PbMn(IV)TeO₆: A New Noncentrosymmetric Layered Honeycomb Magnetic Oxide

PbMnTeO₆, a new noncentrosymmetric layered magnetic oxide was synthesized and characterized. The crystal structure is hexagonal, with space group <i>P</i>6̅2<i>m</i> (No. 189), and consists of edge-sharing (Mn⁴⁺/Te⁶⁺)­O₆ trigonal prisms that form honeycomb-like two-dimensional layers with Pb²⁺ ions between the layers. The structural difference between PbMnTeO₆, with disordered/trigonal prisms of Mn⁴⁺/Te⁶⁺, versus the similar chiral SrGeTeO₆ (space group <i>P</i>312), with long-range order of Ge⁴⁺ and Te⁶⁺ in octahedral coordination, is attributed to a difference in the electronic effects of Ge⁴⁺ and Mn⁴⁺. Temperature-dependent second harmonic generation by PbMnTeO₆ confirmed the noncentrosymmetric character between 12 and 873 K. Magnetic measurements indicated antiferromagnetic order at <i>T</i>_N ≈ 20 K and a frustration parameter (|θ|/<i>T</i>_N) of ∼2.16

Structure and Magnetic Behavior of Layered Honeycomb Tellurates, BiM(III)TeO₆ (M = Cr, Mn, Fe)

New layered honeycomb tellurates, BiM­(III)­TeO₆ (M = Cr, Mn, Fe) were synthesized and characterized. BiM­(III)­TeO₆ (M = Cr, Fe) species crystallize in a trigonal space group, <i>P</i>3̅1<i>c</i> (No. 163), of edge-sharing M³⁺/Te⁶⁺O₆ octahedra, which form honeycomb-like double layers in the <i>ab</i> plane with Bi³⁺ cations located between the layers. Interestingly, the structure of BiMnTeO₆ is similar to those of the Cr/Fe analogues, but with monoclinic space group, <i>P</i>2₁/<i>c</i> (No. 14), attributed to the strong Jahn–Teller distortion of Mn³⁺ cations. The crystal structure of BiM­(III)­TeO₆ is a superstructure of PbSb₂O₆-related materials (ABB′O₆). The Cr³⁺ and Fe³⁺ cations are ordered 80% and 90%, respectively, while the Mn³⁺ ions are completely ordered on the B-site of the ABB′O₆ structure. BiCrTeO₆ shows a broad antiferromagnetic transition (AFM) at ∼17 K with a Weiss temperature (θ) of −59.85 K, while BiFeTeO₆ and BiMnTeO₆ show sharp AFM transitions at ∼11 K with θ of −27.56 K and at ∼9.5 K with θ of −17.57 K, respectively. These differences in the magnetic behavior are ascribed to the different concentration of magnetic nearest versus next-nearest neighbor interactions of magnetic cations due to the relative differences in the extent of M/Te ordering

PbMn(IV)TeO₆: A New Noncentrosymmetric Layered Honeycomb Magnetic Oxide

PbMnTeO₆, a new noncentrosymmetric layered magnetic oxide was synthesized and characterized. The crystal structure is hexagonal, with space group <i>P</i>6̅2<i>m</i> (No. 189), and consists of edge-sharing (Mn⁴⁺/Te⁶⁺)­O₆ trigonal prisms that form honeycomb-like two-dimensional layers with Pb²⁺ ions between the layers. The structural difference between PbMnTeO₆, with disordered/trigonal prisms of Mn⁴⁺/Te⁶⁺, versus the similar chiral SrGeTeO₆ (space group <i>P</i>312), with long-range order of Ge⁴⁺ and Te⁶⁺ in octahedral coordination, is attributed to a difference in the electronic effects of Ge⁴⁺ and Mn⁴⁺. Temperature-dependent second harmonic generation by PbMnTeO₆ confirmed the noncentrosymmetric character between 12 and 873 K. Magnetic measurements indicated antiferromagnetic order at <i>T</i>_N ≈ 20 K and a frustration parameter (|θ|/<i>T</i>_N) of ∼2.16

Low-Temperature Vaterite-Type LuBO₃, a Vacancy-Stabilized Phase Synthesized at High Temperature

Low-temperature vaterite-type LuBO₃ (π-LBO) was prepared by a solid-state reaction method at high temperature. The reasoning of the existence of vacancy-stabilized π-LBO was investigated for the first time using neutron diffraction patterns, Fourier transform infrared (FT-IR) spectra, and high-resolution transmission electron microscopy. The results clearly demonstrated that the B and O vacancies in π-LBO came into being during the heating process. The existence of an open B₃O₉ ring consisting of BO₃ and BO₄ units in π-LBO due to the B and O vacancies was demonstrated by FT-IR. The vacuum ultraviolet–ultraviolet spectroscopic properties of π-LBO were studied in detail. In addition, the luminescence mechanism of Ce³⁺ in π-LBO was put forward and discussed with that of calcite-type LuBO₃ (β-LBO)

Structure and Magnetic Behavior of Layered Honeycomb Tellurates, BiM(III)TeO₆ (M = Cr, Mn, Fe)

New layered honeycomb tellurates, BiM­(III)­TeO₆ (M = Cr, Mn, Fe) were synthesized and characterized. BiM­(III)­TeO₆ (M = Cr, Fe) species crystallize in a trigonal space group, <i>P</i>3̅1<i>c</i> (No. 163), of edge-sharing M³⁺/Te⁶⁺O₆ octahedra, which form honeycomb-like double layers in the <i>ab</i> plane with Bi³⁺ cations located between the layers. Interestingly, the structure of BiMnTeO₆ is similar to those of the Cr/Fe analogues, but with monoclinic space group, <i>P</i>2₁/<i>c</i> (No. 14), attributed to the strong Jahn–Teller distortion of Mn³⁺ cations. The crystal structure of BiM­(III)­TeO₆ is a superstructure of PbSb₂O₆-related materials (ABB′O₆). The Cr³⁺ and Fe³⁺ cations are ordered 80% and 90%, respectively, while the Mn³⁺ ions are completely ordered on the B-site of the ABB′O₆ structure. BiCrTeO₆ shows a broad antiferromagnetic transition (AFM) at ∼17 K with a Weiss temperature (θ) of −59.85 K, while BiFeTeO₆ and BiMnTeO₆ show sharp AFM transitions at ∼11 K with θ of −27.56 K and at ∼9.5 K with θ of −17.57 K, respectively. These differences in the magnetic behavior are ascribed to the different concentration of magnetic nearest versus next-nearest neighbor interactions of magnetic cations due to the relative differences in the extent of M/Te ordering

Structure and Magnetic Behavior of Layered Honeycomb Tellurates, BiM(III)TeO₆ (M = Cr, Mn, Fe)

New layered honeycomb tellurates, BiM­(III)­TeO₆ (M = Cr, Mn, Fe) were synthesized and characterized. BiM­(III)­TeO₆ (M = Cr, Fe) species crystallize in a trigonal space group, <i>P</i>3̅1<i>c</i> (No. 163), of edge-sharing M³⁺/Te⁶⁺O₆ octahedra, which form honeycomb-like double layers in the <i>ab</i> plane with Bi³⁺ cations located between the layers. Interestingly, the structure of BiMnTeO₆ is similar to those of the Cr/Fe analogues, but with monoclinic space group, <i>P</i>2₁/<i>c</i> (No. 14), attributed to the strong Jahn–Teller distortion of Mn³⁺ cations. The crystal structure of BiM­(III)­TeO₆ is a superstructure of PbSb₂O₆-related materials (ABB′O₆). The Cr³⁺ and Fe³⁺ cations are ordered 80% and 90%, respectively, while the Mn³⁺ ions are completely ordered on the B-site of the ABB′O₆ structure. BiCrTeO₆ shows a broad antiferromagnetic transition (AFM) at ∼17 K with a Weiss temperature (θ) of −59.85 K, while BiFeTeO₆ and BiMnTeO₆ show sharp AFM transitions at ∼11 K with θ of −27.56 K and at ∼9.5 K with θ of −17.57 K, respectively. These differences in the magnetic behavior are ascribed to the different concentration of magnetic nearest versus next-nearest neighbor interactions of magnetic cations due to the relative differences in the extent of M/Te ordering

Metastable γ‑Li₂TiTeO₆: Negative Chemical Pressure Interception and Polymorph Tuning of SHG

Intercepting metastable phases by chemical approaches is an important solution to explore structural varieties of functional materials under positive/negative pressure, as paradigmatically exemplified by the polymorph modification in Li2TiTeO6. Here, we stabilized a novel metastable Li2TiTeO6 (denoted as γ-phase) in the ordered-ilmenite-type R3 via facile topotactic reaction from Na2TiTeO6, which was found to crystallize in R3 instead of the reported R3̅ structure. The calculated equilibrium volume of γ-Li2TiTeO6 is larger than that of the ground-state Pnn2-Li2TiTeO6 (denoted as α-phase), indicating that γ-Li2TiTeO6 can only be stabilized under “negative pressure” quantified to be around −6 GPa. The γ-phase irreversibly transforms into the α-phase around 560 °C under ambient pressure, accompanied by a steep increase (∼500 times) of the second harmonic generation (SHG), indicating a potential application of γ-Li2TiTeO6 as an optical thermometer. These findings elegantly show that chemical pressure as well as physical pressure is powerful to tune the polymorphs for metastable phases and exotic properties as paradigmatically exemplified by Li2TiTeO6, which undergoes consecutive polymorph tuning of γ (−6 GPa), α (0 GPa), β (6 GPa, R3-Ni3TeO6 type), and δ (40 GPa, predicted P21/n double perovskite) phases with densified atomic packing

Metastable γ‑Li₂TiTeO₆: Negative Chemical Pressure Interception and Polymorph Tuning of SHG

Intercepting metastable phases by chemical approaches is an important solution to explore structural varieties of functional materials under positive/negative pressure, as paradigmatically exemplified by the polymorph modification in Li2TiTeO6. Here, we stabilized a novel metastable Li2TiTeO6 (denoted as γ-phase) in the ordered-ilmenite-type R3 via facile topotactic reaction from Na2TiTeO6, which was found to crystallize in R3 instead of the reported R3̅ structure. The calculated equilibrium volume of γ-Li2TiTeO6 is larger than that of the ground-state Pnn2-Li2TiTeO6 (denoted as α-phase), indicating that γ-Li2TiTeO6 can only be stabilized under “negative pressure” quantified to be around −6 GPa. The γ-phase irreversibly transforms into the α-phase around 560 °C under ambient pressure, accompanied by a steep increase (∼500 times) of the second harmonic generation (SHG), indicating a potential application of γ-Li2TiTeO6 as an optical thermometer. These findings elegantly show that chemical pressure as well as physical pressure is powerful to tune the polymorphs for metastable phases and exotic properties as paradigmatically exemplified by Li2TiTeO6, which undergoes consecutive polymorph tuning of γ (−6 GPa), α (0 GPa), β (6 GPa, R3-Ni3TeO6 type), and δ (40 GPa, predicted P21/n double perovskite) phases with densified atomic packing

Metastable γ‑Li₂TiTeO₆: Negative Chemical Pressure Interception and Polymorph Tuning of SHG

Intercepting metastable phases by chemical approaches is an important solution to explore structural varieties of functional materials under positive/negative pressure, as paradigmatically exemplified by the polymorph modification in Li2TiTeO6. Here, we stabilized a novel metastable Li2TiTeO6 (denoted as γ-phase) in the ordered-ilmenite-type R3 via facile topotactic reaction from Na2TiTeO6, which was found to crystallize in R3 instead of the reported R3̅ structure. The calculated equilibrium volume of γ-Li2TiTeO6 is larger than that of the ground-state Pnn2-Li2TiTeO6 (denoted as α-phase), indicating that γ-Li2TiTeO6 can only be stabilized under “negative pressure” quantified to be around −6 GPa. The γ-phase irreversibly transforms into the α-phase around 560 °C under ambient pressure, accompanied by a steep increase (∼500 times) of the second harmonic generation (SHG), indicating a potential application of γ-Li2TiTeO6 as an optical thermometer. These findings elegantly show that chemical pressure as well as physical pressure is powerful to tune the polymorphs for metastable phases and exotic properties as paradigmatically exemplified by Li2TiTeO6, which undergoes consecutive polymorph tuning of γ (−6 GPa), α (0 GPa), β (6 GPa, R3-Ni3TeO6 type), and δ (40 GPa, predicted P21/n double perovskite) phases with densified atomic packing

Designing Polar and Magnetic Oxides: Zn₂FeTaO₆ - in Search of Multiferroics

Polar oxides are technically of great interest but difficult to prepare. Our recent discoveries predicted that polar oxides can be synthesized in the corundum-derivative A₂BB′O₆ family with unusually small cations at the A-site and a d⁰ electron configuration ion at B′-site. When magnetic transition-metal ions are incorporated more interesting polar magnetic oxides can form. In this work we experimentally verified this prediction and prepared LiNbO₃ (LN)-type polar magnetic Zn₂FeTaO₆ via high pressure and temperature synthesis. The crystal structure analysis indicates highly distorted ZnO₆ and (Fe/Ta)­O₆ octahedra, and an estimated spontaneous polarization (<i>P</i>_S) of ∼50 μC/cm² along the <i>c</i>-axis was obtained from point charge model calculations. Zn₂Fe³⁺Ta⁵⁺O₆ has a lower magnetic transition temperature (<i>T</i>_<i>N</i> ∼ 22 K) than the Mn₂FeTaO₆ analogue but is less conductive. The dielectric and polarization measurements indicate a potentially switchable component