Phase transition and near-zero thermal expansion of Zr0.5Hf0.5VPO7

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Wang Jun-Ping, Chen Qing-Dong, Li Sai-Lei, Ji Yan-Jun, Mu Wen-Ying, Feng Wei-Wei, Zeng Gao-Jie, Liu You-Wen, Liang Er-Jun. Phase transition and near-zero thermal expansion of Zr_0.5Hf_0.5VPO₇**

Project supported by the National Natural Science Foundation of China (Grant Nos. 11574276, U173112, and 41401384), the Project of Shandong Provincial Higher Educational Science and Technology Program, China (Grant No. J17KB127), the Science and Technology Development Plans of Binzhou City, China (Grant Nos. 2014ZC0307 and 2015ZC0210), and Binzhou University Research Fund Project, China (Grant Nos. BZXYG1513 and BZXYG1706).

. Chinese Physics B, 2018, 27(6): 066501 Copy to clipboard

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Phase transition and near-zero thermal expansion of Zr_0.5Hf_0.5VPO₇**

Wang Jun-Ping^{1, 2}, Chen Qing-Dong², Li Sai-Lei³, Ji Yan-Jun^{1, 2}, Mu Wen-Ying², Feng Wei-Wei², Zeng Gao-Jie³, Liu You-Wen^{1, †}, Liang Er-Jun^{3, ‡}

1College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

2College of Aeronautics and Engineering, Binzhou College, Binzhou 256603, China

3Key Laboratory of Materials Physics of Ministry of Education, School of Physical Science and Engineering, Zhengzhou University, Zhengzhou 450052, China

† Corresponding author. E-mail: ywliu@nuaa.edu.cn

ejliang@zzu.edu.cn

Abstract

The Zr_0.5Hf_0.5VPO₇ is successfully synthesized by the solid-state method with near-zero thermal expansion. Powder x-ray diffraction (XRD), Raman spectroscopy, thermal dilatometry, and scanning electron microscopy (SEM) are used to investigate the structure, the phase transition, and the coefficient of thermal expansion (CTE) of Zr_0.5Hf_0.5VPO₇. The investigation results show that the samples are of the single cubic type with a space group of at room temperature (RT). It can be inferred that the superstructure is transformed from the 3 × 3 × 3 superstructure to the 1 × 1 × 1 ideal crystal in a temperature range between 310 K and 323 K. The CTE is measured by a dilatometer to be 0.59 × 10⁻⁶ K⁻¹ (310 K–673 K). The values of intrinsic (XRD) and extrinsic (dilatometric) thermal expansion are both near zero. The results show that Zr_0.5Hf_0.5VPO₇ has near-zero thermal expansion behavior over a wide temperature range.

PACS: 65.40.De;81.05.Je;61.50.-f;78.30.-j;

Keyword:near-zero thermal expansion;phase transition;x-ray diffraction (XRD);Raman spectrum;

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1. Introduction

Negative thermal expansion (NTE) materials have the characteristics of heat shrinkage and cold rise, which can effectively control the expansion coefficient of material. The NTE material means it is possible to prepare composite materials each with an arbitrary expansion coefficient. The stress caused by thermal expansion and contraction is often the main cause of fatigue, performance decline, failures, and even breaking and falling off of the device. The NTE materials have extensive applications in many fields, such as optics, electronics, optical fiber communications, medicine, daily life, and many other fields. The NTE materials provide an effective way to control the thermal expansion coefficient and temperature range.^[1–9] A series of NTE materials such as AM₂O₈ (A = Zr, Hf; M = W, Mo),^[1–6] AM₂O₇ (A = Zr, Hf; M = V, P),^[7–12] and A₂M₃O₁₂ (A = Y, Sc, Fe; M = W, Mo) has been reported.^[13–19] Recently researchers investigated the materials with near-ZTE, which include Mn₃Cu_0.5A_0.5N (A = Ni, Sn)/Cu composites,^[3] Mn₃Ga_0.5Ge_0.5N,^[4] ZrW₂O₈/A (A = ZrO₂, Al₂O₃, etc.) composites,^[5–7] Zn₄B₆O₁₃,^[20] ZrFeMo₂VO₁₂,^[21] and HfMgMo₃O₁₂.^[22] The materials of MV₂O₇ (M = Zr, Hf) have been extensively explored to reduce the phase transition temperature.^[1,7] The AM₂O₇ family within the cubic space group possesses the coefficient of thermal expansion (CTE) varying linearly from low positive to large negative values. The ZrV₂O₇ is one of the most attractive NTE materials because of its stable structure and isotropic property. However, ZrV₂O₇ shows a two phase transition between 350 K and 375 K. In order to further control the expansion coefficient and the phase transition temperature of ZrV₂O₇, researchers considered the partial substitution of V by other cations. Hisashig et al. substituted Hf⁴⁺ instead of Zr⁴⁺ solid solution of Zr_1−xHf_xV₂O₇. However, the phase transition point cannot be reduced effectively.^[17] Yuan et al.^[10] used Cu²⁺/P⁵⁺ or Fe³⁺/Mo⁵⁺ to replace Zr⁴⁺/V⁵⁺, but they found it was difficult to achieve a high solubility of the dopant. Many efforts have been made to design the composite materials with zero coefficient thermal expansion in a wide range of temperature. We have investigated the effects of substituting Hf⁴⁺/P⁵⁺ instead of Zr⁴⁺/V⁵⁺ and studied the effects of the phase transition and thermal expansion of ZrV₂O₇.

In this paper, a near-zero thermal expansion material of Zr_0.5Hf_0.5VPO₇ is synthesized by the solid state reaction method. Thermal dilatometry, XRD, and Raman spectra are used to investigate the CTE, anisotropy, and phase transition of Zr_0.5Hf_0.5VPO₇. It is found that the materials have the stable near-zero thermal expansion in a wide temperature range.

2. Experimental procedure

Zr_0.5Hf_0.5VPO₇ samples were synthesized by the solid-state reaction method. The stoichiometric quantities of commercial chemicals of ZrO₂ (99%), HfO₂ (99%), (NH₄)H₂PO₄ (99%), and V₂O₅ (99%) were ground together thoroughly with an agate mortar for 2 h. The mixed powder was heated from room temperature (RT) to 1023 K in a furnace at a rate of 5 K/min, maintained at 1023 K for 5 h, and cooled down slowly in the furnace. The sintered powder was reground, pressed into pellets with a thickness of about 10 mm and a diameter of 10 mm, and then sintered again at 1023 K for 5 h. These processes were repeated twice to implement the complete reaction. The obtained compact pellets were kept for measuring their linear CTEs.

The samples were analyzed by XRD with a PANalytical X’Pert PRO x-ray diffractometer to identify the crystalline phase. Variable-temperature x-ray powder data were collected on a Rigaku (Japan, SmartLab 3 KW) diffractometer with Cu Kα (λ = 0.15405 nm) radiation. Unit cell dimensions above the phase transition temperature were determined with software of Powder X. The relative length changes at low temperature were measured with a LINSEIS DIL L75 dilatometer at heating and cooling rates of 5 K/min. The Raman spectra of the samples were tested by a LabRAM HR Evolution Raman spectrometer. The microstructures and energy dispersive spectra of the samples were examined with a scanning electron microscope (SEM, Model Quanta 250).

3. Results

3.1. Crystal structure analysis

Figure 1 shows the Raman spectra of the solid solutions of Zr_0.5Hf_0.5VPO₇. It is obvious that the Raman spectrum of Zr_0.5Hf_0.5VPO₇ is very different from those of ZrV₂O₇ and Zr_0.5Hf_0.5 V₂O₇, which is because the radius of a P⁵⁺ ion (38 pm) is smaller than that of a V⁵⁺ ion (59 pm). Previous studies have shown that the Raman modes in the ranges of 1100 cm⁻¹–910 cm⁻¹, 910 cm⁻¹–700 cm⁻¹, 550 cm⁻¹–450 cm⁻¹, and 400 cm⁻¹–300 cm⁻¹ can be assigned to symmetric stretching (ν₁), asymmetric stretching (ν₃), asymmetric bending (ν₄), and symmetric bending (ν₂) modes of the VO₄ tetrahedron, respectively. The modes below 320 cm⁻¹ originate from lattice modes, caused by Zr atom motion, translational and vibrational modes of VO₄, respectively.^[1,9,23]

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	Fig. 1. (color online) (a) Raman spectra of (curve A) ZrV₂O₇, (curve B) Zr_0.5Hf_0.5VPO₇, and (curve C) Zr_0.5Hf_0.5 V₂O₇. (b) Temperature-dependent Raman spectra of Zr_0.5Hf_0.5VPO₇.

The most obvious change of the Raman spectrum of Zr_0.5Hf_0.5VPO₇ is the splitting of the symmetric stretching and asymmetric stretching modes of VO₄ tetrahedron around 1101 cm⁻¹–700 cm⁻¹ in which there are strong peaks at 1101, 1061, and 972 cm⁻¹ belonging to the symmetric stretching vibrations (ν₁ modes) of VO₄. The peaks at 887, 806, and 700 cm⁻¹ belong to asymmetric stretching (ν₃ modes) of the VO₄. The peaks at 524 cm⁻¹ and 474 cm⁻¹ are ascribed to asymmetric bending (ν₄ modes). While those peaks at 403, 367, 344, 330, and 301 cm⁻¹ originate from symmetric bending (ν₂) of the VO₄. The peaks at 282, 99, and 55 cm⁻¹ originate from lattice modes, which arise from Zr atom motion, translational and librational modes of VO₄.^[19]

Figure 1(b) shows the temperature-dependent Raman spectra of Zr_0.5Hf_0.5VPO₇ in a temperature range from 298 K to 673 K. The modes at about 252, 302, 402, and 700 cm⁻¹ become weakened with temperature increasing. An obvious protrusion appears near 280 cm⁻¹ and becomes broadened as the temperature rises. The Raman peaks do not shift and they do not change their magnitudes, either, which suggests that Zr_0.5Hf_0.5VPO₇ keeps the cubic structure in a temperature range from 298 K to 673 K without phase transition. The results strongly suggest that Zr_0.5Hf_0.5VPO₇ is of a cubic structure and is transformed into the 1 × 1 × 1 ideal crystal structure at RT.

Figure 2 shows the XRD patterns of the solid solutions of Zr_0.5Hf_0.5VPO₇, Zr_0.5Hf_0.5V₂O₇, and ZrV₂O₇ at RT. The diffraction peaks of Zr_0.5Hf_0.5VO₇ and Zr_0.5Hf_0.5V₂O₇ are corresponding to the primary diffraction peaks of ZrV₂O₇ (ICDD-JCPDS-PDFNo. 01-088-0587), which crystallizes into the cubic structure with space group . The diffraction peaks corresponding to the cubic structure recognized for ZrV₂O₇ were observed in Zr_0.5Hf_0.5V₂O₇.^[11] However, those diffraction peaks for the superstructure almost disappear and the secondary peaks in a range of 33°–55° disappear in Zr_0.5Hf_0.5VPO₇. All results show that the 3 × 3 × 3 superstructure of Zr_0.5Hf_0.5VPO₇ has disappeared. However, Zr_0.5Hf_0.5V₂O₇ has such diffraction peaks for the superstructure. These results further prove that P substitution is found to be more effective to suppress the superstructure of ZrV₂O₇.

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	Fig. 2. (color online) (a) XRD patterns of (curve A) Zr_0.5Hf_0.5VPO₇, (curve B) Zr_0.5Hf_0.5 V₂O₇, (curve C) ZrV₂O₇. (b) Magnified part of patterns in panel (a) (symbol ♦ indicates the diffraction peaks of superstructures).

In order to confirm the crystal structure, the diffraction data are analyzed by the Rietveld and Le Bail methods in FullProf. The XRD data are refined in with the cubic structure of ZrV₂O₇ as a starting model. Figure 3 and Table 1 show the Rietveld analysis results of Zr_0.5Hf_0.5VPO₇. It is shown that the Zr_0.5Hf_0.5VPO₇ has a cubic structure with the space group (R_p = 16.7, R_wp = 16.3, R_exp = 5.09, x² = R_wp/R_exp = 3.2). The cell parameters are calculated to be a = b = c = 8.5132 Å and α = β = γ = 90° at RT. The refined results further indicate that Zr_0.5Hf_0.5VPO₇ adopts a cubic structure with the space group (Z = 108).

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	Fig. 3. (color online) Results of the Rietveld analysis of the XRD pattern for Zr_0.5Hf_0.5VPO₇ at room temperature (R_p = 14.1, R_wp = 13.7, R_exp = 5.48, x² = R_wp/R_exp = 2.5).

Table 1.

Detailed XRD refinement results of Zr_0.5Hf_0.5VPO₇.

3.2. Thermal expansion properties

The near-ZTE property of Zr_0.5Hf_0.5VPO₇ is investigated with a dilatometer. Figure 4 shows the variations of the relative length of Zr_0.5Hf_0.5VPO₇ and ZrV₂O₇ with temperatures. The Zr_0.5Hf_0.5VPO₇ presents an excellent stable zero thermal expansion property in the process of heating and cooling after undergoing a transition from superstructure to an ideal structure. The CET of Zr_0.5Hf_0.5VPO₇ is obtained to be 0.59 × 10⁻⁶ K⁻¹ in a temperature from 310 K to 673 K. This result suggests that the Zr_0.5Hf_0.5VPO₇ has a better low thermal expansion property in a relatively low temperature range. It can also be obviously seen that there is no abrupt change below 310 K during the linear thermal expansion of the samples, indicating that the 3 × 3 × 3 superstructure has disappeared and no abrupt thermal expansion is observed, either.

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	Fig. 4. (color online) Relative length changes of (curve a) ZrV₂O₇ and (curve b) Zr_0.5Hf_0.5VPO₇ with temperature increasing from 200 K to 673 K and temperature decreasing from 673 K to 310 K.

Figure 5 shows the temperature dependent XRD patterns of Zr_0.5Hf_0.5VPO₇ in a temperature range from 119 K to 573 K. It is observed that the diffraction peaks of the XRD slightly shift to larger angles with temperature increasing (Fig. 5(b)). There are no obvious peaks appearing/disappearing in the XRD patterns in a temperature range from 119 K to 573 K. These results suggest that the samples keep cubic structures with temperature increasing. The cubic structures are found to be well consistent with space group .

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	Fig. 5. (color online) (a) XRD patterns of Zr_0.5Hf_0.5VPO₇ at different temperatures. (b) Magnified part of patterns in panel (a).

Figure 6 exhibits the variation of the cell volume with temperature, which is calculated by Powder X. It can be clearly seen that there is no abrupt thermal expansion above 400 K. That is because Zr_0.5Hf_0.5VPO₇ keeps a cubic structure in a temperature from 113 K to 673 K without phase transition. The CTE for volume is calculated to be K⁻¹ in a temperature range of 400 K–573 K. This gives rise to a linear , confirming that Zr_0.5Hf_0.5VPO₇ is intrinsically a ZTE material. This result is consistent with that from the Raman spectra, the 3 × 3 × 3 superstructure remarkably disappears for Zr_0.5Hf_0.5VPO₇. The CTE for Zr_0.5Hf_0.5VPO₇ is measured to be 0.59 × 10⁻⁶ K⁻¹ in a temperature range of 364 K–673 K by dilatometry. The difference between CTEs measured by the dilatometer and XRD can relate to the microstructure effect which can add a small negative component to the intrinsic linear expansion coefficient.

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	Fig. 6. (color online) Variation of the cell volume of Zr_0.5Hf_0.5VPO₇ with temperature.

The electronegativity of the cation and the M–O bond strength can be used to analyze the NTE of Zr_0.5Hf_0.5VPO₇. The electronegativity value of P⁵⁺ (2.19) is larger than that of V⁵⁺ (1.63). The bridging oxygen connected with V⁵⁺ or P⁵⁺ gains less negative charges with the content of P⁵⁺ increasing, which reduces the effective negative charge due to the oxygen-oxygen repulsion force and makes the transition harder from superstructure to ideal crystal structure. The partial substitution of V in ZrV₂O₇ by P reduces the symmetry and influences the transverse thermal vibration of V–O–V. The bond angle of V (P)–O–V in ZrV₂O₇ is close to 180° because the transverse motion of bridge O in V–O–V and P–O–V is responsible for the negative thermal expansion of ZrV₂O₇.^[1,17]

Dilatometric results are slightly different from the results from the variable-temperature XRD and variable-temperature Raman spectra. The microstructure effects might be responsible for the cause of such a difference. Raman spectra and XRD reveal the internal molecular structure and vibration mode of the unit cell. The Raman peak will change when the superstructure is disintegrated. The CTE from the dilatometer reflects the thermal expansion of bulk material, in which microcrack and poles are inevitable and may affect the measured results. The differences between intrinsic (XRD) and extrinsic (dilatometric) thermal expansion can be found in Zr_1−xFe_xV_2−xMo_xO₇,^[7] ZrFeMo₂VO₁₂,^[17] etc.

Figure 7 shows the SEM images of the sample. The microstructure of the grain morphology presents a spherical shape, which is closely arranged with the size of single short rod particle in a range from 2 μm to 6 μm. There are a few tiny gaps between rod particles. It should be noted that there are less pores in Zr_0.5Hf_0.5VPO₇ than in other samples even under the same sintering conditions.

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	Fig. 7. SEM images of Zr_0.5Hf_0.5VPO₇ synthesized by solid state reaction.

4. Conclusions

A solid solution of Zr_0.5Hf_0.5VPO₇ with near-ZTE has successfully been synthesized by solid state reaction. Thermal dilatometry, XRD, and Raman spectra are used to investigate the compactness, the coefficient of thermal expansion, isotropy, and phase transition of Zr_0.5Hf_0.5VPO₇. The results show that stable near-zero thermal expansion of the prepared material goes over a wide temperature range. The CTE for Zr_0.5Hf_0.5VPO₇ is measured to be 0.59 × 10⁻⁶ K⁻¹ in a temperature range of 310 K–673 K by dilatometry while it is calculated to be 0.259 × 10⁻⁶ K⁻¹ (400 K–573 K) by XRD. The results from the dilatometer are slightly different from those from the variable-temperature XRD. The differences between the two methods are attributed to the effects of microstructure.

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