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Nam, Gnu,Choi, Woongjin,Jo, Hongil,Ok, Kang Min,Ahn, Kyunghan,You, Tae-Soo American Chemical Society 2017 Chemistry of materials Vol.29 No.3
<P>The solid-solution Zintl compounds with the mixed cations of Ca2+ and Yb2+ in the Ca5-xYbxAl2Sb6 (1.0 <= x <= 5.0) system have been synthesized by high-temperature solid-state reactions. Two slightly different crystal structures of the Ba5Al2Bi6-type and Ca5Ga2Sb6-type phases have been characterized for seven compounds with 2.5 <= x <= 5.0 and three compounds with 1.0 <= x <= 2.0, respectively, by both powder and single-crystal X-ray diffraction analyses. The two title phases adopt the orthorhombic space group Pbam (Z = 2, oP26) with seven independent asymmetric atomic sites and share certain structural similarities, including infinite one-dimensional [Al2Sb8] double chains and isolated space-filling Ca2+/Yb2+ cations. Interestingly, we reveal the crystal-to-crystal solid-state structural transformation of the Yb-rich compound Ca1.5Yb3.5Al2Sb6 from the Ba5Al2Bi6-type to the Ca5Ga2Sb6-type phase through the postannealing process, which can be rationalized as the phase transition from the kinetically more stable structure to the thermodynamically more stable crystal structure on the basis of theoretical calculations. Discrepancies of the local coordination geometries of the anionic [Al2Sb8] units and the geometrical arrangements of structural building moieties in the two distinct phases provoke the different electrical properties of metallic and semiconducting conduction, respectively, for the Ba5Al2Bi6-type and Ca5Ga2Sb6-type phases. Density of states and crystal orbital Hamilton population analyses based on tight-binding linear muffin-tin orbital calculations prove that the band-gap opening in the Ca5Ga2Sb6-type phase should mainly be attributed to an extended bond distance of the bridging Sb-Sb in the [Al2Sb8] unit. A series of thermoelectric (TE) property measurements indicates that the phase transition via the postannealing process eventually results in an enhancement of the TE performance of Yb-rich Ca1.5Yb3.5Al2Sb6.</P>
Gnu Nam,조홍일,옥강민,김종식,유태수 대한화학회 2016 Bulletin of the Korean Chemical Society Vol.37 No.8
Two novel polar intermetallic compounds have been synthesized by a high-temperature reaction method, and their crystal structures have been characterized by both single-crystal and powder X-ray diffractions. La15Ge9Li1 .50(16) crystallizes in the hexagonal space group P63 mc (Z = 2, Pearson code hP52) with nine crystallographically independent sites in the unit cell, and the lattice parameters are a = 15.516(2) and c = 6.895(2) Å. The overall crystal structure can be described as a √3 × √3 × 1 superstructure of the parent compound La5Ge3 adopting the Mn5Si3-type, and such a superstructure transformation should be attributed to the particular ordering patterns of interstitial Li atoms at the Wyckoff 2a and 2b sites with different occupation factors. La7Ge3 also crystallizes in the hexagonal P63 mc space group (Z = 2, Pearson code hP20) with three La and one Ge non-equivalent sites in the unit cell. The lattice parameters are a = 10.666(1) and c = 6.357(1) Å. The crystal structure of La7Ge3 shares some similarities with La15Ge9Li1.50(16) and its parent compound La5Ge3, such as the one-dimensional (1D) zigzag confacial La6 octahedra chains and the 1D zigzag La atomic chains. Theoretical investigations using tight-binding linear muffin-tin orbital (LMTO) method provide a comprehensive understanding about electronic structures and chemical bonding of two title compounds based upon density of states (DOS) and crystal orbital Hamilton population (COHP) analyses. The solid-state 7Li NMR spectrum measurement proves the existence of two independent interstitial Li positions at two distinctive octahedral sites in La15Ge9Li1.50(16).
Nam, Gnu,Choi, Woongjin,Lee, Junsu,Lim, Seong-Ji,Jo, Hongil,Ok, Kang Min,Ahn, Kyunghan,You, Tae-Soo ACS AMERICAN CHEMICAL SOCIETY 2017 Inorganic Chemistry Vol.56 No.12
<P>The Zintl phase solid-solution Ca11-xYbxSb10-yGez (0 <= x <= 9; 0 <= y <= 3; 0 <= z <= 3) system with the cationic/anionic multisubstitution has heen synthesized by molten Sn metal flux and arc-melting methods. The crystal structure of the nine title compounds were characterized by both powder and single-crystal X-ray diffractions and adopted the Ho11Ge10 type structure with the tetragonal space group 14/mmm (Z = 4, Pearson. Code tI84). The overall isotypic structure of the nine title compounds can be illustrated as an assembly of three different types of cationic polyhedra sharing faces with their neighboring polyhedra and the three-dimensional cage-shaped anionic frameworks consisting of the dumbbell-shaped Sb-2 units and the square-shaped Sb-4 or (Sb/Ge)(4) units. During the multisubstitution trials, interestingly, we observed a metal-to-semiconductor transition as the Ca and Ge contents increased in the title system from Yb11Sb10 to Ca9Yb2Sb7Ge3 (nominal compositions) on the basis of a series of thermoelectric property measurements. This phenomenon can be elucidated by the suppression of a bipolar conduction of holes and electrons via an extra hole-carrier doping. The tight-binding linear muffs-tin-orbital calculations using four hypothetical structural models nicely proved that the size of a pseudogap and the magnitude of the density of states at the Fermi level are significantly influenced by substituting elements: as well as their atomic sites in a unit cell. The observed particular cationic/anionic site preferences, the historically known abnormalities of atomic displacement parameters, and the occupation deficiencies of particular atomic sites are further rationalized by the QVAL value criterion on the basis of the theoretical calculations. The results of SEM, EDS, and TGA analyses are also provided.</P>
남근우 ( Gnu Nam ),이연희 ( Yeonhee Lee ),김인자 ( Inja Kim ),전다영 ( Dayoung Jeon ),염혜성 ( Hyeseong Yeom ),신창현 ( Changhyun Shin ),김재영 ( Jae-young Kim ) 한국환경농학회 2023 한국환경농학회 학술대회집 Vol.2023 No.0
본 연구는 실제 암모니아 누출에 따른 화학사고 영향범위를 평가하기 위해 OCA (Off-site consequence analysis) 분석에 따른 사고시나리오 별 영향범위를 평가하고, 사고 반경 내 농작물의 암모니아 오염도를 측정함으로써 암모니아 누출에 화학사고로부터 발생될 수 있는 피해영향범위를 도출하고자 하였다. KORA (Korea off-site risk assessment supporting tool) 소프트웨어를 이용하여 사고영향범위를 분석한 결과, 최악의 사고시나리오는 사고지점으로부터 1.6 km, 대안의 사고시나리오는 666 m의 피해예측 범위로 산정되었고, ALOHA (Areal location of hazardous atmospheres) 소프트웨어 구동 결과는 약 2.4 km를 나타내었다. 한편, 사고지역 당일 풍향(남동풍)에 따라 가시적 농작물 피해범위는 67 m 내외로 확인되었고, 가시적 피해범위 내 고구마 잎의 암모늄 농도는 69.6~218.0 mg/kg 범위로 가시적 피해가 확인되지 않은 지점의 고구마 잎(12.9~23.5 mg/kg)보다 최대 16.9배 높게 나타나는 경향이었다. 반면 OCA 결과를 반영하여 최악의 사고시나리오 내 자생하는 쑥과 대안의 사고시나리오 내 자생하는 쑥을 채취하여 암모늄 함량을 조사한 결과, 62.2~77.8 mg/kg, 49.0~67.8 mg/kg 범위로 가시적 피해지점보다 낮은 경향이었으나, 대체적으로 암모늄 잔류 여부가 확인되었다. 이러한 결과는 가시적 피해를 입은 지점과 인근 주변 환경의 초기 오염도 확인 판단에 도움이 될 지표로 판단되었다. 이와 같은 실제 화학사고 사례연구를 지속적으로 수행하여 기초자료를 축적하고, 나아가 실제 사고 피해 영향범위를 판단하는 기준을 마련하는 연구가 지속적으로 진행되어야 할 것으로 판단된다.