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

Abstract

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 <i>R</i>3̅<i>m</i> symmetry, with space group <i>C</i>2/<i>m</i> and cell parameters <i>a</i> = 5.4011(2), <i>b</i> = 3.1434(1), <i>c</i> = 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 <i>R</i>3̅<i>m</i> and cell parameters <i>a</i> = 3.1103(1) and <i>c</i> = 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.