  In this paper we consider the spin 1/2 highest weight representations for the
6-vertex Yang-Baxter algebra on a finite lattice and analyze the integrable
quantum models associated to the antiperiodic transfer matrix. For these
models, which in the homogeneous limit reproduces the XXZ spin 1/2 quantum
chains with antiperiodic boundary conditions, we obtain in the framework of
Sklyanin's quantum separation of variables (SOV) the following results: I) The
complete characterization of the transfer matrix spectrum
(eigenvalues/eigenstates) and the proof of its simplicity. II) The
reconstruction of all local operators in terms of Sklyanin's quantum separate
variables. III) One determinant formula for the scalar products of separates
states, the elements of the matrix in the scalar product are sums over the SOV
spectrum of the product of the coefficients of the states. IV) The form factors
of the local spin operators on the transfer matrix eigenstates by a one
determinant formula given by simple modifications of the scalar product
formula.
