Valence-ordering structures and magnetic behavior of metallic MMX chain compounds
Valence-ordering structures and magnetic behavior of metallic MMX chain compounds
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DOI：
10.1002/1521-3773(20020802)41:15
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发表时间：
2002-01-01
影响因子：
16.6
通讯作者：
Mitani, T
中科院分区：
文献类型：
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作者：
Mitsumi, M;Kitamura, K;Mitani, T
图1. MMX链状化合物的电子和晶格结构的示意图，其中电子占据Mdz 2轨道。低于80 K的温度被认为是ACP状态。然而，尚未观察到该系统的自旋自由度的损失。最近，我们研究了[{Pt 2（EtCS 2）4 I} I]的晶体结构和固态性质，发现在TM±S ~（205）K处发生了金属±半导体（M±S）跃迁，并在此温度以上观察到桥连碘原子AV态的显著热振动。[4]该化合物显示出漫散射，对应于高于TM±S的Pt-Pt-I单元的两倍重复长度。漫散射在低于140 K时开始转变为超晶格反射。这些超晶格反射被认为起源于CDW或ACP状态。然而，自旋自由度持续下降到2K。报道了一种新型金属MMX链状化合物[{Pt 2（nBuCS 2）4 I} I]（1）的结构相变、价序结构和磁性，以及它的输运性质和一种有效的化学合成方法。该化合物明显地表现出磁化率的突然下降，类似于自旋-Peierls转变，伴随着约210 K的一级相变。我们还进行了[{Pt 2（EtCS 2）4 I} I]（2）在48 K的晶体结构分析，包括超晶格反射，以确定其在低温相中的价序结构。本工作阐明了1和2的晶体结构和磁性之间的相关性。通过缓慢冷却等摩尔量的[Pt 2（nBuCS 2）4]和[Pt 2（nBuCS 2）4 I2]的甲苯±正己烷溶液来生长1的黑色针状晶体。[5]在153±443 K的温度范围内进行了1的差示扫描量热法（DSC）测量。在204±212 K和318±323 K温度范围内观察到对应于一级相变的两个潜热峰，表明存在低温（LT）、室温（RT）和高温（HT）三相。化合物1在室温下表现出相对高的电导率（17±83 S cm ² 1），与[{Pt 2（dta）4 I} I]（约1.5 S cm ² 1）的电导率相当。13 Scm <$1）[2d]和2（5±30 Scm <$1）。[4]电阻率1的温度依赖性表明在高于转变温度TM±S à 325 K的HT相中金属导电。LT和RT相显示半导体行为，激活能分别为134和255 meV。在200±400 K的温度范围内测量了热电势S。HT相显示出几乎与温度无关的S（<$10 mVK <$1）行为，这表明存在半填充金属带。[6]在TM±S ~ 325 K以下，S随温度的降低而略有下降，在270 K附近达到最小值<$16 mVK <$1，然后，如半导体的特性一样，增加。此外，1和S在210 K附近表现出急剧增加。RT和LT阶段1的ORTEP图如图2所示。化合物1经历一级相变，
Figure 1. Schematic representation of electronic and lattice structures of the MMX chain compound, where the electrons occupy the Mdz2 orbitals. below 80 K was concluded to be an ACP state. However, no loss in the spin degree of freedom for this system has yet been observed. Recently, we studied the crystal structure and solidstate properties of [{Pt2 (EtCS2) 4I} I], and revealed a metal±semiconductor (M±S) transition at TM±S à 205 K, above which the remarkable thermal vibration of a bridging iodine atom in the AV state was observed.[4] This compound shows diffuse scattering corresponding to a twofold repetition length of the Pt-Pt-I-unit above TM±S. Diffuse scattering begins to convert into superlattice reflections below 140K. These superlattice reflections are considered to have originated from a CDW or ACP state. The spin degree of freedom, however, persisted down to 2 K. We report the structural phase-transition, valence-ordering structure and magnetic properties of a new metallic MMX chain compound,[{Pt2 (nBuCS2) 4I} I](1), as well as its transport properties and an efficient chemical synthesis. This compound clearly exhibits an abrupt drop in the magnetic susceptibility, similar to the spin-Peierls transition, accompanying a first-order phase transition at about 210 K. We have also performed a crystal structure analysis of [{Pt2 (EtCS2) 4I} I](2) at 48 K which included superlattice reflections to determine its valence-ordering structure in the low-temperature phase. This work has clarified the correlation between the crystal structures and magnetic properties of 1 and 2. Black needle crystals of 1 were grown by the slow cooling of a toluene±n-hexane solution of equimolar amounts of [Pt2 (nBuCS2) 4] and [Pt2 (nBuCS2) 4I2].[5] Differential scanning calorimetry (DSC) measurements of 1 were carried out in the temperature range of 153±443 K. Two peaks of latent heat corresponding to the first-order phase transition were observed in the temperature ranges of 204±212 K and 318±323 K, which revealed the existence of three phases, the low-temperature (LT), room-temperature (RT), and high-temperature (HT) phases. Compound 1 exhibits relatively high electrical conductivity (17±83 S cm¿ 1) at room temperature, comparable to the conductivity of [{Pt2 (dta) 4I} I](ca. 13Scm¿ 1)[2d] and 2 (5±30 Scm¿ 1).[4] The temperature dependence of electrical resistivity 1, indicates metallic conduction in the HT phase above the transition temperature, TM±S à 325K. The LT and RT phases show semiconducting behavior with activation energies of 134 and 255meV, respectively. The thermoelectric power S, was also measured in the temperature range of 200±400 K. The HT phase shows almost temperature-independent behavior of S (¿ 10 mVK¿ 1), which indicates the existence of a half-filled metallic band.[6] Below TM±S à 325 K, S slightly decreases with decreasing temperature, and reaches a minimum value of¿ 16 mVK¿ 1 near 270K and then, as is characteristic of semiconductors, increases. Furthermore, 1 and S exhibit sharp increases at around 210 K. ORTEP diagrams of 1 in the RT and LT phases are shown in Figure 2. Compound 1 undergoes a first-order phase-transi-