Pressure-Induced Phase Transition in Mn(Ta,Nb)2O6: An Experimental Investigation and First-Principle Study

The high-pressure and high-temperature behaviors of manganotantalite Mn(Ta,Nb)(2)O-6 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 (similar to 7.0%) and drastic color change from brownish-yellow to red. The space groups of low-pressure (LP) and highpressure (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 P-T phase diagram of manganotantalite with a positive Clapeyron slope of dP/dT = 0.0073 GPa/K. The P-V data were fitted to a second-order Birch-Murnaghan equation of state with B-0 = 149(4) GPa for the LP phase and B-0 = 188(3) GPa for the HP phase. The isothermal Gruneisen parameters were determined to be 0.23 similar to 2.03 of the LP phase and 0.59 similar to 0.86 of the HP phase for Raman modes. High-pressure behaviors of Mn(Ta,Nb)(2)O-6 indicate that this kind of material is a potential effective pressure sensor to monitor pressure change or warn pressure abnormality.