The Structural And Electrical Transport Properties Study Of SnS, SnTe, Ln₂Se₃and Alq₃under High Pressure

In this paper, by combing the high-pressure synchrotron X-ray diffraction,Raman spectra, electrical transport properties measurements and first-principlescalculations, we have studied systematically the structural and electrical transportproperties of compressed SnS, SnTe, In2Se3and Alq3, to reaveal the related structuraltransition, charge carriers transport mechanism, and the correlation between structuralmodification and abnormal changes of the electrical transport properties, whichestablishe general guidelines in designing or optimizing new practical applications ofthese materials. Detailed experimental and theoretical observations are as following:1. The electrical transport properties of SnS under high pressure has beeninvestigated by the temperature dependence of electrical resistivity measurements,Hall-effect measurements and First-principles calculation, to make in situobservations of the pressure effect on the electrical resistivity, Hall coefficient,charger carriers concentration and free charge carriers mobility. The experimentalresults show that SnS undergoes a semiconductor to semimetal transition at about10.3GPa, and this transition is further substantiated by the First-principles bandstructure calculations. The total and partial density of states results indicate that thesemimetal character of SnS is attributed to the gradually enhanced coupling of theSn-5s, Sn-5p, and S-3p states under compression. With further compression up to12.6GPa, dramatic changes in electrical transport parameters such as the electricalresistivity, the carrier concentration and the carrier mobility are observed, which canbe seen as a concequence of the pressure-induced Pnma-Cmcm structural transition ofSnS. With application of pressure, the lattice parameters of SnS contractanisotropically, which results in the abnormal variation of the Sn-S bonding, andfurther drives the three-coordinated Pnma to five-coordinated Cmcm structural transition.2. By combing the high-pressure Synchrotron X-ray diffraction, pressuredependence of resistivity, and temperature dependence of resisitivity measurements,we have studied the structural and electrical transport properties of SnTe undercompression. SnTe transforms from Fm-3m structure to an intermediate Pnmastructure at~3.7GPa, and then to a new cubic structure with Pm-3m symmetry at31.8GPa. The resistivity of SnTe varies anormalously along with the structural transition.SnTe behaves as a metal at ambient pressure because of its topological insulatingcharacter which drives the overlapping between the valence-band maximum andconduction-band minimum. At high-pressure Pnma and Pm-3m phases, SnTe keepsits metallic character unaffected, which provides fundamental guideline for scientistsof theoretical simulation field to study the the electronic structure of SnTe undercompression.3. We have performed the high-pressure Synchrotron X-ray diffraction, Ramanspectra, and electrical transport properties measurements, in combination of the First-principles theoretical calculations to gain further insight into the structural and chargecarriers transport properties of In2Se3under compression.(1) At room conditions, structure with R-3m symmetry is more suitable than thatwith R3m symmetry for the ambient In2Se3. At~0.8GPa, In2Se3transforms to a newisosymmetric R-3m structure, whilst the volume collapses by~7%. High-pressureRaman spectra measurements further confirm this abnormal isosymmetric structuraltransition. An obvious blue shift in the A1gmode at104cm-1(related to the vibrationsof one layer against the others along c-axis) is observed along with the structuraltransition, which indicates that the interlayer interaction is significantly enhanced inthe high-pressure phase. First-principles calculations results show better agreementwith our experimental observation, and further retionalise the structural transitionfrom the change of the total energy of system. This isosymmetric structural transitioncan be attributed to the relative sliding of the In-Se atoms plane with respect toadjacent planes, in which Se atoms within the sliding plane sit on top of the Se atomsof the adjacent planes before the transition, and slide to the interstitial sites of the Se atoms of neighboring layers along with the structural transition. As the sheets glidewith respect to one another, the sites of Se atoms are replaced by the nearest In atoms,so that the symmetry of In2Se3can be well protected.(2) Within continuous compression up to34.3GPa, a new structure (Phase III)takes form. Rietveld refinement with GSAS software shows that Phase III is found tohave a body-centered cubic (bcc) structure. Furthermore, the structure can be betteridentified with I-43d symmetry with a low Rwpfactor for Phase III. That is In2Se3successfully transforms from two-dimensional layerd structure to three-dimensionalcubic structure with applying pressure.(3) The temperature dependence of resistivity of In2Se3at representativepressures show that, at0.4GPa, the temperature derivative of resistivity (dρ/dT) ofIn2Se3is positive, which is indicative of metallic behavior. At pressure above1.6GPa,a significant decrease in ρ is observed, and the ρ-T curves have negative slopes, dρ/dT <0, characteristic of a semiconductor. However, at40.2GPa and above, ρ showsa weak positive temperature coefficient again (dρ/dT>0), indicating that In2Se3transforms to a new metallic state above40.2GPa. The electrical transport behavior isan integrated result between the in-layer metallic and cross-layer semiconductivestates. Because of the large anisotropy between the in-plane and cross-planeconductivity that the in-plane conductivity is about2-3orders of magnitude higherthan the cross-plane conductivity, σ (T) of In2Se3should be affected significantly byin-plane conductivity (σ∥(T)). Consequently, In2Se3displays a metallic character inPhase I. In Phase II, the electron interaction between adjacent In-Se layers isstrengthened substantially with increasing pressure. The gradually enhanced interlayerinteraction can decrease the carrier transport energy barriers between layers, henceimproving the cross-plane conductivity largely and becomes comparable with thein-plane conductivity. In this case, the cross-layer conductivity increasesexponentially with temperature, which can only be partially compensated by thedecreased in-layer metallic conductivity caused by stronger carrier scattering withincreasing temperature. As a consequence, In2Se3behaves as a semiconductor inphase II. The carrier transport activation energy Egapproaches to zero, as increasing pressure, implying that further increase in carrier concentration would be small, andhence In2Se3shows metallic character. It indicates that In2Se3transforms to a newmetallic state that exhibits metallic character not only in the a-b plane but also acrossthe plane, i. e. a3D metallic state.4. By combing the high-pressure synchrotron X-ray diffraction, Raman spectra,and alternating current impedance spectroscopy measurements, we have investigatedthe structural and electrical transport properties of Alq3under compression. Theexperimental results indicate that, below16.1GPa, there is no obvious variation onthe patterns of Alq3except the relative peak broadening and the collective movementsof all reflection peaks toward lower d-spacing. Above16.1GPa, Alq3becomesamorphous, and the amorphous process is reversible in the compression below17.4GPa but irreversible if compressed to higher pressure, e. g.23.8GPa. At low pressure,the crystalline topology (including the atomic coordination and bonding) is preserved,which retains a “memory” of its original crystal structure and can revert to it. Uponfurther compression, the relative higher density amorphous state can be achievedwhich involves some bonding breaking, and the broken bonding cannot recover to itsoriginal state in decompression. Consequently the pressure-induced amorphizationwill be irreversible. Raman spectra observations reveal that the Al-oxine interactionplays an important role on the electrical transport properties of Alq3. That is besidesthe contribution of π-π orbital overlaps, the Al-oxine interaction is believed alsoimportant on the transport properties on dense Alq3, which provides essential insightinto the correlation between structural modification and electrical properties, andestablishes general guidelines in designing or optimizing new Alq3-based applications.