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1. Stabilization of the Polar Structure and Giant Second‐Order Nonlinear Response of Single Crystal γ‐NaAs 0.95 Sb 0.05 Se 2

   https://doi.org/10.1002/adfm.202211969  

   Iyer, Abishek K. ; He, Jingyang ; Xie, Hongyao ; Goodling, Devin ; Chung, Duck‐Young ; Gopalan, Venkatraman ; Kanatzidis, Mercouri G. ( December 2022 , Advanced Functional Materials)

   The dearth of suitable materials significantly restricts the practical development of infrared (IR) laser systems with highly efficient and broadband tuning. Recently, γ‐NaAsSe₂is reported, and it exhibits a large nonlinear second‐harmonic generation (SHG) coefficient of 590 pm V⁻¹at 2 µm. However, the crystal growth of γ‐NaAsSe₂is challenging because it undergoes a phase transition to centrosymmetric δ‐NaAsSe₂. Herein, the stabilization of non‐centrosymmetric γ‐NaAsSe₂by doping the As site with Sb, which results in γ‐NaAs_0.95Sb_0.05Se₂is reported. The congruent melting behavior is confirmed by differential thermal analysis with a melting temperature of 450 °C and crystallization temperature of 415 °C. Single crystals with dimensions of 3 mm × 2 mm are successfully obtained via zone refining and the Bridgman method. The purification of the material plays a significant role in crystal growth and results in a bandgap of 1.78 eV and thermal conductivity of 0.79 Wm⁻¹K⁻¹. The single‐crystal SHG coefficient of γ‐NaAs_0.95Sb_0.05Se₂exhibits an enormous value of |d₁₁| = 648 ± 74 pm V⁻¹, which is comparable to that of γ‐NaAsSe₂and ≈20× larger than that of AgGaSe₂. The bandgap of γ‐NaAs_0.95Sb_0.05Se₂(1.78 eV) is similar to that of AgGaSe₂, thus rendering it highly attractive as a high‐performing nonlinear optical material.

    

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2. Structural Behavior of C2/m Tremolite to 40 GPa: A High-Pressure Single-Crystal X-ray Diffraction Study

   https://doi.org/10.2138/am-2022-8278  

   Ott, Jason N. ; Kalkan, Bora ; Kunz, Martin ; Berlanga, Genesis ; Yuvali, Ali F. ; Williams, Quentin ( October 2022 , American Mineralogist)

   The high-pressure structure and stability of the calcic amphibole tremolite (Ca2Mg5Si8O22(OH)2) was investigated to ~40 GPa at 300 K by single-crystal X-ray diffraction using synchrotron radiation. C2/m symmetry tremolite displays a broader metastability range than previously studied clinoamphiboles, exhibiting no first-order phase transition up to 40 GPa. Axial parameter ratios a/b and a/c, in conjunction with finite strain versus normalized pressure trends, indicate that changes in compressional behavior occur at pressures of ~5 and ~20 GPa. An analysis of the finite strain trends, using third-order Birch-Murnaghan equations of state, resulted in bulk moduli (𝐾) of 72(7), 77(2), and 61(1) GPa for the compressional regimes from 0-5 GPa (regime I), 5-20 GPa (II), and above 20 GPa (III), respectively, and accompanying pressure-derivatives of the bulk moduli (𝐾′) of 8.6(42), 6.0(3), and 10.0(2). The results are consistent with first-principle theoretical calculations of tremolite elasticity. The axial compressibility ratios of tremolite, determined as 𝛽a : 𝛽b : 𝛽c = 2.22:1.0:0.78 (regime I), 2.12:1.0:0.96 (II), and 1.03:1.0:0.75 (III), demonstrate a substantial reduction of the compressional anisotropy of tremolite at high pressures, which is a notable contrast with the increasingly anisotropic compressibility observed in the high-pressure polymorphs of the clinoamphibole grunerite. The shift in compression-regime at 5 GPa (I-II) transition is ascribed to stiffening along the crystallographic a-axis corresponding to closure of the vacant A-site in the structure, and a shift in the topology of the a-oriented surfaces of the structural I-beam from concave to convex. The II-III regime shift at 20 GPa corresponds to an increasing rate of compaction of the Ca-polyhedra and increased distortion of the Mg-octahedral sites, processes which dictate compaction in both high-pressure compression-regimes. Bond-valence analyses of the tremolite structure under pressure show dramatic overbonding of the Ca-cations (75% at 30 GPa), with significant Mg-cation overbonding as well (40%). These imply that tremolite’s notable metastability range hinges on the calcium cation’s bonding environment. The 8-fold coordinated Ca-polyhedron accommodates significant compaction under pressure, while the geometry of the Ca-O polyhedron becomes increasingly regular and inhibits the reorientation of the tetrahedral chains that generate phase transitions observed in other clinoamphiboles. Peak/background ratio of diffraction data collected above 40 GPa and our equation of state determination of bulk moduli and compressibilities of tremolite in regime III, in concert with the results of our previous Raman study, suggest that C2/m tremolite may be approaching the limit of its metastability above 40 GPa. Our results have relevance for both the metastable compaction of tremolite during impact events, and for possible metastable persistence of tremolite within cold subduction zones within the Earth. 

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3. Atomistic insight into the ferroelastic post-stishovite transition by high-pressure single-crystal X-ray diffraction

   https://doi.org/10.2138/am-2022-8458  

   Zhang, Yanyao ; Chariton, Stella ; He, Jiaming ; Fu, Suyu ; Xu, Fang ; Prakapenka, Vitali B. ; Lin, Jung-Fu ( January 2023 , American Mineralogist)

   Abstract The post-stishovite transition is a classic pseudo-proper typed ferroelastic transition with a symmetry-breaking spontaneous strain. This transition has been studied using high-pressure spontaneous strains, optic modes, and elastic moduli (Cij) based on the Landau modeling, but its atomistic information and structural distortion remain poorly understood. Here we have conducted synchrotron single-crystal X-ray diffraction measurements on stishovite crystals up to 75.3 GPa in a diamond-anvil cell. Analysis of the data reveals atomic positions, bond lengths, bond angles, and variations of SiO6 octahedra across the transition at high pressure. Our results show that the O coordinates split at ~51.4 GPa, where the apical and equatorial Si-O bond lengths cross over, the SiO6 octahedral distortion vanishes, and the SiO6 octahedra start to rotate about the c axis. Moreover, distortion mode analysis shows that an in-plane stretching distortion (GM1+ mode) occurs in the stishovite structure at high pressure while a rotational distortion (GM2+ mode) becomes dominant in the post-stishovite structure. These results are used to correlate with elastic moduli and Landau parameters (symmetry-breaking strain e1–e2 and order parameter Q) to provide atomistic insight into the ferroelastic transition. When the bond lengths of two Si-O bonds are equal due to the contribution from the GM1+ stretching mode, C11 converges with C12, and the shear wave VS1[110] polarizing along [110] and propagating along [110] vanishes. Values of e1–e2 and Q are proportional to the SiO6 rotation angle from the occurrence of the GM1+ rotational mode in the post-stishovite structure. Our results on the pseudo-proper type transition are also compared with that for the proper type in albite and improper type in CaSiO3 perovskite. The symmetry-breaking strain, in all these types of transitions, arises as the primary effect from the structural angle (such as SiO6 rotation or lattice constant angle) and its relevant distortion mode in the low-symmetry ferroelastic phase. 

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4. In situ study of rotating lattice single‐crystal formation in Sb 2 S 3 glass by Laue μXRD

   https://doi.org/10.1111/jace.16905  

   Au‐Yeung, Courtney ; Stan, Camelia ; Tamura, Nobumichi ; Jain, Himanshu ; Dierolf, Volkmar ( November 2019 , Journal of the American Ceramic Society)

   Single‐crystal architectures in glass, formed by a solid‐solid transformation via laser heating, are novel solids with a rotating lattice. To understand the process of lattice formation that proceeds via crystal growth, we have observed in situ Sb₂S₃crystal formation under X‐ray irradiation with simultaneous Laue micro X‐ray diffraction (μXRD) pattern collection. By translating the sample with respect to the beam, we form rotating lattice single (RLS) crystal lines with a consistently linear relationship between the rotation angle and distance from nucleation site. The lines begin with a seed crystal, followed by a transition region comprising of sub‐grain or very similarly oriented grains, followed by the presence of a rotating lattice single crystal of unrestricted length. The results demonstrate that the primary cause of lattice rotation within RLS crystals is the densification accompanying the glass → crystal transformation, rather than stresses produced from the difference in thermal expansion coefficient of the two phases or paraelectric → ferroelectric transition during cooling to ambient temperature.

    

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5. Single crystal neutron and magnetic measurements of Rb 2 Mn 3 (VO 4 ) 2 CO 3 and K 2 Co 3 (VO 4 ) 2 CO 3 with mixed honeycomb and triangular magnetic lattices

   https://doi.org/10.1039/C9DT03389K  

   Smith Pellizzeri, Tiffany M. ; Sanjeewa, Liurukara D. ; Pellizzeri, Steven ; McMillen, Colin D. ; Garlea, V. Ovidiu ; Ye, Feng ; Sefat, Athena S. ; Kolis, Joseph W. ( April 2020 , Dalton Transactions)

   null (Ed.)

   Two new alkali vanadate carbonates with divalent transition metals have been synthesized as large single crystals via a high-temperature (600 °C) hydrothermal technique. Compound I , Rb 2 Mn 3 (VO 4 ) 2 CO 3 , crystallizes in the trigonal crystal system in the space group P 3̄1 c , and compound II , K 2 Co 3 (VO 4 ) 2 CO 3 , crystallizes in the hexagonal space group P 6 3 / m . Both structures contain honeycomb layers and triangular lattices made from edge-sharing MO 6 octahedra and MO 5 trigonal bipyramids, respectively. The honeycomb and triangular layers are connected along the c -axis through tetrahedral [VO 4 ] groups. The MO 5 units are connected with each other by carbonate groups in the ab -plane by forming a triangular magnetic lattice. The difference in space groups between I and II was also investigated with Density Functional Theory (DFT) calculations. Single crystal magnetic characterization of I indicates three magnetic transitions at 77 K, 2.3 K, and 1.5 K. The corresponding magnetic structures for each magnetic transition of I were determined using single crystal neutron diffraction. At 77 K the compound orders in the MnO 6 -honeycomb layer in a Néel-type antiferromagnetic orientation while the MnO 5 triangular lattice ordered below 2.3 K in a colinear ‘up–up–down’ fashion, followed by a planar ‘Y’ type magnetic structure. K 2 Co 3 (VO 4 ) 2 CO 3 ( II ) exhibits a canted antiferromagnetic ordering below T N = 8 K. The Curie–Weiss fit (200–350 K) gives a Curie–Weiss temperature of −42 K suggesting a dominant antiferromagnetic coupling in the Co 2+ magnetic sublattices. 

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