Ex-situ characterization of MnAs after hydrostatic pressure treatment

Manganese arsenide (MnAs) has a unique sequence of phase transitions between its alpha hexagonal and (1 orthorhombic crystal structures, which have been intensely studied for a long time, but there are some coupled issues that still require further understanding. In this work, we investigate the structural and transport properties of bulk compound MnAs after hydrostatic pressure treatment using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy, and resistivity measurements. Before the application of high pressure, a large amount of the polycrystalline MnAs samples was grown under different technological conditions. In addition to the main low-temperature hexagonal alpha phase, a varying amount (up to 5 %) of the orthorhombic (1 phase was detected in the majority of samples. Only one group of samples contained a pure single hexagonal structure. The lattice parameters of the ex-situ XRD studied samples at different pressure reliefs are near the pristine MnAs, and no XRD peaks are associated with a new phase appearance. On the contrary, the high-pressure resistivity behavior indicates a partial recovery of the main hexagonal structure during decompression. SEM images of the depressurized samples indicate densification of the grain boundaries, but with a sufficient deterioration in their quality. The observed results support the reversible scenario of the magnetostructural transitions in MnAs at pressures up to 8.5 GPa, however, the discrepancy between the XRD and resistivity data can be understood in terms of the development of visual defects (cracks) in the pressurized samples.