Electric‐field gradient characterization at ¹⁸¹Ta impurities in sapphire single crystals

Abstract

We report Perturbed-Angular-Correlation (PAC) experiments on corundum Al 2 O 3 single crystals implanted with 181 Hf/ 181 Ta ions at the ISKP at Bonn and measured at La Plata with high efficiency and timeresolution. The magnitude, asymmetry, and orientation (with respect to the crystalline axes) of the electric-field gradient (EFG) tensor were determined measuring the spin-rotation curves as a function of different orientations of the single crystals relative to the detector system. These results are analyzed in the framework of point-charge model and ab initio Full-Potential Linearized-Augmented Plane Wave calculations, and compared with EFG results coming from PAC experiments with 111 In/ 111Cd impurities. This combined study enables the determination of lattice relaxations induced by the presence of the impurity and the state of charge of a deep impurity donor level in the band gap of the semiconductor.

Structure and grain growth of TiO2 nanoparticles investigated by electron and x-ray diffractions and Ta181 perturbed angular correlations

“…The quadrupole frequency is practically independent of the thermal history; heating to and cooling from 1400 K produce the same linear trend q ͑T͒ = q ͑0͒͑1−AT͒ with q ͑0͒ = 798.9͑5͒ MHz and A = 2.96͑7͒ ϫ 10 −5 K −1 . These values are in fair agreement with those obtained for 181 Ta in coarse-grained 13 and single crystal 15 TiO 2 .…”

“…13͒ and in TiO 2 single crystals. 14,15 For the present investigation of nanocrystalline n-TiO 2 , we have chosen 181 Hf/ 181 Ta as probe nucleus for reasons discussed in Sec. III.…”

Site localization of Cd impurities in sapphire

“…These theoretical EFG tensors are then compared to available experimental TDPAC results. 30,31 Although the EFG has already been proved to be a powerful tool to identify impurity sites in metal oxides, 17,19,25,27,28 to our knowledge, a deep study of the Cd localization in Al 2 O 3 through a direct confrontation between measurements and reliable ab initio predictions has not been reported yet. Nevertheless, the situation faced in the present study is complicated by the fact that, in confronting experiments and ab initio EFG results, two possibly equivalent scenarios are found, and to further unravel this issue, we performed energetic studies.…”

Experimental and ab initio study of the nuclear quadrupole interaction of 181 Ta-probes in an α-Fe2O3 single crystal

“…To conclude, we want to comment that also V 33 for other corundum oxides measured with 181 Ta and 111 Cd impurities (probes with very different electronic configurations) are very similar in magnitude and symmetry. For α-Al 2 O 3 :Ta [22] and α-Al 2 O 3 :Cd [23,24] we have (V Cd 33 /V Ta 33 = 0.95), and for Cr 2 O 3 :Ta [25] and for Cr 2 O 3 :Cd [26] we have (1.38 ≤ V Cd 33 /V Ta 33 ≤ 1.03) This behavior is very different to that observed in the extensively studied bixbyite group in which the EFG for 181 Ta is between 2 or 3 times larger than for 111 Cd. New ab initio calculations are in progress in order to understand this different behavior.…”