EXAFS and EPR analysis of the local structure of Mn-doped Li₂B₄O₇

"EXAFS and EPR analysis of the local structure of Mn-doped Li 2 B 4 O 7 " (2013). Peter Dowben Publications. 264. The local structure of Mn-doped Li 2 B 4 O 7 (001) was investigated using extended X-ray absorption fine structure (EXAFS) at the Mn K edge and electron paramagnetic resonance (EPR). The location of the Mn dopant in a lithium tetraborate crystal is consistent with occupation of a site with strong oxygen coordination. The Mn–O bond lengths are similar to those observed with Mn doping of the icosahedral based boron carbide where Mn is in a substitutional dopant in one of the cage sites. From EXAFS, the manganese does not appear to greatly alter the overall tetragonal form of lithium tetraborate, with the dopant most likely substituting for one of the two B sites and with placement of some of the Mn in a Li site still possible. The EPR spectra agree with the literature examined resolving multiple Mn species in the crystal lattice. 1 Introduction As illustrated in Fig. 1, the oxide lithium tetraborate (I4 1 cd, a ¼ 9.479 A ˚ , c ¼ 10.290 A ˚) is a complex tetragonal crystal with 104 atoms in a unit cell [1–5]. The pyroelectric and piezoelectric properties of the lithium borates require excellent dielectric properties [6] in the crystals of Li 2 B 4 O 7 , particularly along the polar [001] direction. This material has been measured to have undoped resistivity of 10 10 V cm or more [7, 8]; consequently, for applications such as scintillation or use in solid state devices, doping of lithium tetraborate may be essential. For example, in semiconducting boron carbides the resistivity problem has been circumvented by filling impurity bands [9, 10]. This appears to lead to dramatic increases in carrier concentrations in materials with otherwise very large carrier effective masses [10]. Similarly, carrier mobility and carrier concentrations both may be increased in lithium tetraborates by the addition of impurities [6, 7]. Lithium borates [8, 11– 13] are among the boron rich materials (e.g., BN [14–16], BP [17–20], and BC [21–29]) that have been considered as possible materials for effective solid state neutron detectors. Applying extended X-ray absorption fine structure (EXAFS) techniques to successfully determine the physical