By tuning the tolerance factor, t, of the Ruddlesden-Popper oxide Ca 2 MnO 4 through isovalent substitutions, we show that the uniaxial coefficient of linear thermal expansion (CLTE) of these systems can be systematically changed through large negative to positive values. Highresolution X-ray diffraction measurements show that the magnitude of uniaxial negative thermal expansion (NTE) increases as t decreases across the stability window of the NTE phase. Transitions to phases with positive

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... al expansion (PTE) are found to occur at both the high-t and low-t limits of stability. First-principles calculations demonstrate that reducing t enhances the contribution to thermal expansion from the lowest frequency phonons, which