Phase Transition, Hygroscopicity And Thermal Expansion Property Of （HfMG）_xCr_2-2xMo₃O₁₂ And （ZrMG）_xY_2-2xMo₃O₁₂

The thermal stress, which is produced by thermal expansion, can lead to materialcracks and even failure that brings us very great inconvenience. Considerable recentinterest in thermal expansion has centred on the investigation and application ofnegative thermal expansion (NTE) materials, which make it possible for us to get nearzero thermal expansion to reduce thermal stress. Of the NTE materials, compoundswith general formula A2M3O12, where A is a trivalent cation, the mixture ofmonovalent ions and bivalent ions or bivalent ions and tetravalent ions, and M iseither W or Mo, are an important stable family exhibiting larger NTE. It has beenrationalized that NTE properties of orthorhombic molybdates and tungstates arerelated to the size of the A26+cation. However, with increasing the metal ion A26+radius the openings in the network become large enough to admit water moleculesthat inhibit the flexibility as well as the internal vibrations of the polyhedra which areessential for NTE. Moreover, part of A2M3O12(such as Al2M3O12、Cr2M3O12、Fe2M3O12) show monoclinic phase at room temperature, and it can becomeorthorhombic at a certain temperature.On one hand, Cr2M3O12show monoclinic phase at room temperature, and it canbecome orthorhombic at673K that restricts the wide spread use. In this paper, wesynthesized ceramics of (HfMg)xCr2-2xMo3O12(0≤x≤1). It is found that the phasetransition temperature of (HfMg)xCr2-2xMo3O12can be effectively decreases byincreasing the content of (HfMg)6+. And (HfMg)xCr2-2xMo3O12(0.7≤x≤1.0) withorthorhombic phase show low thermal expansion in a wide temperature range.On the other hand, Y2Mo3O12has a typical structure of A2M3O12withorthorhombic symmetry and exhibits an average NTE coefficient of-9.36×10-6K-1from2to1073K, whereas it possesses obvious crystal water at room temperature inair. The crystal water interacts strongly with the polyhedra and hinders their motions.To reduce the crystal water in Y2Mo3O12, some ions such as Fe3+, Cr3+, and La3+haveadopted to substitute Y3+. It was found that the crystal water can be reduced obviouslybut not completely by substitution of Fe3+and Cr3+in order to holding theorthorhombic phase. In this paper, we synthesized ceramics of (ZrMg)xY2-2xMo3O12(0≤x≤1) aiming at realizing NTE but without hygroscopicity. It is found that thenumber of water species in per formula are reduced obviously with increasing thesubstitution of (ZrMg)6+(ion radius72×10-12m) for Y3+(ion radius90×10-12m). The overall linear thermal expansion coefficients change notably with increasing thecontent of (ZrMg)6+in (ZrMg)xY2-2xMo3O12, and when x=0.7, the material showsnear zero thermal expansion: αx=0.7=-0.12×10-6K-1(430-870K). Meanwhile, pureZrMgMo3O12shows low negative thermal expansion, whose thermal expansioncoefficient’s absolute value is smaller than3×10-6K-1, and low softening temperature(at a temperature, atoms are easy far away from their equilibrium positions, andcrystal appears low hardness and abrupt contraction when further increasingtemperature or pressure). After substituting Y3+for (ZrMg)6+(x=0.8), the softeningtemperature increases remarkably and it presents near zero thermal expansion.