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

[Image: see text] Layered CaTaN(2) and MgTa(2)N(3) and cubic Mg(2)Ta(2)N(4) were prepared by direct solid state reaction from the binary nitrides Ta(3)N(5) and A(3)N(2) (A: Mg, Ca). CaTaN(2) 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)°. Ca(2+) and Mg(2+) deintercalation was investigated in the three compounds both chemically and electrochemically. No significant Mg(2+) extraction could be inferred for MgTa(2)N(3) and Mg(2)Ta(2)N(4), neither after reaction with NO(2)BF(4) nor after electrochemical oxidation at 100 °C in alkyl carbonate electrolytes. Rietveld refinement of the X-ray powder diffraction pattern of chemically oxidized Ca(0.89)TaN(2) 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 CaTaN(2) 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.