Effect of strontium substitution on growth and piezoelectric properties of calcium magnesium silicate Ca₂MgSi₂O₇ single crystals

Piezoelectric materials have attracted attention for medical, civil engineering, and information technology applications. ¹⁾ Piezoelectric materials can be broadly classified into polar and nonpolar piezoelectrics. For example, BaTiO₃, Pb(Zr,Ti)O₃, and (K,Na)NbO₃ are polar piezoelectrics, and α-SiO₂ (quartz) and langasite-family are non-polar piezoelectrics. Polar piezoelectric materials generally have a high dielectric constant and high piezoelectric constant d_ij; hence, they have been used in medical transducer and actuator devices. ^2–6) On the other hand, nonpolar piezoelectric materials show a low dielectric constant and low d_ij, but have higher phase transition temperatures. Therefore, their possible applications include gas sensors, ⁷⁾ microbalance, ⁸⁾ and combustion pressure sensors, ⁹⁾ which operate at temperatures higher than 300 °C. Promising candidates are langasite La₃Ga₅SiO₁₄-type crystals, ¹⁰⁾ rare-earth calcium oxoborate ReCa₄O(BO₃)₃ (Re = Y, La–Lu) crystals, ^11,12) gallium phosphate GaPO₄, ^8,13) and aluminum nitride AlN and its related nitride thick films, ^14–17) which have been investigated vigorously. These crystals and thin films have one or more shortcomings in applications, as mentioned in Ref. 18.

In the last ten years, melilite-type crystals such as Ca₂Al₂SiO₇ (CAS) ¹⁹⁾ and CaYAl₃O₇ crystal ²⁰⁾ have been studied. The melilite-type crystals possess the tetragonal symmetry and belong to $P\bar{4}{2}_{1}m$ space group. The crystal has two kinds of piezoelectric constants (d₁₄ and d₃₆) in shear mode. In compression mode with the piezoelectric d'₃₁ constant used for pressure sensor applications, piezoelectric responses are obtained by using crystal substrates with both (XYt)θ° rotation cut for d₁₄ and (ZXt)θ° rotation cut for d₃₆. ^21,22) The melilite-type single crystal exhibits both high resistivity and stable piezoelectric properties at high temperatures. For the CAS crystal, because d₁₄ is higher than d₃₆, the compressive strength of the (XYt)45° cut was measured in Ref. 18. However, the compressive strength of the (XYt)45° cut of the CAS crystal is 140 MPa, and this value is insufficient for practical applications. There is clear cleavage plane (001) normal to the crystallographic c-axis in the melilite-type crystals. Because the shear stress direction on the (XYt)45° cut is in c-plane (001), this cause a low mechanical strength. Therefore, another piezoelectric d'₃₁ constant, originating from d₃₆ on the (ZXt)45° cut is attractive because the compressive strength of the (ZXt)45° cut is greater than 800 MPa. ²³⁾ Among melilite-type crystals, calcium magnesium silicate Ca₂MgSi₂O₇ (CMS), known as the mineral akermanite, shows a d₃₆ value of more than 4 pC/N ^24,25); hence, d'₃₁ should be comparable to d₁₁ (2.20 pC/N ²⁶⁾) of quartz crystals. CMS crystals are grown from a melt with stoichiometric composition using the conventional Czochralski (Cz) method. ^25,27) Because CMS shows a commensurate-incommensurate phase transition occurring at approximately 85 °C, ²⁸⁾ strontium substitution into CMS was successfully applied. Although the phase transition temperature decreases to less than −50 °C, ²³⁾ the effect of Sr substitution on the crystal structure and electrical properties remains unknown. A previous report ²⁹⁾ demonstrated that a Sr-substituted CMS (Ca_1−x Sr_x MgSi₂O₇: CSMS100x) single crystal with x up to 0.64 was obtained. However, the details of the crystal growth have not been reported. Another report ²³⁾ showed that colorless and transparent CSMS100x single crystals for x = 0.3 were grown by the Cz method. Therefore, there have been no reports on the growth of CSMS100x single crystals for 0.3 < x < 0.64, and their electrical properties are unknown.

In this study, the effect of Sr substitution on the growth, structural, and electrical properties of CMS piezoelectric single crystals was investigated. In addition, the relationship between the crystal structure and the piezoelectric properties of melilite-type crystals was demonstrated.

Crystal growth of CMS and CSMS100x was conducted using the RF-heating Cz method. As starting materials, powders of CaCO₃, SrCO₃, MgO, and SiO₂ with 99.99% purity were used. We prepared these powders with stoichiometric amounts corresponding to CMS and CSMS100x. In this study, the chemical composition of CSMS100x with x = 0.3–0.6 was selected based on previous studies. ^23,29) The powders were then mixed in ethanol, dried, and calcined at 1350 °C for 5 h in air. Powder X-ray diffraction (XRD) was conducted to determine the phases of the calcined powders. The calcined powder was compacted to became pellet. The pellets were charged in a platinum crucible (diameter and height: 50 mm). The growth atmosphere consisted of argon gas flow and oxygen gas flow at rates of 5 × 10⁻⁴ and 5 × 10⁻⁶ m³ min⁻¹, respectively. These gases were mixed in the furnace. By using a CSMS10 single crystal reported in Ref. 23, rectangular bars with dimensions of 3 mm × 3 mm × 30 mm were fabricated and used as the seeds. The rotation rate and pulling rate were 10 rpm and 1.0 mm h⁻¹, respectively.

Phase identification and the chemical composition analysis of the as-grown crystals was performed using powder XRD analysis and an electron probe microanalyzer (EPMA), respectively. Density of the as-grown crystals was calculated using the lattice parameter determined by the powder XRD analysis. The crystals were cut from the grown boules and were crushed to size of less than 100 μm. These crystals were used on single-crystal XRD analysis. Using a Rigaku automated four-circle diffractometer, we collected the diffraction intensities at RT. Using the Rigaku program, the intensities were corrected for the absorption, polarization, and Lorentz factors. We refined the crystal structure using the full-matrix least-squares program SHELXL97. ³⁰⁾ The electro-acoustical constants (dielectric, piezoelectric, and elastic compliance constants) of single crystals made of CMS, CSMS30, and CSMS40 were determined using an NF LCR meter ZM2376. We fabricated equivalent (ZXt)45° cut plate resonators with dimensions 12 mm × 3 mm × 0.8 mm. In this study, the piezoelectric constant ${d^{\prime} }_{31}$ expressed by the equation ${d^{\prime} }_{31}=\left({d}_{36}\sin 2\theta \right)/2$ using the piezoelectric constant d₃₆ and rotation angle θ of the substrate was focused and evaluated by measuring the resonant and anti-resonant frequencies of the resonators in length-extensional mode.

We have grown the CSMS100x crystals with x = 0.0 (CMS) and x = 0.3 – 0.6 (CSMS30, CSMS40, CSMS50 and CSMS60) by the Czochralski method. The solidification fraction g = W_crystal/W_initial, where W_crystal and W_initial are the weights of the grown crystal and starting melt, respectively, was calculated to be approximately 0.30 for all grown crystals. The as-grown CMS, CSMS40, and CSMS60 crystals are shown in Fig. 1. The CMS and CSMS30 crystals were transparent and colorless. Moreover, these crystals had smooth surfaces and no inclusions. The CSMS40 crystal was transparent and colorless but had many bubbles from the middle to the end of the crystal boule, as shown in Fig. 1(b). The CSMS50 crystal was also transparent but had numerous bubbles throughout the crystal. On the other hand, the CSMS60 crystal shown in Fig. 1(c) was composed of a mixture of transparent and opaque parts. From these results, CMS, CSMS30, and CSMS40 crystals were selected to determine electro-acoustical constants.

Zoom In Zoom Out Reset image size

Figure 2 shows the XRD profiles of the CSMS100x powders obtained by pulverizing the crystal samples. From CMS to CSMS50, all peaks in the powder XRD patterns of the boules were identified to be those of akermanite Ca₂MgSi₂O₇ phase without any impurity phases. With increasing Sr-content x, all peaks were gradually shifted toward to lower diffraction angles. This result is reasonable from the viewpoint of the ionic size [Ca²⁺ (r^VIII = 1.12 Å) and Sr²⁺ (1.26 Å)] ³¹⁾ and suggested that the Sr²⁺ cation was introduced into the Ca²⁺ site. In the CSMS60, akermanite and mervinite Ca₃MgSi₂O₈ phases were observed.

Zoom In Zoom Out Reset image size

Figure 3 shows photographs and scanning electron microscope (SEM) image of the cross-section of the upper part of the CSMS60 crystal. As mentioned above, the CSMS60 crystal is composed of a mixture of transparent and opaque parts. Comparing Figs. 3(c) and 3(d), the transparent and opaque parts are named areas A and B, respectively. The chemical compositions of areas A and B determined by EPMA were (Ca+Sr):Mg:Si ratios 2.11:1.01:2 and 3.33:1.02:2, respectively. This result indicates that the opaque parts (area B) are Sr-substituted mervinite Ca₃MgSi₂O₈ phase. Therefore, it was found that the solubility limit of Sr in CMS is x = 0.5, under the growth conditions set in this study. This observation is consistent with that reported in Ref. 29 and phase diagram of the Ca₂MgSi₂O₇–Sr₂MgSi₂O₇ binary system. ³²⁾

Zoom In Zoom Out Reset image size

The lattice parameters a and c of the CSMS100x crystals were determined using 2θ_dif values of the corresponding diffraction peaks shown in Fig. 2. Figure 4 shows Sr-content x in CSMS100x crystals versus lattice parameter a, c and crystallographic a/c ratio. Both the a and c values increased with the Sr content x, but the crystallographic a/c ratio decreased with increasing x. This result indicates that the CSMS100x crystal lattice is enlarged anisotropically by introducing Sr²⁺ cations into the Ca²⁺ site; that is, the c-axis expands more easily than the a-axis.

Zoom In Zoom Out Reset image size

Figure 5 shows the frequency dependences of impedance |Z| and phase θ_p measured in k₃₁ mode for the CSMS40 crystals at RT. The phase value θ_p was found to be on the order of 88.7° and no spurious mode of vibration was shown. Electromechanical coupling factor k^' ₃₁ and piezoelectric constant ${d^{\prime} }_{31}$ were evaluated by measuring the mechanical series resonance frequency f_s and parallel resonance frequency f_p of resonators. The electro-acoustic properties of the CSMS100x crystals are listed in Table I.

Zoom In Zoom Out Reset image size

The influence of the Sr content x in CSMS100x crystals on the piezoelectric constant d₃₆ is shown in Fig. 6. The d₃₆ value decreased with increasing Sr content, x. The relationship between the piezoelectric constants (d_14, d₃₆) and the polyhedral distortion Δ for the 8-coordinated site of melilite-type crystals was reported in our previous report. ²³⁾ Δ is defined as ${\rm{\Delta }}=\left(1/N\right){\sum }_{i}^{N}{\left(\left({R}_{i}-\bar{R}\right)/\bar{R}\right)}^{2}$ where N is the coordination number, R_i is the individual bond length, and $\overline{R}$ is the average bond length. ³³⁾ In the reports, ²³⁾ the piezoelectric constant d₁₄ increases with Δ, on the other hand, d₃₆ slightly decreases. By single-crystal structure analysis, the Δ values for the 8-coordinated site of the CMS and CSMS30 crystals were determined to be 2.7 × 10⁻³ and 2.3 × 10⁻³, respectively. This means that the Δ value decreases with increasing Sr content x. Hence, in the CSMS100x crystals, d₃₆ increases with increasing the Δ value. This result is inconsistent with the trend reported in Ref. 23, another relationship must be determined.

Zoom In Zoom Out Reset image size

Figure 7 shows the dependence of d₃₆ on the crystallographic a/c ratio in the melilite-type crystals. The d₃₆ and the crystallographic a/c ratio data were obtained from the references (Ba₂MgGe₂O₇, ²⁴⁾ Sr₂MgGe₂O₇, ²⁴⁾ Sr₂MgSi₂O₇, ²⁴⁾ Ca₂ZnSi₂O₇, ²⁴⁾ Ba₂ZnGe₂O₇, ²⁴⁾ Sr₂ZnGe₂O₇, ²⁴⁾ BaLaGa₃O₇, ³⁴⁾ CaNdGa₃O₇, ³⁴⁾ Ca₂MgSi₂O₇, ²⁵⁾ Ca₂Ga₂SiO₇, ²⁵⁾ SrNdGa₃O₇, ³⁴⁾ Ca₂Al₂SiO₇ ³⁵⁾). From the figure, an increase in the crystallographic a/c ratio, that is, high tetragonality, leads to an increase in the piezoelectric constant d₃₆. This tendency can be explained as follows: an increase in the crystallographic a/c ratio implies that the a-axis is more easily enlarged than the c-axis. The ${d^{\prime} }_{31}$ ($=\left({d}_{36}\sin 2\theta \right)/2$) value of melilite-type crystals with tetragonal symmetry indicates the magnitude of the electric charge along the c-axis developed by the applied stress normal to the c-axis, that is, along direction rotated by θ from the a-axis. By increasing the a/c ratio, the cations can move more easily in the c-plane consisting of the a-axis of the crystal. This explanation is not universal because a positive correlation between d₁₄ and Δ in the melilite-type crystals is still observed, as reported in Ref. 23. These tendencies will provide a hint on how to improve piezoelectric properties of melilite-type crystals.

Zoom In Zoom Out Reset image size

The crystal growth of CMS and Sr-substituted CMS (CSMS100x) was conducted using the Cz method. In this study, the CSMS100x crystals without impurity phases and bubbles were grown up to x = 0.5 (CSMS50) and x = 0.4 (CSMS40), respectively. Powder XRD measurements clarified that both crystallographic axis c and a increased with Sr content x, and the crystallographic axis ratio a/c decreased. Single-crystal structure analysis showed that the polyhedral distortion of the Ca²⁺ site was relaxed by Sr substitution. The piezoelectric d₃₆ constants were obtained using CMS, CSMS30, and CSMS40 crystals. It was found that the piezoelectric constant d₃₆ decreased with increasing Sr content. We found that the independent piezoelectric constant, d₃₆, in the melilite-type crystal increased with an increase in a/c.

Part of this study was financially supported by JSPS KAKENHIs 20H05879 and 22H02162.