Microscopic, structural, transport, and thermodynamic measurements of single crystalline Ba͑Fe 1−x TM x ͒ 2 As 2 ͑TM= Ni and Cu͒ series, as well as two mixed TM= Cu/ Co series, are reported. In addition, high-magnetic field, anisotropic H c2 ͑T͒ data were measured up to 33 T for the optimally Ni-doped BaFe 2 As 2 sample. All the transport and thermodynamic measurements indicate that the structural and magnetic phase transitions at 134 K in pure BaFe 2 As 2 are monotonically suppressed and increasingly separated in a similar manner by these dopants. In the Ba͑Fe 1−x Ni x ͒ 2 As 2 ͑x Յ 0.072͒, superconductivity, with T c up to 19 K, is stabilized for 0.024Յ x Յ 0.072. In the Ba͑Fe 1−x Cu x ͒ 2 As 2 ͑x Յ 0.356͒ series, although the structural and magnetic transitions are suppressed, there is only a very limited region of superconductivity: a sharp drop of the resistivity to zero near 2.1 K is found only for the x = 0.044 samples. In the Ba͑Fe 1−x−y Co x Cu y ͒ 2 As 2 series, superconductivity, with T c values up to 12 K ͑x ϳ 0.022 series͒ and 20 K ͑x ϳ 0.047 series͒, is stabilized. Quantitative analysis of the detailed temperature-dopant concentration ͑T − x͒ and temperature-extra electrons ͑T − e͒ phase diagrams of these series shows that there exists a limited range of the number of extra electrons added, inside which the superconductivity can be stabilized if the structural and magnetic phase transitions are suppressed enough. Moreover, comparison with pressure-temperature phase diagram data, for samples spanning the whole doping range, further re-enforces the conclusion that suppression of the structural/magnetic phase transition temperature enhances T c on the underdoped side, but for the overdoped side T C max is determined by e. Therefore, by choosing the combination of dopants that are used, we can adjust the relative positions of the upper phase lines ͑structural and magnetic phase transitions͒ and the superconducting dome to control the occurrence and disappearance of the superconductivity in transition metal, electron-doped BaFe 2 As 2 .

The discovery of a new family of high-T(C) materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent: for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked; however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal-to-orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2.

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