Structure And Physical Properties Of Several Layered Compounds Containing Sn(Si)

The layered compounds,owning the flexible structure and abundant physical properties,usually show excellent performances in the heat,electricity,light,magnetism and other aspects and have a variety of potential applications.Exploring new layered compounds will help us to improve these properties and reveal physical mechanisms behind the observed phenomenas in the materials.In this dissertation,several new layered compounds with SnAs layers were designed and their low thermal conductivities were revealed,which provides a new idea for the discovery and design of new materials with low thermal conductivities.In addition,doping effects of V and Cr on layered compounds with AlSi layers are studied comparatively to promote the understanding of the layered superconductors.The main results are as follows:Firstly,the compounds containing double lone pair electrons as NaSnAs and NaSnP are reported at the first time and physical mechanisms behind their low thermal conductivities were revealed.The measured minimum thermal conductivities of narrow band gap semiconductors NaSnAs and NaSnP are 0.62 W/m/K and 0.58 W/m/K,respectively,which are close to the calculated minimum.Especially,their thermal conductivities are about 40% lower than that of NaSn₂As₂,which has similar SnAs and Na structural layers and less lone pair electrons.It is revealed NaSnAs and NaSnP have double lone pair electrons is the origin of their low conductivities.Double lone pair electrons induces stronger anharmonic vibration,higher Grüneisen parameters ??? and lower thermal conductivity.These findings provide ways for exploring new materials with low thermal conductivities.Secondly,a new compound Li_1.67Sn_0.33 As was designed to study the relationship between its structure and the property of low thermal conductivity.Two different structures of Li_1.67Sn_0.33 As compounds were designed.Their space groups are Fm3???m and Ia3???,corresponding to the high temperature phase and low temperature phase of Li_1.67Sn_0.33 As.All of them exhibit semiconductor behavior with carrier activation energies of 0.93 e V and 1.04 eV,respectively.At the temperature of 500 K,the lattice thermal conductivities of these two phases are 0.91 W/m/K and 1.00 W/m/K,respectively.The intrinsic low thermal conductivities of these two materials are attributed to the random occupation of lightatoms?Liatoms?and heavyatoms?Snatoms?at the same sites in the lattice.It is found that the higher degree of random occupation of Liatoms and Snatoms,the stronger the phonon scattering and the lower lattice thermal conductivity.In addition,a new compound Li1.76Sn0.24 As was found with adjusting Li and Sn ratios.Its space group belongs to Pa3??? and its lattice thermal conductivity at 500 K is 0.78 W/m/K.Thirdly,doping effects of the physical decision layer and the carrier layer of NaSn₂As₂ were studied.On the one hand,the doping experiments of Patoms shows the maximum doping amount of P is about 6%.NaSn₂As₂-xPx compounds show metal behavior and their resistivities increase with the increasing doping amount.Compounds show paramagnetism and no superconductivity.On the other hand,Sratoms can be introduced into the Na-positions of NaSn₂As₂,forming the continuous solid solution compounds.They are paramagnetic metal materials.With the increasing amount of Sratoms,the carrier type changes from holes to electrons,which may be due to its multiple band structure at the Fermi level and the introduction of electrons with doping.Here,tuning chemical pressure and carrier concentration failed to induce the superconductivity of NaSn₂As₂.Finally,a comparative study of doping effects of V and Cr on SrAlSi superconductor were performed.The actual solid solution limits of V and Cr in SrAlSi are 16 at.% and 13 at.%,respectively.Particular attention was paid on the carrier concentrations and the variation trends of Tc of SrAl1-xMx Si with doping.In SrAl1-xVx Si,V doping decreases the carrier concentration by about three orders of magnitude and leads to the quenching of superconductivity at a V dopant level of x = 0.2.The lower N?EF?is responsible for the decreased Tc onset and the quenched superconductivity in SrAl1-x VxSi.In contrast,Cr doping does not significantly change the carrier concentration with increasing x till to x = 0.3,which results in almost the same value of N?EF?of SrAlSi and makes the Tc onset decrease only 0.6 K.These results reported here indicate the valence electrons of transition metal dopants have a profound effect on superconductivity and will offer clues to better understanding the doping effect on superconductors and other related functional materials.