## 2 Experimental Section  
The experimental samples were synthesized via solid-phase sintering. The initial components were zirconium oxide (Sigma Aldrich, Burlington, MA, USA, CAS Registry number 1314-23-4, particle size \(<\) 5 \(\upmu\)m, density 5.89 g/cm\({}^{3}\), purity \(\geq\) 99.99%) and calcium carbonate (Sigma Aldrich, Burlington, MA, USA, CAS Registry Number 471-34-1, particle size \(<\) 5 \(\upmu\)m, density 2.93 g/cm\({}^{3}\), purity \(\geq\) 99.99%). In the initial mixture, the concentrations of CaCO\({}_{3}\) (dopant) expressed through the weight fraction of C\({}_{\rm w}\) were 0.05, 0.10, 0.15, 0.20, and 0.25. In the first step, the initial mixture was homogenized via dry milling. Prior to the milling process, the initial powders were weighed using high-precision scales according to the weight fraction. For homogenization, the planetary mill PULVERISETTE 6 classic line (Fritsch International, Idar-Oberstein, Germany) with tungsten carbide balls was used. Dry milling of the charge was carried out for 30 min at 250 rpm; the ratio of balls to mixture was 2:1. In the second step, the resulting homogenized powder was mixed with the binder of an aqueous solution of polyvinyl alcohol (JSC EuroChem, Kovdor, Russia, 16/1 grade, CAS Registry Number 9002-89-5, molecular weight 31,000-50,000) in the required ratio (about 1 wt.% of the polyvinyl alcohol in the dry residue). In the third step, green pellets from pressed powders were formed via pressing. A hydraulic press and a stainless steel mold were used to produce green pellets of the resulting powder in the form of tablets with a thickness of \(\sim\)1 mm and a diameter of 12.1 mm by applying a pressure of \(\sim\)200 MPa. In the final step, the samples were sintered in the air at 1500 \({}^{\circ}\)C for 5 h in a muffle furnace using a corundum crucible.  
The obtained ceramic tablets were investigated via X-ray powder diffraction (XRD) using a D8 Advance Eco diffractometer (Bruker GmbH, Mannheim, Germany), Raman spectroscopy using an Enspectr M532 spectrometer (Spectr-M LLC, Chernogolovka, Russia), and dielectric spectroscopy using a HIOKI IM3533 LCR meter (HIOKI E.E. Corporation, Ueda, Japan). To perform dielectric measurements, silver paste was applied to both sides of the tablet surface to create a plain capacitor structure. Micrographs were taken on cross-sections of the tablets using a Hitachi TM 3030 scanning electron microscope (SEM) (Hitachi Ltd., Tokyo, Japan) at an accelerating voltage of 15 kV. Elemental analysis and elemental mapping were performed using a Quantax 70 unit (Bruker GmbH, Mannheim, Germany). The morphological parameters (the grains' size and distribution) were evaluated using ImageJ software 1.54f. Before grain size calculation, the digital images obtained via SEM were adjusted to obtain a clear, high-contrast view of the grains. After that, automatic and manual defining of the grain outlines with calculation of the Feret diameter were carried out. The microhardness of the obtained ceramics was determined using the Vickers method with a diamond indenter on a METKON Duroline-M microhardness tester (Metkon Instruments Inc., Bursa, Turkey) under a load of 0.2 kgf at a dwell time of 10 s. Before the hardness test, the surface of the tablets was gently ground and polished according to the procedures in DIN EN ISO 6507-2:2018 [23] and ISO 14705:2016 [24].