Tunable Negative Thermal Expansion in Layered Perovskite Ba₃Zr₂S₇

We simulated the thermal expansion coefficient (TEC) of the layered perovskite sulfide Ba₃Zr₂S₇ (<i>P</i>4₂/<i>mnm</i> 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 <i>n</i> = 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 <i>P</i>2/<i>c</i> 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.