Cite this Article
Li Tao, Liu Xian-Sheng, Cheng Yong-Guang, Ge Xiang-Hong, Zhang Meng-Di, Lian Hong, Zhang Ying, Liang Er-Jun, Li Yu-Xiao. Zero and controllable thermal expansion in HfMgMo 3 − x W x O 12 . Chinese Physics B, 2017, 26(1): 016501  
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Zero and controllable thermal expansion in HfMgMo 3 − x W x O 12
Li Tao1, Liu Xian-Sheng2, Cheng Yong-Guang1, 3, Ge Xiang-Hong1, Zhang Meng-Di1, Lian Hong1, Zhang Ying1, Liang Er-Jun1, †, Li Yu-Xiao1, ‡
School of Physical Science & Engineering and Key Laboratory of Materials Physics of Ministry of Education, Zhengzhou University, Zhengzhou 450052, China
Henan Key Laboratory of Photovoltaic Materials and School of Physics and Electronics, Henan University, Kaifeng 475004, China
School of Physical Science & Engineering and Key Laboratory of Materials Physics of Ministry of Education, Zhengzhou University, Zhengzhou 450052, China

 

† Corresponding author. E-mail: ejliang@zzu.edu.cn liyuxiao@zzu.edu.cn

Abstract

HfMgMo W x O12 with , 1.0, 1.5, 2.0, and 2.5 are developed with a simple solid state method. With increasing the content of W, solid solutions of HfMgMo W x O12 crystallize in an orthorhombic structure for and a monoclinic structure for . A near-zero thermal expansion (ZTE) is realized for HfMgMo W O12 and negative coefficients of thermal expansion (NCTE) are achieved for other compositions with different values. The ZTE and variation of NCTE are attributed to the difference in electronegativity between W and Mo and incorporation of a different amount of W, which cause variable distortion of the octahedra and softening of the MoO4 tetrahedra, and hence an enhanced NCTE in the a- and c-axis and reduced CTE in the b-axis as revealed by Raman spectroscopy and x-ray diffraction.

PACS: 65.40.De;81.05.Je;61.50.–f;78.30.–j
Keyword:zero thermal expansion;negative thermal expansion;orthorhombic structure,;solid solution;Raman spectroscopy
1. Introduction

Negative thermal expansion (NTE) materials are becoming very attractive due to their potential applications in tailoring the coefficient of thermal expansion (CTE) to reduce thermal stress.[1–12] To date, different families of NTE materials based on various mechanisms, such as the phonon effect,[2, 13–16] magnetovolume effect,[5] spontaneous ferroelectric polarization,[4] and charge transfer,[17, 18] have been reported.

Of the NTE materials, the O12 ( , Al, Cr, Y, Yb, Er, etc. , W) family has received considerable interests due to its chemical flexibility and hence tailorable NTE. A modified structure of ABM3O12 was also possible to present the NTE property. HfMgW3O12 was first reported by Suzuki and Omote in 2004[19] and identified as an orthorhombic structure with space group at room temperature (RT) and above.[19] Nevertheless, Gindhart and Lind[20] through variable temperature neutron and x-ray diffraction characterization showed that HfMgW3O12 was crystallized in monoclinic structure with space group P21/a below 400 K and was transformed to an orthorhombic structure with space group Pnma at higher temperatures. Its linear CTE is in the orthorhombic phase. HfMgMo3O12 with a low linear CTE of from 298 to 1013 K was reported by Marinkovic et al. in 2008.[21] It crystallizes in orthorhombic symmetry with space group or (33)[21] and transforms to monoclinic structure at 175 K.[22] Baiz et al.[23] reported non-hydrolytic sol-gel synthesis of HfMgW3O12 and ZrMgW3O12 without referring to the structure and property. ZrMgMo3O12 was shown to be an orthorhombic structure without phase transition from 113 K to 1200 K and the linear CTE is about (300–1000 K),[24] while ZrMgW3O12 was reported to be highly hygroscopic, and its linear CTE is (440–975 K) after releasing crystal water.[25]

As mentioned above, HfMgMo3O12 and HfMgW3O12 have a low positive and NTE, respectively. In the paper, we report the synthesis and thermal expansion properties of the solid solutions of HfMgMo W x O12 with , 1.0, 1.5, 2.0, and 2.5 to achieve zero and controllable thermal expansion without hygroscopicity.

2. Experimental details

Analytical grade reagents of HfO2, MgO, WO3, MoO3 were used as raw materials, which were weighted and ground according to the molar ratios of the destination materials HfMgMo W x O12 with , 1.0, 1.5, 2.0, and 2.5. The mixed raw materials were pressed into cylinders using a uni-axis tablet machine with pressure about 200 MPa. The pressed pellets were sintered at 1073–1323 K for 3 h in a muffle furnace in air. For comparison, HfMgMo3O12 and HfMgW3O12 were also prepared.

X-ray diffraction (XRD) measurements were carried out with an x-ray diffractometer (Model X’Pert PRO) to identify the crystalline phase. Raman spectra were recorded with a Jobin-Yvon LabRAM HR Evolution Raman spectrometer equipped with a TMS 94 heating/freezing stage (an accuracy of ±0.1 K). A laser excitation wavelength of 633 nm was used. The differential scanning calorimetry (DSC) measurements were carried out on a Netzsch STA 449F3 simultaneous thermal analyzer in the temperature range of 293–1073 K with the heating rates of 10 K/min. The microstructures and energy dispersive spectra of the samples were examined with a scanning electron microscope (SEM, Model Quanta 250). The linear CTEs were measured with a dilatometer (Linseis DIL L76).

3. Results and discussion
3.1. Crystal structure analysis

Figure 1 shows the XRD patterns of HfMgMo W x O12 with , 1.0, 1.5, 2.0, and 2.5 at RT. It was reported that HfMgMo3O12 crystallizes in an orthorhombic structure with space group Pnma.[21] There are not obvious changes in the XRD patterns with increasing the content of W. This is probably the reason that HfMgW3O12 was identified as an orthorhombic structure with space group Pnma[19] or a monoclinic structure with space group below 400 K[20] by different groups.

Fig. 1. (color online) XRD patterns of HfMgMo W x O12 with , 1.0, 1.5, 2.0, and 2.5. For comparison, the XRD patterns of HfMgMo3O12 and HfMgW3O12 are also presented.

In Fig. 2 we show Raman spectra of HfMgMo W x O12 with x = 0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3 for the external librational and translational vibrations of the polyhedra (a) and internal symmetric stretching vibrations of the MoO4/WO4 tetrahedra (b), respectively. Figure 2(a) indicates that the Raman spectra for have similar features in the low wave number region while those for are different as indicated by the disappearance of the Raman modes between 60 and 120 cm . The insert shows the temperature-dependent Raman spectra of HfMgW3O12. It is obvious that a Raman mode at about 92 cm appears at 423 K and exhibits a red-shift with increasing temperature. Besides, a Raman mode at about 33 cm at RT shifts to blue with increasing temperature till 373 K, and a Raman mode at about 38 cm appears at 423 K which shifts a little with increasing temperature. The low wavenumber external librational and translation vibrations are the origins for NTE. These results suggest strongly that (i) HfMgW3O12 adopts a monoclinic structure at RT and transforms to an orthorhombic one above 400 K; (ii) HfMgMo W x O12 adopts an orthorhombic phase for and a monoclinic structure for .

Fig. 2. (color online) Raman spectra of HfMgMo W x O12 with x = 0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3 for the low wavenumber (a) and high wavenumber (b) regions, respectively. The insert shows the temperature-dependent Raman spectra of HfMgW3O12. The sharp peak denoted by an asterisk (*) is the sparkle from air plasma.

The most distinct change of the Raman spectra in the symmetric stretching region is the appearance of the Raman band at about 1021 cm for which increases in intensity and shifts progressively to 1028 cm with increasing the contents of W. This band can be assigned unambiguously to the symmetric stretching vibrations. The blue shift indicates a hardening of the WO4 tetrahedra with increasing the contents of W. Another distinct change is the obvious red shift of the symmetric stretching band at about 990 cm for HfMgMo3O12 which shifts to 983 cm for and to 978 cm for , indicating a progressive weakening of MoO4 tetrahedra with increasing the contents of W. In the orthorhombic structure, each HfO6/MgO6 octahedron shares its corners with six MoO4/WO4 tetrahedra and each MoO4/WO4 tetrahedron shares its corners with four HfO6/MgO6 octahedra. Since the electronegativity of W (2.36) is larger than that of Mo (2.16), it has a higher ability to attract electrons to the bond, leading to weakening of the bond if they share the same HfO6/MgO6 octahedron. Due to the difference in electronegativity and cation radius between W and Mo, a distortion for the octahedra is expected.

To confirm the crystal structure, structure analysis was performed by the Pawley refinement of XRD patterns with the Academic software of TOPAS 4.0. Figure 3(a) shows the results for HfMgMo W O12. It is shown that HfMgMo W O12 adopts an orthorhombic structure with space group Pnma (62), with acceptable values of %, %, and %. The obtained lattice constants for the a-, b-, and c-axes at RT are , , and , respectively. The samples for , 1.5, 2.0, and 2.5 are analyzed with the same method. It is shown that structure for , 1.5, 2.0 can be better refined with space group Pnma (orthorhombic) while that for can be better refined with space group (monoclinic). For example, the obtained R-values for are , , and with space group Pnma and , and with space group . The results confirm our Raman analysis above. Figure 3(b) shows the lattice constants and volume for . It is evident that the lattice constants increase with the contents of W but with different ratios in different directions. For example, with respect to HfMgMo3O12, the lattice constants of HfMgMo W O12 and HfMgMo W O12 are increased by 0.077%, 0.175%, and 0.104% and by 0.182%, 0.375%, and 0.232% for the a-, b-, and c-axes, respectively. It is possibly a result of the distortion of the octahedra and bond angle changes caused by partial substitution of W for Mo.

Fig. 3. (color online) (a) Structure refinement by the Pawley method with space group Pnma. Tick marks (bottom) indicate positions of the HfMgMo W O12 diffraction lines and the curve in the middle is the difference between the experimental and fitted results. (b) Lattice constants and volume change of HfMgMo W x O12 with , 1.0, 1.5, and 2.0.
3.2. Micro morphology of HfMgMo W x O12

Figure 4(a) shows the SEM images ((a1)–(a7)) of HfMgMo W x O12 with x = 0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3. For the samples with x = 0 and 0.5, the grain morphology presents a spherical shape about 1 m. With increasing the content of W from 1.0 to 2.0, the average grain sizes increase to near 3 with unobvious porosity. Especially, when the content of W increases to 2.5 and 3, the grain morphology transforms from sphere to rod. Figure 4(b) presents the EDS spectra ((b1)–(b7)) of HfMgMo W x O12 with x = 0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3. When the content of W increases to 2.5 and 3, i.e., the content of W is larger than that of 2Mo, the relative intensity of the spectra present notable change: the intensity of W is higher than that of Hf, which corresponds to the formation of rod morphology grains.

Fig. 4. (color online) SEM images (a) and EDS spectra (b) of HfMgMo W x O12 with , 1.0, 1.5, 2.0, and 2.5. For comparison, SEM images (a) and EDS spectra (b) of HfMgMo3O12 and HfMgW3O12 are also presented.
3.3. Thermal expansion property

The relative length changes of HfMgMo W x O12 with , 1.0, 1.5, 2.0, and 2.5 were measured with a dilatometer (Fig. 5). It is found that all the samples of HfMgMo W x O12 with , 1.0, 1.5, 2.0, and 2.5 present low CTE or NTE property. Especially, the sample of HfMgMo W O12 presents near-zero thermal expansion (ZTE) with . The largest NTE is observed for HfMgMo W O12 which is about .

Fig. 5. (color online) Relative length changes of HfMgMo W x O12 with , 1.0, 1.5, 2.0, and 2.5.

In order to see whether the ZTE for HfMgMo W O12 is intrinsic or not, we measured the XRD of the samples at different temperatures and calculated axial and volume parameters at each temperature. Figure 6(a) shows the selected temperature-dependent XRD patterns for HfMgMo W O12. Figure 6(b) shows the changes of the lattice constants and volume with temperature. It is evident that the a- and c-axes contract while the b-axis expands with increasing temperature, giving rise to near-ZTE in volume. The CTEs for the a-, b-, and c-axes and volume are calculated to be , , , , respectively. This gives rise to a linear CTE , confirming that HfMgMo W O12 is intrinsically a ZTE material. Compared to HfMgMo3O12,[21] the NTEs for the a- and c-axes are enhanced by about 49% and 117%, while the CTE for the b-axis is increased by only 1.9%. This explains why a ZTE is realized by the Mo:W ratio = 5:1 instead of 1:1. With increasing the contents of W, the NTEs for the a- and c-axes will be more enhanced and a linear NTE with different values is expected for the orthorhombic phase. The ZTE and variation of NCTE are attributed to the difference in electronegativity between W and Mo and the incorporation of a different amount of W, which cause variable distortion of the octahedra and softening of the MoO4 tetrahedra, and hence an enhanced NCTE in the a- and c-axis and reduced CTE in the b-axis as revealed by Raman spectroscopy and XRD. The enhanced NTE result is consistent with the report about controllable thermal expansion by chemical modifications.[26]

Fig. 6. (color online) (a) XRD patterns of HfMgMo W O12 at different temperatures; (b) changes of lattice constants and volume of HfMgMo W O12 with temperature.
4. Conclusions

HfMgMo W x O12 with , 1.0, 1.5, 1.5, 2.0, and 2.5 are synthesized by the solid solution method. Raman spectroscopic study indicates that the samples for crystallize in orthorhombic structure while that for adopts a monoclinic structure at RT. This is confirmed by structure refinements. A ZTE material is realized with the composition ratio of Mo:W = 5:1 and NTEs with different coefficients are obtained with increasing the contents of W. The ZTE and variation of NCTE are attributed to the difference in electronegativity between W and Mo and incorporation of a different amount of W which cause variable distortion of the octahedra and softening of the MoO4 tetrahedra, and hence an enhanced NCTE in the a- and c-axis and reduced CTE in the b-axis as revealed by Raman spectroscopy and XRD.

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