Anomalous Structural Variation And Electrical Transport Behaviors In Compressed Zn₂SnO₄: Effect Of Interface

Nano/submicro-meter materials have a great deal of grain boundary elements andthey are derived from the surface of ultrafine particles in the process of materialformation. The grain boundary is similar with the structure of the gas and has highactivity and mobility. For these reasons, these materials have many novel properties.Interface effect has been recognized as one of the most important factors that arestrongly correlated with the properties of nano/submicro-meter materials and affectthe transformation of the materials under pressure. However, thus far, to characterizethe sample interface changes and the related properties under high pressure is still achallenging work because of the experimental restrictions. In this paper, we proposeda strategy by combing in situ high pressure techniques with transmission electronmicroscopy measurements which can be successfully used to characterize theinterface changes of materials, and establish a link between the interface and thestructural transformation and electrical transport behaviors.Here, we choosed tetrakaidecahedral Zn2SnO4single crystals as research objects.Then, we employed alternating current impedance based on diamond anvil cell tostudy the electrical transport behaviors of Zn2SnO4under pressure. At the same time,we employed X-ray diffraction measurements and transmission electron microscopyto reveal the interface effect on the structural and electrical transport behaviors of thematerial under pressure. Two important results have been obtained from our study.1. By X-ray diffraction measurements, we find that tetrakaidecahedral Zn2SnO4single crystals undergo a new phase transition pathway under pressure.In thispaper, Zn2SnO4takes place a structural transition from cubic to hexagonalstructure at29.8GPa. These conclusions are different from the results of recenttheoretical and experimental studies. In our high pressure XRD experiments,Zn2SnO4single crystals have a tetrakaidecahedral morphology with the average particle size of~800nm whose crystal size and morphology aredifferent from the samples in recent theoretical and experimental studies. Thus,we think that the differences between our experimental results and othertheoretical and experimental results are caused by the different crystal size andmorphology of the samples. This comparison also provides a direct proof aboutthat the initial crystal size and morphology of the materials can affect the phasetransition.2. In situ high pressure alternating current impedance measurement and HRTEMobservations on the decompressed samples demonstrate that the interfacechanges can be characterized upon compression, which explains the anomalousresistance changes observed in the sample under pressure. Through theseexperiments,we find that three obvious changes in the slope of the pressuredependence of the total resistance can be clearly seen at9.0,18.7and30.6GPa,respectively. In the pressure range of0~12.5GPa, there are two overlappedsemicircles on the complex impedance planes. This shows that the bulk and thegrain boundary both contribute to the total resistance of Zn2SnO4under theeffect of pressure. However, only the arcs of bulk stay there after12.5GPawhich indicates that bulk conduction dominates the electrical transportproperties of Zn2SnO4and the grain boundary effect disappears. By HRTEMexperiments, we find that grain refinement takes place between8.0GPa to13.0GPa. This also clarifies that the transition at12.5GPa is related to the grainrefinement of the sample, but not a structural transition as reported in a recentliterature.We believe our strategy can also be extended to uncover the granular interfacechanges that might hide in the high pressure studies on other materials, and furthershed light on the underlying mechanism for the related transitions.