11B and 89Y solid state MAS NMR spectroscopic investigations of the layered borides YTB4 (T = Mo, W, Re)

Abstract

The YCrB₄ type borides YTB₄ (T = Mo, W, Re) were synthesized from the elements by arc-melting and subsequent annealing. The structures were refined from single crystal X-ray diffractometer data: Pbam, a = 602.84(8), b = 1164.5(1), c = 361.20(4) pm, wR₂ = 0.0404, 624 F² values, 26 variables for YMoB₄, a = 603.00(7), b = 1165.1(1), c = 360.63(6) pm, wR₂ = 0.0487, 474 F² values, 26 variables for YWB₄, and a = 596.67(6), b = 1154.4(1), c = 360.21(4) pm, wR₂ = 0.0465, 544 F² values, 26 variables for YReB₄. The boron atoms build up planar networks which are a tessellation of slightly distorted pentagons and heptagons. Adjacent networks coordinate the transition metal and yttrium atoms in the form of pentagonal and heptagonal prisms, respectively. The four crystallographically distinct boron sites are differentiated by high-resolution ¹¹B solid state isotropic magnetic shifts and nuclear electric quadrupolar coupling constants. Partial site assignments are possible based on comparisons with electric field gradient calculations using the WIEN2k code. ⁸⁹Y MAS NMR spectroscopic studies suggest substantially weaker Knight shift contributions to the resonance frequencies when compared to other intermetallic yttrium compounds, including other ternary yttrium boride compounds measured previously.