Synthesis and study of structural and electrophysical characteristics of piezoceramic section (1 − x)(0.8PbMg1/3Nb2/3O3⋅0.2BiScO₃) ⋅ x(0.8PbTiO3⋅0.2BiScO₃) of the ternary system BiScO₃ – PbTiO₃ – PbMg1/3Nb2/3O₃