High resolution neutron diffraction study of structural anomalies in K_1-x(ND₄)_xD₂PO₄ mixed crystals

Mixtures of some ferroelectric and antiferroelectric crystals in a certain composition range undergo a transition to a dipolar glass state at low temperatures. Mixed crystals based on KH₂PO₄ and NH₄H₂PO₄ are the example. In the present paper, results of high resolution neutron diffraction experiments to investigate the properties of powder sample of deuterated K_0.36(ND₄)_0.64D₂PO₄ are discussed. The temperature dependence of the diffraction pattern was studied over the temperature range from 5K to 300K. It was found that the investigated sample structure is characterised by two sublattices over this temperature range. One of these sublattices is tetragonal and the other is orthorhombic. This is not a simple mechanical mixture of two phases, however. The relative intensities of the diffraction lines of the tetragonal and orthorhombic sublattices show coherent temperature variations. In addition, for both sublattices an unusual temperature behaviour of the lattice constants and diffraction line widths was observed. The observed effects are reversible. A possible mechanism of the observed anomalies is discussed.