## 1 Introduction  
The use of renewable energy from different technologies to make a contribution to the world's total energy supply has grown fast. Among different energy conversion technologies, piezoelectric energy-harvesting has received considerable attention for powering small electronic components. The piezoelectric effect is a unique property that allows materials to convert mechanical energy to electrical energy or vice versa. Piezoelectric material selection is crucial in the design of piezoelectric energy harvesters. Numerous researchers are working in this area, but most of the solutions being employed have low power (nW) and high frequency [1,2]. Among the various piezoelectric materials, lead zirconate titanate (PZT) based ceramic is one of the most widely used because of its fast electro-mechanical response, relatively low power requirements, high generative force, and inherent durability [3, 4, 5, 6, 7]. Extensive research and development of PZT-based ceramics have been conducted to find such piezoelectric materials [1, 7, 8, 9, 10]. A variety of parameters is used to compare piezoelectric materials for energy-harvesting, such as electromechanical coupling factor, piezoelectric charge constant, output voltage, power density, energy-conversion efficiency, and cost and frequency of operation. Among them, energy-conversion efficiency is the most important when designing piezoelectric energy harvesters for real applications [11]. [PERSON] et al. proposed a new figure of merit (FOM), which combines the transduction efficiency and the energy-conversion capacity, for comparing piezoelectric materials for energy-harvesting applications [11]. [PERSON] et al. proposed a FOM for a polycrystalline piezoelectric energy-harvesting device, which can be expressed by the following Equations (1) and (2) [12]:  
\[u=1/2\ (d.g)\ (\mathrm{F}/\mathrm{A})^{2} \tag{1}\]  
\[g=d/\varepsilon_{33}^{\mathrm{T}} \tag{2}\]  
where \(d\) is the piezoelectric charge constant, \(g\) is the piezoelectric voltage constant, \(\varepsilon_{33}^{\mathrm{T}}\) is the dielectric permittivity, F is the applied force, and A is the area. According to Equations (1) and (2), a material with a high \(d\) and a lower \(\varepsilon_{33}^{\mathrm{T}}\) will generate high FOM. In general, the piezoelectric charge constant and the dielectric constant increase or decrease concurrently upon modification with dopants or processing techniques [7; 8; 9; 10]. However, by modifying the fabrication technique and choosing suitable dopants, it is possible to produce high \(d_{33}\) with low \(\varepsilon_{33}^{\mathrm{T}}\). Previous work has reported on such a piezoelectric ceramic composition of \((1-x)\)Pb(Zr\({}_{0.53}\)Ti\({}_{0.47}\))O\({}_{3}\)-xBiYO\({}_{3}\) [PZT-BY] and its fabrication process [13]. In the present study, PZT-BY composition is further tuned using antimony oxide (both Sb\({}_{2}\)O\({}_{3}\) and Sb\({}_{2}\)O\({}_{5}\)) to improve the FOM. The Sb\({}_{2}\)O\({}_{3}\)- and Sb\({}_{2}\)O\({}_{5}\)-doped PZT has had a long history whereby Sb\({}_{2}\)O\({}_{3}\)/Sb\({}_{2}\)O\({}_{5}\) was doped with other oxides for tailoring piezoelectric/dielectric properties [14; 15; 16; 17; 18; 19; 20; 21; 22]. [PERSON] et al. reported a high electrical output in the lanthanum-doped Pb(Ni\({}_{1/3}\)Sb\({}_{2/3}\))-PbZrTiO\({}_{3}\) system which was comparable to the PZT type 5A [16]. [PERSON] et al. reported a high electrical output in the Sb\({}_{2}\)O\({}_{3}\)-doped Pb(Zr\({}_{0.54}\)Ti\({}_{0.46}\))O\({}_{3}\) system [14]. They explained that the ionic radius of Sb\({}^{3+}\) is smaller than that of other trivalence \"soft\" ions; this may be the most important factor for improving the piezoelectric activity. [PERSON] et al. reported a high electromechanical coupling factor and dielectric constant with relatively higher \(T_{\mathrm{c}}\) in the \(x\)PbZrO\({}_{3}\)-\((90-x)\)PbTiO\({}_{3}\)-\(10\)Pb(Ni\({}_{1/3}\), Sb\({}_{2/3}\))O\({}_{3}\) ternary system [17]. [PERSON] et al.