We report a sudden reversal in the pressure dependence of T c in the iron-based superconductor CsFe 2 As 2 , similar to that discovered recently in KFe 2 As 2 [Tafti et al., Nat. Phys. 9, 349 (2013)]. As in KFe 2 As 2 , we observe no change in the Hall coefficient at T → 0, again ruling out a Lifshitz transition across the critical pressure P c . We interpret the T c reversal in the two materials as a phase transition from one pairing state to another, tuned by pressure, and we investigate which parameters control this transition. Comparing samples of different residual resistivity ρ 0 , we find that a sixfold increase in impurity scattering does not shift P c . From a study of x-ray diffraction on KFe 2 As 2 under pressure, we report the pressure dependence of lattice constants and As-Fe-As bond angle. The pressure dependence of the various lattice parameters suggests that P c should be significantly higher in CsFe 2 As 2 than in KFe 2 As 2 , but we find on the contrary that P c is lower in CsFe 2 As 2 , indicating that other factors control T c . Resistivity measurements under pressure reveal a change of regime across P c , suggesting a possible link between inelastic scattering and pairing symmetry.

Using high resolution sound velocity measurements we have obtained a very precise magnetic phase diagram of Ba3CoSb2O9 a material that is considered to be an archetype of the spin 1/2 triangular-lattice antiferromagnet. Results obtained for the field parallel to the basal plane (up to 18 T) show three phase transitions, consistent with predictions based on simple 2D isotropic Heisenberg models and previous experimental investigations. The phase diagram obtained for the field perpendicular to the basal plane clearly reveals an easy-plane character of this compound and in particular, our measurements show a single first order phase transition at Hc1 = 12.0 T which can be attributed to a spin-flop between an umbrella-type configuration and a coplanar V-type order where spins lie in a plane perpendicular to the ab-plane. A low temperatures softening of the lattice within some of the ordered phases is also observed and may be a result of residual spin fluctuations.

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