I. Introduction
Barium titanate, the prototype of ferroelectric perovskites, has attracted since its discovery a significant and constant interest, due to its outstanding performances useful for practical devices, as high permittivity materials for multilayer ceramic capacitors, high electrostrictive coefficients materials for electronic, electrostriction and pulse generating devices, transducers, infrared detectors, positive temperature coefficient thermistors, and tunable elements in wireless communication [1]–[3]. In particular, remarkable tailoring of functional characteristics can be achieved by substitution with foreign ions on the A, B, or in both sites of the BaTiO3 perovskite unit cell. Isovalent substitution with M4+ ions such as Zr [4]–[6], Sn [7]–[10], and Ce [11]–[13] onto the Ti4+ site gave rise to enhanced material constants (permittivity and pyro- and piezo-electric constants), high thermal stability in a large temperature range, and a diminished and tunability hysteresis loops. Isovalent substitution on B-site with larger ions than Ti4+, such as Zr4+ or Sn4+ induce strong modifications of the structural-phase transitions with phase polymorph overlapping for specific compositions, and a ferroelectric–relaxor crossover when increasing the dopant addition [4]–[10], while the substitution with smaller radius ions (such as Si or Ge), far less investigated, apparently does not modify the structural transition temperatures, room temperature tetragonality, and ferroelectric character. There are only few reports concerning the role of Ge or Si addition, which mostly were used as sintering aids to promote a better densification of BaTiO3-based ceramics at a lower sintering temperature [14]–[16]. Early studies reported the first properties of BaTi0.9Si0.1O3 and BaTi0.9Ge0.1O3 ceramics [17] and the role of additives with limited solubility in BaTiO3 matrix [18]. In these reports, no shift of the ferro–para transition temperature and a reduction of permittivity with respect to values of pure BaTiO3 was determined. A systematic microstructural analysis and the phase equilibria diagram for the system BaTiO3–BaGeO3 was proposed in [19], in which the solubility limit of BaGeO3 in BaTiO3 was determined around 1.8 mol% and the formation of a eutectic composition at 68 mol% with a melting temperature of 1120 °C. No electrical characterization was performed. More recent articles reported the shrinkage mechanism and phase evolution of specific compositions of Ba(Ge,Ti)O3 ceramics produced by wet methods [20], [21]. By the Ge addition, the low-field dielectric analysis indicated a reduction of permittivity and a transformation of the ferro–para dielectric peak into a smeared one with respect with properties of pure BaTiO3 [17], [18], [20], [21], while an increased permittivity was reported in [22]. Higher permittivity than BaTiO3 was also reported for Si or Ge-addition, but usually being accompanied by higher losses [23]. No reports concerning the high field properties (e.g., ferroelectric switching and tunability) for Ge–BaTiO3 ceramic compounds were found, except some polarization and coercivity values in the old paper [17], without showing the loops. In the present work, we report the dielectric, tunability, and ferroelectric properties of BaTiO3 ceramics with addition of Ge: , 0.01, 0.018, 0.1, 0.68, and 1 prepared by solid-state reaction.