Zero Thermal Expansion in ZrMgMo₃O₁₂: NMR Crystallography Reveals Origins of Thermoelastic Properties

The coefficient of thermal expansion of ZrMgMo₃O₁₂ has been measured and was found to be extremely close to zero over a wide temperature range including room temperature (αl = (1.6 ± 0.2) × 10^–7 K^–1 from 25 to 450 °C by X-ray diffraction (XRD)). ZrMgMo₃O₁₂ belongs to the family of AMgM₃O₁₂ materials, for which coefficients of thermal expansion have previously been reported to range from low-positive to low-negative. However, the low thermal expansion property had not previously been explained because atomic position information was not available for any members of this family of materials. We determined the structure of ZrMgMo₃O₁₂ by nuclear magnetic resonance (NMR) crystallography, using ⁹¹Zr, ²⁵Mg, ⁹⁵Mo, and ¹⁷O magic angle spinning (MAS) and ¹⁷O multiple quantum MAS (MQMAS) NMR in conjunction with XRD and density functional theory calculations. The resulting structure was of sufficient detail that the observed zero thermal expansion could be explained using quantitative measures of the properties of the coordination polyhedra. We also found that ZrMgMo₃O₁₂ shows significant ionic conductivity, a property that is also related to its structure