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1. Structural phase purification of bulk HfO ₂ :Y through pressure cycling

   https://doi.org/10.1073/pnas.2312571121  

   Musfeldt, J. L.; Singh, Sobhit; Fan, Shiyu; Gu, Yanhong; Xu, Xianghan; Cheong, S.-W.; Liu, Z.; Vanderbilt, David; Rabe, Karin M. (January 2024, Proceedings of the National Academy of Sciences)

   We combine synchrotron-based infrared absorption and Raman scattering spectroscopies with diamond anvil cell techniques and first-principles calculations to explore the properties of hafnia under compression. We find that pressure drives HfO ${}_2$:7%Y from the mixed monoclinic ( $P{2}_1/c$) $+$antipolar orthorhombic ( $\mathit{Pbca}$) phase to pure antipolar orthorhombic ( $\mathit{Pbca}$) phase at approximately 6.3 GPa. This transformation is irreversible, meaning that upon release, the material is kinetically trapped in the $\mathit{Pbca}$metastable state at 300 K. Compression also drives polar orthorhombic ( $Pca{2}_1$) hafnia into the tetragonal ( $P{4}_2/nmc$) phase, although the latter is not metastable upon release. These results are unified by an analysis of the energy landscape. The fact that pressure allows us to stabilize targeted metastable structures with less Y stabilizer is important to preserving the flat phonon band physics of pure HfO ${}_2$. 

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2. Zintl Phase Compounds Mg3Sb2−xBix (x = 0, 1, and 2) Monolayers: Electronic, Phonon and Thermoelectric Properties From ab Initio Calculations

   https://doi.org/10.3389/fmech.2022.876655  

   Chang, Zheng; Ma, Jing; Yuan, Kunpeng; Zheng, Jiongzhi; Wei, Bin; Al-Fahdi, Mohammed; Gao, Yufei; Zhang, Xiaoliang; Shao, Hezhu; Hu, Ming; et al (April 2022, Frontiers in Mechanical Engineering)

   The Mg 3 Sb 2− x Bi x family has emerged as the potential candidates for thermoelectric applications due to their ultra-low lattice thermal conductivity ( κ L ) at room temperature (RT) and structural complexity. Here, using ab initio calculations of the electron-phonon averaged (EPA) approximation coupled with Boltzmann transport equation (BTE), we have studied electronic, phonon and thermoelectric properties of Mg 3 Sb 2− x Bi x (x = 0, 1, and 2) monolayers. In violation of common mass-trend expectations, increasing Bi element content with heavier Zintl phase compounds yields an abnormal change in κ L in two-dimensional Mg 3 Sb 2− x Bi x crystals at RT (∼0.51, 1.86, and 0.25 W/mK for Mg 3 Sb 2 , Mg 3 SbBi, and Mg 3 Bi 2 ). The κ L trend was detailedly analyzed via the phonon heat capacity, group velocity and lifetime parameters. Based on quantitative electronic band structures, the electronic bonding through the crystal orbital Hamilton population (COHP) and electron local function analysis we reveal the underlying mechanism for the semiconductor-semimetallic transition of Mg 3 Sb 2-− x Bi x compounds, and these electronic transport properties (Seebeck coefficient, electrical conductivity, and electronic thermal conductivity) were calculated. We demonstrate that the highest dimensionless figure of merit ZT of Mg 3 Sb 2− x Bi x compounds with increasing Bi content can reach ∼1.6, 0.2, and 0.6 at 700 K, respectively. Our results can indicate that replacing heavier anion element in Zintl phase Mg 3 Sb 2− x Bi x materials go beyond common expectations (a heavier atom always lead to a lower κ L from Slack’s theory), which provide a novel insight for regulating thermoelectric performance without restricting conventional heavy atomic mass approach. 

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3. Optical dielectric properties of HfO ₂ -based films

   https://doi.org/10.1116/6.0001651  

   Dou, Hongyi; Strkalj, Nives; Zhang, Yizhi; MacManus-Driscoll, Judith L.; Jia, Quanxi; Wang, Haiyan (May 2022, Journal of Vacuum Science & Technology A)

   We report the dielectric Properties of HfO 2 -based films in the optical–high frequency range. The demonstrated tunability of the optical dielectric constant of HfO 2 -based compounds is of great relevance for optoelectronic applications, e.g., high-refractive index dielectrics for nanoantenna and optical coatings for electronic displays. Since the optical dielectric constant of HfO 2 is determined by the electronic structure and its crystal environment, we tune the physical properties of HfO 2 films on MgO by adding different dopants. In this work, we aim to determine the influence of doping together with the resulting crystal structure on the optical dielectric constant. Hence, we studied 20 mol. % Y-doped HfO 2 (HYO), Hf 0.5 Zr 0.5 O 2 (HZO), and Hf 0.5 Ce 0.5 O 2 (HCO). Among the dopants, Y 2 O 3 has the lowest, ZrO 2 an intermediate, and CeO 2 the highest real part of the optical dielectric constant. The optical dielectric constant is found to be lowest in the cubic HYO films. An intermediate dielectric constant is found in HZO films that is predominantly in the monoclinic phase, but additionally hosts the cubic phase. The highest dielectric constant is observed in HCO films that are predominantly in the cubic phase with inclusions of the monoclinic phase. The observed trend is in good agreement with the dominant role of the dopant type in setting the optical dielectric constant. 

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4. Experimental and computational investigations of TiIrB: a new ternary boride with Ti _1+x Rh _2−x+y Ir _3−y B ₃ -type structure

   https://doi.org/10.1515/znb-2021-0121  

   Scheifers, Jan P.; Gibson, Kate A.; Fokwa, Boniface P. (October 2021, Zeitschrift für Naturforschung B)

   Abstract A new ternary phase, TiIrB, was synthesized by arc-melting of the elements and characterized by powder X-ray diffraction. The compound crystallizes in the orthorhombic Ti 1+ x Rh 2− x + y Ir 3− y B 3 structure type, space group Pbam (no. 55) with the lattice parameters a  = 8.655(2), b  = 15.020(2), and c  = 3.2271(4) Å. Density Functional Theory (DFT) calculations were carried out to understand the electronic structure, including a Bader charge analysis. The charge distribution of TiIrB in the Ti 1+ x Rh 2− x + y Ir 3− y B 3 -type phase has been evaluated for the first time, and the results indicate that more electron density is transferred to the boron atoms in the zigzag B 4 units than to isolated boron atoms. 

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5. The intrinsic thermal transport properties of the biphenylene network and the influence of hydrogenation: a first-principles study

   https://doi.org/10.1039/D1TC04154A  

   Zhang, Pei; Ouyang, Tao; Tang, Chao; He, Chaoyu; Li, Jin; Zhang, Chunxiao; Hu, Ming; Zhong, Jianxin (December 2021, Journal of Materials Chemistry C)

   Utilizing first-principles calculations combined with phonon Boltzmann transport theory up to fourth-order anharmonicity, we systematically investigate the thermal transport properties of the biphenylene network [BPN, recently synthesized experimentally by Fan et al. , Science , 2021, 372 , 852–856] and hydrogenated BPN (HBPN). The calculations show that four-phonon scattering significantly affects the lattice thermal conductivity ( κ ) of BPN. At room temperature, the κ of BPN is reduced from 582.32 (1257.07) W m −1 K −1 to 309.56 (539.88) W m −1 K −1 along the x ( y ) direction after considering the four-phonon scattering. Moreover, our results demonstrate that the thermal transport in BPN could also be greatly suppressed by hydrogenation, where the κ of HBPN along the x ( y ) direction is merely 16.62% (10.14%) of that of pristine BPN at 300 K. The mechanism causing such an obvious decrease of κ of HBPN is identified to be due to the enhanced phonon scattering rate and reduced group velocity, which is further revealed by the increased scattering phase space and weakened C–C bond. The results presented in this work shed light on the intrinsic thermal transport features of BPN and HBPN, which will help us to understand the phonon transport processes and pave the way for their future developments in the thermal field. 

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