On the Study of Ca and Mg Deintercalation from Ternary Tantalum Nitrides

Layered CaTaN₂ and MgTa₂N₃ and cubic Mg₂Ta₂N₄ were prepared by direct solid state reaction from the binary nitrides Ta₃N₅ and A₃N₂ (A: Mg, Ca). CaTaN₂ showed a slight Ca deficiency (0.11 moles per formula), and a monoclinic distortion from previously reported R3̅m symmetry, with space group C2/m and cell parameters a = 5.4011(2), b = 3.1434(1), c = 5.9464(2) Å and β = 107.91(3)°. Ca²⁺ and Mg²⁺ deintercalation was investigated in the three compounds both chemically and electrochemically. No significant Mg²⁺ extraction could be inferred for MgTa₂N₃ and Mg₂Ta₂N₄, neither after reaction with NO₂BF₄ nor after electrochemical oxidation at 100 °C in alkyl carbonate electrolytes. Rietveld refinement of the X-ray powder diffraction pattern of chemically oxidized Ca_0.89TaN₂ indicates a decrease of the Ca content to 0.34 concomitant to the disappearance of the monoclinic distortion and expansion of the interlayer space from 5.658 to 5.762 Å, space group R3̅m and cell parameters a = 3.1103(1) and c = 17.287(1) Å. Deintercalation in this compound was also achieved electrochemically at 100 °C. Results of density functional theory calculations seem to indicate that reaction mechanisms for CaTaN₂ oxidation additional and/or alternative to deintercalation are taking place, which is likely related to the loss of crystallinity observed upon oxidation and the irreversibility of the process.