High-Pressure Synthesis and Crystal Growth, Characterization and Thermodynamic Properties of Iron-Pnictide Superconductors and Related Systems

这一项目的重点是合成高质量的铁磷酸盐和相关超导体样品，并实现对其结构、磁性和超导性质的详细了解。将特别强调高压合成-一种在这类超导体中获得高T_c材料的有前途的技术-以及最高分辨率的热力学研究，如比热、热膨胀和超声波传播。后两种技术以及电阻率测量可以与流体静压(4He-气体)相结合。为了实现这些目标，我们将沿着紧密相连的主要路线前进：i)常压和高压(AP/HP)合成以及晶体生长；ii)材料表征以及在常压和静水压力下对选定化合物的热力学和传输测量。本论文的目的是在高压/高温条件下合成LnFeAsO_(1-x)Fy(Ln=La，Ce，Tb和Dy)的单相样品，在AP-和HP-条件下生长AeFe2As2(Ae=Ca，Ba)及其掺杂变体和FeSe1-x样品。在ii)中，将通过使用标准技术，如X射线衍射、电阻率和磁测量来进行结构和物理表征。这些结果是优化合成条件不可缺少的输入参数。将在选定的高质量材料上对它们的热力学性质进行详细研究。特别是，由于高压制备将对结构、磁性和超导性质产生不同的影响，这些研究旨在更深入地了解这些性质的相互作用。

The focus of this project rests on the synthesis of high-quality samples of Fe-pnictide and related superconductors, and the achievement of a detailed understanding of their structural, magnetic and superconducting properties. Special emphasis will be placed on high-pressure synthesis – a promising technique for obtaining high-Tc materials in this class of superconductors -, and highest-resolution thermodynamic studies, such as specific heat, thermal expansion and ultrasound propagation. The latter two techniques as well as resistivity measurements can be combined with hydrostatic (4He-gas) pressure. To reach these goals, we will proceed along to main lines, which are closely interconnected: i) Ambient- and high-pressure (AP/HP) synthesis as well as crystal growth; ii) Material characterization as well as thermodynamic and transport measurements at ambient and hydrostatic pressure on selected compounds. In i) the aim is to synthesize monophasic samples of LnFeAsO1-x Fy (Ln = La, Ce, Tb and Dy) at HP/high-temperature (HT) conditions; AP- and HP-crystal growth of AeFe2As2 (Ae = Ca, Ba) and their doped variants and samples of FeSe1-x both at AP- and HP-conditions. In ii), the structural and physical characterization will be performed by employing standard techniques, such as X-ray diffraction, resistivity and magnetic measurements. These results are indispensable input parameters for optimizing the synthesis conditions in i). Detailed studies of their thermodynamic properties will be carried out on selected high-quality materials. In particular, since the HP preparation will affect the structural, magnetic and superconducting properties differently, these studies aim at gaining a deeper insight into the mutual interplay of these properties.