Near‐Zero Thermal Expansion in In(HfMg)_0.5Mo₃O₁₂

Abstract

In(HfMg)0.5Mo3O12, which can be considered as a 1:1 mole ratio solid solution of the low‐positive thermal expansion material HfMgMo3O12 and the low‐negative thermal expansion (NTE) material In2Mo3O12 was prepared. From DSC and XRPD results, we show that In(HfMg)0.5Mo3O12 exists in a monoclinic (P21/a) structure at low temperature and undergoes a phase transition at ~425 K to an orthorhombic phase (Pnma), with an associated enthalpy change of 0.89 kJ mol−1. Thermal expansion is large and positive in the low‐temperature monoclinic phase (average αℓ = 16 × 10−6 K−1 and 20 × 10−6 K−1, from dilatometry and XRPD, respectively). Remarkably, this material has a near‐zero thermal expansion (ZTE) coefficient over the temperature range ~500 to ~900 K in the high‐temperature orthorhombic phase, both intrinsically and for the bulk sample. The average linear intrinsic (XRPD) value is αℓ = −0.4 × 10−6 K−1, and the average bulk (dilatometric) value is αℓ = 0.4 × 10−6 K−1 with an uncertainty of ± 0.2 × 10−6 K−1. The slight difference between intrinsic and bulk thermal expansion is attributed to microstructural effects. XRPD results show that the thermal expansion is more isotropic than for the parent compounds HfMgMo3O12 and In2Mo3O12.

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

“…54 It is important to estimate these factors when refining crystal structures using experimental EFG tensor parameters and PAW DFT calculations, as otherwise the polyhedral distortions would be under-or overestimated. Our calibration curves for 17 O, 25 Mg, 91 Zr, and 95 Mo showed that the calculated EFG tensor parameters are scaled by 1.029 ± 0.007 [uncertainties correspond to the standard error of the fit] for 17 O, 1.14 ± 0.04 for 25 Mg, 0.93 ± 0.04 for 91 Zr, and 1.11 ± 0.02 for 95 Mo, when compared to experiment. The small deviations on the calibration factors, each of the order of a few percent at most, indicate the precision of this method.…”

“…Reactive sintering of the pellets for 12 h at 700°C in air formed ZrMgMo 3 O 12 , as confirmed by X-ray diffraction (XRD; >99% pure). Samples were synthesized using this method for all of the reported experiments, except for the 17 5 (Alfa Aesar, 99.6%) was sealed with a magnetic stir bar in a dry round-bottom flask. The flask was then placed in an ice bath on a magnetic stir plate.…”

“…The standard uncertainty of the instrument in Δ / 0 was ±1%. 17,63 Thermal Analysis. Differential scanning calorimetry (DSC) was used to delineate any phase transitions in ZrMgMo 3 O 12 with a 20.43 mg sample in an aluminum pan using a TA Instruments Q200 series differential scanning calorimeter equipped with a liquid N 2 cooling head.…”

“…17 O MAS NMR spectra were acquired at 9.4, 16.5, and 21.1 T magnetic field strengths ( 17 O NMR Larmor frequencies of 54, 95, and 122 MHz, respectively). The samples were spun up to 13.5 kHz using 4 mm MAS probes (9.4 and 16.5 T instruments) and up to 26 kHz at 21.1 T using a 2.5 mm MAS probe.…”

“…23−25 Their CTEs are chemically tunable by aliovalent substitution of a trivalent A′ cation to produce A′ 2−2x (AMg) x M 3 O 12 materials. 17,18 They are also of interest because of the high ionic conductivity reported for HfMgW 3 O 12 . 26,27 AMgM 3 O 12 materials display low-magnitude thermal expansion in an orthorhombic space group, which had previously been determined through indexing analysis to be either Pna2 1 or Pnma.…”