Physical Properties Of Pristine And Potassium-doped P-oligophenyls

p-oligophenyls（POPs）molecules all have the structure with a certain number of phenyl rings connecting at the para positions.Due to the special structure and excellent properties,these materials play very important roles in the fields of industry and scientific research.Recently,discovery of the superconductivity with the transition temperatures between 7 K and 120 K in the potassium doped p-terphenyl has aroused plenty of interest in exploring high temperature organic superconductors.The early studies focused on the structure of the POPs discovered that the molecular conformation and structure of POPs are generally changed at low temperature.The torsion Angles between the benzene rings in the POP molecule reappears at low temperature,and the structure transforms from triclinic to monoclinic.However,there are still some deficiencies in the current researches.The exact structural transition temperatures of these POPs still need to be confirmed,and the chain length effects and the influence of structural transition on the Raman vibrational properties are still unclear.On the other hand,other POPs almost have the same properties with p-terphenyl,whether the superconductivity exists in other POPs and whether the ubiquitous structural transition contributes to the occurrence of superconductivity are still shrouded in mystery.Therefore,study on the POP materials is very desired.In this paper,our work includes the following three parts:（1）Raman scattering spectra of benzene,biphenyl,p-terphenyl,p-quaterphenyl,p-quinquephenyl,and p-sexiphenyl at ambient conditions are presented.The vibrational properties in a frequency range from lattice vibrations to the high frequency of the C-H vibrations are analysed in detail.The vibration modes result from the interaction among benzene rings in POPs can be clearly distinguished with comparing to the benzen.The two indicators for estimating the molecular conjugation in POPs are examined here.In addition,we find that the frequency of the lowest energy peak also shows regular changes with increasing molecular chain length.Thus,this mode can also be used to indicate the molecular conjugation.（2）Raman spectra of POPs from benzene to p-sexiphenyl at different temperatures are obtained.We find that the structural transition in POPs has great influence on the internal Raman vibrational properties.For biphenyl with" displacive" type transition,the Raman spectra show several new peaks below transition temperature and these modes blue shift with decreasing temperature.Forp-terphenyl and other materials with"ordering-disorder" type transition,no any new peaks appear at low temperature.Such a phenomenon is also the difference between these two structural transition types.Almost all vibration modes show significant anomalies in the frequency,line width,and intensity below the transition temperature,especially the lowest energy peak,1220 and 1280 cm^-1 peaks,and the two peaks near 1600 cm^-1.Therefore,these peaks can be used to be indicators to estimate the structure changes of POPs at low temperature.In addition,we also find that the degree of the anomalies of the internal vibration modes gradually weakens with increasing chain length.This indicates that the torsion angles of the POP molecules gradually decrease with increasing chain length.A phase diagram of the melting point and the structural transition temperature is also established.They all increase with increasing chain length,and the extrapolated melting point for the polymer with infinite chain length is greater than 1000 K,and the transition temperature is about 350 K.（3）Potassium doped biphenyl and p-terphenyl are synthesiezed.After the magnetic susceptibility measurements,we find superconducting phases with the transition temperature of 7.2 K in the potassium doped biphenyl,3.5 K and 7.2 K in the potassium doped p-quaterphenyl,and a weak diamagnetic signal at 120 K in the pestled p-quaterphenyl.The Raman scattering spectra of these materials are also measured,and the vibrational modes related to the bipolar are observed in those samples.The forma-tion of the bipolar can be used to explain the occurrence of the superconducting cooper pairs in those samples.In addition,the structures of the superconducting samples are performed,as well as some relevant theoretical calculations.These superconducting phases in these materials are suggested to be K₂C₁₂H₁₀ and K₂C₂₄H₁₈,respectively.