Chemistry, Symmetry, and Negative Thermal Expansion in Empty NaSICON-Type Phosphates

Abstract Crystal structures of NaSICON-type phosphates M′M”(PO4)3 (M′M” = Al0.5Nb1.5, Fe0.5Nb1.5, Sc0.5Nb1.5, TiTa, TiNb, TiSb, SnNb) were studied over a wide temperature range using X-ray and neutron powder diffraction to establish the relationship between composition and thermal expansion. Surprisingly, the size and mass of M′ and M” have negligible effects, with the compositions crystallizing in the high-symmetry R3̅c form (M′ = Al, Ti, Sc, Fe; M” = Nb, Ta) all showing similar negative coefficients of thermal expansion (CTE, α) from 3 K to ∼1000 K. In contrast, the compositions distorted to the R3̅ form (M′ = Ti, Sn; M” = Nb, Sb) display positive expansion in the low-symmetry region, with CTE reducing above the R3̅ to R3̅c phase transition. Conventional (positive) CTE is thus a signature of the low-symmetry R3̅ form in M′M”(PO4)3. The results suggest that compositions with near-zero thermal expansion can be designed by selecting M′M” to balance the stability of the R3̅ and R3̅c forms with positive and negative CTE, respectively. Ab initio molecular dynamics simulations confirmed the critical role of the temperature evolution of the M–O–P angles in R3̅ vs R3̅c forms.