are isostructural with the parent RMnO₃ manganites. Compared with RMnO₃, (R_0.667Mn_0.333)MnO₃ exhibits enhanced Néel temperatures of about T_N1 = 106-110 K and ferrimagnetic or canted antiferromagnetic properties. Compounds with R = Er and Tm show additional magnetic transitions at about T_N2 = 9-16 K. (Tm_0.667Mn_0.333)MnO₃ exhibits a magnetization reversal or negative magnetization effect with a compensation temperature of about 16 K.

Enhancement of Ferroelectricity for Orthorhombic (Tb0.861Mn0.121)MnO3-δ by Copper Doping

Copper-doped (Tb_0.861Mn_0.121)MnO_3-δ has been synthesized by the conventional solid state reaction method. X-ray, neutron, and electron diffraction data indicate that they crystallize in Pnma space group at room temperature. Two magnetic orderings are found for this series by neutron diffraction. One is the ICAM (incommensurate canted antiferromagnetic) ordering of Mn with a wave vector q_Mn = (∼0.283, 0, 0) with a ≈ 5.73 Å, b ≈ 5.31 Å, and c ≈ 7.41 Å, and the other is the CAM (canted antiferromagnetic) ordering of both Tb and Mn in the magnetic space group Pn'a2₁' with a ≈ 5.73 Å, b ≈ 5.31 Å, and c ≈ 7.41 Å. A dielectric peak around 40 K is found for the samples doped with Cu, which is higher than that for orthorhombic TbMnO₃.

Synthesis, structure and magnetic properties of (Eu1−xMnx)MnO3−δ

The solid solution (Eu_1−xMn_x)MnO_3−δ (0 ≤ x ≤ 0.126) has been synthesized using a conventional solid-state method.

Multiferroicity Broken by Commensurate Magnetic Ordering in Terbium Orthomanganite

TbMnO3 is an important multiferroic material with strong coupling between magnetic and ferroelectric orderings. Incommensurate magnetic ordering is suggested to be vital for this coupling in TbMnO3 , which can be modified by doping at the site of Tb and/or Mn. Our study shows that a self-doped solid solution Tb1-x Mny MnO3 (y≤x) can be formed with Mn doped into the site of Tb of TbMnO3 . When y is small Tb1-x Mny MnO3 shows both ferroelectric and incommensurate magnetic orders at low temperature, which is similar to TbMnO3 . However, if y is large enough, a commensurate antiferromagnetic ordering appears along with the incommensurate magnetic ordering to prevent the appearance of multiferroicity in Tb1-x Mny MnO3 . That is to say, the magnetoeletric coupling can be broken by the co-existence of a commensurate antiferromagnetic ordering. This finding may be useful to the study of TbMnO3 .

Copper doped EuMnO3: synthesis, structure and magnetic properties

Solid solution EuMn_1−xCu_xO_3−δ (0 ≤ x ≤ 0.316) crystallizes in the space group Pnma and shows magnetic transitions from about 52 to 20 K.

Negative thermal expansion in functional materials: controllable thermal expansion by chemical modifications

Negative thermal expansion (NTE) is an intriguing physical property of solids, which is a consequence of a complex interplay among the lattice, phonons, and electrons. Interestingly, a large number of NTE materials have been found in various types of functional materials. In the last two decades good progress has been achieved to discover new phenomena and mechanisms of NTE. In the present review article, NTE is reviewed in functional materials of ferroelectrics, magnetics, multiferroics, superconductors, temperature-induced electron configuration change and so on. Zero thermal expansion (ZTE) of functional materials is emphasized due to the importance for practical applications. The NTE functional materials present a general physical picture to reveal a strong coupling role between physical properties and NTE. There is a general nature of NTE for both ferroelectrics and magnetics, in which NTE is determined by either ferroelectric order or magnetic one. In NTE functional materials, a multi-way to control thermal expansion can be established through the coupling roles of ferroelectricity-NTE, magnetism-NTE, change of electron configuration-NTE, open-framework-NTE, and so on. Chemical modification has been proved to be an effective method to control thermal expansion. Finally, challenges and questions are discussed for the development of NTE materials. There remains a challenge to discover a "perfect" NTE material for each specific application for chemists. The future studies on NTE functional materials will definitely promote the development of NTE materials.