The high-pressure and high-temperature behaviors of manganotantalite Mn(Ta,Nb)2O6 have been investigated by single-crystal X-ray diffraction and Raman spectroscopy combined with diamond anvil cell technique, as well as first-principle calculations. A pressure-induced reversible phase transition of manganotantalite occurs at 9.5 GPa and room temperature, accompanied by a large volume collapse (∼7.0%) and drastic color change from brownish-yellow to red. The space groups of low-pressure (LP) and high-pressure (HP) phases are the same (Pbcn), but the coordination numbers of Mn increase from six to eight and Ta increase from six to seven, respectively. The band gap becomes narrow from 2.37 to 1.59 eV. We determined the PT phase diagram of manganotantalite with a positive Clapeyron slope of dP/dT = 0.0073 GPa/K. The PV data were fitted to a second-order Birch–Murnaghan equation of state with B0 = 149(4) GPa for the LP phase and B0 = 188(3) GPa for the HP phase. The isothermal Grüneisen parameters were determined to be 0.23∼2.03 of the LP phase and 0.59∼0.86 of the HP phase for Raman modes. High-pressure behaviors of Mn(Ta,Nb)2O6 indicate that this kind of material is a potential effective pressure sensor to monitor pressure change or warn pressure abnormality.

Yungui Liu, Shengxuan Huang, Xiang Li, Haipeng Song, Jingui Xu, Dongzhou Zhang, and Xiang Wu, Inorganic Chemistry Article ASAP, DOI: 10.1021/acs.inorgchem.0c02571 abstract 

Examples of the two-dimensional XRD patterns of manganotantalite (a) at 0.3 and (b) 11.3 GPa. Indices are given for manganotantalite reflections (underlined by boxes).  Large black spots are due to diamond reflections. The diffraction rings are due to gasket (Re) reflections.