Contents I. Introduction
MgB2 has many properties (at the phenomenological level) intermediate between the high cuprate and conventional low superconductors [1]. For MgB2, Mg nonstoichiometry was found to lead to different dependencies on the applied pressure, therefore, one could expect different behaviors as a function of small Mg nonstoichiometry [2]. Established ordered substitution of oxygen atoms (20–100 nm precipitates) for boron lattice sites can positively influence and SC transition behavior of MgB2 (the basic bulk MgB2 crystal structure and orientation are preserved, oxygen precipitates may act as pinning centers) [3]. The periodicity of the oxygen ordering was dictated by the oxygen concentration in the precipitates and primarily occurred in the (010) plane. The positive influence of Ti and Zr addition (about 10%) on of MgB2 wires and on their water resistance (due to TiB2 or ZrB2 formation) have been observed in [4], [5]. We have established [6] the positive influence of the Ta presence during the HP synthesis of MgB2. Here we present a more detailed study of this phenomenon and the results of investigations of HP-synthesis and sintering of MgB2 and MgB2–Ta materials.
