Phase stability of AlYB₁₄sputtered thin films

Abstract

AlYB(14) (Imma) thin films were synthesized by magnetron sputtering. On the basis of x-ray diffraction, no phases other than crystalline AlYB(14) could be identified. According to electron probe microanalysis, energy dispersive x-ray analysis and elastic recoil detection analysis, the Al and Y occupancies vary in the range of 0.73-1.0 and 0.29-0.45, respectively. Density functional theory based calculations were carried out to investigate the effect of occupancy on the stability of Al(x)Y(y)B(14) (x,y = 0.25, 0.5, 0.75, 1). The mean effective charge per icosahedron and the bulk moduli were also calculated. It is shown that the most stable configuration is Al(0.5)YB(14), corresponding to a charge transfer of two electrons from the metal atoms to the boron icosahedra. Furthermore, it is found that the stability of a configuration is increased as the charge is homogeneously distributed within the icosahedra. The bulk moduli for all configurations investigated are in the range between 196 and 220 GPa, rather close to those for known hard phases such as α- Al(2)O(3).

Bonding and elastic properties of amorphous AlYB

“…The calculated bulk modulus is 195 GPa and all elastic constants are as follows: C 11 ¼ 399 GPa, C 12 ¼93 GPa and C 44 ¼154 GPa. As the bulk modulus for crystalline Al x Y y B 14 (x, y¼0.25, 0.5, 0.75, 1.00) was reported to be in the range 196-220 GPa [8], the obtained value is relatively large for an amorphous counterpart, but acceptable. We can also estimate the degree of isotropy and hence assess self-consistently if our supercell is sufficiently large to describe this amorphous solid properly.…”

“…Bulk modulus of AlMgB 14 was measured to be in the range of 196-264 GPa [6]. Kölpin et al performed ab initio calculations on XMgB 14 (X¼ Al, C, Si, Ge, Mg, Sc, Ti, V, Zr, Hf, Nb, Ta) [7] as well as Al x Y y B 14 (x, y¼0.25, 0.5, 0.75, 1.00) [8] and reported bulk moduli in the range from 196 to 228 GPa. Similar values were calculated for AlMgB 14 and (Al,Si) MgB 14 [9,10].…”

Correlative theoretical and experimental investigation of the formation of AlYB14 and competing phases

“…10 The influence of such phases, with similar chemical composition to B 14 -phases, on the synthesis of B 14 -phases has not been investigated in a systematic way. Besides synthesis in bulk form 16,17 also thin film synthesis of icosahedral boron-rich solids 15,18 has been reported. The icosahedrally bonded borides should not be confused with other icosahedral phases such as quasicrystals.…”

“…So far, only post-annealing of films with the composition of AlMgB 14 resulted in the formation of crystalline AlMgB 14 at temperatures of 1000 C. 15 Tian et al observed three distinct peaks at 13.85 , 27.89 , and 42.37 corresponding to (011), (022), and (033) planes, respectively. 15 For Al-Y-B, direct synthesis of crystalline AlYB 14 thin films at a substrate temperature of T S ¼ 800 C was reported by K€ olpin et al 18 In that work, a combination of direct current (DC) and radio frequency (RF)-MS was used, but the mechanisms which trigger the growth of crystalline AlYB 14 remain unclear. 18 The phase formation was reported based on the presence of 6 diffraction peaks which are consistent with the AlYB 14 structure: 25 18 Yan et al 27 reported the formation of AlMgB 14 nanocrystals in an amorphous Al-Mg-B matrix in the as deposited state at a substrate temperature of 200 C based on diffraction data reported in 2013.…”

“…15 For Al-Y-B, direct synthesis of crystalline AlYB 14 thin films at a substrate temperature of T S ¼ 800 C was reported by K€ olpin et al 18 In that work, a combination of direct current (DC) and radio frequency (RF)-MS was used, but the mechanisms which trigger the growth of crystalline AlYB 14 remain unclear. 18 The phase formation was reported based on the presence of 6 diffraction peaks which are consistent with the AlYB 14 structure: 25 18 Yan et al 27 reported the formation of AlMgB 14 nanocrystals in an amorphous Al-Mg-B matrix in the as deposited state at a substrate temperature of 200 C based on diffraction data reported in 2013. 28 Yan et al utilized an AlMg compound target which was operated in DC mode and two B targets which were according to the authors sputtered using 350 kHz pulses.…”