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1. Multifunctional Cu ₂ TSiS ₄ (T = Mn and Fe): Polar Semiconducting Antiferromagnets with Nonlinear Optical Properties

   https://doi.org/10.1021/acs.inorgchem.2c03754  

   Messegee, Zachary T; Cho, Jun Sang; Craig, Andrew J; Garlea, V Ovidiu; Xin, Yan; Kang, Chang-Jong; Proffen, Thomas E; Bhandari, Hari; Kelly, Jordan C; Ghimire, Nirmal J; et al (January 2023, Inorganic Chemistry)

   Cu2TSiS4 (T = Mn and Fe) polycrystalline and single-crystal materials were prepared with high-temperature solid-state and chemical vapor transport methods, respectively. The polar crystal structure (space group Pmn21) consists of chains of corner-sharing and distorted CuS4, Mn/FeS4, and SiS4 tetrahedra, which is confirmed by Rietveld refinement using neutron powder diffraction data, X-ray single-crystal refinement, electron diffraction, energy-dispersive X-ray spectroscopy, and second harmonic generation (SHG) techniques. Magnetic measurements indicate that both compounds order antiferromagnetically at 8 and 14 K, respectively, which is supported by the temperature-dependent (100–2 K) neutron powder diffraction data. Additional magnetic reflections observed at 2 K can be modeled by magnetic propagation vectors k = (1/2,0,1/2) and k = (1/2,1/2,1/2) for Cu2MnSiS4 and Cu2FeSiS4, respectively. The refined antiferromagnetic structure reveals that the Mn/Fe spins are canted away from the ac plane by about 14°, with the total magnetic moments of Mn and Fe being 4.1(1) and 2.9(1) μB, respectively. Both compounds exhibit an SHG response with relatively modest second-order nonlinear susceptibilities. Density functional theory calculations are used to describe the electronic band structures. 

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2. Zn ₂ MnSbO ₆ and Zn ₂ FeSbO ₆ : Two New Polar High-Pressure Ordered Corundum-Type Compounds

   https://doi.org/10.1021/acs.chemmater.4c02146  

   Provino, Alessia; Emge, Thomas J; Walker, David; Frank, Corey E; Yoshida, Suguru; Gopalan, Venkatraman; Croft, Mark; Deng, Zheng; Jin, Changqing; Manfrinetti, Pietro; et al (December 2024, Chemistry of Materials)

   Two new compounds, Zn2FeSbO6 and Zn2MnSbO6, have been synthesized under high-pressure and high-temperature conditions. The synthesis, single-crystal and powder X-ray diffraction, X-ray absorption near-edge spectroscopy (XANES), optical second harmonic generation (SHG), and magnetic and heat capacity measurements were carried out for both compounds and are described. The lattice parameters are a = 5.17754(6) Å and c = 13.80045(16) Å for Zn2FeSbO6 and a = 5.1889(10) Å and c = 14.0418(3) Å for Zn2MnSbO6. Single-crystal X-ray diffraction analyses indicate that Zn2FeSbO6 consists of a cocrystal of superimposed Ni3TeO6 (NTO) and ordered ilmenite (OIL) components with a ratio of approximately 2:1 and Zn2MnSbO6 contains two nearly identical, but noncrystallographically related, OIL components in a ratio of approximately 6:1. 

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3. Hidden anomalies in topological $t\text{−}{\mathrm{PtBi}}_{2-x}$ probed by second harmonic generation

   https://doi.org/10.1103/PhysRevB.108.224104  

   Chavez, Bryan L; Kharal, Govinda; Xing, Lingyi; Crittenden, Scott; Crawford, Thomas M; Wu, Yanwen; Jin, Rongying (December 2023, Physical Review B)

   The noncentrosymmetric Weyl semimetal PtBi2−x (t-PtBi2−x) exhibits various interesting technologically important physical properties. We report the experimental investigation of PtBi1.6 via second harmonic generation (SHG), single-crystal x-ray diffraction, magnetic susceptibility, and electrical resistivity measurements. While bulk structural, magnetic, and electrical properties show no phase transitions below room temperature, the temperature dependence of the SHG intensity reveals two anomalies: one at T ∗ ∼ 60 K and another at Tx ∼ 200 K. Quantitative analysis indicates that the SHG signal results from both the buckled Bi1 surface termination with the 3m symmetry and flat Bi2 surface termination with the m symmetry. However, the anomalies are mainly driven by Bi1 on the surface: (1) T ∗ marks the onset of surface states which is also manifested in the c-axis resistivity drop and (2) Tx corresponds to the lowest thermal contraction of the structure and enhanced magnetic susceptibility. This study demonstrates that SHG is a powerful technique for probing surface properties even for noncentrosymmetric materials. 

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4. Crystal Growth and Magnetism of Transition Metal Pyrochlore Fluorides

   https://doi.org/10.1021/acs.inorgchem.3c01491  

   Masachchi, Lakshani W; Keerthisinghe, Navindra; Morrison, Gregory; Berseneva, Anna A; Smith, Mark D; zur_Loye, Hans-Conrad (August 2023, Inorganic Chemistry)

   Geometric magnetic frustration arises when the geometry of a structure prevents the simultaneous fulfillment of nearest-neighbor antiferromagnetic interactions and is commonly observed in lattices that exhibit a triangular topology, such as those found in the pyrochlore structure. Via a mild hydrothermal route, we have synthesized seven quaternary β-pyrochlore related fluorides AxM2+xM3+(2–x)F6, (A = Cs and Rb; M2+ = Co2+, Ni2+ and Zn2+; and M3+ = V3+ and Fe3+). Crystal structures and compositions were determined using a combination of single-crystal X-ray diffraction and energy-dispersive spectroscopy. After adjusting the reaction conditions, phase pure products of AxM2+xM3+(2–x)F6 were obtained. The magnetic susceptibility and isothermal magnetization data for all seven compounds were collected to interpret the magnetic behavior, which ranged from paramagnetic to antiferromagnetic with and without a ferromagnetic component. We found that the magnetic behavior of the AxM2+xV3+(2–x)F6 pyrochlore structures strongly depends on the presence or absence of unpaired electrons on the M2+ position. The titled pyrochlore compounds, with the exception of the Zn-analog, can be considered frustrated materials, with frustration indices in the range of 6 –13. 

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5. Room temperature flash of single crystal titania: Electronic and optical properties

   https://doi.org/10.1111/jace.18798  

   Yadav, Devinder; Yuan, Yakun; Gopalan, Venkatraman; Raj, Rishi; Jo, Seohyeon (September 2022, Journal of the American Ceramic Society)

   Abstract A single‐crystal specimen of rutile (titania) was flashed repetitively, while increasing the electric field after each cycle. As expected, the flash onset temperature continued to drop modestly at higher fields. However, when the field was increased from 400 to 450 V cm^–1, the flashed onset fell dramatically down to room temperature. We have investigated the electrical and optical properties of this room temperature flashed specimen (called SZ). The specimen was electronically conducting. Optical absorption spectroscopy revealed a narrow band of new energy levels that were generated just below the conduction band. The gap between the conduction band and this flash‐induced energy level agreed with the peak in the electroluminescence spectrum. Optical second harmonic generation (SHG) is reported. The flash‐on condition significantly lowered the SHG, which rebounded when the flash was turned off. This result suggests that the structure becomes more centrosymmetric in the state of flash, which may represent a disordered state of defects. The possibility of studying flash behavior at room temperature, without a furnace (as in SZ type specimens), opens a considerable simplification for in‐situ characterization of flash behavior. For example, a possible relationship between memristor physics and the flash phenomenon can be studied. 

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