Elastic Anomalies in the 2D Van Der Waals Ti3C2Tx MXene: Possible Sliding Ferroelectricity and Ordering of Intercalated Halides

MXenes are 2D carbides and nitrides of transition metals whose layers are kept by van derWaals forces. These materials are generally metallic and easily intercalate several types ofmolecules and ions, so that they are studied for various types of applications, from energystorage to corrosion resistance. We measured the dynamic Young’s modulus of self-standingfilms of Ti3C2Tx with the free flexural resonance and with the DMA, finding evidence of a previouslyunnoticed phase transition at T C = 350 K. In a previous paper it is argued that the transition should be related to the onset of sliding ferroelectricity. This is a new type of ferroelectricity known in bilayers of graphene, BN and few chalcogenides, with which ultrafast andstable ferroelectric memories have already been realized. If confirmed, Ti 3 C 2 T x would the firstMXene exhibiting sliding ferroelctricity and metallic conduction. The present work is focusedon the effect of the intercalatation of molecules, mainly water, in the intralayer space. Theinterlayer nature of the transition at 350 K is demonstrated by suppressing it through intercalationof H2O. Preliminary results are also presented on composite Ti 3 C 2 Cl 2 -KBr, where giantsoftening upon heating up to 800 K and a final stabilization with elastic anomalies around250 K might be due to intercalation of K and its subsequent ordering/disordering. This mightopen new perspectives in the study of the behaviour of intercalated ions in electrodes forsupercapacitors and batteries.