Structural studies of rare-earth activated cubic boron nitride micropowders

Abstract

Rare-earth (RE) activated cBN-based materials were produced as cBN–Tb, cBN–Ce, cBN–Eu, and cBN–Tm micropowders with 0.1 at.% RE in HPHT conditions. Their X-ray diffraction spectra are compared with those of a standard unactivated cBN micropowder. It is established that structurally cBN–RE materials are interstitial solid solutions based on interstitial solid solutions because there is initial distortion in the unactivated cBN due to its nonstoichiometry. The atomic displacements in the cBN crystal lattice caused by interstitial RE ions are calculated from XRD reflection intensities. The displacements correlate with the cBN lattice distortions evaluated from the regularly nonlinear relationship between the cBN–RE lattice parameters (calculated for each reflection) and the Nelson–Riley function, unlike the unactivated cBN micropowder where this relationship is linear. The degree of nonlinearity depends on the RE content of cBN and the size of RE ions and can correlate with larger shifts of the (111), (220), and (331) reflections toward smaller angles, compared with the shift of (331). The XRD and Raman spectra of cBN micropowders with 0.1 at.% RE of the same kind reveal no stacking faults in the crystal lattice. However, it is not improbable that such faults might form in cBN in which the concentration of RE of one kind is higher than 0.1 at.% or there are different kinds of RE ions. The latter is supported by the TO–LO splitting in the Raman spectra of cBN–Ce–Tb (~0.1 at.% RE) micropowders, which has never been observed in cBN. The RE concentration in cBN is no higher than 0.1 at.%, which is less than that (10 at.% Er) in cBN nanopowders achieved so far by another scientific group using the plasma method. Therefore, studies aimed at obtaining light-emitting materials based on cBN will be continued to reach higher luminescence of RE ions in cBN. Since no data have been previously provided on the crystal structure of light-emitting materials based on RE-activated cBN, it is concluded that structurally new cBN-based materials have been produced.