Enhancement of pyroelectricity in Mn-doped (011) 71Pb(Mg1/3Nb2/3)O3–6PbZrO3–23PbTiO3 single crystals Cite as: Appl. Phys. Lett. 119, 152903 (2021); https://doi.org/10.1063/5.0064776 Submitted: 27 July 2021 • Accepted: 02 October 2021 • Published Online: 14 October 2021 Atul Thakre, Seunguk Mun, Panithan Sriboriboon, et al. Appl. Phys. Lett. 119, 152903 (2021); https://doi.org/10.1063/5.0064776 © 2021 Author(s). 119, 152903 Applied Physics Letters ARTICLE scitation.org/journal/apl Enhancement of pyroelectricity in Mn-doped (011) 71Pb(Mg1/3Nb2/3)O3–6PbZrO3–23PbTiO3 single crystals Cite as: Appl. Phys. Lett. 119, 152903 (2021); doi: 10.1063/5.0064776 Submitted: 27 July 2021 . Accepted: 2 October 2021 . Published Online: 14 October 2021 Atul Thakre,1 Seunguk Mun,2 Panithan Sriboriboon,2 Shashank Priya,3 Yunseok Kim,2,a) and Jungho Ryu1,4,a) AFFILIATIONS 1 School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 38541, South Korea 2 School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon, Gyeonggi-do 16419, Republic of Korea 3 Materials Research Institute, Pennsylvania State University, Pennsylvania, Pennsylvania 16801, USA 4 Institute of Materials Technology, Yeungnam University, Gyeongsan 38541, South Korea a) Authors to whom correspondence should be addressed: yunseokkim@skku.edu and jhryu@ynu.ac.kr ABSTRACT Single crystals of 71PMN-6PZ-23PT [71Pb(Mg1/3Nb2/3)O3-6PbZrO3-23PbTiO3] oriented along the thickness direction (011) with and without Mn doping were grown by a solid-state single-crystal growth method, and pyroelectric properties of the crystals were investigated. Though the pyroelectric coefficient of a Mn doped crystal is not significantly higher than the un-doped one at room temperature (RT), a large enhancement was observed after 0.7 mol. % Mn doping at high temperatures (>100  C). Furthermore, the FoMs for practical applications at RT, the Mn doped crystal showed large enhancement as compared to the un-doped one. The presented single crystals also yielded excellent figure of merit (FoM) values for pyroelectricity: Fi, Fv, and FD were 3.5  1010 m V1, 0.02 m2 C1, and 2.68  105 Pa1/2, respectively, at RT. The large pyroelectric response in the Mn-doped single crystal is attributed to the large ferroelectric polarization and low dielectric constant and dielectric loss. The demonstrated pyroelectric response in the Mn-doped 71PMN-6PZ-23PT single crystal shows that it exhibits excellent potential for various thermal sensor applications. Published under an exclusive license by AIP Publishing. https://doi.org/10.1063/5.0064776 In modern life, pyroelectric materials have extensive applications such as sensors, imaging devices, and detectors.1 The degree of pyroelectricity of the material is characterized by its pyroelectric coefficient (p). The value of p reaches its maximum near the ferroelectric phase transition [ferroelectric (FE) to relaxor (RFE) or antiferroelectric (AFE) at depolarization temperature (Td) or FE to paraelectric at phase transition temperature (Tm)], where the maximum rate of change in the polarization (dPr/dT) occurs.1–5 Therefore, the enhancement of p can be achieved by obtaining an abrupt phase transition in FE, AFE, or RFE.1 Perovskite polycrystalline ceramics and single crystals have received significant attention from the scientific community and have been extensively studied as functional materials owing to their excellent ferroelectric properties.6–10 Over the past few decades, pyroelectric characteristics of several materials, such as barium strontium titanate (BaxSr1-xTiO3),11,12 lithium tantalate (LiTaO3),13,14 sodium-bismuth titanate (Na0.5Bi0.5TiO3),15 and triglycine sulfate (TGS),16 have been investigated and used as pyroelectric detectors. To improve the Appl. Phys. Lett. 119, 152903 (2021); doi: 10.1063/5.0064776 Published under an exclusive license by AIP Publishing pyroelectric response of materials, various efforts, such as controlling their microstructure and/or composition, have been made to enhance the figures of merit (FoM) and reduce the dielectric constant and dielectric loss based on the theory, as expressed in Eq. (1).17 (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3(PMN-xPT) single crystals, grown by the Bridgman technique, have been extensively studied and implemented for detector applications.18 To improve its pyroelectric characteristics and decrease the dielectric loss, various acceptors, such as Mn, have been incorporated into this system. Li et al. demonstrated the decreased dielectric loss of Mn-doped PMN-PT single crystals, 00 which was attributed to the generation of Mn Ti-Vo dipole defects, resulting in the pinning of domain walls and hardening of the crystal.19 Another group demonstrated similar high performance of infrared detection in Mn-doped PMN-PT(72–28).20 Recently, Neumann et al. reported excellent pyroelectric coefficient (750 lC m2 K1) and FoMs of Mn doped PIN-PMN-PT single crystals.21,22 Although Mndoped PMN-PT based ferroelectric single crystals have been studied in 119, 152903-1 Applied Physics Letters ARTICLE terms of their excellent piezoelectric properties, their role in pyroelectricity and the effect of Mn doping concentration on their pyroelectric characteristics have not been explored in detail. Recently, we reported the enhanced electromechanical quality factor (Qm) of a 0.7 mol. % Mn-doped 71Pb(Mg1/3Nb2/3)O3-6PbZrO3-23PbTiO3(71PMN-6PZ-23PT) single crystal, which was attributed to the dominant pinning of the domain walls.23 The pinning of the domain walls resulted in an enlarged internal bias electric field, which can significantly reduce the dielectric constant and dielectric loss. The significant decrease in the dielectric constant and dielectric loss in the 0.7 mol. % Mn doped 71PMN-6PZ-23PT single crystals indicates the enhancement in the pyroelectric FoMs. In this work, we report a comprehensive study of the impact of Mn doping on the pyroelectric properties of 71PMN-6PZ-23PT single crystals. Owing to its superior control over dopant concentration and homogeneity, the solid-state single-crystal growth (SSCG) method was used to prepare the single-crystal specimens. Among the Mn doping concentrations used, the maximum FoM, i.e., k2 Qm, was obtained for 0.7 mol. % Mn-doped 71PMN-6PZ-23PT single crystals. The obtained high FoM and decreased dielectric constant and dielectric loss indicate an excellent pyroelectric response, as described in Eq. (1), for the 0.7 mol. % Mn-doped 71PMN-6PZ-23PT single crystals.10 To evaluate pyroelectric materials for sensor applications, the FoM sensitivity, that is, FD, is very important. FD is given as p FD ¼  ; CV ðer e0 tan dÞ1=2 (1) where p is the pyroelectric coefficient, CV is the volume specific heat, er and e0 are the relative permittivity and absolute permittivity of free space, respectively, and tan d is the dielectric loss. Mn-doped71PMN-6PZ-23PT single crystals were grown by employing the SSCG method and cut (011) in the plane along the thickness direction.10,23,24 The single crystals were prepared at Ceracomp Co., Ltd. Details of the preparation of the single-crystal specimen are given elsewhere.24 To obtain the electrical characterization, Au/Ti electrodes were deposited using the DC sputtering technique. The temperature-dependent dielectric (at 100 Hz) and ferroelectric (at 10 Hz) properties of the single crystals were measured using an impedance analyzer (E4990A, Keysight Technology) and an aixACCT TF-2000 ferroelectric evaluation system coupled with a test oven in the temperature range of 25–250  C. The pyroelectric coefficient (p) measurements were performed over a temperature range of 25–150  C with a constant heating rate of 1  C min1 using a Keithley electrometer (6517 B) coupled with a high-temperature test oven (PolyK Tech., PA, USA) interfaced with LabView software. Piezoresponse force microscopy (PFM) measurements were conducted using a commercial instrument (NX10, Park Systems) in conjunction with a lock-in amplifier (SR830, Stanford Research Systems) using Pt/Cr-coated conductive atomic force microscopy (AFM) tips (Multi75E-G, Budget-Sensors). To calibrate the PFM amplitude, the tip used for the PFM measurement was calibrated with the forcedistance (F-D) curve measurement. The slope of the F-D curve was used to calculate the calibration factor of the inverse optical lever sensitivity (OLS) and was adapted to the PFM amplitude signal. As a result, the PFM amplitude was calibrated with the OLS factor and used to calculate the picometer per modulation voltage unit of pm/V. An AC Appl. Phys. Lett. 119, 152903 (2021); doi: 10.1063/5.0064776 Published under an exclusive license by AIP Publishing scitation.org/journal/apl modulation voltage of 1 V at 17 kHz was applied to the tip for the PFM measurements. The temperature-dependent dielectric characteristics of the undoped and Mn-doped 71PMN-6PZ-23PT single crystals were measured at 100 Hz over a wide temperature range of 25–250  C, and the results are shown in Fig. 1. The single crystals underwent a phase transition from rhombohedral to tetragonal at TRT and then from tetragonal to cubic at the phase transition temperature, Tm.24 The depolarization temperature (Td) occurred between TRT and Tm. The Mn-doped 71PMN-6PZ-23PT single crystals exhibited increased Td and Tm (by 27  C and 35  C, respectively) compared to their un-doped counterparts. To improve the pyroelectric FoM, that is, the detectivity FD, the dielectric constant and dielectric loss should be reduced, as described in Eq. (1). The Mn-doped single crystal showed significant decreases in the dielectric constant (from 2643 to 1915 at RT and from 41 000 to 23 500 at near Td) and the dielectric loss at the phase transition temperatures, which clearly indicates an improved FoM, that is, FD, as shown in Table I. Owing to the significant decrease in the dielectric constant and dielectric loss, the Mn-doped 71PMN-6PZ-23PT single crystals showed a higher pyroelectric response. To further investigate the ferroelectric phase transition in detail, the temperature-dependent polarization vs electric field (P–E) hysteresis loops at 10 Hz and 20 kV cm1 electric fields were measured over a temperature range of 25–150  C for doped and un-doped 71PMN6PZ-23PT single crystals (Fig. 2). The un-doped and doped single crystals showed maximum polarizations (Pmax) of nearly 32.6 and 34.3 lC cm2, respectively, whereas the recorded remnant polarizations (Pr) were 26.7 and 27.8 lC cm2, respectively. It can be noted here that the doping of Mn into the 71PMN-6PZ-23PT single crystal resulted in a significant shift in the P–E hysteresis loop toward the positive electric field, and the resultant shift in the internal bias electric field (Ei) for the 0.7 mol. % Mn-doped single crystal was measured as approximately 0.5 kV/cm. In our previous report, Ei was investigated for varying concentrations of Mn (0–1 mol. %) in 71PMN-6PZ-23PT single crystals.23 The significant value of Ei indicates the formation of oxygen vacancies, which eventually results in domain wall pinning FIG. 1. Temperature-dependent dielectric constant and loss tangent of the undoped and 0.7 mol. % Mn-doped (011)-oriented 71PMN-6PZ-23PT single crystals at 100 Hz over a wide temperature range of 25–250  C. 119, 152903-2 Applied Physics Letters ARTICLE scitation.org/journal/apl TABLE I. Comparison of the pyroelectric properties of [011]-oriented Mn-doped 71PMN-6PZ-23PT single crystals with those reported in the literature at RT. Composition 71PMN-6PZ-23PT Mn: 71PMN-6PZ-23PT Mn:PIMNT(29/31/41) PLZT(4/86/14)—ceramic 0.01Mn:0.025PMN-0.125PT-PZ ceramic Mn-PIN-PMN-PT crystals Mn-KNN-BKT LiTaO3 crystals Td Tm p (lC m2 K1) at RT ( C) ( C) err (at 100 Hz) tand Fi Fv FD (1010 mV1) (m2 C1) (105 Pa1/2) Reference 120 147   118 124 158 216   440 870 730  316 2643 1915 350 667 196 0.25 0.01 0.000 5 0.011 0.014 1.76 3.5 2.92 2.9 - 0.001 0.02 0.094 0.048 0.073 0.23 2.67 23.5 3.54 4.33 This work 210 620    600 218 230 335 980 47 0.000 7 0.035 0.000 5 2.4 0.994 0.72 0.08 0.011 0.17 19.8 0.571 15.7 7 6 1 owing to the doping of Mn2þ ions at the B-site of the 71PMN-6PZ23PT single-crystal lattice. These oxygen vacancies accumulate at the domain walls, which further reduce the domain wall motion and cause the pinning effect. The domain wall pinning effect eventually decreases the dielectric constant and dielectric loss.25 At room temperature (RT, 25  C), the 71PMN-6PZ-23PT single crystals exhibited a typical hysteresis shape for the ferroelectric phase. As the temperature increased, the P–E hysteresis loops for both types of single crystals, the 0.7 mol. %-doped single crystals show a much sharper phase transition than the un-doped one. The Pr value decreased for both types of single crystals, and the P–E hysteresis loops became extremely slim. The drastic change in the polarization of the Mn-doped 71PMN-6PZ23PT single crystals as the temperature changed clearly indicates the potential for a higher pyroelectric response. To further investigate the ferroelectric domain information, PFM measurements were performed.24,25 From the PFM analysis results, the amplitude and the phase signal can be interpreted as the magnitude of the piezoresponse and the direction of polarization, respectively. As shown in Figs. 3(a)–3(f), the PFM phase and the amplitude were significantly different from each other. For the 0.7 mol. % Mndoped 71PMN-6PZ-23PT single crystal, the amplitude, that is, the 9 8 29 piezoresponse, was somewhat higher than that of the un-doped one. Furthermore, although the phase for the un-doped 71PMN-6PZ-23PT single crystal appeared to be rather noisy, the phase for the 0.7 mol. % Mn-doped 71PMN-6PZ-23PT single crystal became clear. These results can be clearly observed in the histograms of the PFM amplitude and phase, shown in Figs. 3(g) and 3(h), respectively. The PFM amplitude of the 0.7 mol. % Mn-doped 71PMN-6PZ-23PT single crystal was higher than that of the un-doped crystal. We note that the average value of the amplitude increased from 3.93 to 4.88 pm/Vac after Mn doping. The enhanced value of the PFM amplitude confirms the enhanced piezoresponse in the Mn-doped 71PMN-6PZ-23PT single crystals. Thus, the relatively higher amplitude and clear phase contrast might be responsible for the enhanced polarization. Therefore, when subjected to a temperature change, the 0.7 mol. % Mn doped single crystal exhibits higher rate of change in the polarization with a change in temperature (dP/dT). Furthermore, it eventually results in the enhanced pyroelectric response in the Mn-doped 71PMN-6PZ-23PT single crystals.26 The pyroelectric coefficient, p, can also be determined using the temperature-dependent P–E hysteresis loops by calculating the rate of change of remnant polarization (Pr) with respect to temperature (T), FIG. 2. Temperature-dependent P–E hysteresis loops of the un-doped and 0.7 mol. % Mn-doped [011]-oriented 71PMN-6PZ-23PT single crystals at 10 Hz over a wide temperature range of 25–150  C. Appl. Phys. Lett. 119, 152903 (2021); doi: 10.1063/5.0064776 Published under an exclusive license by AIP Publishing 119, 152903-3 Applied Physics Letters ARTICLE scitation.org/journal/apl FIG. 3. PFM response and statistic histogram of un-doped and 0.7 mol. % Mn doped [001] oriented 71PMN-6PZ-23PT single crystals (scan size 10  10 lm2). Topography of (a) the un-doped and (d) the 0.7 mol. % doped. PFM amplitude of the (b) un-doped and the (e) 0.7 mol. % doped. PFM phase of the (c) un-doped and (f) the 0.7 mol. % doped. (g) and (h) Histogram of the PFM amplitude extracted from the amplitude of (b) and (e), respectively. that is, dPr/dT, as shown in Fig. 4(a).27 In this measurement, the temperature was controlled manually using an oil bath and a hot plate; therefore, the rate of temperature change (dT) could not be maintained 1  C/min. Therefore, the obtained p values were lower than the values obtained in the thermally stimulated discharge current (TSDC) measurement. This corresponds to the pyroelectric coefficients of the un-doped and Mn-doped single crystals. The single crystals without Mn doping showed a comparatively gradual decrease in the remnant polarization (Pr) over the temperature range, whereas a higher rate of change in Pr was observed in the 0.7 mol. % Mn-doped single crystals, as shown in Fig. 4(a). The single crystals underwent a sharp phase transition near the Td and Tm temperatures and entered the paraelectric phase. Because the Mn-doped 71PMN-6PZ-23PT single crystals Appl. Phys. Lett. 119, 152903 (2021); doi: 10.1063/5.0064776 Published under an exclusive license by AIP Publishing exhibited excellent enhancement in dPr/dT, the actual temperaturedependent pyroelectric coefficient was measured by recording the TSDC generated by temperature fluctuations in the temperature range of 25–150  C with a uniform heating rate of 1  C/min, as shown in Fig. 4(b). The pyroelectric responses of the single crystals were consistent with the dPr/dT plots. At room temperature, the p values of the un-doped and Mn-doped 71PMN-6PZ-23PT single crystals were 540 and 871 lC m2 K1, respectively. However, at near Td, the p value obtained from the un-doped single crystal was 108 155 lC m2 K1, whereas the Mn-doped single crystal exhibited a large enhancement to 205 631 lC m2 K1. The large enhancement in the Mn-doped 71PMN-6PZ-23PT single crystals can be attributed to the larger polarization and the lower dielectric constant and dielectric loss. It should 119, 152903-4 Applied Physics Letters ARTICLE scitation.org/journal/apl FIG. 4. Temperature vs pyroelectric coefficients of the un-doped and 0.7 mol. % Mn-doped [011]-oriented 71PMN-6PZ-23PT single crystals calculated from (a) temperaturedependent P–E hysteresis loops at 10 Hz (dPr/dT) over a wide temperature range of 25–150  C and (b) thermally stimulated depolarization current. (c) and (d) The temperature dependent FoMs (Fv and FD) plots for the un-doped and 0.7 mol. % Mn-doped [011]-oriented 71PMN-6PZ-23PT single crystals at 100 Hz. be emphasized that these pyroelectric coefficients were calculated from the TSDC measurement performed on heating only. Therefore, these TSDC values may also have contributions from the release of trapped charges and nonreversible effects due to domain wall motions.27 The latter effects are likely to occur near Td and generally account for the very large coefficients in this region. Thus, the RT pyroelectric coefficients used for the calculations of the FoM values were not demonstrated experimentally to be truly reversible values, which would also need measurements on cooling to verify them and should only be taken as indicative values at this stage. The obtained pyroelectric properties of the 71PMN-6PZ-23PT single crystals were compared with those reported in the literature, and the results are listed in Table I. The FoM values, such as current responsivity Fi, voltage responsivity Fv, and detectivity FD [Eq. (1)], were calculated using the following equations: p ; qcP p ; Fv ¼ qcP e0 er Fi ¼ (2) (3) where cP is the specific heat capacity (2.5 J K1 cm3), q is the density of the pyroelectric material, e0 is the absolute permittivity of free space (8.854  1012 F m1), and er is the relative dielectric permittivity of the pyroelectric material.7,27,28 Table I shows that the Mn-doped 71PMN6PZ-23PT single crystals exhibited excellent enhancement in the pyroelectric properties and FoM values compared to the un-doped single crystal. The FoM values at RT for the Mn-doped single crystals were Appl. Phys. Lett. 119, 152903 (2021); doi: 10.1063/5.0064776 Published under an exclusive license by AIP Publishing Fi ¼ 3.5  1010m/V, Fv ¼ 0.02 m2/C, and FD ¼ 2.67  105 Pa1/2. The temperature dependent pyroelectric FoMs, such as Fv and FD, calculated at 100 Hz have been shown in Figs. 4(c) and 4(d). It can be clearly observed from Figs. 4(c) and 4(d) that the 0.7 mol. % Mn doped 71PMN-6PZ-23PT single crystals exhibit enhanced values after doping. The effects of Mn doping on 71PMN-6PZ-23PT single crystals in terms of their dielectric, ferroelectric, and pyroelectric properties were comprehensively investigated. The doping of 0.7 mol. % Mn into the 71PMN-6PZ-23PT single crystals significantly reduced the dielectric constant and dielectric loss and enhanced the polarization, which eventually resulted in an enhanced pyroelectric response and corresponding FoM values. As confirmed by the PFM amplitude and phase data, the Mn-doped single crystals showed improved piezoresponse, which resulted in enhanced polarization. The Mndoped single crystals exhibited a higher depolarization temperature and a higher pyroelectric coefficient at higher temperatures (>100  C). Owing to the reduced dielectric constant and dielectric loss, the FoMs for practical applications at RT, the Mn doped crystal showed large enhancement as compared to the un-doped one. FoM values at RT with Fi ¼ 3.5  1010 mV1, Fv ¼ 0.02 m2 C1, and FD ¼ 2.67  105 Pa1/2. The present study demonstrated that Mndoped 71PMN-6PZ-23PT single crystals have excellent potential for pyroelectric applications. This study was supported by the National Research Foundation of Korea (Nos. NRF-2019R1A2B5B01070100 and NRF2021R1A2C2009642) and a National Research Council of Science 119, 152903-5 Applied Physics Letters and Technology (NST) grant by the Korean Government (MSIP) (No. CAP-17-04-KRISS). DATA AVAILABILITY The data that support the findings of this study are available from the corresponding authors upon reasonable request. REFERENCES 1 R. W. Whatmore, Rep. Prog. Phys. 49, 1335 (1986). A. Thakre, A. Kumar, H.-C. Song, D.-Y. Jeong, and J. Ryu, Sensors 19, 2170 (2019). N. M. Shorrocks, A. 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