Tunable Negative Thermal Expansion in Layered Perovskite Ba3Zr2S7

We simulated the thermal expansion coefficient (TEC) of the layered perovskite sulfide Ba3Zr2S7 (P42/mnm symmetry) from first principles. The calculated ambient pressure and room-temperature volumetric TEC is 38 × 10-6 K-1, which makes the material suitable for use in conventional PV devices. We further predicted low-temperature, pressure-tunable negative thermal expansion (NTE) in Ba3Zr2S7 that arises from a quasi-2D vibration mechanism shared by other n = 2 Ruddlesden-Popper oxides Ca3Ti2O7, Ca3Zr2O7, and Sr3Zr2O7. We computationally found a pressure-induced phase transition to a structure in the monoclinic crystal system. Experimental investigation of this system as a function of pressure supported by in situ diffraction studies in a diamond anvil cell confirmed a phase change at high pressures to a new polymorph that likely exhibits P2/c symmetry. Our simulations show that the quasi-2D mechanism and proximity to a mechanochemical transition enhance the NTE response. These features may be used to design NTE in other layered perovskites.