Negative Thermal Expansion Properties of Sm_0.85Sr_0.15MnO_3-δ

Abstract

A novel negative thermal expansion (NTE) material composed of Sm_0.85Sr_0.15MnO_3-δ was synthesized using the solid-state method. By allowing Sr²⁺ to partially replace Sm³⁺ in SmMnO₃, the ceramic material Sm_0.85Sr_0.15MnO_3-δ exhibits NTE properties between 360K and 873K, and its average negative thermal expansion coefficient was −10.08 × 10⁻⁶/K. The structure of Sm_0.85Sr_0.15MnO_3-δ is orthogonal, the space group is pbnm, the morphology is regular, and the grain size is uniform. The results of X-ray diffraction and XPS (X-ray photoelectron spectroscopy) suggest that the NTE phenomenon is related to the electron transfer of Mn ions. With the increase in temperature, Mn⁴⁺ is rapidly transformed into Mn³⁺, accompanied by Mn⁴⁺O₆ octahedron distortion and oxygen defects. It was found that the sample volume continually decreased at the same time.

1. Introduction

We know that most instruments are composed of various materials, but with increases in temperature, different thermal expansion coefficients of various constituent materials may lead to thermal mismatches, and small cracks in the equipment can lead to performance failures and even instrument damage. NTE materials have attracted considerable research attention in the production of composites with accurately controllable positive, negative, or zero coefficients of thermal expansion [1,2,3,4,5,6,7,8,9,10,11].

A great number of NTE materials have been found, such as oxides (Cu_1.5Mg_0.5V₂O₇, Cu₂V₂O₇, and HfMnMo₃O₁₂, etc.) [12,13,14,15,16], antiperovskite Mn₃XN, and perovskite (BiNiO₃, Gd_1-xSr_xMnO_3-σ, and Er_0.7Sr_0.3NiO_3-δ, etc.) [17,18,19,20,21]. However, each material has limitations because of some defects. ZrW₂O₈ is a metastable phase at room temperature (RT), which is difficult to prepare due to it readily decomposing [1]. ZrV₂O₇ exists as a phase transformation at 375K [2]. Y₂Mo₃O₁₂ has a water-absorbing quality at RT. Although antiperovskite (Mn₃Cu(Ge)N, Mn₃NiN, and Mn₃ZnN, etc.) possesses the properties of superconductivity, giant magnetoresistance, magnetocaloric effects, and constant electrical resistivity [8], the NTE temperature range is usually under RT, and its preparation conditions are very strict. Mn₃Cu(Ge)N needs to be grown on a silicon surface with high pressure and argon gas protection. The NTE perovskite ABO₃ (A = Gd, Er, and Bi, etc.; B = Mn, Er, Sr, Ni and Sr, etc.) not only shows NTE properties in a large temperature range above RT but also has simple preparation conditions.

Kurimamachiya-chouses conducted research on Sr²⁺ partly substituting Gd³⁺ in GdMnO₃. They pointed out that Gd_1-xSr_xMnO_3-δ had excellent NTE properties [18]. L. J. Fu reported the NTE material of Er_0.7Sr_0.3NiO_3-δ with Sr²⁺ partly substituting Er³⁺ in ErNiO₃ [19]. These studies suggest that the substituting method is an effective way to prepare new kinds of NTE materials with excellent properties [16,18,19,20]. In the present study, we conducted research on Sr²⁺ partly substituting Sm³⁺ in SmMnO₃. The thermal properties are discussed.

2. Experimental Procedures

The sample was prepared according to the conventional solid-state method. Analytic-grade Sm₂O₃ (purity 99.5%), SrO (purity 99.5%), and MnO₂ powder were used as raw materials. Using MnO₂ as the raw material, Mn₂O₃ powder was prepared by burning in a 923 K furnace for 10 h.

Sm₂O₃, SrO, and Mn₂O₃ powders were mixed according to the mole ratio of Sm:Sr:Mn = 0.85:0.15:1. The mixtures were ground using an agate mortar for 1 h and then ground with ethanol for 2 h. The obtained mixtures were then dried for 1 h at 353 K in a baking oven. Afterward, the mixtures were pressed into cylindrical-shape compacts (Ø10 × 5 mm) using a powder pellet machine (769YP-15A, 200 MPa). The compacts were initially sintered in a pipe furnace (AY-BF-555-180) at 1273 K for 10 h in air and subsequently sintered at 1623 K for 10 h. The sample was allowed to cool in the furnace naturally.

The linear thermal expansion coefficient was measured using a dilatometer Linseis L76 (heating and cooling rates of 5 K/min). The XRD measurement was carried out using Bruker D8 Advance with CuKα radiation. The XRD pattern of the sample was analyzed using X’Pert HighScore Plus software. The lattice constants a, b, and c and the unit cell volume of the sample were calculated using powderX software and the least square method. The surface morphology of the sample was observed using the FEI Quanta 250 scanning electron microscopy (SEM), and the EDS energy spectrum was obtained using an Appllo XP. The TGA and DSC were tested using a LabsysTM thermal analyzer. The XPS (X-ray photoelectron spectroscopy) was performed using a Thermo Scientific K-Alpha instrument for the valence analysis of the Mn element. The BET tests were performed to determine the size and volume of the holes using an ASAP2460 device.

3. Results and Discussion

3.1. Phase Analysis

Figure 1a is the XRD pattern of the sample at RT. Comparing the XRD pattern with the JCPDS cards for SmMnO₃ (00-025-0747), Eu_0.9Sr_0.1MnO₃ (No. 00-051-0252), and Eu_0.8Sr_0.2MnO₃ (00-051-0251), we found that the diffraction peaks were similar to those of the JCPDS cards, except for some shifts, which suggests that the as-prepared sample had similar structure to that of SmMnO₃, Eu_0.9Sr_0.1MnO₃, and Eu_0.8Sr_0.2MnO₃. It can be confirmed that the ceramic Sm_0.85Sr_0.15MnO_3-δ crystallizes in an orthorhombic structure. As the ionic radius of Sr²⁺ (ionic radius 1.18 Å) is bigger than that of Sm³⁺ (ionic radius 0.958 Å), the difference in the ionic radius may cause lattice distortion. As Sr²⁺ partly substitutes for Sm³⁺, the diffraction peaks also shift.

Figure 2a shows the SEM image of the sample. We found that the ceramic sample was composed of homogenous spherical or elliptic spherical particles with some obvious agglomerations. There were pores and microcracks in the sintered body. The size of the particles was uniform, with an average grain size of about 1~2 μm. The EDS analysis of the sample revealed the primary elements of Sm, Sr, Mn, and O, and their atomic ratio (Sm:Sr:Mn:O) was about 0.85:0.15:1:3 (seeing

Table 1. Atomic ratio of Sm, Sr, Mn, and O in Sm_0.85Sr_0.15MnO_3-δ by EDS.

Element	Sm	Sr	Mn	O
(at.%)	14.46	2.39	16.20	66.95

). Combined with the XRD analysis, we identified the composition of the samples as being Sm_0.85Sr_0.15MnO₃.

3.2. Thermal Expansion Property

Figure 3a–c show the relative length (dL/L) with the temperature increases of SmMnO₃, SrMnO₃, and Sm_0.85Sr_0.15MnO_3-δ, respectively. SmMnO₃ (Figure 3a) and SrMnO₃ (Figure 3b) showed positive thermal expansion. Calculating according to the curve, the expansion coefficients were 5.24 × 10⁻⁶/K and 12.7 × 10⁻⁶/K, respectively. When the temperature was below 360 K, the ceramic Sm_0.85Sr_0.15MnO_3-δ showed a positive thermal expansion of 0.46875 × 10⁻⁶/K. As the temperature increased, the ceramic Sm_0.85Sr_0.15MnO_3-δ showed an NTE property in the range of 360 to 873 K. The average linear expansion coefficient was −10.08 × 10⁻⁶/K.

Figure 4 shows the high-temperature XRD patterns of ceramic Sm_0.85Sr_0.15MnO_3-δ from RT to 873 K. As the temperature increased, the diffraction peaks of Sm_0.85Sr_0.15MnO_3-δ moved slightly to small angles, except three diffraction peaks (31.54°, 33.79°, and 52.65°) that moved to a large angle.

Figure 5 shows the variation in the Sm_0.85Sr_0.15MnO_3-δ lattice parameters and volume with temperature increases, which was calculated using the powderX software. In a, c in Figure 5, the increase occurred gradually, while in b in Figure 5, it decreased as the temperature increased gradually. We believe that the thermal expansion of Sm_0.85Sr_0.15MnO_3-δ was due to anisotropy. We can see that from RT to 360 K, Sm_0.85Sr_0.15MnO_3-δ showed a positive expansion property. As the temperature increased to 360~873 K, Sm_0.85Sr_0.15MnO_3-δ showed an NTE property with the average linear expansion coefficient of −3.33 × 10⁻⁶/K. However, the original calculation of the negative thermal expansion coefficient of Sm_0.85Sr_0.15MnO_3-δ in this temperature range was −10.08 × 10⁻⁶/K, according to Figure 3. As can be seen from Figure 2 above, there were pores and microcracks in the crystal. Therefore, we believe that when the temperature rises, the crystal squeezes the open space, namely, these pores and microcracks, which is another reason for the negative thermal expansion.

Table 2. Pore size, pore volume, and BET surface area of the sample.

Sample	Pore Size (nm)	Pore Volume (cm3/g)	BET Surface Area (m2/g)
Sm0.85Sr0.15MnO3-δ	15.7842	0.002563	0.6351

shows the pore size, pore volume, and BET surface area of the sample. The specific surface of the material itself is large, and the general level of adsorption is good. When the pore structure of carbon materials is more complex, it is easy to have a flexible hole, and the pore size becomes larger after gas adsorption. With the doping of Sr²⁺, oxygen defects are caused, and the gas is adsorbed in the pores. With the further doping of Sr²⁺, the adsorption oxygen saturation does not change. With the increase in temperature, the gas is sintered out, the b-axis shrinks at the same time, and the pore size becomes smaller, resulting in the negative expansion property.

Figure 6a is the XPS spectrum of Sm_0.85Sr_0.15MnO_3-δ; the characteristic peaks of Sm, Sr, Mn, and O are shown in the figure, respectively. The surface of the sample was free from any pollutants, and element C was used for the calibration of the XPS atlas. Figure 6b,c show the XPS spectra of Mn. In the XPS spectrum, the sample had a bimodal structure, which indicates that the Mn elements on the sample surface existed in two forms: Mn³⁺ and Mn⁴⁺, which led to the oxygen vacancy. The presence of the oxygen vacancy facilitated the movement of electrons between Mn⁴⁺ and Mn³⁺. The oxygen vacancy also led to the shortening of the bond length of the Mn-O bond, which led to lattice distortion and generated internal stress; this reduced the bond angle of Mn-O-Mn and increased the double-exchange effect.

3.3. Discussion

SmMnO₃ is a typical manganite perovskite structure. The structure of SmMnO₃ is shown in Figure 7. As for the MnO₆ octahedron in SmMnO₃, the distortion was caused by a change in the length of the Mn-O bond.

There are three kinds of common modes for this change [22,23,24,25,26] as follows. (1) The surface tension contract model Q₁, as shown in Figure 8a. Six oxygen atoms of the unit cell move close to or far away from the manganese atom at the same time, making the Mn-O bond length decrease or increase significantly. This model can increase the energy of the system, which is not conducive to the system energy being able to decrease and makes the system extremely unstable in turn. (2) The plane distortion model Q₂, as shown in Figure 8b. In a unit, two oxygen atoms in the horizontal plane leave a manganese atom, while the other two oxygen atoms become close to the manganese atom. The location of the two oxygen atoms in the vertical plane remains unchanged. (3) The expansion mode, or inspiratory mode Q₃, which is shown in Figure 8c. In a MnO₆ octahedron, the two oxygen atoms in the vertical plane leave manganese atoms, while the four oxygen atoms in the horizontal plane become close to the manganese atom simultaneously. For a MnO₆ octahedron, the Q₁ and Q₂ models normally exist. Since the Q₁ model is unstable, the distortion of the MnO₆ octahedron is mainly the Q₂ model, also called the plane distortion model.

We used MnO₂, Sm₂O₃, and SrO as the raw materials to prepare Sm_0.85Sr_0.15MnO_3-δ. In the reaction process, there was a reciprocal transformation between Mn³⁺ and Mn⁴⁺.

When Sr²⁺ substitutes the Sm³⁺ in SmMnO₃, Sr²⁺ will occupy the position of Sm³⁺. To maintain the valence balance, electron transfer occurs in the Mn³⁺ converting into Mn⁴⁺ in Sm_0.85Sr_0.15MnO_3-δ. Additionally, the p electron of O²⁻ will migrate to the orbit of the nearby Mn⁴⁺, and the d electron of Mn³⁺ will migrate to the orbit of the nearby Mn³⁺. Thus, this mechanism results in the electronic conduction and position exchanges of Mn⁴⁺ and Mn³⁺ ions. The system energy remains unchanged throughout. This process is known as the double exchange [27]. The structure of Mn³⁺-O²⁻-Mn⁴⁺ forms in the process. However, according to the theory of Zener [28], the route of electron transfer between two Mn³⁺ changes between Mn³⁺ and Mn⁴⁺. In order to keep the electron transfer between two Mn³⁺, the magnetic moment between Mn³⁺ and Mn⁴⁺ ions should be parallel to each other. In this situation, it is favorable for there to be more electron transfer between Mn³⁺ and Mn⁴⁺ ions.

According to the analysis of the variable-temperature XRD data, we considered that the thermal property of Sm_0.85Sr_0.15MnO_3-δ might be related to the interaction of the lattice vibration and electron transfer between Mn³⁺ and Mn⁴⁺. As the temperature rose, the lattice vibrated dramatically and Mn⁴⁺ converted into Mn³⁺. Moreover, the electron transfer rate increased between the Mn³⁺ and Mn⁴⁺ ions. The number of Mn³⁺ ions that can cause the Jahn–Teller [29] effect increased. The oxygen ions in the Mn³⁺O₆ octahedron became slant, or even produced oxygen defects, making the unit cell volumes decrease. From RT to 360 K, the unit cell volume increased. The reason is that the contribution of the lattice vibration to the thermal expansion exceeded that of the MnO₆ octahedral distortion and oxygen defects. As the temperature increased, Sm_0.85Sr_0.15MnO_3-δ showed a low positive thermal expansion property, and above 360 K, the unit cell volume decreased. With more Mn⁴⁺ ions converting into Mn³⁺, the Mn³⁺O₆ octahedral distortion was enhanced and oxygen defects occurred. These contributed more to the thermal expansion than the lattice vibration. Therefore, Sm_0.85Sr_0.15MnO_3-δ shows a negative thermal expansion property between 360 K and 873 K.

The DSC and TGA results of ceramic Sm_0.85Sr_0.15MnO_3-δ also support the above statements. Figure 9a presents the DSC curve of ceramic Sm_0.85Sr_0.15MnO_3-δ. In the curve, Sm_0.85Sr_0.15MnO_3-δ has an endothermic peak at about 360 K. This shows that more Mn⁴⁺ ions were converted to Mn³⁺ with the increase in temperature. Thus, Mn³⁺O₆ octahedral distortion was enhanced and oxygen defects occurred. The unit cell volume began to decrease, which is consistent with the results calculated by the high-temperature XRD (seeing b in Figure 5). As electron transfer occurred between the Mn³⁺ and Mn⁴⁺ ions, the amount of Mn⁴⁺ decreased, and oxygen ions in the Mn³⁺O₆ octahedron became slant or even produced oxygen defects. The TGA results of Sm_0.85Sr_0.15MnO_3-δ confirm the existence of oxygen defects. In Figure 8b, the weight of the Sm_0.85Sr_0.15MnO_3-δ sample decreased when the temperature increased from RT to 873 K. In addition, the variable-temperature XRD (seeing Figure 5) showed that there was no phase transition with the increase in temperature. As electron transfer occurred between the Mn³⁺ and Mn⁴⁺ ions, Mn⁴⁺O₆ converted into Mn³⁺O₆ and oxygen defects appeared. Therefore, we consider that the loss of the weight can be ascribed to the oxygen defects.

Moreover, the non-stoichiometric ratio of Sm_0.85Sr_0.15MnO_3-δ caused the mole ratio mismatch of the Sm, Mn, and O atoms. Some lattice vacancies and interstitials existed in the crystal lattice, making the lattice distortion continuous. In the structure analysis, the crystal distortion was found to have a direct impact on the bond length and angle of the MnO₆ octahedron. As for ABO₃, when we conducted the substitution in the A position with a different ionic radius, especially in the non-stoichiometric ratio manganese perovskite, the size mismatch effects of the A position ion together with lattice space and interstitial caused a difference in the crystal structure. These eventually led to a great change in the lattice parameters and unit cell size [30,31,32].

4. Conclusions

(1)

A novel negative thermal expansion material composed of Sm_0.85Sr_0.15MnO_3-δ was synthesized using the solid-state method with an NTE coefficient of −10.08 × 10⁻⁶/K from 360 to 873 K.

(2)

The particles were homogenous spherical or elliptic–spherical particles with a uniform particle size of about 1~2 μm.

(3)

The ceramic Sm_0.85Sr_0.15MnO_3-δ crystallized in an orthorhombic structure with the space group Pbnm. When Sr²⁺ substituted the Sm³⁺ in SmMnO₃, Sr²⁺ occupied the position of Sm³⁺. To maintain the valence balance, electronic transfer occurred in the Mn³⁺, converting into Mn⁴⁺ in Sm_0.85Sr_0.15MnO_3-δ. The Mn³⁺-O²⁻-Mn⁴⁺ structure formed in the process.

(4)

The thermal property of Sm_0.85Sr_0.15MnO_3-δ is considered to be related to the interaction of the lattice vibration and electron transfer between Mn ions. As the temperature rise, the lattice vibrated dramatically and more Mn³⁺ converted into Mn⁴⁺. Additionally, the electron transfer rate increased between the Mn³⁺ and Mn⁴⁺ ions as the temperatures increased. The number of Mn³⁺ ions that can cause the Jahn–Teller effect increasesd. The oxygen ions in the Mn³⁺O₆ octahedron became slant or even produced oxygen defects. The contributions of the lattice vibrations and electron transfer between Mn³⁺ and Mn⁴⁺ to the thermal expansion changed with the increasing temperature.

(5)

The pore energy in the sintered body partially absorbed the expansion of the a-axis a and the c-axis; the negative expansion phenomenon can be explained from the perspective of the contraction of the b-axis. The abnormal thermal expansion behavior of the Sm_0.85Sr_0.15MnO_3-δ perovskite system is caused by the presence of pores in the sintered body combined with the negative expansion of the b-axis in the perovskite system.