Thermal expansion and spontaneous strain associated with the normal-incommensurate phase transition in melilites

The linear thermal expansions of åkermanite (Ca₂MgSi₂O₇) and hardystonite (Ca₂ZnSi₂O₇) have been measured across the normal-incommensurate phase transition for both materials. Least-squares fitting of the high temperature (normal phase) data yields expressions linear in T for the coefficients of instantaneous linear thermal expansion, $$alpha _1 = frac{1}{l}frac{{dl}}{{dT}}$$ for åkermanite: $$begin{gathered} alpha _{[100]} = 6.901(2) times 10^{ - 6} + 1.834(2) times 10^{ - 8} T hfill alpha _{[100]} = - 2.856(1) times 10^{ - 6} + 11.280(1) times 10^{ - 8} T hfill end{gathered} $$ for hardystonite: $$begin{gathered} alpha _{[100]} = 15.562(5) times 10^{ - 6} - 1.478(3) times 10^{ - 8} T hfill alpha _{[100]} = - 11.115(5) times 10^{ - 6} + 11.326(3) times 10^{ - 8} T hfill end{gathered} $$ Although there is considerable strain for temperatures within 10° C of the phase transition, suggestive of a high-order phase transition, there appears to be a finite ΔV of transition, and the phase transition is classed as “weakly first order”.