From Weak- to Strong-Coupling Superconductivity in the AlB2-Type Solid Solution SrGa1-xAlxGe with Honeycomb Layers.

We report on the structure and the superconducting properties of 9-electron 111 compounds with honeycomb layers, namely SrGaGe, SrAlGe, and the SrGa1-x AlxGe solid solution. By means of single-crystal X-ray diffraction we show that, on one hand, SrGaGe crystallizes into the centrosymmetric P6/mmm space group (a = 4.2555(2) ˚A, c = 4.7288(2) ˚A) with statistical disorder in the [GaGe]2- 6 honeycomb layers. On the other hand, we confirm that SrAlGe crystallizes in a non-centrosymmetric space group, namely P6m2 (a = 4.2942(1) ˚A, c = 4.7200(2) ˚A) with fully ordered [AlGe]2- 6 honeycomb layers. By using magnetization and specific heat measurements, we show that the superconducting properties of SrGaGe and SrAlGe differ significantly from each other. SrGaGe is a superconductor with a critical temperature of Tc = 2.6 K falling into the weak coupling limit, while SrAlGe has a Tc = 6.7 K and can be classified in the strong coupling limit. By realizing the SrGa1-x AlxGe solid solution, we were able to investigate the transition between the different crystal structures as well as the evolution of the electronic properties. We show that the transition from the weak to the strong coupling superconductivity in this system is likely associated with the disorder-to-order transition of the honeycomb layer, along with the loss of the inversion center in the crystal structure.