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.

已对ZrMgMo3O12的热膨胀系数进行了测量，结果显示其在包括室温在内的宽温度区间内热膨胀系数近乎为零：通过X射线衍射（X-ray diffraction，XRD）表征，在25至450 ℃区间内其线性热膨胀系数α_l = (1.6 ± 0.2) × 10^–7 K^–1。ZrMgMo3O12属于AMgM3O12材料家族，该家族材料此前被报道的热膨胀系数范围为低正值至低负值。然而由于该家族任何成员的原子位置信息均未获取，其低热膨胀特性此前始终无法得到合理解释。本研究借助91Zr、25Mg、95Mo与17O的魔角旋转（magic angle spinning，MAS）核磁共振（nuclear magnetic resonance，NMR）、17O多量子魔角旋转（multiple quantum MAS，MQMAS）NMR技术，结合X射线衍射与密度泛函理论（density functional theory，DFT）计算，通过核磁共振晶体学方法解析了ZrMgMo3O12的晶体结构。所获结构具备足够精细的细节，可通过配位多面体性质的定量分析，对观测到的零热膨胀特性作出合理解释。本研究同时发现，ZrMgMo3O12表现出显著的离子导电性，该特性同样与其晶体结构密切相关。