Study On The Electrical Transport Properties Of BaMnO₄ Under High Pressure

As an excellent material,BaMnO₄is often used as an oxidant or catalyst in organic chemical reactions;at the same time,because of the high REDOX potential of Mn O₄^2-,BaMnO₄is also used in alkaline dry cells.As a thermodynamic parameter independent of temperature and chemical composition,pressure provides a new dimension for the study of substances.Under the action of pressure,the distance between the atoms in the material will be reduced,the lattice structure will undergo transformation,physical and chemical properties will also undergo unexpected changes,producing strange phenomena that do not exist under normal pressure,therefore,pressure is the source of the creation of new materials,new theories and new concepts.In this paper,we conducted in situ studies on the electrical transport and structural properties of BaMnO₄materials under high pressure,and obtained the following results:1.The AC impedance impedance spectra of BaMnO₄were measured in the pressure range of 0-30.7 GPa,and a suitable effective circuit was selected to fit the impedance spectra.From 0 to 2.1 GPa,the impedance spectrum of BaMnO₄consists of a semi-circular arc and a straight line rising 45°,indicating that the electrical transport characteristic of BaMnO₄at this time is typical ionic conduction;when the pressure exceeds 2.1 GPa,the impedance spectrum only consists of semi-circular arc,and the imaginary part of impedance tends to zero stably with the decrease of frequency,at this time,the sample BaMnO₄turns into electron conduction.This indicates that the ion transport channel of BaMnO₄is compressed under the action of pressure,and the ions are confined to the rigid point of the crystal lattice.By analyzing the fitted equivalent circuit,it is found that the resistance decreases with the increase of pressure before 13 GPa,however,after 13 GPa,the decreasing trend of resistance becomes obviously weak.In addition,in the experimental pressure range of0-30.7 GPa,the response law of grain size and grain boundary resistance to pressure is consistent,and the grain boundary resistance is always greater than the grain resistance.2.High pressure synchrotron radiation XRD experiments were carried out on BaMnO₄in the pressure range of 0-31 GPa.Under the action of pressure,each diffraction peak of BaMnO₄moves to a higher Angle with the increase of pressure,the diffraction peak is obviously widened and the peak strength becomes weak,but,there is no new diffraction peak and no disappearance of old diffraction peak.In the pressure range of 31 GPa,the orthogonality structure of BaMnO₄is always stable.3.The spacing d of each crystal plane and cell volume of BaMnO₄under high pressure were calculated by Calculate abc V software,it is obtained that the relationship between the spacing d of the characteristic crystal faces（111）,（200）,（021）,（210）,（121）and（211）with the pressure.The results showed that before and after 13 GPa,the spacing of BaMnO₄crystal planes showed two different trends with the increase of pressure.At 0-13 GPa,the d value decreases rapidly with the increase of pressure,and the crystal structure becomes denser and denser;when the pressure exceeds 13 GPa,the decreasing trend of the crystal plane spacing is no longer obvious.This indicates that the sample BaMnO₄has an isostructural phase transition at 13 GPa.The variation trend of the spacing between the crystal faces with the pressure is exactly consistent with the variation trend of the resistance,indicating that the variation of the electrical properties of the samples is caused by the piezotropic isostructural phase transition of BaMnO₄.