Neutron interferometry is a method of high-accuracy determination of neutron scattering length bc. The phase shift acquired by a neutron wave that propagates through a sample inserted in one of the beams of an interferometer is proportional to the neutron wavelength. A knowledge of it precisely defines the accuracy of the determination of bc. As a solution of this problem, the non-dispersive sample geometry was proposed. In this case the phase shift is λ-independent but the very precise orientation of the sample relative to the crystal planes of the interferometer is required. To overcome this obstacle a new nondispersive phase difference method was developed which achieves an accuracy in the determination of bc of even better than 10 -4, i.e. at the level of 10 -5. This method was used for the high-accuracy measurement of the coherent scattering length of Si. Experiments carried out in HMI Berlin and NIST Gaithersburg and NPI Rez on different interferometric facilities at different neutron wavelengths proved that the coherent scattering lengths can be measured with a relative uncertainty of 0.002%.